Prof. Dr.-Ing Andreas Markus Kempf

Combustion and Gas Dynamics
University of Duisburg-Essen

Contact

Hub
  • Carrier-phase DNS of detailed NOx formation in early-stage pulverized coal combustion with fuel-bound nitrogen
    Shamooni, A. and Debiagi, P. and Wang, B. and Luu, T.D. and Stein, O.T. and Kronenburg, A. and Bagheri, G. and Stagni, A. and Frassoldati, A. and Faravelli, T. and Kempf, A.M. and Wen, X. and Hasse, C.
    Fuel 291 (2021)
    Carrier-phase direct numerical simulation of detailed NOx formation in pulverized coal flames (PCC) with fuel-bound nitrogen is conducted in a 3D temporally evolving mixing layer setup where Lagrangian particles (Colombian bituminous coal) in an air stream (upper half of the domain) mix with the products of lean volatile/air combustion in the lower stream. The release of fuel-N is represented by ammonia, hydrogen cyanide, and a lumped nitrogenated tar (pyridine). Devolatilization is modeled by fitting a 2-step pyrolysis approach to the detailed heterogeneous PoliMi kinetics. A comprehensive homogeneous mechanism including all standard pathways of NOx and pyridine oxidation is adopted. Results show a partition of NO in two distinct branches of scatter plots of NO mass fraction vs. volatile mixture fraction after flame establishment, corresponding to NO in the lower stream flame region and hot spots near the upper stream. The contribution of NO2, prompt, and thermal mechanisms to total NOx is limited in the early stages of PCC. The main source of NO is fuel-N, with NH being the most important precursor. Pyridine plays an important role for NO production in the upper stream through CN formed from CHCHCN. CN and ammonia oxidation have the highest contribution to NH production. Regarding NO destruction, NO reactions with HCCO, CHi and C through the reburn process constitute the largest share. NO conversion to N2O by NH followed by conversion of N2O to N2 and NO+N→N2+O are the two most important pathways directly reducing NO to N2. © 2020 Elsevier Ltd
    view abstract10.1016/j.fuel.2020.119998
  • Detailed simulations of the DLR auto-igniting pulsed jet experiment
    Inanc, E. and Lipkowicz, J.T. and Kempf, A.M.
    Fuel 284 (2021)
    Numerical simulations of an auto-igniting pulsed jet in a vitiated co-flow experiment by DLR (German Aerospace Center) are conducted by highly-resolved large-eddy simulations using direct chemistry with an augmented reduced mechanism. The experiments consist of two operation modes: continuous injection used for code-verification and pulsed injection utilized for fundamental investigation of auto-ignition dynamics. Initially, reference one-dimensional self-igniting counter-flow flames are investigated. Then, a grid convergence study has been performed. It is shown that even a coarser grid would be sufficient to describe the ignition chemistry since the ignition kernel appears at low velocities and fuel-lean conditions in zones of low scalar dissipation rates. For the statistically steady jet, numerical predictions are in a very good agreement with the experiments, giving confidence in the applied models. For the pulsed jet, all of the predicted ignition delay times and locations are in the range of the experimental observations. Time-resolved statistics reveal that thermochemical properties of the gas in a pulsed jet achieve states that are impossible to reproduce in laminar conditions. For further analysis, hydroxyl and formaldehyde are chosen as a marker for the established flame and for the ignition, respectively. In laminar conditions, these two species are perfectly correlated. However, the unsteady dynamics of the pulsed jet invalidates the correlation between the minor species chemistry prior to ignition. This yields the discrepancy in the auto-ignition delay time and the location of the ignition kernel between different pulses, as the thermochemical state needed for the ignition occurs in a random manner. © 2020 Elsevier Ltd
    view abstract10.1016/j.fuel.2020.118947
  • Investigation of Turbulent Pulverized Solid Fuel Combustion with Detailed Homogeneous and Heterogeneous Kinetics
    Wang, B. and Shamooni, A. and Stein, O.T. and Kronenburg, A. and Kempf, A.M. and Debiagi, P. and Hasse, C.
    Energy and Fuels (2021)
    A comprehensive Euler-Lagrange framework for pulverized coal combustion using detailed multi-step heterogeneous kinetics is presented. The heterogeneous kinetics employ the POLIMI model that involves 37 species (22 solid species and 15 gas species) and 49 reactions to describe detailed pyrolysis as well as char oxidation, gasification, and annealing for a wide range of coals. The porous structure of the coal particles is considered, and the heterogeneous reactions are assumed to occur throughout the entire particle in a volume-based approach. The ordinary differential equations of the heterogeneous kinetics are integrated on each Lagrangian coal particle and predict the conversion of the raw coal components to light volatile hydrocarbons, heavy tar species, and char off-gases. Hence, the composition of the solid fuel components and the released gas changes dynamically in space and time, providing high-fidelity predictions of solid fuel combustion. The chemical conversion of the released species in the gas phase is described by a homogeneous kinetic mechanism with 76 species and 973 reactions that was reduced from the comprehensive CRECK-G-1407 kinetic mechanism. The new modeling framework is employed within carrier-phase direct numerical simulations (CP-DNS) of pulverized coal combustion in a three-dimensional turbulent mixing layer. This configuration includes the additional physics of turbulence and particle group combustion by mixing solid fuel particles suspended in a primary oxidizer stream with the products from lean volatile combustion in a secondary stream. The CP-DNS results are analyzed with and without the available set of 14 char conversion reactions, and a low degree of char conversion indicated by an increased rate of CO production is captured for particles with temperatures higher than 1800 K. The CP-DNS results from the detailed POLIMI approach feature a distinct bimodal shape of the volatile release curve and multi-regime combustion. The POLIMI data are used to evaluate the predictive capability of simpler pyrolysis models. The original competing two-step model (C2SM) by Kobayashi is investigated and shown to predict heavily delayed ignition. A new competing two-step devolatilization approach is proposed as an alternative model reduction suitable for fitting bimodal volatile release rates, such as that predicted by POLIMI. The CP-DNS using the alternative pyrolysis model faithfully captures the onset of ignition and multi-regime flame branches. Differences arise in the local tar species compositions in the gas phase as a result of the time-varying (POLIMI) and fixed (new C2SM) volatile compositions for the respective models. The flame structure is further analyzed by chemical explosive mode analysis (CEMA), and the occurrence of premixed and non-premixed flames zones is confirmed, whereas a simpler flame index analysis fails to correctly indicate the multi-regime nature of the flame. This recognition of multi-regime combustion serves as a guidance for selecting suitable conditioning variables for flamelet and other combustion submodels in large eddy simulation. © 2021 American Chemical Society.
    view abstract10.1021/acs.energyfuels.0c03479
  • Numerical Investigation of Remote Ignition in Shock Tubes
    Lipkowicz, J.T. and Nativel, D. and Cooper, S. and Wlokas, I. and Fikri, M. and Petersen, E. and Schulz, C. and Kempf, A.M.
    Flow, Turbulence and Combustion 106 (2021)
    Highly resolved two- and three-dimensional computational fluid dynamics (CFD) simulations are presented for shock-tube experiments containing hydrogen/oxygen (H2/O2) mixtures, to investigate mechanisms leading to remote ignition. The results of the reactive cases are compared against experimental results from Meyer and Oppenheim (Proc Combust Inst 13(1): 1153–1164, 1971. https://doi.org/10.1016/s0082-0784(71)80112-1) and Hanson et al. (Combust Flame 160(9): 1550–1558, 2013. https://doi.org/10.1016/j.combustflame.2013.03.026). The results of the non-reactive case are compared against shock tube experiments, recently carried out in Duisburg and Texas. The computational domain covers the end-wall region of the shock tube and applies high order numerics featuring an all-speed approximate Riemann scheme, combined with a 5th order interpolation scheme. Direct chemistry is employed using detailed reaction mechanisms with 11 species and up to 40 reactions, on a grid with up to 2.2 billion cells. Additional two-dimensional simulations are performed for non-reactive conditions to validate the treatment of boundary-layer effects at the inlet of the computational domain. The computational domain covers a region at the end part of the shock tube. The ignition process is analyzed by fields of localized, expected ignition times. Instantaneous fields of temperature, pressure, entropy, and dissipation rate are presented to explain the flow dynamics, specifically in the case of a bifurcated reflected shock. In all cases regions with locally increased temperatures were observed, reducing the local ignition-delay time in areas away from the end wall significantly, thus compensating for the late compression by the reflected shock and therefore leading for first ignition at a remote location, i.e., away from the end wall where the ignition would occur under ideal conditions. In cases without a bifurcated reflected shock, the temperature increase results from shock attenuation. In cases with a bifurcated reflected shock, the formation of a second normal shock and shear near the slip line is found to be crucial for the remote ignition to take place. Overall, the two- and three-dimensional simulations were found to qualitatively explain the occurrence of remote ignition and to be quantitatively correct, implying that they include the correct physics. © 2020, The Author(s).
    view abstract10.1007/s10494-020-00219-w
  • Robust dynamic adaptation of the Smagorinsky model based on a sub-grid activity sensor
    Hasslberger, J. and Engelmann, L. and Kempf, A. and Klein, M.
    Physics of Fluids 33 (2021)
    The present study deals with the application of a sub-grid activity sensor to an eddy viscosity type base model in the context of Large Eddy Simulation (LES). The coherent structure function is used to build this sensor in combination with explicit test filtering. The proposed sensor features two main advantages: First, it attenuates the sub-grid scale dissipation of the base model for transitional flows. Depending on local conditions, the sensor is essentially blending the eddy viscosity between zero and the value resulting from the standard Smagorinsky model. Second, the sensor rectifies the incorrect near-wall scaling of the standard Smagorinsky model. Application of the sensor requires no averaging (in the homogeneous direction) and is easy to implement, and the additional computational cost is insignificant. In order to assess the model, three different configurations have been examined: laminar-to-turbulent transition in the Taylor-Green vortex, wall-dominated channel flows, and a free planar jet flow including passive scalar mixing. Based on a posteriori LES, it has been found that the new sensor-enhanced Smagorinsky model often outperforms established eddy viscosity models from the literature, such as the standard Smagorinsky model and the sigma model, as well as the LES without the explicit sub-grid model. © 2021 Author(s).
    view abstract10.1063/5.0032117
  • A comprehensive study of flamelet tabulation methods for pulverized coal combustion in a turbulent mixing layer — Part I: A priori and budget analyses
    Wen, X. and Rieth, M. and Scholtissek, A. and Stein, O.T. and Wang, H. and Luo, K. and Kempf, A.M. and Kronenburg, A. and Fan, J. and Hasse, C.
    Combustion and Flame 216 (2020)
    In this work, a comprehensive study of flamelet tabulation methods for pulverized coal combustion in a turbulent mixing layer is conducted. At first, a priori analyses are conducted to evaluate the suitability of the premixed and non-premixed flamelet models for the studied pulverized coal flame with multiple combustion modes. Then, to clarify why a certain flamelet model does work or not work in certain regions, a more in-depth investigation of the premixed and non-premixed flamelet models is conducted through a budget analysis. The results show that the first and second derivatives in physical space can be well reproduced by the tabulated manifolds in trajectory variable space for both the premixed and non-premixed flamelet models, between which the non-premixed flamelet model performs slightly better. For the time derivative, large discrepancies can be observed, although the predicted variation trend overall follows the reference results. Through the analysis of the individual budget terms in the trajectory variable space, the individual trajectory variable's contributions to the convection and diffusion of thermo-chemical variables are quantified. Through the analysis of the individual budget terms for the sensible enthalpy and the CO mass fraction governing equations, the influences of the space transformation on the individual transport process (e.g., convection, diffusion, etc.) are clarified. Overall, the findings obtained from the budget analyses are consistent with those obtained from the a priori analyses. © 2019 The Combustion Institute
    view abstract10.1016/j.combustflame.2019.05.046
  • A simple post-processing method to correct species predictions in artificially thickened turbulent flames
    Gruhlke, P. and Inanc, E. and Mercier, R. and Fiorina, B. and Kempf, A.M.
    Proceedings of the Combustion Institute (2020)
    An efficient method for accurate species (such as CO) concentration predictions on artificially thickened flames is proposed and demonstrated for the Cambridge swirled flame. This method relies on corrections of the (side-) effects introduced by the (dynamic) flame thickening and wrinkling models, applied as an LES post-processing step. This technique provides better predictions of minor species in the inner reaction zone wherever a thickened flame model is used - provided that the flame burns in the flamelet regime, and that the interest is on in-flame data rather than on post-flame data. A demonstration for CO concentrations is given and compared to experimental evidence. It is concluded that the thickening factor correction is universal and can be applied for all species, whereas the efficiency factor correction must be adjusted for each species, and heat loss level separately. Both corrections were found to reduce the grid and code dependencies of the results. As this technique is easy to understand, easy to implement and suitable as a correction for available data, we recommend its use in all applications of thickened flame models. © 2020 The Combustion Institute.
    view abstract10.1016/j.proci.2020.06.215
  • Analysis of mixture stratification effects on unstrained laminar flames
    Inanc, E. and Chakraborty, N. and Kempf, A.M.
    Combustion and Flame 219 (2020)
    Detailed one-dimensional computations of unsteady unstrained laminar flames subjected to sinusoidal equivalence ratio perturbations are presented. The responses of the flame thickness, flame speeds, species concentrations and the species reaction rates to equivalence ratio variations are investigated. The effect of stratification is quantified by comparing the structure of a stratified flame to that of an equivalent homogeneous mixture flame at an equivalence ratio that the stratified flame experiences. The difference between a flame burning into a leaner mixture or a richer mixture yields hysteresis for consumption speed and flame thickness in the equivalence ratio space, which becomes more prominent with stronger stratification, especially when the flame propagates towards a negative equivalence ratio gradient under fuel-lean conditions. The displacement speed and its components are analysed, with the diffusion, reaction rate and cross-dissipation components all showing a strong hysteresis, but with different signs, partially cancelling each other. The phase space responses of the scalars are compared and the different phase shifts are evaluated. Interestingly, these observations were not affected by the choice of the reaction mechanism. The effects of equal and mixture-average diffusivity assumptions on the results are tested, where the latter caused two times stronger hysteresis effects: the thermo-diffusive effects of heat and products behind the flame were found to play a significant role for laminar flame propagation in stratified mixtures, even for unstrained flames. The stratified flame shows significant alteration in the species concentrations and reaction rates, especially for the minor and product species. The response of the sinusoidal oscillations is compared against cases with linear mixture stratification. Even with the absence of the compressible strain, it is demonstrated that the stratification effects heavily influence the flame properties and the treatment of thermo-physical transport properties has been demonstrated to be pivotal to the accurate prediction of this behaviour. © 2020 The Combustion Institute
    view abstract10.1016/j.combustflame.2020.06.009
  • Detailed analysis of early-stage NOx formation in turbulent pulverized coal combustion with fuel-bound nitrogen
    Wen, X. and Shamooni, A. and Stein, O.T. and Cai, L. and Kronenburg, A. and Pitsch, H. and Kempf, A. and Hasse, C.
    Proceedings of the Combustion Institute (2020)
    A carrier-phase direct numerical simulation (CP-DNS) of pulverized coal combustion in a mixing layer is performed, considering three NOx formation mechanisms (fuel-NOx, thermal-NOx and prompt-NOx). Detailed analyses, including reaction path analysis, chemical timescale analysis, and a priori and budget analyses are conducted to investigate the NOx production mechanisms and the performance of the flamelet model. Considering the high computational cost of CP-DNS, this work focuses on the early phase governed by devolatilization, where char reactions are less important. The reaction path analyses show that the principal thermal-NO reaction contributes to the net consumption of NO in fuel-bound nitrogen pulverized coal flames, which is essentially different from fuel-nitrogen-free flames. The chemical timescale analyses show that the production rates of NOx species are faster than those of major species, which confirms the suitability of the flamelet tables. The a priori analyses show that the gas temperature and major/intermediate species can be predicted well by the flamelet model, while the NOx species show significant discrepancies in certain regions. Finally, the budget analyses explain why the flamelet model performs differently for major/intermediate and NOx species. © 2020 The Combustion Institute.
    view abstract10.1016/j.proci.2020.06.317
  • Direct numerical simulations of nanoparticle formation in premixed and non-premixed flame-vortex interactions
    Cifuentes, L. and Sellmann, J. and Wlokas, I. and Kempf, A.
    Physics of Fluids 32 (2020)
    Direct numerical simulations (DNSs) of nanoparticle formation in reactive flows are challenging, and only greatly simplified DNS test-cases are possible, which help clarify the turbulence-particle-dynamics interaction and guide the necessary modeling efforts. As a basis for such studies, a new DNS database is introduced, which resolves the smallest relevant scales of the nanoparticle concentration field to obtain insights into the statistics of nanoparticle formation in reactive flows. Formation and evolution of iron oxide nanoparticles in premixed and non-premixed flames wrapped-up by a vortex have been investigated using the sectional model and direct chemistry. The DNSs capture the "engulfing"and local dilution of the particle fields. Different zones of high particle number concentration have been found in every flame, and it was shown that the thickness of these zones decreases with increasing Schmidt number, which confirms that in simulations of nanoparticle-forming turbulent reacting flows, the grid resolution has to be very fine to resolve the characteristic scale for high sections. The contributions to the change in particle concentration due to diffusion, coagulation, and nucleation have been analyzed in detail, and dominant contributions across the particle number concentration layers and across the flames have been identified. This analysis has also been carried out in terms of flat, concave, and convex iso-surface geometries, induced by the flame-vortex interaction and characterized by the curvature of the particle number concentration fields and also by the flame curvature. The results demonstrate that the flame curvature effects cannot be ignored in modeling strategies. The probability density functions for the particle number concentrations have been analyzed and quantified in terms of Shannon information entropy, which illustrates the effect of fast diffusion (and entropy production) of the smaller particles and slow diffusion (and entropy production) of the largest particles with high Schmidt numbers. In addition, the unclosed filtered or averaged agglomeration term was evaluated as a basis for future modeling efforts, showing that agglomeration rates will be underestimated by orders of magnitude unless suitable models are developed. © 2020 Author(s).
    view abstract10.1063/5.0020979
  • Experimental and numerical investigation of iron-doped flames: FeO formation and impact on flame temperature
    Nanjaiah, M. and Pilipodi-Best, A. and Lallane, M. and Fjodorow, P. and Schulz, C. and Cheskis, S. and Kempf, A. and Wlokas, I. and Rahinov, I.
    Proceedings of the Combustion Institute (2020)
    Gas-phase iron compounds strongly affect the flame structure already at very low concentrations, which implies the control of combustion efficiency, pollution formation, and materials synthesis in flames. The impact of iron pentacarbonyl on low-pressure premixed flames was investigated experimentally and numerically for a broad range of equivalence ratios. The burner was operated in top-to-bottom orientation, causing a strong effect of buoyancy on the flow field, a configuration, also known as buoyancy-opposed flame. The application of ultra-sensitive broadband intracavity laser absorption diagnostics enabled path-integrated measurements of gas-phase FeO in the particle-laden flow. Spatially-resolved temperature distributions were measured via OH laser-induced fluorescence. The measurements were complemented by detailed simulations of the down-firing flame to determine the (one-dimensional) flow field on the centerline of the burner. The experimental findings were the basis for extension of existing reaction schemes for iron-doped flames and a new skeletal scheme was proposed. Measured temperatures and normalized FeO concentrations were used to validate both the detailed and the skeletal scheme. The results of the optimization and reduction procedure helped to improve the understanding of the structure of the iron-doped flame and the role of iron-cluster formation in the interaction mechanisms which cause the flame inhibition or promotion by iron-compounds. © 2020 The Combustion Institute. Published by Elsevier Inc.
    view abstract10.1016/j.proci.2020.07.006
  • Fast Flow Field Estimation for Various Applications with A Universally Applicable Machine Learning Concept
    Leer, M. and Kempf, A.
    Flow, Turbulence and Combustion (2020)
    This paper presents an approach for the prediction of incompressible laminar steady flow fields over various geometry types. In conventional approaches of computational fluid dynamics (CFD), flow fields are obtained by solving model equations on computational grids, which is in general computationally expensive. Based on the ability of neural networks to intuitively identify and approximate nonlinear physical relationships, the proposed method makes it possible to eliminate the explicit implementation of model equations such as the Navier–Stokes equations. Moreover, it operates without iteration or spatial discretization of the flow problem. The method is based on the combination of a minimalistic multilayer perceptron (MLP) architecture and a radial-logarithmic filter mask (RLF). The RLF acts as a preprocessing step and its purpose is the spatial encoding of the flow guiding geometry into a compressed form, that can be effectively interpreted by the MLP. The concept is applied on internal flows as well as on external flows (e.g. airfoils and car shapes). In the first step, datasets of flow fields are generated using a CFD-code. Subsequently the neural networks are trained on defined portions of these datasets. Finally, the trained neural networks are applied on the remaining unknown geometries and the prediction accuracy is evaluated. Dataset generation, neural network implementation and evaluation are carried out in MATLAB. To ensure reproducibility of the results presented here, the trained neural networks and sample applications are made available for free download and testing. © 2020, The Author(s).
    view abstract10.1007/s10494-020-00234-x
  • Geometric modeling and analysis of detonation cellular stability
    Crane, J. and Shi, X. and Lipkowicz, J.T. and Kempf, A.M. and Wang, H.
    Proceedings of the Combustion Institute (2020)
    A geometric model with a low computational complexity capable of simulating detonation behavior in physical systems is proposed. In support of the geometric model development, a series of cylindrical 1D simulations with a variable size initiation kernel are performed in hydrogen-oxygen mixtures. From these 1D simulations a detonation cell stabilization mechanism is identified. The stabilization mechanism is predicated on the size of the gap between the pressure and temperature fronts at the point where the average pressure front velocity along one cell length is equal to the CJ velocity. This gap, in a multidimensional detonation, is the ignition kernel of a subsequent blast, and dictates the formation of the subsequent cell. Serial analysis of blasts in this context leads to a unique stable blast kernel size for any mixture, which, within the uncertainty of the initial kernel state, can predict the experimental cell length for mixtures considered in this study. Using a tabulation of the 1D simulations as an input, a formulation and sample results of the geometric model are shown. The geometric model can reproduce both qualitative and quantitative features of experimental detonation cellular structure. © 2020 The Combustion Institute. © 2020 The Combustion Institute.
    view abstract10.1016/j.proci.2020.06.278
  • Investigation of a High Karlovitz, High Pressure Premixed Jet Flame with Heat Losses by LES
    Gruhlke, P. and Janbazi, H. and Wlokas, I. and Beck, C. and Kempf, A.M.
    Combustion Science and Technology 192 (2020)
    Large-eddy simulations (LES) are presented for a lean preheated high pressure jet flame experiment for which detailed in situ data is available, using a finite rate chemistry (FRC) approach in a gas-turbine model combustor at high Karlovitz number. The impact of the different combustion models on the flame stabilization in the simulation is investigated and the predicted carbon monoxide (CO) and nitric oxide (NO x) emissions are analyzed. For the FRC approach, the DRM19 reaction mechanism and a new inhouse skeletal mechanism are applied. The more detailed DRM19 mechanism is extended to include OH* species, the new skeletal mechanism includes CO and NO x reaction paths. An industry relevant tabulated chemistry approach is assessed on the ability to predict this lifted flame, where the flamelet tables are calculated from the detailed GRI-3.0 reaction mechanism. A dynamic thickened flame approach is applied to resolve the flame on the numerical grid including a model for the turbulence chemistry interaction. Adiabatic and non-adiabatic simulations are compared, where the impact of heat losses due to chamber cooling and thermal radiation are considered. Velocities, temperatures, fuel mass fractions and CO and NO x mass fractions at different axial locations are in good agreement to the experiments when heat losses are considered. The significant flame lift was correctly predicted by the FRC approach with DRM19 chemistry when non-adiabatic boundary conditions were applied. This provides evidence that the flame is stabilized by flame propagation assisted by auto ignition and that ignition-delay times of mixtures composed of fresh and burnt gases need to be captured by the applied models. © 2020 Taylor & Francis Group, LLC.
    view abstract10.1080/00102202.2020.1781101
  • Particle history from massively parallel large eddy simulations of pulverised coal combustion in a large-scale laboratory furnace
    Rabaçal, M. and Costa, M. and Rieth, M. and Kempf, A.M.
    Fuel 271 (2020)
    A study on the coal particle history during combustion in a large-scale furnace using large eddy simulation is presented. The massively parallel execution produces a high-resolution representation of the fluid mixing and particle dispersion throughout the whole computational domain. The coal combustion is modelled using well-established, cost-effective combustion models. A specific feature of the devolatilization model is the optimisation of the kinetic constants for the furnace operating condition, which were obtained through an iterative procedure between particle heating rates from full large eddy simulation runs and the advanced model Chemical Percolation Devolatilization. In a previous work, we showed that the classical coal combustion models, when used in a high-resolution massively parallel large eddy simulation, lead to satisfactory predictions of the in-flame gas properties, namely gas temperature and gas species concentrations. In this work, we went beyond the comparisons between gas phase measurements and predictions. Single particles were tracked over time and instantaneous ensambles were collected to obtain a better understanding of the conditions that coal particles are subjected to in the investigated test case. The particles trajectory, combustion history and instantaneous state distribution were analysed. The volatile flame features were related with the characteristic trajectory of different sized particles. The combustion history revealed that particles are subjected to large variations of heating rates, including very short sequential periods alternating between heating and cooling during the early stages of combustion, due to the high turbulence intensity in the near burner region. Finally, the state distribution of the ensamble provided a global picture of the instantaneous coal combustion process. © 2020 Elsevier Ltd
    view abstract10.1016/j.fuel.2020.117587
  • Regularized, parameter free scale similarity type models for Large Eddy Simulation
    Klein, M. and Ketterl, S. and Engelmann, L. and Kempf, A. and Kobayashi, H.
    International Journal of Heat and Fluid Flow 81 (2020)
    The fidelity of Large Eddy Simulations (LES) depends strongly on the closures of the sub-grid scale (SGS) stress tensor. Although it is well known that the SGS stresses in LES are not aligned with the strain rate tensor, the most widely used models are still of eddy viscosity type, due to their robust behavior in LES and reasonable performance in a posteriori testing. The unstable behavior of more advanced anisotropic models, that is typically found in LES, has been attributed to either the fact that these models provide backscatter or to the fact that they do not provide a sufficient amount of dissipation. Based on recent advances in the field, an alternative modeling strategy is suggested, which can be used to regularize an arbitrary anisotropic (e.g. scale similarity type) model. The resulting model is easy to implement, can be written in compact form and is free of model parameters. The model has been tested a-posteriori and results are presented for a Taylor-Green-Vortex, a free plane jet and a turbulent channel flow of friction Reynolds numbers 395, 590 and 934. The results are compared to well-known eddy viscosity models and when applicable, to simulations without explicit LES model. The new model exhibits good performance for a variety of mesh resolutions and for all configurations. Furthermore, a-priori analysis results in the context of liquid atomization indicate that the model might be suitable as well in more complex physical scenarios. The a-priori analysis performance of the model is found to be nearly equivalent to the underlying structural anisotropic model in terms of its correlation coefficient, but the model is free of backscatter and provides good stability in LES. © 2019 Elsevier Inc.
    view abstract10.1016/j.ijheatfluidflow.2019.108496
  • 3D evolutionary reconstruction of scalar fields in the gas-phase
    Unterberger, A. and Kempf, A. and Mohri, K.
    Energies 12 (2019)
    An evolutionary reconstruction technique (ERT) was developed for three-dimensional (3D) reconstruction of luminescent objects, in particular turbulent flames for the first time. The computed tomography (CT) algorithm is comprised of a genetic algorithm (GA) and a ray-tracing software. To guide the reconstruction process, a mask is introduced. It uses a Metropolis algorithm (MA) to sample locations where specific genetic operators can be applied. Based on an extensive parameter study, performed on several types of phantoms, the ability of our algorithm for 3D reconstructions of fields with varying complexities is demonstrated. Furthermore, it was applied to three experiments, to reconstruct the instantaneous chemiluminescence field of a bunsen flame, a highly turbulent swirl flame and the turbulent Cambridge-Sandia stratified flame. Additionally, we show direct and quantitative comparison to an advanced computed tomography of chemiluminescence (CTC) method that is based on an algebraic reconstruction technique (ART). The results showed good agreement between CTC and ERT using both phantom data from flame simulations, and experimental data. © 2019 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).
    view abstract10.3390/en12112075
  • A hybrid flamelet finite-rate chemistry approach for efficient LES with a transported FDF
    Rieth, M. and Chen, J.-Y. and Menon, S. and Kempf, A.M.
    Combustion and Flame 199 (2019)
    A hybrid method combining flamelet tabulation with transported filtered density function (FDF) finite rate chemistry has been developed and applied to large eddy simulation (LES) of the Sydney/Sandia piloted turbulent flame with inhomogeneous inlets. Aiming to improve the efficiency while maintaining accuracy, the hybrid method applies the computationally expensive Lagrangian particles representing FDF transport and direct chemistry only in selected, dynamically varying locations. The rest of the domain is treated with flamelet chemistry based on the Eulerian fields and a presumed top-hat PDF closure. The method relies on consistency between Eulerian and Lagrangian fields through robust, accurate coupling and consistent modeling. The performance of the hybrid model is verified by an extensive comparison against the experiment and the ‘pure’ models, i.e., the (a) flamelet LES with presumed FDF, (b) flamelet LES with transported FDF, and (c) direct chemistry LES with transported FDF. Finite rate chemistry is found to improve species predictions over flamelet chemistry and the hybrid method is found to reproduce these improvements by using particles with finite rate chemistry only at locations where the flamelet is not sufficient, promising a reduced computational cost. © 2018
    view abstract10.1016/j.combustflame.2018.09.019
  • Detailed simulation of iron oxide nanoparticle forming flames: Buoyancy and probe effects
    Sellmann, J. and Rahinov, I. and Kluge, S. and Jünger, H. and Fomin, A. and Cheskis, S. and Schulz, C. and Wiggers, H. and Kempf, A. and Wlokas, I.
    Proceedings of the Combustion Institute 37 (2019)
    Nanoparticle formation in flames is strongly influenced by the residence-time-temperature history inside the flame. We study how the temperature history can be intentionally modified by orienting flames either in an upward-firing or downward-firing configuration. We also investigate the influence of unintended residence-time modifications caused by sampling nozzles. These phenomena are investigated by experiments and simulations for the synthesis of iron oxide nanoparticles from premixed iron-pentacarbonyl-doped hydrogen/oxygen flat flames. The experiments apply molecular-beam sampling with a particle mass spectrometer to measure particle sizes and a quartz microbalance to detect the presence of condensed matter. The simulations rely on a finite-rate chemistry approach with species-specific diffusion, particle dynamics are described by a bi-modal population balance model. It is demonstrated that the downward-burning flame forms a detached stagnation point, causing longer residence times at elevated temperature than an upward- or horizontally firing flame, permitting the growth of larger particles. These iron oxide particles are eventually formed in the recombination zone of the flame, but no condensed matter was found in the reaction zone. The experiments also observed the formation of particles in the preheat zone, but their composition and all aspects of their disappearance remain uncertain. Current models do, however, suggest the formation of iron particles and their subsequent evaporation and combustion. © 2018 The Combustion Institute.
    view abstract10.1016/j.proci.2018.06.041
  • Evaluation of a flamelet/progress variable approach for pulverized coal combustion in a turbulent mixing layer
    Rieth, M. and Kempf, A.M. and Stein, O.T. and Kronenburg, A. and Hasse, C. and Vascellari, M.
    Proceedings of the Combustion Institute 37 (2019)
    A steady flamelet/progress variable (FPV) approach for pulverized coal flames is employed to simulate coal particle burning in a turbulent shear and mixing layer. The configuration consists of a carrier-gas stream of air laden with coal particles that mixes with an oxidizer stream of hot products from lean combustion. Carrier-phase DNS (CP-DNS) are performed, where the turbulent flow field is fully resolved, whereas the coal is represented by Lagrangian point particles. CP-DNS with direct chemistry integration is performed first and provides state-of-the-art validation data for FPV modeling. In a second step the control variables for FPV are extracted from the CP-DNS and used to test if the tabulated manifold can correctly describe the reacting flow (a priorianalysis). Finally a fully coupled a posteriori FPV simulation is performed, where only the FPV control variables are transported, and the chemical state is retrieved from the table and fed back to the flow solver. The a priori results show that the FPV approach is suitable for modeling the complex reacting multiphase flow considered here. The a posteriori data is similarly in good agreement with the reference CP-DNS, although stronger deviations than a priori can be observed. These discrepancies mainly appear in the upper flame (of the present DNS), where premixing and highly unsteady extinction and re-ignition effects play a role, which are difficult to capture by steady non-premixed FPV modeling. However, the present FPV model accurately captures the lower, more stable flame that burns in non-premixed mode. © 2018 The Combustion Institute.
    view abstract10.1016/j.proci.2018.05.150
  • Evolutionary Camera Pose Estimation of a Multi-Camera Setup for Computed Tomography
    Unterberger, A. and Menser, J. and Kempf, A. and Mohri, K.
    Proceedings - International Conference on Image Processing, ICIP 2019-September (2019)
    We have developed an optimization procedure based on a genetic algorithm (GA) to estimate the extrinsic camera parameters of a multi-camera setup with a 3-dimensional (3D) target. Computed tomography (CT) is an imaging technique that uses projections (images) of a 3D field, to infer volumetric information about the measured entity. The CT inversion process relies on an accurately calibrated measurement arrangement in relation to a common reference coordinate system. Here, simultaneous projections of a 3D calibration target are obtained from a setup containing 30 CCD cameras. The calibration procedure was tested in a numerical study, and a CT experiment was conducted for assessing the quality of the calibration. Reconstructions of a turbulent flame were achieved using the calibration procedure and the computed tomography of chemiluminescence (CTC) technique. © 2019 IEEE.
    view abstract10.1109/ICIP.2019.8804398
  • Flamelet tabulation methods for solid fuel combustion with fuel-bound nitrogen
    Wen, X. and Debiagi, P. and Stein, O.T. and Kronenburg, A. and Kempf, A.M. and Hasse, C.
    Combustion and Flame 209 (2019)
    This work develops flamelet tabulation methods for solid fuel combustion with and without fuel-bound nitrogen based on computations in particle-laden counterflow flames. The proposed flamelet tabulation methods for NO prediction are evaluated for two different coal types. The results show that extracting the NO mass fraction from the flamelet library directly results in significant discrepancies in the studied fuel nitrogen-free flame, while reasonable predictions can be obtained by using the same method in the fuel-bound nitrogen flames for both coal types. In contrast, solving a transport equation for the NO mass fraction tends to improve the NO prediction. The NO concentration is well predicted by splitting the NO source term into a formation part and a rescaled consumption part. The best NO prediction is obtained by extracting the formation part and the rescaled consumption part with a modified reaction progress variable. © 2019 The Combustion Institute
    view abstract10.1016/j.combustflame.2019.07.039
  • Fully-resolved simulations of coal particle combustion using a detailed multi-step approach for heterogeneous kinetics
    Tufano, G.L. and Stein, O.T. and Kronenburg, A. and Gentile, G. and Stagni, A. and Frassoldati, A. and Faravelli, T. and Kempf, A.M. and Vascellari, M. and Hasse, C.
    Fuel (2019)
    Fully-resolved simulations of the heating, ignition, volatile flame combustion and char conversion of single coal particles in convective gas environments are conducted and compared to experimental data (Molina and Shaddix, 2007). This work extends a previous computational study (Tufano et al., 2016) by adding a significant level of model fidelity and generality, in particular with regard to the particle interior description and heterogeneous kinetics. The model considers the elemental analysis of the given coal and interpolates its properties by linear superposition of a set of reference coals. The improved model description alleviates previously made assumptions of single-step pyrolysis, fixed volatile composition and simplified particle interior properties, and it allows for the consideration of char conversion. The results show that the burning behavior is affected by the oxygen concentration, i.e. for enhanced oxygen levels ignition occurs in a single step, whereas decreasing the oxygen content leads to a two-stage ignition process. Char conversion becomes dominant once the volatiles have been depleted, but also causes noticeable deviations of temperature, released mass, and overall particle conversion during devolatilization already, indicating an overlap of the two stages of coal conversion which are usually considered to be consecutive. The complex pyrolysis model leads to non-monotonous profiles of the combustion quantities which introduce a minor dependency of the ignition delay time τign on its definition. Regardless of the chosen extraction method, the simulations capture the measured values of τign very well. © 2018 The Authors
    view abstract10.1016/j.fuel.2018.11.139
  • Large-eddy simulation of acoustic flame response to high-frequency transverse excitations
    Sharifi, V. and Kempf, A.M. and Beck, C.
    AIAA Journal 57 (2019)
    The objective of this numerical study is to analyze the excitation mechanisms of high-frequency thermoacoustic instabilities. Two distinct acoustic wave/flame interaction scenarios are considered, which can arise in a technical combustion system. First, an acoustic oscillation is enforced with the pressure antinode at the burner centerline, termed as pressure forcing. Second, a transverse acoustic oscillation is enforced with the velocity antinode at the burner centerline, termed as velocity forcing. The excitations are created by the interaction of two incident acoustic waves for a range of excitation frequencies of practical relevance. The susceptibility of the flame to acoustic oscillations is quantified using the Rayleigh index. Detailed postprocessing is performed to investigate the underlying thermoacoustic mechanisms and to quantify the contributions of each possible mechanism to the total Rayleigh index of the system as a function of different excitation frequencies. It is observed that the main driving mechanism with pressure forcing can be related to the density oscillation and that the main driving mechanism with velocity forcing can be related to the flame surface oscillation relative to the position of the acoustic pressure node. For both forcing cases, the laminar flame speed is varied to change the length of the flame. With pressure forcing, the highest Rayleigh index is found for a specific flame length, associated to a specific laminar flame speed. With velocity forcing, the Rayleigh index increases when the flame speed is reduced and the flame length is increased, respectively. © 2018 by the American Institute of Aeronautics and Astronautics, Inc.
    view abstract10.2514/1.J056818
  • Large-Eddy Simulation of Sandia Flame D with Efficient Explicit Filtering
    Bertels, A. and Kober, B. and Rittler, A. and Kempf, A.
    Flow, Turbulence and Combustion (2019)
    A uniform Gaussian filter has been applied explicitly to the LES conservation equations for mass, momentum and mixture to simulate a piloted non-premixed methane-air flame (Sandia Flame D). Using a basic property of exponential functions, the three dimensional Gaussian filter is decomposed into the product of three one dimensional filters, greatly reducing the cost of filtering. Seven simulations on three different grids have been performed to investigate the influence of grid refinement with a purely implicit filter, the effects of explicit filtering with increasing filter width and the effect of grid refinement at constant filter-size. Overall, consistent results have been achieved at a cost that is moderate with implicit or explicit filtering, so that explicit filtering can be applied in cases where the numerical error should be independent of the modelling error. © 2019, Springer Nature B.V.
    view abstract10.1007/s10494-018-9997-0
  • Local entrainment velocity in a premixed turbulent annular jet flame
    Cifuentes, L. and Kempf, A. and Dopazo, C.
    Proceedings of the Combustion Institute 37 (2019)
    The local entrainment velocity of the enstrophy interfaces of a methane-air turbulent premixed turbulent annular jet flame stabilized on a bluff-body burner has been investigated using a high-fidelity flame-resolved three-dimensional simulation. The enstrophy (inner and outer) and the scalar interfaces have been defined and characterized by their propagation speeds, VE and Sd, relative to the fluid flow. Mean values (?Sd?|c? and ?VE?|c?) conditioned on the reaction progress variable c have been obtained. A thin layer (near the enstrophy interfaces) has been used to compute mean values (?VE?|E?, ?Sd?|E?, and its different contributions) conditional upon enstrophy E. At the inner interface, results indicate that ?Sd?|c?>?0 and ?Sd?|E?>?0 (entrainment of fresh reactants into the flame front and hot products), while ?VE?|c?<?0 and ?VE?|E?<?0 (entrainment of hot products into the reacting jet across the inner enstrophy interface). The outer enstrophy interface displays ?VE?|E?>?0 (ambient gases are predominantly entrained into the jet of reactants), which implies a lean mixture in its neighborhood. These preliminary results aim at understanding the physical mechanisms of flame anchoring, in terms of entrainments of either hot products or fresh reactants into the diffusive-reactive region. Local geometries of the inner and outer interfaces have also been examined, through the computation of joint probability density functions of the mean curvature km and Gauss curvature kg of the iso-enstrophy surfaces, and through ?VE?|km, kg? at the inner and outer interfaces. This information has subsequently been used to discuss the physics of how the turbulent entrainment process affects premixed flames.
    view abstract10.1016/j.proci.2018.07.031
  • Numerical study of a pulsed auto-igniting jet flame with detailed tabulated chemistry
    Inanc, E. and Kempf, A.M.
    Fuel 252 (2019)
    This work investigates an auto-igniting impulsively started jet flame issuing into hot and vitiated co-flow by Large Eddy Simulation (LES)with multidimensional detailed tabulated chemistry. The experiment from DLR (German Aerospace Center, Stuttgart)is reproduced. The combustion model includes the scalar dissipation rate effects and pressure dependency, where the pressure is used to couple the tabulated chemistry to the compressible solver. We identify the effect of (weak)pressure perturbations and analyze auto-ignition (AI)properties in pulsed flames and their variations on different realizations. For the former task, two separate simulations are performed in density- and pressure-based formulations with the same boundary conditions. A good agreement was achieved between both of the solvers and the experiments for the statistically steady jet, providing evidence that the combustion model is suitable for the pulsed jet. Both solvers managed to describe the mixing dynamics well. However, they slightly underpredicted the lift-off height of the flame, where we suspect a minor overestimation of the flame propagation speeds. The transient jet was simulated for 20 pulse cycles to achieve pulse statistics of AI. The estimated delay times and the location of the AI matched experimental observations. The AI kernels always emerged on the lean-side of the mixing layer at the so-called most-reactive mixture fraction, and at various heights above the burner for each pulse cycle. It was observed that AI was extremely sensitive to the local temperature field, where a slight temperature variation caused a significant change in AI delay time and location. Finally, the statistics showed slight variations between the pulses for the most-reactive mixture fraction, likely caused by turbulent fluctuations of the flow field. Overall, the results show that the tabulated chemistry model is able to reproduce the AI delay time and location with a satisfactory agreement. © 2019 Elsevier Ltd
    view abstract10.1016/j.fuel.2019.04.074
  • SpraySyn-A standardized burner configuration for nanoparticle synthesis in spray flames
    Schneider, F. and Suleiman, S. and Menser, J. and Borukhovich, E. and Wlokas, I. and Kempf, A. and Wiggers, H. and Schulz, C.
    Review of Scientific Instruments 90 (2019)
    In many scientific communities, the definition of standardized experiments has enabled major progress in process understanding. The investigation of the spray-flame synthesis of nanoparticles at a well-defined standard burner by experiment and simulation makes it possible to produce a comprehensive data set with various established and novel measuring methods. In this work, we introduce the design of the SpraySyn burner as a new standard for a free-jet type burner that offers well-defined and simulation-friendly boundary conditions and geometries as well as accessibility for optical diagnostics. A combustible precursor solution is fed through a centrally located capillary and aerosolized with an oxygen dispersion gas flow. The spray flame is stabilized by a premixed flat methane/oxygen pilot flame fed via a porous bronze matrix surrounded by a stabilizing nitrogen coflow emanating through the same porous matrix, providing easy-to-calculate boundary conditions for simulations. This burner design enables the use of a wide choice of solvents, precursors, and precursor combinations. Best-practice operating instructions and parameters are given, and large-eddy simulations are performed demonstrating the suitability of the SpraySyn burner for computational fluid dynamics simulations. For ensuring reproducible operation across labs, we define a consumer-camera-based flame characterization scheme for the quantitative assessment of the flame geometry such as flame length, diameter, tilt angle, and photometric distribution of visible chemiluminescence along the center axis. These parameters can be used for benchmarking the pilot and spray flame by each user of the SpraySyn burner with the reference flames. © 2019 Author(s).
    view abstract10.1063/1.5090232
  • Studying transient jet flames by high-resolution LES using premixed flamelet chemistry
    Inanc, E. and Proch, F. and Kempf, A.M.
    ERCOFTAC Series 25 (2019)
    A transient piloted turbulent non-premixed methane jet flame approaching its blow-off limit is numerically studied by high-resolution Large-Eddy Simulations (LES). In the statistically steady jet phase, the high turbulence intensity leads to local flame extinction and re-ignition events. During the transient phase, the pulsation leads to a global flame extinction soon after the blow-off velocity is reached. The flame then re-ignites when the strain is relaxed. To model turbulent combustion with a minimum set of equations in order to reduce the computational effort, a tabulated detailed chemistry approach is tested. © Springer Nature Switzerland AG 2019.
    view abstract10.1007/978-3-030-04915-7_32
  • Analysis of flame curvature evolution in a turbulent premixed bluff body burner
    Cifuentes, L. and Dopazo, C. and Sandeep, A. and Chakraborty, N. and Kempf, A.
    Physics of Fluids 30 (2018)
    The physical mechanisms responsible for flame curvature evolution of a methane-air premixed flame attached to a bluff-body burner have been investigated using a high-fidelity flame-resolved three-dimensional simulation database. The contributions to the mean curvature generation due to the fluid flow motion and due to a combination of flow and flame propagation induced strain rates have been analyzed in detail and dominant contributions in different zones (reactants, flame, and products) of the flame have been identified. The effect of fluid flow on the mean curvature evolution is important on the unburned gas side, whereas the flame propagation dominates the mean curvature evolution in the reaction region and toward the hot products. The statistical contributions of the mean curvature transport equation have been analyzed in terms of the iso-scalar surface geometry, characterized by the mean and Gauss curvatures. This information has subsequently been used to provide physical insights into the dominant mechanisms of curvature evolution for different flame topologies. © 2018 Author(s).
    view abstract10.1063/1.5044525
  • Analysis of mild ignition in a shock tube using a highly resolved 3D-LES and high-order shock-capturing schemes
    Lipkowicz, J.T. and Wlokas, I. and Kempf, A.M.
    Shock Waves (2018)
    A highly resolved three-dimensional large-eddy simulation (LES) is presented for a shock tube containing a stoichiometric hydrogen–oxygen (H 2/O 2) mixture, and the results are compared against experimental results. A parametric study is conducted to test the effects of grid resolution, numerical scheme, and initial conditions before the 3D simulations are presented in detail. An approximate Riemann solver and a high-order interpolation scheme are used to solve the conservation equations of the viscous, compressible fluid and to account for turbulence behind the reflected shock. Chemical source terms are calculated by a finite-rate model. Simultaneous results of pseudo-Schlieren, temperature, pressure, and species are presented. The ignition delay time is predicted in agreement with the experiments by the three-dimensional simulations. The mechanism of mild ignition is analysed by Lagrangian tracer particles, tracking temperature histories of material particles. We observed strongly increased temperatures in the core region away from the end wall, explaining the very early occurrence of mild ignition in this case. © 2018, Springer-Verlag GmbH Germany, part of Springer Nature.
    view abstract10.1007/s00193-018-0867-4
  • Carrier-Phase DNS of biomass particle ignition and volatile burning in a turbulent mixing layer
    Rieth, M. and Rabaçal, M. and Kempf, A.M. and Kronenburg, A. and Stein, O.T.
    Chemical Engineering Transactions 65 (2018)
    Direct numerical simulations (DNS) of a three-dimensional turbulent mixing layer are performed to study the volatile ignition and combustion behavior of biomass under conditions relevant for industrial applications. The DNS is designed such that it resolves the flame and all turbulent scales, but reverts to a point-particle description to avoid the resolution of individual particle boundary layers. The biomass particles are seeded in an air stream and mix with hot products from a second stream. The particles are heated up as they mix with the hot gases in the developing turbulent mixing layer. This is followed by devolatilization and volatile combustion. The volatile gas composition is modeled based on the composition and properties of the biomass particles. The gas phase is described by a reduced mechanism with 59 species and 462 reactions derived from the CRECK primary reference fuel and biomass mechanisms. The simulations are performed with the in-house LES/DNS code PsiPhi, coupled to Cantera to evaluate gas phase kinetics. The data is analyzed in terms of instantaneous contour plots of relevant quantities as well as spatially averaged statistics. The results are compared to a case of coal burning in the identical mixing layer setup such that differences and similarities between the ignition and burning behavior of biomass and coal can be analyzed. Both cases feature only small amounts of char conversion, hence the focus lies on volatile combustion. While zero-dimensional reactor calculations predict an earlier ignition of the volatiles from biomass for some conditions and biomass particles heat up faster, it is found that biomass ignites later than coal in the mixing layer. This is associated with the higher stoichiometric mixture fraction and lower heat release of the volatile-air mixture from biomass. Copyright © 2018, AIDIC Servizi S.r.l.
    view abstract10.3303/CET1865007
  • Carrier-phase DNS of pulverized coal particle ignition and volatile burning in a turbulent mixing layer
    Rieth, M. and Kempf, A.M. and Kronenburg, A. and Stein, O.T.
    Fuel 212 (2018)
    Direct numerical simulations (DNS) of a three-dimensional turbulent mixing layer are performed, where coal particles seeded in an air stream mix with hot lean combustion products in a second stream. This case mimicks conditions of a pulverized coal flame stabilized by hot products such as found in industrial furnaces. Particles are heated up by the hot gases and devolatilize, followed by volatile combustion in the gas phase. The carrier-phase DNS resolves all relevant scales of the fluid phase except the boundary layers around individual particles. The simulation results are assessed in terms of instantaneous contour plots of relevant quantities, spatially averaged statistics, scatter plots and (pseudo-)flamelets. The analysis provides insight into the mechanisms of solid particle ignition and burning stabilized by hot combustion products, as well as the flame structure and combustion mode. It is shown that ignition initially occurs at very lean conditions when particles are entrained in the hot gases. Subsequently volatile combustion proceeds in non-premixed as well as premixed combustion modes, characterized by means of the flame index, with an overall higher heat release in non-premixed zones. At late times two flames can be clearly distinguished, an upper flame burning into the air carrying the particles and a lower flame burning into the lean products. The latter flame shows a pure non-premixed behavior, while the former illustrates a complex flame structure with both premixed and non-premixed modes, as well as flame quenching. The DNS database and initial analysis lay the foundation for future systematic studies in similar configurations and support the development of models suitable for the combustion of solid fuel particles. © 2017 Elsevier Ltd
    view abstract10.1016/j.fuel.2017.09.096
  • Coal particle volatile combustion and flame interaction. Part I: Characterization of transient and group effects
    Tufano, G.L. and Stein, O.T. and Wang, B. and Kronenburg, A. and Rieth, M. and Kempf, A.M.
    Fuel 229 (2018)
    The heat-up, devolatilization, ignition and volatile combustion of single coal particles and particle arrays in laminar and turbulent flow are investigated by direct numerical simulation (DNS). The first part of the paper considers the transient evolution and group effects in laminar flows, the second part will report the effects of particle Reynolds number and turbulence. The conditions for DNS are extracted from an accompanying large eddy simulation (LES) of a semi-industrial coal furnace. The DNS fully resolves the particle boundary layers, the flame thickness and the smallest flow scales. Particle heat-up is captured by solving for intra-particle heat transfer, while devolatilization and volatile combustion are described by specific particle boundary conditions and detailed homogeneous chemistry. The transient physico-chemical processes around the particle(s) are characterized for a single particle first, before the interactions of several particles are studied. The analysis of arrays of infinite particle layers shows a strong dependence of the flame interaction on the inter-particle distance Lx. In particular, different combustion regimes are observed for different Lx (for a fixed particle Reynolds number Rep), ranging from isolated burning of the particles for large Lx to group combustion for small Lx, and spanning a wide range of global equivalence ratios from very lean (large Lx) to very rich (small Lx). The regime transition affects the surface temperature and devolatilization rate history of the particles. Models for the mixture fraction distributions in the particle wake are provided, based on the analogous problem of droplet evaporation. © 2018 Elsevier Ltd
    view abstract10.1016/j.fuel.2018.02.105
  • Coal particle volatile combustion and flame interaction. Part II: Effects of particle Reynolds number and turbulence
    Tufano, G.L. and Stein, O.T. and Wang, B. and Kronenburg, A. and Rieth, M. and Kempf, A.M.
    Fuel 234 (2018)
    Direct numerical simulation (DNS) is performed to characterize volatile combustion of isolated coal particles and closely spaced particle ensembles in laminar and turbulent flow. In part I of the paper the transient evolution of devolatilization and group flame effects were studied, Tufano et al. (2018). This analysis was limited to laminar flows and relatively low particle Reynolds numbers Rep. Here, we investigate the effect of large Rep and considerable levels of turbulence on the devolatilization and burning behavior of the particles. The complex physico-chemical interactions during PCC are characterized for laminar flow conditions first, before arrays of infinite particle layers are subjected to turbulence. Increasing Rep in laminar flow leads to delayed ignition of single particles with local extinction due to high upstream scalar dissipation rates and the formation of wake flame structures downstream of the particle. An attempt is made to recover the single particle results with a standard steady laminar flamelet approach, which is shown to work well at low Rep, but fails at high Reynolds numbers, where multi-dimensional effects occur and must be incorporated into flamelet modeling. It is found that applying standard film theory to model the effect of convection on devolatilization rates can lead to qualitatively wrong trends and up to 66% error in the peak devolatilization rate compared to the DNS results at Rep=8. The analysis of particle arrays in laminar flow shows a strong dependence of flame interactions on the values of Rep due to the different extent of the particle wakes. The occurrence of significant levels of turbulence introduces a wide range of additional chemical states due to the randomness of the turbulent fluctuations that can either act to increase or decrease the local strain, in turn weakening or enhancing particle interactions. For the studied conditions turbulence slightly promotes the mass release from the most upstream particle set, but considerably delays the volatile release from the downstream particles, which is explained by the different extents and degrees of interaction of the up- and downstream volatile flames. The present results are considered useful for the development of LES sub-grid scale combustion models for pulverized coal flames, such as flamelets and others. © 2018 Elsevier Ltd
    view abstract10.1016/j.fuel.2018.07.054
  • Instantaneous 3D flame imaging by background-oriented schlieren tomography
    Grauer, S.J. and Unterberger, A. and Rittler, A. and Daun, K.J. and Kempf, A.M. and Mohri, K.
    Combustion and Flame 196 (2018)
    We apply background-oriented schlieren (BOS) imaging with computed tomography to reconstruct the instantaneous refractive index field of a turbulent flame in 3D. In BOS tomography, a network of cameras are focused through a variable index medium (such as a flame) onto a background of patterned images. BOS data consist of pixel-wise “deflections” between a reference and distorted image, caused by variations in the refractive index along the path between the camera and background. Multiple simultaneous BOS images, each from a unique perspective, are combined with a tomography algorithm to reconstruct the refractive index distribution (or optical density) in the probe volume. This quantity identifies the edges of the wrinkled turbulent flame surface. This is the first application of BOS imaging to flame tomography, setting the stage for low-cost 3D flame thermometry. Tomography is carried out within the Bayesian framework, using Tikhonov and total variation (TV) priors. The TV prior is more compatible with the abrupt spatial variation in the refractive index field caused by the flame front. We demonstrate the suitability of TV regularization using a proof-of-concept simulation of BOS tomography on an LES phantom. The technique was then used to reconstruct the instantaneous 3D refractive index field of an unsteady natural gas/air flame from a Bunsen burner using a 23-camera setup. Our results show how BOS tomography can capture and visualize 3D features of a flame and provide benchmark data for simulations. © 2018 The Combustion Institute
    view abstract10.1016/j.combustflame.2018.06.022
  • Modelling heat loss effects in high temperature oxy-fuel flames with an efficient and robust non-premixed flamelet approach
    Wollny, P. and Rogg, B. and Kempf, A.
    Fuel 216 (2018)
    The non-premixed steady flamelet model is extended by two simple, robust and effective heat loss modelling approaches. The heat release damping (HRD) approach decreases the chemical source term in the energy equation by a constant factor, while the artificial radiation (AR) approach introduces an augmented temperature dependent radiative source term. The models are tested in a simulation of the 0.78 MWth IFRF (International Flame Research Foundation) pilot scale, non-premixed, natural gas/oxygen flame. Both approaches are applied in steady laminar flamelet calculations with detailed chemistry, tabulating the thermo-chemical state as a function of mixture fraction and normalised heat loss. The turbulence-chemistry interaction is modelled using the β-PDF approach. An enthalpy transport equation is solved to keep track of the heat loss, while radiative heat transfer is calculated by the P-1 model. We observe that the major species, temperature, velocities and velocity fluctuations show a good agreement with the available experimental data. The heat loss modelling yields a significant improvement over the adiabatic model. Interestingly, both heat loss models (HRD and AR) show negligible differences in the simulations of the turbulent flame and permit to apply the steady laminar flamelet model to oxy-fuel processes in a simple, robust and user friendly manner. © 2017 Elsevier Ltd
    view abstract10.1016/j.fuel.2017.11.127
  • Prediction of CO and NOx pollutants in a stratified bluff body burner
    Gruhlke, P. and Proch, F. and Kempf, A.M. and Dederichs, S. and Beck, C. and Mahiques, E.I.
    Journal of Engineering for Gas Turbines and Power 140 (2018)
    The major exhaust gas pollutants from heavy duty gas turbine engines are CO and NOx. The difficulty of predicting the concentration of these combustion products originates from their wide range of chemical time scales. In this paper, a combustion model that includes the prediction of the carbon monoxide and nitric oxide emissions is tested. Large eddy simulations (LES) are performed using a compressible code (OpenFOAM). A modified flamelet generated manifolds (FGM) approach is applied with an artificially thickened flame approach (ATF) to resolve the flame on the numerical grid, with a flame sensor to ensure that the flame is only thickened in the flame region. For the prediction of the CO and NOx emissions, pollutant species transport equations and a second, CO based, progress variable are introduced for the flame burnout zone to account for slow chemistry effects. For the validation of the models, the Cambridge burner of Sweeney et al. (2012, "The Structure of Turbulent Stratified and Premixed Methane/Air Flames-I: Non-Swirling Flows," Combust. Flame, 159, pp. 2896-2911; 2012, "The Structure of Turbulent Stratified and Premixed Methane/Air Flames-II: Swirling Flows," Combust. Flame, 159, pp. 2912-2929.) is employed, as both carbon monoxide and nitric oxide [Apeloig et al. (2016, "PLIF Measurements of Nitric Oxide and Hydroxyl Radicals Distributions in Swirl Stratified Premixed Flames," 18th International Symposium on the Application of Laser and Imaging Techniques to Fluid Mechanics, Lisbon, Portugal, July 4-7.)] data are available. Copyright © 2018 by ASME.
    view abstract10.1115/1.4039833
  • Response surface and group additivity methodology for estimation of thermodynamic properties of organosilanes
    Janbazi, H. and Hasemann, O. and Schulz, C. and Kempf, A. and Wlokas, I. and Peukert, S.
    International Journal of Chemical Kinetics 50 (2018)
    Group additivity methods simplify the determination of thermodynamic properties of a wide range of chemically related species involved in detailed reaction schemes. In this paper, we expand Benson's group additivity method to organosilanes. Based on quantum-chemical calculations, the thermodynamic data of 22 stable silicon-organic species are calculated, presented in the form of NASA polynomials, and compared to the available experimental data. Based on this theoretical database, a complete set of 24 Si- and C-atom-centered, single-bonded and nonradical group additivity values for enthalpy of formation, standard entropy, and heat capacity at temperatures from 200 to 4000 K is derived through unweighted multivariate linear regression. © 2018 Wiley Periodicals, Inc.
    view abstract10.1002/kin.21192
  • Statistics of strain rates and surface density function in a flame-resolved high-fidelity simulation of a turbulent premixed bluff body burner
    Sandeep, A. and Proch, F. and Kempf, A.M. and Chakraborty, N.
    Physics of Fluids 30 (2018)
    The statistical behavior of the surface density function (SDF, the magnitude of the reaction progress variable gradient) and the strain rates, which govern the evolution of the SDF, have been analyzed using a three-dimensional flame-resolved simulation database of a turbulent lean premixed methane-Air flame in a bluff-body configuration. It has been found that the turbulence intensity increases with the distance from the burner, changing the flame curvature distribution and increasing the probability of the negative curvature in the downstream direction. The curvature dependences of dilatation rate a u→ and displacement speed Sd give rise to variations of these quantities in the axial direction. These variations affect the nature of the alignment between the progress variable gradient and the local principal strain rates, which in turn affects the mean flame normal strain rate, which assumes positive values close to the burner but increasingly becomes negative as the effect of turbulence increases with the axial distance from the burner exit. The axial distance dependences of the curvature and displacement speed also induce a considerable variation in the mean value of the curvature stretch. The axial distance dependences of the dilatation rate and flame normal strain rate govern the behavior of the flame tangential strain rate, and its mean value increases in the downstream direction. The current analysis indicates that the statistical behaviors of different strain rates and displacement speed and their curvature dependences need to be included in the modeling of flame surface density and scalar dissipation rate in order to accurately capture their local behaviors. © 2018 Author(s).
    view abstract10.1063/1.5029931
  • A flamelet/progress variable approach for modeling coal particle ignition
    Vascellari, M. and Tufano, G.L. and Stein, O.T. and Kronenburg, A. and Kempf, A.M. and Scholtissek, A. and Hasse, C.
    Fuel 201 (2017)
    In this work an extended flamelet/progress variable (FPV) approach is presented with the aim of simulating the homogeneous auto-ignition of single coal particles during devolatilization. The FPV approach allows chemistry to be decoupled from the solution of the flow field, using a pre-computed strained flamelet look-up table. In this way, the complex chemistry governing coal particle ignition can be accurately accounted for. In particular, one-dimensional unsteady laminar diffusion flamelet (ULDF) simulations are used to generate the table, considering different values of the mixture fraction and the scalar dissipation rate on the particle surface. A new analytical formulation for the scalar dissipation rate is used to solve the diffusion flamelet equations in mixture fraction space, characterized by maximum values on the particle side, rapidly decreasing to zero towards the oxidizer. The time of the ULDF simulations is mapped to progress variable to characterize the ignition process. The FPV approach is then used in laminar resolved CFD simulations of the reactive boundary layer around a single coal particle. The flamelet table look-up is performed using the cell values of the mixture fraction and progress variable along with the particle surface values of the mixture fraction and scalar dissipation rate. Results from the new approach are finally compared with results from a simulation fully resolving transport processes and the detailed chemistry by means of . a priori and . a posteriori analyses. The results show that the FPV method is able to correctly reproduce the flame structure before and during the ignition process. © 2016 Elsevier Ltd.
    view abstract10.1016/j.fuel.2016.09.005
  • A Simple Approach for Specifying Velocity Inflow Boundary Conditions in Simulations of Turbulent Opposed-Jet Flows
    Tirunagari, R.R. and A. Pettit, M.W. and Kempf, A.M. and Pope, S.B.
    Flow, Turbulence and Combustion 98 (2017)
    A new methodology is developed to specify inflow boundary conditions for the velocity field at the nozzle exit planes in turbulent counterflow simulations. The turbulent counterflow configuration consists of two coaxial opposed nozzles which emit highly-turbulent streams of varying species compositions depending on the mode considered. The specification of velocity inflow boundary conditions at the nozzle exits in the counterflow configuration is non-trivial because of the unique turbulence field generated by the turbulence generating plates (TGPs) upstream of the nozzle exits. In the method presented here, a single large-eddy simulation (LES) is performed in a large domain that spans the region between the TGPs of the nozzles, and the time series of the velocity fields at the nozzle exit planes are recorded. To provide inflow boundary conditions at the nozzle exit planes for simulations under other conditions (e.g., different stream compositions, bulk velocity, TGP location), transformations are performed on the recorded time series: the mean and r.m.s. (root-mean-square) quantities of velocity, as well as the longitudinal integral length scale on the centerline, at the nozzle exits in simulations are matched to those observed in experiments, thereby matching the turbulent Reynolds number Ret. The method is assessed by implementing it in coupled large-eddy simulation/probability density function (LES/PDF) simulations on a small cylindrical domain between the nozzle exit planes for three different modes of the counterflow configuration: N2 vs. N2; N2 vs. hot combustion products; and CH4/N2 vs. O2. The inflow method is found to be successful as the first and second moments of velocity from the LES/PDF simulations agree well with the experimental data on the centerline for all three modes. This simple yet effective inflow strategy can be applied to eliminate the computational cost required to simulate the flow field upstream of the nozzle exits. It is also emphasized that, in addition to the predicted time series data, the availability of experimental data close to the nozzle exit planes plays a key role in the success of this method. © 2016, Springer Science+Business Media Dordrecht.
    view abstract10.1007/s10494-016-9743-4
  • CoFlaVis: A visualization system for pulverized coal flames
    Michalski, M. and Rieth, M. and Kempf, A. and Krüger, J.
    Computing in Science and Engineering 19 (2017)
    One of the problems with which researchers of different domains, such as chemistry and fluid dynamics, are concerned is the optimization of coal combustion processes to increase the efficiency, safety, and cleanliness of such systems. The coal combustion process is reproduced by using complex simulations that normally produce highly complex data comprising many characteristics. Such datasets are employed by scientists to validate their hypotheses or to present new hypotheses, and the data analysis is mostly restricted to time-consuming workflows only capable of a portion of the data's full spectrum. To support the experts, interactive visualization and analysis tools have been developed by different suppliers to manage and understand multivariate data. © 1999-2011 IEEE.
    view abstract10.1109/MCSE.2017.3971156
  • Dilute suspensions in annular shear flow under gravity: Simulation and experiment
    Schröer, K. and Kurzeja, P. and Schulz, S. and Brockmann, P. and Hussong, J. and Janas, P. and Wlokas, I. and Kempf, A. and Wolf, D.E.
    EPJ Web of Conferences 140 (2017)
    A dilute suspension in annular shear flow under gravity was simulated using multi-particle collision dynamics (MPC) and compared to experimental data. The focus of the analysis is the local particle velocity and density distribution under the influence of the rotational and gravitational forces. The results are further supported by a deterministic approximation of a single-particle trajectory and OpenFOAM CFD estimations of the overcritical frequency range. Good qualitative agreement is observed for single-particle trajectories between the statistical mean of MPC simulations and the deterministic approximation. Wall contact and detachment however occur earlier in the MPC simulation, which can be explained by the inherent thermal noise of the method. The multi-particle system is investigated at the point of highest particle accumulation that is found at 2/3 of the particle revolution, starting from the top of the annular gap. The combination of shear flow and a slowly rotating volumetric force leads to strong local accumulation in this section that increases the particle volume fraction from overall 0.7% to 4.7% at the outer boundary. MPC simulations and experimental observations agree well in terms of particle distribution and a close to linear velocity profile in radial direction. © The Authors, published by EDP Sciences, 2017.
    view abstract10.1051/epjconf/201714009034
  • Flame resolved simulation of a turbulent premixed bluff-body burner experiment. Part I: Analysis of the reaction zone dynamics with tabulated chemistry
    Proch, F. and Domingo, P. and Vervisch, L. and Kempf, A.M.
    Combustion and Flame 180 (2017)
    Results from a highly resolved simulation are presented for a turbulent lean premixed methane-air bluff-body burner investigated experimentally at Cambridge University and Sandia National Laboratories. The Cartesian computational grid consists of 1.6 billion cells with a resolution of 100μm, which is sufficient to capture the laminar (thermal) flame thickness of 500μm. The combustion process is modeled with premixed flamelet generated manifolds (PFGM). The quality of the simulation is assessed by investigating the resolution of the flame- and velocity scales, it is demonstrated that the relevant scales are resolved in a direct numerical simulation (DNS) sense in the flame. The simulation is validated by comparing temporal statistics of velocity, temperature and major species mass fractions against experimental results. It is shown that the combustion regime varies with the distance from the burner and the progress of the reaction. Ensemble-averaged statistics, conditional means and averages along turbulent flamelets are compared against reference data from unstrained premixed one-dimensional flames. The analysis is carried out with respect to previous findings from DNS of much simpler flame configurations featuring synthetic turbulence. It is concluded that the major physical properties are comparable. In other words, most of the findings from previous DNS studies for canonical cases are relevant, at least for the lab-scale jet-flame examined here. The flame normal strain is found to be aligned with the most compressive strain rate. The mean principal curvature of the progress variable iso-surfaces is predominantly zero and skewed towards positive values, the turbulent flame structure is mainly slightly thinned compared to the laminar one. The displacement speed of the flame is found to take partially negative values. The lack of correlation between the displacement speed and the consumption speed is also reported, the correlation being achieved considering the normal component of the diffusive flux only. © 2017 The Combustion Institute
    view abstract10.1016/j.combustflame.2017.02.011
  • Flame resolved simulation of a turbulent premixed bluff-body burner experiment. Part II: A-priori and a-posteriori investigation of sub-grid scale wrinkling closures in the context of artificially thickened flame modeling
    Proch, F. and Domingo, P. and Vervisch, L. and Kempf, A.M.
    Combustion and Flame 180 (2017)
    Dynamic sub-filter closures for artificially thickened flame (ATF) combustion models for large eddy simulation (LES) are investigated with consistent a-priori and a-posteriori analyses. The analyses are based on a flame resolved simulation (quasi DNS) and large eddy simulations of the bluff body burner experiment by Hochgreb and Barlow with premixed flamelet generated manifolds (PFGM). Both flame resolved simulation and LES are performed under the conditions of a single real flame experiment, using the same domain size, filter sizes, boundary conditions and numerics, all with an additional validation by comparison to experimental data. For the evaluation of the sub-filter wrinkling factor, the well-established model by Charlette et al. (2002) in the modified version by Wang et al. (2011) is used with a static and with a dynamic model parameter, a new dynamic power-law model is discussed additionally. In the a-priori analysis, the probability density functions (PDFs) of the sub-grid scale (SGS) wrinkling factor are compared against the modeled ones based on the flame resolved simulation data. These a-priori modeled wrinkling factor PDFs are then compared against the a-posteriori ones from the LES results, where a similar shape is observed for all models. The static model tends to over-predict the wrinkling factor, a better agreement is found for the dynamic models for the medium and small filter width, where the new formulation improves the results for the latter. For the largest filter width, the wrinkling factor is under-predicted by the dynamic models. This under-prediction is, however, compensated by larger gradients of the progress variable field so that the mean flame surface density conditioned on the progress variable is in closer agreement with the flame resolved simulation than the wrinkling factor PDFs are. Finally, radial profiles of the time-averaged temperature from the LES, flame resolved simulation and experiment are compared against each other. With the dynamic SGS wrinkling models, the LES results converge with grid refinement against the flame resolved simulation results. © 2017 The Combustion Institute
    view abstract10.1016/j.combustflame.2017.02.012
  • Flame surface density based modelling of head-on quenching of turbulent premixed flames
    Sellmann, J. and Lai, J. and Kempf, A.M. and Chakraborty, N.
    Proceedings of the Combustion Institute 36 (2017)
    The near-wall behaviour of the generalised flame surface density (FSD) transport in the context of Reynolds Averaged Navier-Stokes (RANS) simulations has been analysed for different values of global Lewis number using three-dimensional Direct Numerical Simulation (DNS) data of head-on quenching of statistically planar turbulent premixed flames by an isothermal inert wall. It has been found that the statistical behaviour of the FSD based reaction rate closure and the terms of the FSD transport equation are significantly affected by the presence of the wall and by the global Lewis number. The near-wall predictions of the standard FSD based mean reaction rate closure and existing sub-models for the unclosed terms of the FSD transport equation have been found to be inadequate based on . a-priori DNS assessment, and modifications to these models have been suggested so that the predictions of modified models for reaction rate closure and FSD transport remain satisfactory, both close to the wall and away from it over a wide range of global Lewis number. © 2016 The Authors.
    view abstract10.1016/j.proci.2016.07.114
  • Flamelet LES modeling of coal combustion with detailed devolatilization by directly coupled CPD
    Rieth, M. and Clements, A.G. and Rabaçal, M. and Proch, F. and Stein, O.T. and Kempf, A.M.
    Proceedings of the Combustion Institute 36 (2017)
    A large eddy simulation (LES) with direct CPD devolatilization modeling and gas phase combustion modeling through a new flamelet approach is presented for the CRIEPI flame by Hwang et al., 2005 [1]. The devolatilization rates are directly determined from CPD for each coal particle. The flamelet is generated from non-premixed one-dimensional gaseous flames and is based on mixture fractions for volatiles and methane as well as on enthalpy and scalar dissipation rate. A transport equation for mixture fraction variance is combined with an assumed pdf approach for modeling turbulence-chemistry interaction. Special emphasis is put on the influence of devolatilization, with a comparison of LES with direct CPD coupling to empirical models with fitted and standard rate constants. The results are further analyzed by scatter plots and phase space trajectories of the quantities of interest. The results show that large deviations between CPD and the fitted model exist on the instantaneous particle level. It is shown that the direct use of CPD in the LES is feasible and that the flamelet model is able to perform well. Some weaknesses specific to the CRIEPI flame are also discussed. © 2016 The Combustion Institute.
    view abstract10.1016/j.proci.2016.06.077
  • Highly resolved flamelet les of a semi-industrial scale coal furnace
    Rieth, M. and Proch, F. and Clements, A.G. and Rabaçal, M. and Kempf, A.M.
    Proceedings of the Combustion Institute 36 (2017)
    A highly resolved large eddy simulation (LES) of the semi-industrial IFRF coal furnace [1,2] employing the steady flamelet model is presented. The flamelet table is based on mixture fractions of volatile and char off-gases as well as on enthalpy and scalar dissipation rate. Turbulence-chemistry interaction is treated with an assumed pdf approach, with the variance obtained from a transport equation. Radiation is computed by the discrete ordinates method and the grey weighted sum of grey gases model. The simulation is conducted with the massively parallel "PsiPhi" code on up to 1.7 billion cells and with 40 million particles. Results are processed and compared against the comprehensive set of experiments to (i) validate the new flamelet model and the simulation method and to (ii) gain further insight into the combustion process that is not available from the experiment. The simulation results show that the flamelet LES approach can successfully describe the flow field and combustion inside the furnace; major species and velocities are found in good agreement with the experiment. The results are further analyzed with a focus on the processes of particle heating, devolatilization, char combustion and flame stabilization in a highly turbulent environment. Additionally, the relative importance of scalar dissipation rate is highlighted, showing a large separation of mixing scales between volatile and char off-gas combustion due to the long residence time and generally much lower scalar dissipation rates than typical for lab-scale experiments. © 2016 The Combustion Institute. Published by Elsevier Inc.
    view abstract10.1016/j.proci.2016.08.089
  • Inline coating of silicon nanoparticles in a plasma reactor: Reactor design, simulation and experiment
    Münzer, A. and Sellmann, J. and Fortugno, P. and Kempf, A. and Schulz, C. and Wiggers, H.
    Materials Today: Proceedings 4 (2017)
    Gas-phase synthesis of nanoparticles (NPs) in hot plasmas is a promising approach to produce pure, highly specific, and complex nanomaterials at large production rates. Post-processing of the material by particle coating, embedding, or surface functionalization is often required to adjust the materials' properties with respect to their utilization in functional structures. Due to the high surface-to-volume ratio, the nanoparticles' surface properties strongly influence the processing and thus their applicability. We report on a scalable and continuous gas-phase synthesis process of silicon nanoparticles by a high-temperature single-step plasma process with subsequent inline coating. Our process requires a two-stage supply of process gases: First, silicon nanoparticles (Si-NPs) are formed from the gaseous precursor monosilane (SiH4) after its decomposition in the plasma zone. Secondly, the coating agent ethylene (C2H4) is mixed with the hot, particle-laden gas flow downstream of the plasma zone via a specifically-designed coating nozzle. To facilitate a homogeneous intermixing of C2H4 and the nanoparticle-laden gas stream, fluid dynamics simulations were performed to design and optimize the geometry of the coating nozzle. The process conditions can be varied to tune the decomposition process of gaseous C2H4 in respect to coating the Si-NP surface. As a result, we are able to tune the composition of the nanoparticles. Product characterization by X-ray diffraction, Raman, FTIR and X-ray photoelectron spectroscopy revealed that either SiC, or silicon with a carbon-like or a polyethylene-like shell is produced respectively, with increasing distance of the coating nozzle from the plasma. For all process conditions, spherical, coated particles with a highly-crystalline silicon core were observed as indicated by TEM measurements.
    view abstract10.1016/j.matpr.2017.09.176
  • Instantaneous 3D imaging of highly turbulent flames using computed tomography of chemiluminescence
    Mohri, K. and Göers, S. and Schöler, J. and Rittler, A. and Dreier, T. and Schulz, C. and Kempf, A.
    Applied Optics 56 (2017)
    The computed tomography of chemiluminescence (CTC) technique was applied for the first time to a real highly turbulent swirl flame setup, using a large number of CCD cameras (N q = 24 views), to directly reconstruct the three-dimensional instantaneous and time-averaged chemiluminescence fields. The views were obtained from a 172.5° region (in one plane) around the flame, and the CTC algorithm [Floyd et al., Combust. Flame 158, 376 (2011)] was used to reconstruct the flame by discretizing the domain into voxels. We investigated how the reconstructions are affected by the views’ arrangement and the settings of the algorithm, and considered how the quality of reconstructions should be assessed to ensure a realistic description of the capabilities of the technique. Reconstructions using N q ≤ 12 were generally better when the cameras were distributed more equiangularly. When N q was severely low (e.g., 3), the reconstruction could be improved by using fewer voxels. The paper concludes with a summary of the strengths and weaknesses of the CTC technique for examining a real turbulent flame geometry and provides guidance on best practice. © 2017 Optical Society of America.
    view abstract10.1364/AO.56.007385
  • Investigation of Numerical Effects on the Flow and Combustion in les of ICE
    Nguyen, T. and Kempf, A.M.
    Oil and Gas Science and Technology 72 (2017)
    This work investigates the influence of numerical dissipation on the modeled combustion in Large Eddy Simulations (LES). It is well known that capturing the dynamics of the in-cylinder flow is crucial for engine simulations, as it strongly affects flame propagation. The flame propagation during the power-stroke highly depends on the turbulence level that is developed throughout compression. This turbulence level will be strongly influenced by the accuracy of the numerical schemes employed. Even a small extent of upwinding, filtering, low-order implicit time-stepping, cell-stretching or mapping between grids may affect the flow field, the turbulence level, and hence the turbulent flame speed and the pressure curve. To provide a reference, the LES in-house code PsiPhi is used, which ensures a minimum of dissipation due to high order explicit time-stepping, homogeneous and isotropic filters and cells. Good stability of the code permits the use of a second-order Central Differencing Scheme (CDS) for the transport of momentum, avoiding numerical dissipation. To analyse the effect of numerical dissipation, simulations of a fired engine are performed using different numerical schemes for the convection of momentum. Physical quantities including the total kinetic energy, the velocity gradient, the turbulent viscosity, the in-cylinder pressure, the flame propagation or the burning rate of different test cases are evaluated and compared to each other to show the numerical effects on combustion. Furthermore, the suitability of common LES quality criteria including an energy criterion and viscosity ratio is discussed based on the comparison of simulations with less and more accurate numerics. It is shown that these LES quality indicators can be highly misleading. © T. Nguyen and A.M. Kempf, published by IFP Energies nouvelles, 2017.
    view abstract10.2516/ogst/2017023
  • Large eddy simulation of particle aggregation in turbulent jets
    Pesmazoglou, I. and Kempf, A.M. and Navarro-Martinez, S.
    Journal of Aerosol Science 111 (2017)
    Aggregation is an inter-particle process that involves a multitude of different physical and chemical mechanisms. Aggregation processes often occur within turbulent flows; for example in spray drying, soot formation, or nanoparticle formation. When the concentration of particles is very large, a direct simulation of individual particles is not possible and alternative approaches are needed. The present work follows the stochastic aggregation modelling based on a Lagrangian framework by Pesmazoglou, Kempf, and Navarro-Martinez (2016) and implements it in the Large Eddy Simulation context. The new coupled model is used to investigate particle aggregation in turbulent jets. Two cases are considered: an existent Direct Numerical Simulation of nanoparticle agglomeration in a planar jet and an experimental configuration of SiO2 nanoparticles in a round jet. The results show a good agreement in both cases, demonstrating the advantages of the Lagrangian framework to model agglomeration and it capacity to describe the full particle size distribution. © 2017 Elsevier Ltd
    view abstract10.1016/j.jaerosci.2017.06.002
  • Large eddy simulations of nanoparticle synthesis from flame spray pyrolysis
    Rittler, A. and Deng, L. and Wlokas, I. and Kempf, A.M.
    Proceedings of the Combustion Institute 36 (2017)
    Large eddy simulations of the nanoparticle synthesis from flame spray pyrolysis are presented. A standard reactor is investigated, with ethanol/hexamethyldisiloxane (HMDSO) mixture as spray/precursor composition and oxygen as dispersion gas for the production of silica nanoparticles. Spray evaporation, ignition and stabilisation of the flame are achieved by a premixed methane/oxygen pilot flame. The gas, spray and nanoparticle phases are modelled with Eulerian, Lagrangian and Eulerian approaches, respectively. A modified tabulated chemistry model, adapted from the premixed flamelet generated manifold approach (PFGM) with artificial flame thickening (ATF) is proposed, tested and applied for the system. The control variables are the element mass fractions of hydrogen and carbon together with a joint progress variable. The population balance equation of the nanoparticles is modelled in terms of number, volume and surface area concentration, its subfilter distribution is modelled with a delta function. The combustion of HMDSO and formation of silica particle monomers is described by a two-step global mechanism. The nucleation source term is tabulated as a function of the control variables. The submodels for spray and combustion are validated separately to compensate for the shortage in detailed experimental data for nanoparticle spray flames. Subsequently, simulation results for the particles are presented and discussed, in particular the polydisperse particle size distributions resulting from turbulence. © 2016 The Combustion Institute.
    view abstract10.1016/j.proci.2016.08.005
  • On the Evolution of the Flow Field in a Spark Ignition Engine
    Janas, P. and Wlokas, I. and Böhm, B. and Kempf, A.
    Flow, Turbulence and Combustion 98 (2017)
    The development of the turbulent flow field inside a spark ignition engine is examined by large-eddy simulation (LES), from the intake flow to the tumble break-down. Ten consecutive cold flow engine cycles on a coarse and twenty cycles on a fine grid are simulated and compared to experiments of the same engine. The turbulent subgrid scales are modeled by the standard Smagorinsky and by the recently developed Sigma model. A comparison of the intake flow is made against Particle Image Velocimetry (PIV) measurements along horizontal and vertical lines and to an LES simulation performed by the Darmstadt group. Furthermore, we show the first LES comparison to Magnetic Resonance Velocimetry (MRV conducted by Freudenhammer et al.) measurements, which provided the 3D flow field inside a full scale dummy of the entire upper cylinder head including the valve seat region, at a time which mimics inflow conditions of the corresponding engine. Our LES is in good qualitative and quantitative agreement with the simulation and the experiments, with the notable exception of the measured in-cylinder pressure, which is discussed in detail and compared to 0D simulations and simulations from other groups. A criterion is proposed for estimating the number of cycles needed in a simulation, if experimental data is available. We put emphasis on the flow in the valve seat region, where turbulence is generated, and discuss the formation of the large scale tumble motion, including a comparison of the radial velocity fields on rolled-up planes around the valve seat. Here, spots of high velocities were found in the under flow region, which cannot been seen by the ensemble averaged MRV measurement. Within the compression stroke, a 2D vortex center identification algorithm is applied on slices inside the combustion chamber, yielding a 3D visualization of the tumble vortex, which is found to have a “croissant-like” shape. The tumble vortex trajectory is plotted on the symmetry plane and compared to measurements. Finally, we consider a modified definition of the (turbulent) integral length scale that provided further insight to the tumble break-down process. © 2016, Springer Science+Business Media Dordrecht.
    view abstract10.1007/s10494-016-9744-3
  • Prediction of CO and NOx pollutants in a stratified bluff body burner
    Gruhlke, P. and Proch, F. and Kempf, A.M. and Mahiques, E.I. and Dederichs, S. and Beck, C.
    Proceedings of the ASME Turbo Expo 4A-2017 (2017)
    The major exhaust gas pollutants from heavy duty gas turbine engines are CO and NOx. The difficulty of predicting the concentration of these combustion products originates from their wide range of chemical time scales. In this paper, a combustion model that includes the prediction of the carbon monoxide and nitric oxide emissions is tested. Large eddy simulations (LES) are performed using a compressible code (OpenFOAM). A modified flamelet generated manifolds (FGM) approach is applied with a thickened flame approach (ATF) to resolve the flame on the numerical grid, with a flame sensor to ensure that the flame is only thickened in the flame region. For the prediction of the CO and NOx emissions, pollutant species transport equations and a second, CO based, progress variable are introduced for the flame burnout zone to account for slow chemistry effects. For the validation of the models, the Cambridge burner of Sweeney and Hochgreb [1,2] is employed, as both carbon monoxide and nitric oxide [3] data is available. Copyright © 2017 Siemens Energy, Inc.
    view abstract10.1115/GT201763203
  • Energy loss in intergalactic pair beams: Particle-in-cell simulation
    Kempf, A. and Kilian, P. and Spanier, F.
    Astronomy and Astrophysics 585 (2016)
    Aims. The change in the distribution function of electron-positron pair beams determines whether GeV photons can be produced as secondary radiation from TeV photons. We will discuss the instabilities driven by pair beams. Methods. The system of a thermal proton-electron plasma and the electron-positron beam is collision free. We have, therefore, used the particle-in-cell simulation approach. It was necessary to alter the physical parameters, but the ordering of growth rates has been retained. Results. We were able to show that plasma instabilities can be recovered in particle-in-cell simulations, but their effect on the pair distribution function is negligible for the beam-background energy density ratios typically found in blazars. © ESO, 2016.
    view abstract10.1051/0004-6361/201527521
  • Flamelet LES of a semi-industrial pulverized coal furnace
    Rieth, M. and Proch, F. and Rabaçal, M. and Franchetti, B.M. and Cavallo Marincola, F. and Kempf, A.M.
    Combustion and Flame 173 (2016)
    The goal of this work is to introduce the flamelet model into large eddy simulation (LES) of realistic coal furnaces. A flamelet table based on two mixture fractions (for volatile and char off-gases) and enthalpy is generated and used in a massively parallel LES of the semi-industrial IFRF coal furnace (Weber et al. 1992 [1, 2]) for which comprehensive experimental data is available enabling the validation of the flamelet model under realistic conditions. Comparison between experiment and simulation is shown by means of averaged quantities of velocities, species concentrations and temperature. Overall good agreement between experiment and simulation could be obtained, giving evidence for the suitability of the flamelet model. The results of the LES are further analyzed, focussing on instantaneous particle and gas phase data to gain additional insight into the coal conversion process inside the furnace. © 2016 The Combustion Institute
    view abstract10.1016/j.combustflame.2016.07.013
  • High-resolution LES of a starting jet
    Inanc, E. and Nguyen, M.T. and Kaiser, S. and Kempf, A.M.
    Computers and Fluids 140 (2016)
    An impulsively started jet is investigated by a highly resolved large-eddy simulation (LES). The numerical calculations are presented, analyzed and compared to the experimental data by Soulopoulos et al. (2014). Different inlet velocity profiles and their turbulence intensity are employed in the simulations to identify the appropriate boundary conditions by comparing the obtained mixture fraction and its dissipation rate against the experimental data. A sensitivity analysis of the numerical calculations to the different filter sizes is performed, and the ramp-up functions of the inlet flow are investigated. A satisfactory agreement between the simulation and the experiment is achieved. It is found that the potential core, which is observed in our calculations, was absent in the experiments, likely due to premature mixing in the nozzle. Two parameters, namely the mixture fraction and its dissipation rate, are chosen for the statistical evaluation, which is presented for both the calculation and the experiment. It is clear that the initial/boundary conditions influence the flow dynamics, thus considerable differences in the statistics can be observed. The comparison of the resolved scales shows that the simulation resolves structures smaller than those from the experiment by a factor of two. However, this does not lead to the discrepancy of the numerical and experimental statistics, which is independent of the resolution. It is observed that the high scalar dissipation rate (SDR) values are mainly located in the mixing layer, however the vortex ring is occupied by the considerably lower SDR values. © 2016 Elsevier Ltd
    view abstract10.1016/j.compfluid.2016.10.022
  • Large Eddy Simulation of a 100 kWth swirling oxy-coal furnace
    Franchetti, B.M. and Cavallo Marincola, F. and Navarro-Martinez, S. and Kempf, A.M.
    Fuel 181 (2016)
    Large Eddy Simulation (LES) has been applied to the swirling 100 kWth OXYCOAL-AC test facility of Aachen University. A set of models to represent devolatilisation, volatile combustion, char combustion and radiation for oxy-coal combustion in an LES framework has been implemented and tested. A qualitative analysis of the flow behaviour and the overall coal combustion processes occurring within the furnace was made. The LES results for the flow field were compared to axial and tangential mean velocity measurements, showing good agreement, particularly in the upstream regions of the flame. The LES results were also compared to oxygen concentrations (vol.) and gas temperature. Overall good agreement was observed in the upstream central regions of the flame, whilst downstream the LES overestimated the combustion rates. It was also found that the recirculation zones are sensitive to char combustion, not just to the rate of devolatilisation as one might expect. An interesting problem occured in the prediction of the velocity profiles, for which the measurements were taken based on coal-particles, so that the outer-most stream remained invisible in the experiments (but not the LES), due to being free from particles. The results show the potential of using LES for more complex oxy-coal combustion burners and opens the way for applications to industrial furnaces. © 2016 Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.fuel.2016.05.015
  • Large Eddy Simulation of an Internal Combustion Engine Using an Efficient Immersed Boundary Technique
    Nguyen, T.M. and Proch, F. and Wlokas, I. and Kempf, A.M.
    Flow, Turbulence and Combustion 97 (2016)
    This paper presents highly resolved large eddy simulations (LES) of an internal combustion engine (ICE) using an immersed boundary method (IBM), which can describe moving and stationary boundaries in a simple and efficient manner. In this novel approach, the motion of the valves and the piston is modeled by Lagrangian particles, whilst the stationary parts of the engine are described by a computationally efficient IBM. The proposed mesh-free technique of boundary representation is simple for parallelization and suitable for high performance computing (HPC). To demonstrate the method, LES results are presented for the flow and the combustion in an internal combustion engine. The Favre-filtered Navier-Stokes equations are solved for a compressible flow employing a finite volume method on Cartesian grids. Non-reflecting boundary conditions are applied at the intake and the exhaust ports. Combustion is described using a flame surface density (FSD) model with an algebraic reaction rate closure. A simplified engine with a fixed axisymmetric valve (see Appendix A) is employed to show the correctness of the method while avoiding the uncertainties which may be induced by the complex engine geometry. Three test-cases using a real engine geometry are investigated on different grids to evaluate the impact of the cell size and the filter width. The simulation results are compared against the experimental data. A good overall agreement was found between the measurements and the simulation data. The presented method has particular advantages in the efficient generation of the grid, high resolution and low numerical dissipation throughout the domain and an excellent suitability for massively parallel simulations. © 2015, Springer Science+Business Media Dordrecht.
    view abstract10.1007/s10494-015-9683-4
  • Numerical Investigation of Third-Body Behavior in Dry and Wet Environments under Plane Shearing
    Pieper, C. and Oschmann, T. and Markauskas, D. and Kempf, A. and Fischer, A. and Kruggel-Emden, H.
    Chemical Engineering and Technology 39 (2016)
    A coupled discrete element method and computational fluid dynamics (DEM-CFD) approach to model and assess third-body behavior in dry and wet environments under plane shearing is presented. DEM is used to model the granular media, while the fluid side of the system is simulated with CFD, which is based on the finite volume method. The applied model is extended to consider buoyancy as well as lubrication effects. The third body is confined and compressed between two walls, which are sheared in opposite direction with a constant velocity. The influence of different shear velocities, fluid viscosities, and gravity orientations on particle and fluid rheology is investigated. Obtained results of both dry and lubricated systems are compared regarding velocity and porosity distribution across the gap, sliding friction, and particle interaction. Copyright © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/ceat.201500647
  • Resolved flow simulation of pulverized coal particle devolatilization and ignition in air- and O2/CO2-atmospheres
    Tufano, G.L. and Stein, O.T. and Kronenburg, A. and Frassoldati, A. and Faravelli, T. and Deng, L. and Kempf, A.M. and Vascellari, M. and Hasse, C.
    Fuel 186 (2016)
    A resolved laminar flow simulation approach is used to investigate the effect of enhanced oxygen levels on single coal particle ignition, comparing the numerical results against experimental data for well-defined conditions (Molina and Shaddix, 2007). Devolatilization is described by a generic boundary condition at the particle surface that accounts for both convective and diffusive phenomena during pyrolysis. The heating rate history of the particle is obtained by solving for intra-particle heat transfer and heat exchange between the particle and its surroundings. The time evolution of volatile release is captured by using the particle mean temperature to calculate the devolatilization rate from a single kinetic rate law with CPD-fitted parameters. The assumed volatile composition includes both light gases and larger hydrocarbons to represent tars. A skeletal kinetic mechanism for pyrolysis and oxidation of hydrocarbon and oxygenated fuels containing 52 species and 452 reactions is used to accurately describe homogeneous chemistry. Particle heat-up, pyrolysis, ignition and envelope flame stabilization are characterized in four gas atmospheres differing in oxygen content and the use of either N2 or CO2 as balance gas. In agreement with the experimental evidence, enhanced oxygen levels shorten ignition delay time τign and result in a higher intensity of the combustion process according to temperature and radical production peaks for all studied mixtures. For the studied oxy-mixtures the presence of CO2 in substitution of N2 delays ignition. The observed behavior is coherent with the different thermo-physical properties of the gas mixtures. The sensitivity of predicted ignition delay to a set of uncertainties is also discussed. It is found that while the absolute values of predicted ignition delay time are functions of potential particle preheating, particle Reynolds number and the chosen criterion to extract ignition delay, the relative trends among the gas mixtures remain in line with the experimental evidence. © 2016 Elsevier Ltd
    view abstract10.1016/j.fuel.2016.08.073
  • Stochastic modelling of particle aggregation
    Pesmazoglou, I. and Kempf, A.M. and Navarro-Martinez, S.
    International Journal of Multiphase Flow 80 (2016)
    Aggregation is an inter-particle process which involves a multitude of different physicochemical mechanisms. In the present work, particles in the nano-scale are considered, with such concentration that renders their direct simulation as individual particles intractable. A stochastic aggregation model is presented for large particle populations in a Lagrangian framework. The model allows for simultaneous collisions between numerical parcels present in a certain volume of interaction (e.g. computational cell) and can be directly coupled to an unsteady numerical solver of a continuous flow. The model performance is evaluated against analytic solutions for a sum (Golovin) and constant aggregation kernel. © 2015 Elsevier Ltd.
    view abstract10.1016/j.ijmultiphaseflow.2015.12.004
  • A Genetic Algorithm-Based Method for the Optimization of Reduced Kinetics Mechanisms
    Sikalo, N. and Hasemann, O. and Schulz, C. and Kempf, A. and Wlokas, I.
    International Journal of Chemical Kinetics 47 (2015)
    This paper describes an automatic method for the optimization of reaction rate constants of reduced reaction mechanisms. The optimization technique is based on a genetic algorithm that aims at finding new reaction rate coefficients that minimize the error introduced by the preceding reduction process. The error is defined by an objective function that covers regions of interest where the reduced mechanism may deviate from the original mechanism. The mechanism's performance is assessed for homogeneous reactor or laminar-flame simulations against the results obtained from a given reference - the original mechanism, another detailed mechanism, or experimental data, if available. The overall objective function directs the search towards more accurate reduced mechanisms that are valid for a given set of operating conditions. An optional feature to the objective function is a penalty term that permits to minimize the change to the reaction coefficients, keeping them as close as possible to the original value. This means that the penalty function can be used to constrain the reaction rates modifications during the optimization if needed. It is demonstrated that the penalty function is successful and can be combined with predefined uncertainty bounds for each reaction of the mechanism. In addition, the penalty function can be modified to achieve a further reduction of the mechanism. The algorithm is demonstrated for the optimization of a previously reduced variant of the GRI-Mech 3.0, a tert-butanol combustion mechanism by Sarathy et al. (Combust. Flame, 2012, 159, 2028-2055) and a hydrogen mechanism by Konnov (Combust. Flame, 2008, 152, 507-528), for which the complete uncertainty vector is known. The method has shown to be, robust, flexible, and suitable for a wide range of operating conditions by using multiple criteria simultaneously. © 2015 Wiley Periodicals, Inc.
    view abstract10.1002/kin.20942
  • Challenging modeling strategies for LES of non-adiabatic turbulent stratified combustion
    Fiorina, B. and Mercier, R. and Kuenne, G. and Ketelheun, A. and Avdić, A. and Janicka, J. and Geyer, D. and Dreizler, A. and Alenius, E. and Duwig, C. and Trisjono, P. and Kleinheinz, K. and Kang, S. and Pitsch, H. and Proch, F. and Cavallo Marincola, F. and Kempf, A.
    Combustion and Flame 162 (2015)
    Five different low-Mach large eddy simulations are compared to the turbulent stratified flame experiments conducted at the Technical University of Darmstadt (TUD). The simulations were contributed by TUD, the Institute for Combustion Technology (ITV) at Aachen, Lund University (LUND), the EM2C laboratory at Ecole Centrale Paris, and the University of Duisburg-Essen (UDE). Combustion is modeled by a premixed flamelet tabulation with local flame thickening (TUD), a premixed flamelet progress variable approach coupled to a level set method (ITV), a 4-steps mechanism combined with implicit LES (LUND), the F-TACLES model that is based on filtered premixed flamelet tabulation (EM2C), and a flame surface density approach (UDE). An extensive comparison of simulation and experimental data is presented for the first two moments of velocity, temperature, mixture fraction, and major species mass fractions. The importance of heat-losses was assessed by comparing simulations for adiabatic and isothermal boundary conditions at the burner walls. The adiabatic computations predict a flame anchored on the burner lip, while the non-adiabatic simulations show a flame lift-off of one half pilot diameter and a better agreement with experimental evidence for temperature and species concentrations. Most simulations agree on the mean flame brush position, but it is evident that subgrid turbulence must be considered to achieve the correct turbulent flame speed. Qualitative comparisons of instantaneous snapshots of the flame show differences in the size of the resolved flame wrinkling patterns. These differences are (a) caused by the influence of the LES combustion model on the flame dynamics and (b) by the different simulation strategies in terms of grid, inlet condition and numerics. The simulations were conducted with approaches optimized for different objectives, for example low computational cost, or in another case, short turn around. © 2015 The Combustion Institute.
    view abstract10.1016/j.combustflame.2015.07.036
  • Initial reaction steps during flame synthesis of iron-oxide nanoparticles
    Kluge, S. and Deng, L. and Feroughi, O. and Schneider, F. and Poliak, M. and Fomin, A. and Tsionsky, V. and Cheskis, S. and Wlokas, I. and Rahinov, I. and Dreier, T. and Kempf, A. and Wiggers, H. and Schulz, C.
    CrystEngComm 17 (2015)
    Premixed, laminar H2/O2/Ar and CH4/O2/N2 low-pressure flat flames doped with iron pentacarbonyl (Fe(CO)5) were used to investigate the initial steps towards the formation of iron-oxide nanoparticles. The particles were extracted from the flame using a molecular beam sampling probe and the mass flow rate of condensed material was measured by a quartz crystal microbalance (QCM). It was observed that particles are already formed on the cold side of the flame, and vanish quickly once they pass through the flame front. To understand the process and assess the perturbations caused by the sampling probe, spatially resolved laser-based measurements of temperature, Fe and FeO concentration as well as molecular-beam sampling with particle mass spectrometry (PMS) were carried out. Numerical flow simulations of the synthesis flames, the reactor, and the sampling were performed and the simulations confirmed the experimental findings of very early particle formation. The detailed knowledge of the perturbation caused by invasive probing enabled further insight into the iron-oxide nanoparticle formation mechanism. From the results it is concluded that neither Fe atoms nor FeO molecules belong to the growth species of iron-oxide nanoparticles from flame synthesis. © The Royal Society of Chemistry.
    view abstract10.1039/c5ce00456j
  • Investigation of the sampling nozzle effect on laminar flat flames
    Deng, L. and Kempf, A. and Hasemann, O. and Korobeinichev, O.P. and Wlokas, I.
    Combustion and Flame 162 (2015)
    Sampling probes used for the mass spectrometric sampling of a flame can affect the flame's flow field. Although this effect is already compensated for by heuristic correction functions, state of the art 3-D simulations may permit an even better consideration of this effect. This work has investigated the perturbations induced by sampling probes in burner-stabilized, laminar, flat flames using numerical simulations. Any deviations in the flow and temperature fields from the ideal, one-dimensional flat flame were generated here by a perforated burner plate; they are also examined. Corresponding mass spectrometric measurements were performed in flames of CH4/O2/Ar and H2/O2/N2, burning under atmospheric conditions. In the present study, heat transfer from the flame to the sampling nozzle was studied with a conjugate heat transfer model. Combustion was described using a finite rate chemistry model, employing a detailed reaction mechanism for a H2/O2/N2 flame and a reduced mechanism for a CH4/O2/Ar flame. Compared to the ideal, one-dimensional, and unperturbed flame, the probe was found to affect the measurements of the concentrations of some species by up to 50%. The results highlight the value of supporting numerical simulations of both the flow and combustion for such measurements with invasive probing. © 2014 The Combustion Institute.
    view abstract10.1016/j.combustflame.2014.11.035
  • Investigations on the effect of different subgrid models on the quality of LES results
    Proch, F. and Pettit, M.W.A. and Ma, T. and Rieth, M. and Kempf, A.M.
    ERCOFTAC Series 20 (2015)
    view abstract10.1007/978-3-319-14448-1_19
  • Large Eddy Simulation of coal combustion in a large-scale laboratory furnace
    Rabaçal, M. and Franchetti, B.M. and Marincola, F.C. and Proch, F. and Costa, M. and Hasse, C. and Kempf, A.M.
    Proceedings of the Combustion Institute 35 (2015)
    A detailed Large Eddy Simulation (LES) of pulverised coal combustion in a large-scale laboratory furnace is presented. To achieve a detailed representation of the flow, mixing and particle dispersion, a massively parallel LES was performed. Different phenomenological network models were applied and compared to each other in order to obtain the most adequate devolatilization kinetic data for the LES. An iterative procedure allowed to optimise the devolatilization kinetic data for the studied coal and operating conditions. The particle combustion history is studied by analysing particle instantaneous properties giving a perspective on coal combustion that currently is not available by other means than LES. Predicted major species and temperature were compared with measurements and a good agreement was obtained. The finely resolved near burner region revealed that the flame is stabilised very close to the burner. Furthermore, two distinct zones of CO2 production were found - one in the internal recirculation zone (IRZ) due to gaseous combustion, and one downstream of the vortex breakdown, due to intense char combustion. It was found that particle properties are inhomogeneous within the IRZ, whereas in the external recirculation zone (ERZ) and downstream of the vortex breakdown they were found to be homogeneous. © 2014 The Combustion Institute. Published by Elsevier Inc. All rights reserved.
    view abstract10.1016/j.proci.2014.06.023
  • Large Eddy Simulations of a turbulent premixed swirl flame using an algebraic scalar dissipation rate closure
    Butz, D. and Gao, Y. and Kempf, A.M. and Chakraborty, N.
    Combustion and Flame 162 (2015)
    Large Eddy Simulations (LES) of a swirl-stabilised turbulent premixed flame in the well-known TECFLAM burner configuration have been carried out by solving transport equations of Favre-filtered reaction progress variable and mixture fraction. A recently proposed closure for the Scalar Dissipation Rate (SDR) is used for the modelling of the filtered reaction rate of reaction progress variable, whereas the Favre filtered mixture fraction is used to account for mixture stratification due to entrainment. The computational results are utilised to analyse the nature of stratification at representative locations in the swirl flame to gain physical insight into the flame structure. Additionally, two algebraic Flame Surface Density (FSD) closures, which were found to perform well in a previous analysis (Ma et al., 2013), are used for the modelling of the filtered reaction rate of reaction progress variable. The predictions of SDR closure are compared to the corresponding results obtained from algebraic FSD closures. The predictions of SDR based simulations show reasonably good agreement with experimental findings; the level of accuracy is at least comparable to that achieved with algebraic FSD models and to the results reported in the literature. © 2015 The Authors.
    view abstract10.1016/j.combustflame.2015.05.003
  • LES of the Sydney piloted spray flame series with the PFGM/ATF approach and different sub-filter models
    Rittler, A. and Proch, F. and Kempf, A.M.
    Combustion and Flame 162 (2015)
    Detailed numerical investigations of the Sydney spray flame series [1] are presented for ethanol flames referred to as "EtF3, EtF6 and EtF8", which feature identical ethanol mass flow rates but different carrier gas mass flow rates. Large eddy simulations (LES) are performed, where the gaseous and liquid phases are modeled by an Eulerian/Lagrangian approach. The turbulent sub-filter stresses (sgs) are modeled with Nicoud's sigma model [2] on grids with two different resolutions. Combustion is modeled with the premixed flamelet generated manifold approach (PFGM), which is combined with the artificially thickened flame (ATF) method. The sub-filter distributions of the control variables are modeled with (a) a β function (β-fdf) and (b) a top-hat function (TH). First, the influence of the variance in the mixture fraction and reaction progress variable is investigated separately, where the variances are either determined from an algebraic model or a transport equation model. Subsequently, the TH model is used to account for the joint impact of Z and Yp. The results are compared against the experimental measurements and reference simulations without sub-filter model. The particle statistics are in good agreement with the experimental data. The variances predicted by the two algebraic models are quite similar, whereas the transport equation model predicts variances which are one order of magnitude higher. The results obtained with the TH and the β-fdf model are comparable. It is found that the impact of the sgs models for the mixture fraction and the progress variable increases with an increasing carrier gas mass flow rate. © 2014 The Combustion Institute.
    view abstract10.1016/j.combustflame.2014.11.025
  • Modeling heat loss effects in the large eddy simulation of a model gas turbine combustor with premixed flamelet generated manifolds
    Proch, F. and Kempf, A.M.
    Proceedings of the Combustion Institute 35 (2015)
    Large eddy simulation results are presented for a model gas turbine combustion chamber, which is operated with a premixed and preheated methane/air mixture. The off-center position of the high axial momentum confined jet burner causes a strong outer recirculation, which stabilizes the flame. Turbulent combustion is modeled by the premixed flamelet generated manifolds (PFGM) technique, which is combined with the artificial thickened flame (ATF) approach. The influence of different heat loss modeling strategies on flame propagation and structure is investigated. Besides the established method of using burner-stabilized flames as basis for the non-adiabatic tabulation, an alternative approach based on freely propagating flames with heat loss inclusion by scaling of the energy equation source term is presented. Different grid resolutions are applied to study the impact of cell size and filter width on the results, the effect of subfilter modeling is also examined. The simulation setup and the modeling approach are validated by comparison of computed statistics against measurements. A good overall agreement between simulation and experiment is observed. However, the length of the flame was slightly under-predicted; it is shown that a simple method for consideration of strain effects on the flame has the potential to improve the predictions here. The effect of heat loss on the combustion process is then characterized further based on probability density functions obtained from the simulation results. © 2014 The Combustion Institute. Published by Elsevier Inc. All rights reserved.
    view abstract10.1016/j.proci.2014.07.036
  • Multi-directional 3D flame chemiluminescence tomography based on lens imaging
    Wang, J. and Song, Y. and Li, Z.-H. and Kempf, A. and He, A.-Z.
    Optics Letters 40 (2015)
    Flame chemiluminescence tomography (FCT) has been widely used in flame diagnostics for three-dimensional (3D), spatially resolved measurements of instantaneous flame geometry and, to some extent, of species concentrations. However, in most studies, tomographic reconstructions are based on a traditional parallel projection model. Due to the light collection characteristics of a lens, a parallel projection model is not appropriate for the practical optical setups that are used for emission imaging, particularly at small F-numbers. Taking the light collection effect of the lens into account, this Letter establishes a complete and novel tomographic theory for a multi-directional tomography system consisting of a lens and CCD cameras. A modified camera calibration method is presented first. It determines the exact spatial locations and intrinsic parameters of the cameras. A 3D projection model based on the lens imaging theory is then proposed and integrated into the multiplicative algebraic reconstruction technique (MART). The new approach is demonstrated with a 12-camera system that is used to reconstruct the emission field of a propane flame, thereby resolving space and time. © 2015 Optical Society of America.
    view abstract10.1364/OL.40.001231
  • Numerical investigation of the process steps in a spray flame reactor for nanoparticle synthesis
    Weise, C. and Menser, J. and Kaiser, S.A. and Kempf, A. and Wlokas, I.
    Proceedings of the Combustion Institute 35 (2015)
    The synthesis of titanium dioxide nanoparticles from titanium tetraisopropoxide (TTIP) in a nanoparticle spray flame reactor was investigated. The nanoparticle properties are affected by different processes: (a) the break-up of the liquid jet from the spray nozzle, (b) the combustion of the spray and in the pilot flame and (c) the formation and growth of the nanoparticles. The spray process of the injected liquid was analyzed by volume of fluid (VOF) calculations and validated by shadowgraphy imaging which provided the size distribution and the mean velocity of the droplets. The spray angle was determined by a side illuminated long exposure image of the spray. The resulting spray properties (droplet sizes, velocity, and spray angle) served as injector boundary conditions for the downstream combustion simulations. Spray and gas phase of the flame were simulated using an Euler-Lagrange approach, turbulence was modeled by the RNG k-epsilon model, and turbulent combustion was described as a partially stirred reactor (PaSR). For the formation and growth of the nanoparticles within the synthesis reactor, the population balance equation was solved coupled to the spray combustion using a monodisperse model. The findings from experiment and simulation are discussed in terms of flow, species, temperature, and nanoparticle formation inside the reactor. The effect of the spray droplet properties as droplet size, angle, mean velocity and the dispersion behavior on the nanoparticle synthesis process are investigated and discussed, confirming the observation that this type of spray reactor is a robust design overall. © 2014 Published by Elsevier Inc. on behalf of The Combustion Institute.
    view abstract10.1016/j.proci.2014.05.037
  • PICPANTHER: A simple, concise implementation of the relativistic moment implicit particle-in-cell method
    Kempf, A. and Kilian, P. and Ganse, U. and Schreiner, C. and Spanier, F.
    Computer Physics Communications 188 (2015)
    A three-dimensional, parallelized implementation of the electromagnetic relativistic moment implicit particle-in-cell method in Cartesian geometry (Noguchi et al., 2007) is presented. Particular care was taken to keep the C++11 codebase simple, concise, and approachable. GMRES is used as a field solver and during the Newton-Krylov iteration of the particle pusher. Drifting Maxwellian problem setups are available while more complex simulations can be implemented easily. Several test runs are described and the code's numerical and computational performance is examined. Weak scaling on the SuperMUC system is discussed and found suitable for large-scale production runs. © 2014 Elsevier B.V.
    view abstract10.1016/j.cpc.2014.11.010
  • A genetic algorithm-based method for the automatic reduction of reaction mechanisms
    Sikalo, N. and Hasemann, O. and Schulz, C. and Kempf, A. and Wlokas, I.
    International Journal of Chemical Kinetics 46 (2014)
    An automatic method for the reduction of chemical kinetic mechanisms under specific physical or thermodynamic conditions is presented. The method relies on the genetic algorithm search logic to gradually reduce the number of reactions from the detailed mechanism while still preserving its ability to describe the overall chemistry at an acceptable error. Accuracy of the reduced mechanism is determined by comparing its solution to the solution obtained with the full mechanism under the same initial and/or physical conditions. However, not only the chemical accuracy and the size of the mechanism are considered but also the time for its solution which helps to avoid stiff and slow converging mechanisms, thus preferring the fast solutions. The reduction method is demonstrated for a detailed mechanism for methane combustion, GRI-Mech 3.0, which was reduced from 325 reactions and 53 species to 58 reactions and 26 species, and for an iron oxide formation mechanism from iron pentacarbonyl doped flames by Wlokas et al. (Int J Chem Kinet 2013, 45(8), 487-498), originally consisting of 144 reactions and 34 species, which was reduced to 37 reactions and 24 species. The performance of the reduced mechanisms is shown for homogeneous constant pressure reactors and for burner-stabilized flames. The results show a good agreement between reduced and full mechanisms for both the reactor and flame cases. The presented method is flexible and can be easily adjusted to either yield more accurate (but bigger) or smaller (but less accurate) reduced mechanisms, depending on the user's preference. © 2013 Wiley Periodicals, Inc.
    view abstract10.1002/kin.20826
  • A posteriori testing of the flame surface density transport equation for LES
    Ma, T. and Stein, O.T. and Chakraborty, N. and Kempf, A.M.
    Combustion Theory and Modelling 18 (2014)
    Flame Surface Density (FSD) models for Large Eddy Simulation (LES) are implemented and tested for a canonical configuration and a practical bluff body stabilised burner, comparing common algebraic closures with a transport equation closure in the context of turbulent premixed combustion. The transported method is expected to yield advantages over algebraic closures, as the equilibrium of subgrid production and destruction of FSD is no longer enforced and resolved processes of strain, propagation and curvature are explicitly accounted for. These advantages might have the potential to improve the ability to capture large-scale unsteady flame propagation in situations with combustion instabilities or situations where the flame encounters progressive wrinkling with time. The initial study of a propagating turbulent flame in wind-tunnel turbulence shows that the Algebraic Flame Surface Density (FSDA) method can predict an excessively wrinkled flame under fine grid conditions, potentially increasing the consumption rate of reactants to artificially higher levels. In contrast, the Flame Surface Density Transport (FSDT) closure predicts a smooth flame front and avoids the formation of artificial flame cusps when the grid is refined. Five FSDA models and the FSDT approach are then applied to the LES of the Volvo Rig. The predicted mean velocities are found to be relatively insensitive to the use of the FSDT and FSDA approaches, whereas temperature predictions exhibit appreciable differences for different formulations. The FSDT approach yields very similar temperature predictions to two of the tested FSDA models, quantitatively capturing the mean temperature. Grid refinement is found to improve the FSDT predictions of the mean flame spread. Overall, the paper demonstrates that the apparently complicated FSD transport equation approach can be implemented and applied to realistic, strongly wrinkled flames with good success, and opens up the field for further work to improve the models and the overall FSDT approach. © 2014 ALSTOM Technologie AG. Published by Taylor & Francis Group. All Rights Reserved.
    view abstract10.1080/13647830.2013.848383
  • Aerosol nucleation in a turbulent jet using Large Eddy Simulations
    Pesmazoglou, I. and Kempf, A.M. and Navarro-Martinez, S.
    Chemical Engineering Science 116 (2014)
    This work presents a model for particle nucleation and transport in the context of Large Eddy Simulation. A turbulent Dibutyl-Phthalate-laden Nitrogen jet diffusing in atmospheric air is used for validation. The proposed nucleation model treats the process in a probabilistic manner where the frequency of events is determined from local equilibrium conditions. Two methodologies for the sub-grid influence on nucleation rates are implemented: the presumed β-PDF and the source expansion method. Good agreement is found with respect to the experimental results for particle concentrations. The differences between using mean and instantaneous values for the evaluation of the nucleation rate are shown. For the grid spacing used, the unresolved scales seem to have little influence on the calculated particle concentrations. It is concluded that the use of instantaneous nucleation rates is advantageous and therefore it is important to consider a particle coupling that allows for the full use of instantaneous values. © 2014 Elsevier Ltd.
    view abstract10.1016/j.ces.2014.05.022
  • Comparison of the Sigma and Smagorinsky LES models for grid generated turbulence and a channel flow
    Rieth, M. and Proch, F. and Stein, O.T. and Pettit, M.W.A. and Kempf, A.M.
    Computers and Fluids 99 (2014)
    Context: A new and promising Large-Eddy simulation (LES) subgrid model, the Sigma model, has been developed by Nicoud, Baya-Toda and co-workers. Its performance in different codes and test cases compared to the Smagorinsky model is of interest.Objective: The present work investigates how suitable different subgrid stress (SGS) models, i.e. the static and dynamic Smagorinsky and in particular Sigma models are for a Turbulent Opposed Jet (TOJ) configuration and evaluates the differences between the models for a TOJ and channel flow configurations.Method: The Sigma model has been implemented in a dedicated LES/DNS code and is tested against Direct Numerical Simulation (DNS) data from channel flow and grid turbulence data obtained from DNS and from measurements in a TOJ configuration. The flow through the turbulence generating plate (TGP) of the TOJ configuration constitutes a very sensitive test case for fluid flow simulations. Hence, it is a suitable case for a comparison of different SGS models for LES.To compare the SGS models, only the isothermal flow through one of the opposed nozzles, including the TGP has been simulated. LES and DNS have been performed using different grid resolutions down to a grid spacing smaller than the Kolmogorov length scale estimated for the region between the nozzles. The DNS results are being compared to experimental results, while LES results are compared to the DNS data in turn.To underline the differences between the SGS models and to show the general applicability of the newly implemented Sigma model, simulations of the turbulent channel flow have been performed additionally.Results: The TOJ and channel flow simulations show good agreement between DNS and LES. It has been found, that the Sigma model is a better alternative to the static Smagorinsky model with comparable results to the dynamic Smagorinsky model for most of the settings examined. © 2014 Elsevier Ltd.
    view abstract10.1016/j.compfluid.2014.04.018
  • Explanation of the local galactic cosmic ray energy spectra measured by Voyager 1. I. Protons
    Schlickeiser, R. and Webber, W.R. and Kempf, A.
    Astrophysical Journal 787 (2014)
    Almost exactly 100 yr after the original discovery of cosmic rays, the V1 spacecraft has observed, for the first time, the local interstellar medium energy spectra of cosmic ray H, He, C/O nuclei at nonrelativistic kinetic energies, after leaving the heliosphere modulation region on 2012 August 25. We explain these observations by modeling the propagation of these particles in the local Galactic environment with an updated steady-state spatial diffusion model including all particle momentum losses with the local interstellar gas (Coulomb/ionization, pion production, adiabatic deceleration, and fragmentation interactions). Excellent agreement with the V1 cosmic ray H observations is obtained if the solar system resides within a spatially homogeneous layer of distributed cosmic ray sources injecting the same momentum power law p -s with s = 2.24 ± 0.12. The best fit to the V1 H observations also provides an estimate of the characteristic break kinetic energy TC = 116 ± 27 MeV, representing the transition from ionization/Coulomb energy losses at low energies to pion production and adiabatic deceleration losses in a Galactic wind at high energies. As the determined value is substantially smaller than 217 MeV in the absence of adiabatic deceleration, our results prove the existence of a Galactic wind in the local Galactic environment. © 2014. The American Astronomical Society. All rights reserved..
    view abstract10.1088/0004-637X/787/1/35
  • LES of flow processes in an SI engine using two approaches: Openfoam and psiphi
    Nguyen, T. and Janas, P. and Lucchini, T. and D'Errico, G. and Kaiser, S. and Kempf, A.
    SAE Technical Papers 1 (2014)
    In this study two different simulation approaches to large eddy simulation of spark-ignition engines are compared. Additionally, some of the simulation results are compared to experimentally obtained in-cylinder velocity measurements. The first approach applies unstructured grids with an automated meshing procedure, using OpenFoam and Lib-ICE with a mapping approach. The second approach applies the efficient in-house code PsiPhi on equidistant, Cartesian grids, representing walls by immersed boundaries, where the moving piston and valves are described as topologically connected groups of Lagrangian particles. In the experiments, two-dimensional two-component particle image velocimetry is applied in the central tumble plane of the cylinder of an optically accessible engine. Good agreement between numerical results and experiment are obtained by both approaches. Copyright © 2014 SAE International.
    view abstract10.4271/2014-01-1121
  • Numerical analysis of the Cambridge stratified flame series using artificial thickened flame LES with tabulated premixed flame chemistry
    Proch, F. and Kempf, A.M.
    Combustion and Flame 161 (2014)
    Detailed comparisons of LES results against measurement data are presented for the turbulent lean and rich stratified Cambridge flame series. The co-annular methane/air burner with a central bluff body for flame stabilization has been investigated experimentally by Sweeney et al. [1,2]. Three cases with varying levels of stratification in the lean and rich combustion regime are taken into account. Turbulent combustion is modeled by using the artificial thickened flame (ATF) approach in combination with flamelet generated manifolds (FGM) lookup tables. The model is adapted for stratified combustion and an alternative formulation for the flame sensor is presented. Three different grids are used to investigate the influence of the filter width and the sub-filter modeling on the overall results. Velocities, temperatures, equivalence ratios, and major species mass fractions predictions are compared with measurements for three different stratification rates and an overall good overall agreement was found between simulation and experiment. Some deviations occur near the bluff body, which are analyzed further by evaluation of atomic and species mass fractions. The stratified combustion process was further investigated and characterized by probability density functions extracted from the simulation results. © 2014 The Combustion Institute.
    view abstract10.1016/j.combustflame.2014.04.010
  • OxyCAP UK: Oxyfuel combustion - Academic programme for the UK
    Chalmers, H. and Al-Jeboori, M. and Anthony, B. and Balusamy, S. and Black, S. and Cavallo Marincola, F. and Clements, A. and Darabkhani, H. and Dennis, J. and Farrow, T. and Fennell, P. and Franchetti, B. and Gao, L. and Gibbins, J. and Hochgreb, S. and Hossain, M. and Jurado, N. and Kempf, A. and Liug, H. and Lu, G. and Ma, L. and Navarro-Martinez, S. and Nimmo, W. and Oakey, J. and Pranzitelli, A. and Scott, S. and Snape, C. and Sun, C.-G. and Sun, D. and Szuhánszki, J. and Trabadela, I. and Wigley, F. and Yan, Y. and Pourkashanian, M.
    Energy Procedia 63 (2014)
    The OxyCAP-UK (Oxyfuel Combustion - Academic Programme for the UK) programme was a £2M collaboration involving researchers from seven UK universities, supported by E.On and the Engineering and Physical Sciences Research Council. The programme, which ran from November 2009 to July 2014, has successfully completed a broad range of activities related to development of oxyfuel power plants. This paper provides an overview of key findings arising from the programme. It covers development of UK research pilot test facilities for oxyfuel applications; 2-D and 3-D flame imaging systems for monitoring, analysis and diagnostics; fuel characterisation of biomass and coal for oxyfuel combustion applications; ash transformation/deposition in oxyfuel combustion systems; materials and corrosion in oxyfuel combustion systems; and development of advanced simulation based on CFD modelling. © 2014 The Authors. Published by Elsevier Ltd.
    view abstract10.1016/j.egypro.2014.11.055
  • Validation and implementation of algebraic LES modelling of scalar dissipation rate for reaction rate closure in turbulent premixed combustion
    Ma, T. and Gao, Y. and Kempf, A.M. and Chakraborty, N.
    Combustion and Flame 161 (2014)
    The closure of the filtered reaction rate of the reaction progress variable using an algebraic model for Favre-filtered Scalar Dissipation Rate (SDR) N~c in turbulent premixed combustion has been assessed in the context of Large Eddy Simulations (LES). This assessment consists of a priori Direct Numerical Simulation (DNS) analysis based on freely propagating statistically planar turbulent premixed flames and a posteriori analysis, involving the LES simulations of a well-documented rectangular dump combustor configuration with sudden expansion (i.e. ORACLES burner) and a premixed flame stabilised on a triangular bluff body flame holder (i.e. Volvo Rig). It has been found that the newly developed SDR model satisfactorily captures N~c obtained from explicitly filtered DNS data. The predictions of this SDR based LES closure in the ORACLES burner and Volvo Rig configurations exhibit good agreement with experimental results without requiring any major modification to the model parameters. The predictions of the SDR model for the LES of the ORACLES burner and Volvo Rig have been compared to those of two algebraic Flame Surface Density (FSD) models, which yielded satisfactory agreement with experimental data in a previous analysis. The performance of the SDR based closure remains either comparable to or better than the FSD based closures for the two test configurations considered in this analysis. © 2014 The Combustion Institute.
    view abstract10.1016/j.combustflame.2014.05.023
  • A dynamic model for the lagrangian stochastic dispersion coefficient
    Pesmazoglou, I. and Kempf, A.M. and Navarro-Martinez, S.
    Physics of Fluids 25 (2013)
    A stochastic sub-grid model is often used to accurately represent particle dispersion in turbulent flows using large eddy simulations. Models of this type have a free parameter, the dispersion coefficient, which is not universal and is strongly grid-dependent. In the present paper, a dynamic model for the evaluation of the coefficient is proposed and validated in decaying homogeneous isotropic turbulence. The grid dependence of the static coefficient is investigated in a turbulent mixing layer and compared to the dynamic model. The dynamic model accurately predicts dispersion statistics and resolves the grid-dependence. Dispersion statistics of the dynamically calculated constant are more accurate than any static coefficient choice for a number of grid spacings. Furthermore, the dynamic model produces less numerical artefacts than a static model and exhibits smaller sensitivity in the results predicted for different particle relaxation times. © 2013 AIP Publishing LLC.
    view abstract10.1063/1.4848855
  • A posteriori testing of algebraic flame surface density models for LES
    Ma, T. and Stein, O.T. and Chakraborty, N. and Kempf, A.M.
    Combustion Theory and Modelling 17 (2013)
    In the application of Large Eddy Simulation (LES) to premixed combustion, the unknown filtered chemical source term can be modelled by the generalised flame surface density (FSD) using algebraic models for the wrinkling factor Ξ. The present study compares the behaviour of the various models by first examining the effect of sub-grid turbulent velocity fluctuation on Ξ through a one-dimensional analysis and by the LES of the ORACLES burner (Nguyen, Bruel, and Reichstadt, Flow, Turbulence and Combustion Vol. 82 [2009], pp. 155-183) and the Volvo Rig (Sjunnesson, Nelsson, and Max, Laser Anemometry, Vol. 3 [1991], pp. 83-90; Sjunnesson, Henrikson, and Löfström, AIAA Journal, Vol. 28 [1992], pp. AIAA-92-3650). Several sensitivity studies on parameters such as the turbulent viscosity and the grid resolution are also carried out. A statistically 1-D analysis of turbulent flame propagation reveals that counter gradient transport of the progress variable needs to be accounted for to obtain a realistic flame thickness from the simulations using algebraic FSD based closure. The two burner setups are found to operate mainly within the wrinkling/corrugated flamelet regime based on the premixed combustion diagram for LES (Pitsch and Duchamp de Lageneste, Proceedings of the Combustion Institute, Vol. 29 [2002], pp. 2001-2008) and this suggests that the models are operating within their ideal range. The performance of the algebraic models are then assessed by comparing velocity statistics, followed by a detailed error analysis for the ORACLES burner. Four of the tested models were found to perform reasonably well against experiments, and one of these four further excels in being the most grid-independent. For the Volvo Rig, more focus is placed upon the comparison of temperature data and identifying changes in flame structure amongst the different models. It is found that the few models which largely over-predict velocities in the ORACLES case and volume averaged in a previous a priori DNS analysis (Chakraborty and Klein, Physics of Fluids, Vol. 20 [2008], p. 085108), deliver satisfactory agreement with experimental observations in the Volvo Rig, whereas a few of the other models are only able to capture the experimental data of the Volvo Rig either quantitatively or qualitatively. © 2013 Copyright Taylor and Francis Group, LLC.
    view abstract10.1080/13647830.2013.779388
  • Buoyancy induced limits for nanoparticle synthesis experiments in horizontal premixed low-pressure flat-flame reactors
    Weise, C. and Faccinetto, A. and Kluge, S. and Kasper, T. and Wiggers, H. and Schulz, C. and Wlokas, I. and Kempf, A.
    Combustion Theory and Modelling 17 (2013)
    Premixed low-pressure flat-flame reactors can be used to investigate the synthesis of nanoparticles. The present work examines the flow field inside such a reactor during the formation of carbon (soot) and iron oxide (from Fe(CO)5) nanoparticles, and how it affects the measurements of nanoparticle size distribution. The symmetry of the flow and the impact of buoyancy were analysed by three-dimensional simulations and the nanoparticle size distribution was obtained by particle mass spectrometry (PMS) via molecular beam sampling at different distances from the burner. The PMS measurements showed a striking, sudden increase in particle size at a critical distance from the burner, which could be explained by the flow field predicted in the simulations. The simulation results illustrate different fluid mechanical phenomena which have caused this sudden rise in the measured particle growth. Up to the critical distance, buoyancy does not affect the flow, and an (almost) linear growth is observed in the PMS experiments. Downstream of this critical distance, buoyancy deflects the hot gas stream and leads to an asymmetric flow field with strong recirculation. These recirculation zones increase the particle residence time, inducing very large particle sizes as measured by PMS. This deviation from the assumed symmetric, one-dimensional flow field prevents the correct interpretation of the PMS results. To overcome this problem, modifications to the reactor were investigated; their suitability to reduce the flow asymmetry was analysed. Furthermore, 'safe' operating conditions were identified for which accurate measurements are feasible in premixed low-pressure flat-flame reactors that are transferrable to other experiments in this type of reactor. The present work supports experimentalists to find the best setup and operating conditions for their purpose. © 2013 Copyright Taylor and Francis Group, LLC.
    view abstract10.1080/13647830.2013.781224
  • Large Eddy simulation of a pulverised coal jet flame
    Franchetti, B.M. and Cavallo Marincola, F. and Navarro-Martinez, S. and Kempf, A.M.
    Proceedings of the Combustion Institute 34 (2013)
    Large Eddy simulation (LES) has been applied to the pulverised coal jet flame studied at the Japanese Central Research Institute of Electric Power (CRIEPI). A working set of models to represent coal combustion, Lagrangian particle transport and radiative heat transfer in an LES framework has been implemented and tested. The simulation results of the flow field were compared to experimental data for both a reactive and non-reactive case, and an overall good agreement emerged. A simple method for replicating pyrometer measurements was developed for the LES and results obtained from the method were compared to the experimental data. Finally the species concentrations were compared to experimental results for CO2, O2 and N2. The results show the potentials of using LES for pulverised coal combustion and open the way for further developments on the coal combustion models and the applications to more complex burners. © 2012 The Combustion Institute. Published by Elsevier Inc. All rights reserved.
    view abstract10.1016/j.proci.2012.07.056
  • Large eddy simulations of the Darmstadt turbulent stratified flame series
    Marincola, F.C. and Maa, T. and Kempf, A.M.
    Proceedings of the Combustion Institute 34 (2013)
    Large-eddy simulations of the Darmstadt turbulent stratified flame (TSF) burner are presented. The TSF burner is operated under a wide variety of conditions. A case without stratification or shear, and one with stratification but no shear, are considered for validation purposes. A combined mixture fraction and reaction progress variable approach is used, with the reaction rate modelled by a fractal flame wrinkling flame surface density model, where the laminar flame speed is obtained as a function of the mixture fraction. The simulation results and the suitability of the modelling approach are verified and validated through comparison of mean and variance of axial and radial velocity, temperature and mixture fraction with experimental data. The effects of stratification on the flame flow field are then studied by comparing the two reactive simulations, and studying the impact of the cross-scalar dissipation rate term. Finally, the occurrence of potential back- or front-supported stratification is examined through statistical analysis of the flame normal directions. The modelling approach employed was found to produce very good predictions of the TSF burner, with both back- and front-supported stratification modes occurring, the former being considerably more likely. © 2012 The Combustion Institute.
    view abstract10.1016/j.proci.2012.08.001
  • Mechanism of iron oxide formation from iron pentacarbonyl-doped low-pressure hydrogen/oxygen flames
    Wlokas, I. and Faccinetto, A. and Tribalet, B. and Schulz, C. and Kempf, A.
    International Journal of Chemical Kinetics 45 (2013)
    A chemical reaction mechanism was developed for the formation of iron oxide (Fe2O3) from iron pentacarbonyl (Fe(CO)5) in a low-pressure hydrogen-oxygen flame reactor. In this paper, we describe an extensive approach for the flame-precursor chemistry and the development of a novel model for the formation of Fe2O3 from the gas phase. The detailed reaction mechanism is reduced for the implementation in two-dimensional, reacting flow simulations. The comprehensive simulation approach is completed by a model for the formation and growth of the iron oxide nanoparticles. The exhaustive and compact reaction mechanism is validated using experimental data from iron-atom laser-induced fluorescence imaging. The particle formation and growth model are verified with new measurements from particle mass spectrometry. © 2013 Wiley Periodicals, Inc.
    view abstract10.1002/kin.20786
  • Note on the use of Yee-lattices in (semi-) implicit particle-in-cell codes
    Kempf, A. and Ganse, U. and Kilian, P. and Spanier, F.
    Journal of Computational Physics 237 (2013)
    A modification of the implicit algorithm for particle-in-cell simulations proposed by Petrov and Davis (2011) [1] is presented. The original lattice arrangement is not inherently divergence-free, possibly leading to unphysical results. This arrangement is replaced by a staggered mesh resulting in a reduction of the divergence of the magnetic field by several orders of magnitude. © 2012 Elsevier Inc.
    view abstract10.1016/j.jcp.2012.11.045
  • Oxidation of divalent rare earth phosphors for thermal history sensing
    Rabhiou, A. and Kempf, A. and Heyes, A.
    Sensors and Actuators, B: Chemical 177 (2013)
    Knowledge of component temperatures under the extreme conditions in industrial prime movers is of great practical importance, but very hard to obtain. Thermal indicating paints offer one possible and practical way, but they have many disadvantages. A novel concept for utilising phosphorescent coatings as thermal history sensors was proposed by Feist et al. [1] in 2007. These phosphor coatings undergo irreversible changes when exposed to high temperatures that affect their photoluminescent emission properties in such a way that off-line analysis of the emission at room temperature can reveal the temperature history of the coating. In this paper, an investigation of the thermally activated oxidation of 2+ ions in phosphors such as BaMgAl10O 17:Eu2+, BaMgAl10O17:Eu 2+, Mn2+ and SrAl14O25:Eu 2+ is reported and used to demonstrate the potential for a phosphorescent thermal history sensor based on a new physical process. Phosphor powders were annealed at temperatures up to 1400 °C, and characterised using photoluminescence spectroscopy. An intensity ratio temperature measurand was defined and it was shown that the dynamic range of a thermal history sensor based on SrAl14O25:Eu2+ could provide a dynamic range extending from 600 °C to 1300 °C. © 2012 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.snb.2012.10.012
  • Thermal history sensing with thermographic phosphors
    Heyes, A.L. and Rabhiou, A. and Feist, J.P. and Kempf, A.
    AIP Conference Proceedings 1552 8 (2013)
    The ability to measure temperatures on high thermal loaded components in gas turbines and similar prime movers is critical during the design phase if the performance of cooling strategies is to be confirmed. Restricted access and the extreme environment mean that on-line temperature measurement is not always possible and that off-line temperature techniques employing thermal history sensors are sometimes necessary. The authors have developed a new type of sensor based on ceramic phosphors. These show bright narrow band emission that is easily detected and distinguished from the background. Crystallization, phase change and diffusion are all temperature dependent processes that affect the emission characteristics and that, with proper calibration, can be used to form a phosphor based thermal history sensor. Results from the calibration of crystallization in Y2SiO5:Tb and its application in the form of a temperature indicating paint are reviewed. A new embodiment of the phosphor thermal history sensor concept is then presented comprising a YSZ/YAG:Dy composite applied using air plasma spraying in the form of a thermal barrier coating. The coating is shown to function as a thermal history sensor albeit with a limited dynamic range. © 2013 AIP Publishing LLC.
    view abstract10.1063/1.4821412
  • Towards comprehensive coal combustion modelling for les
    Stein, O.T. and Olenik, G. and Kronenburg, A. and Cavallo Marincola, F. and Franchetti, B.M. and Kempf, A.M. and Ghiani, M. and Vascellari, M. and Hasse, C.
    Flow, Turbulence and Combustion 90 (2013)
    Large eddy simulations of pulverised coal combustion (PCC-LES) stabilised on a laboratory-scale piloted jet burner are carried out. The joint simulation effort of three research groups at Freiberg University (FG), Imperial College (IC) and Stuttgart University (ST) is presented, and the details of the comprehensive coal combustion models and their numerical implementation in three different computer programs are discussed. The (standard) coal sub-models and parameters used by the different groups are unified wherever possible. Differences amongst the groups are a different code basis and an Eulerian treatment of the coal particles by IC, while FG and ST use the Lagrangian framework for particle transport. The flow modelling is first validated for the corresponding non-reacting case and all LES calculations accurately capture the experimental trends. Velocity field statistics for the PCC case are in good accordance with the experimental evidence, but scalar statistics illustrate the complexity of coal combustion modelling. The results show notable differences amongst the groups that cannot only be attributed to the different treatment of the particle phase, and they highlight the difficulty to assess and interpret the quality of specific modelling approaches, and a need for further work by the research community. The present study is the first to compare three originally independent transient coal simulations and a step towards comprehensive PCC-LES. © 2012 Springer Science+Business Media Dordrecht.
    view abstract10.1007/s10494-012-9423-y
  • An efficient, parallel low-storage implementation of Klein's turbulence generator for LES and DNS
    Kempf, A.M. and Wysocki, S. and Pettit, M.
    Computers and Fluids 60 (2012)
    Klein's popular method for the generation of 'artificial' inflow turbulence for application in LES and DNS computations has been modified to reduce computational effort and memory requirement, and improve parallel scaling performance. An exponential filter kernel is applied to a field of random noise, where the width of the filter is chosen such that a prescribed integral length-scale is recovered from the filtered field. We generate the random noise as a unique function of physical time and space in logical coordinates, such that any parallel process may generate the same random number for any location within the domain. The filtering operation is also decomposed into the three coordinate directions. These modifications reduce the required computational effort by several orders of magnitude, drastically decrease the memory footprint of the method, and negate any inter-process communication. It thus becomes possible to generate non-periodic pseudo-turbulent inflow conditions at very little cost for computation and code implementation. © 2012 Elsevier Ltd.
    view abstract10.1016/j.compfluid.2012.02.027
  • LES of lifted flames in a gas turbine model combustor using top-hat filtered PFGM chemistry
    Olbricht, C. and Stein, O.T. and Janicka, J. and Van Oijen, J.A. and Wysocki, S. and Kempf, A.M.
    Fuel 96 (2012)
    Progress variable approaches permit the efficient large eddy simulation (LES) of complex industrial combustion systems, where assumed shape filtered density functions (β-FDFs) are widely used to account for subgrid scale effects. In this study a new modelling approach for the LES of partially premixed combustion is introduced, which is based on top-hat filtered premixed flamelet-generated manifolds (TH-PFGM) which are consistent with the LES methodology. Due to the top-hat filtering the resulting lookup tables require fewer dimensions than conventional β-integrated tables, permitting a low-storage representation. In the present paper TH-PFGM is applied to a lifted swirl flame in a model gas turbine combustor. The paper presents the underlying TH-PFGM modelling theory, its extension to any number of dimensions, and simulation results from the LES of the model combustor. Results show that TH-PFGM accurately captures the flame lift off dynamics governed by a low frequency penetration of the flame into the fuel supply, which leads to fluid expansion and in turn flame lift off. The statistical data for flow and species concentration fields from LES are in good accordance with the experimental evidence, as well as results from a comparable LES study. © 2012 Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.fuel.2012.01.018
  • Phosphor based temperature indicating paints
    Heyes, A.L. and Rabhiou, A. and Feist, J.P. and Kempf, A.M.
    Proceedings of the ASME Turbo Expo 1 (2012)
    The ability to measure temperature in extreme environments such as the hot sections of gas turbines is critically important. Several on-line techniques exist but it is often not possible to measure in real-time the temperature of all surfaces of interest. Indeed, some surfaces are so inaccessible as to require complex, costly and intrusive instrumentation for on-line temperature measurement. Here, off-line sensors, also called thermal history sensors, can be used to record the temperatures to which they are exposed, in such a way that they can be extracted later off-line, at room temperature. Probably the best-known types of thermal history sensor are the colour changing thermal paints, that are widely used in gas turbine development. These have been valuable tools of engine developers for many years, but their use presents a number of challenges so that alternatives would be welcome. This paper reports the latest developments of a thermal history sensor based on phosphors that undergo permanent changes in their luminescence properties when exposed to high temperatures. Such thermal history sensors have several advantages over and address many of the shortcomings of existing sensors. The paper contains details of the application of a phosphor-based temperature indicating paint based on Y2SiO5:Tb suspended in a chemical binder. The binder was found to influence the optical properties of the phosphor but despite this, a viable sensor paint for temperatures in the range 400°C to 900°C was formed. A thermal history coating was installed using a thermal barrier coating architecture, applied on various components of a Royce-Rolls Viper 201 engine owned by STS and operated for a number of hours at Cranfield University. Post-operation analysis revealed a temperature distribution on the surfaces/components and enabled hotspots to be identified. Overall the results suggest that phosphor-based temperature indicating paints have the potential to surpass the capability of existing paints. Copyright © 2012 by ASME.
    view abstract10.1115/GT2012-69811
  • Simultaneous temperature,mixture fraction and velocity imaging in turbulent flows using thermographic phosphor tracer particles
    Fond, B. and Abram, C. and Heyes, A.L. and Kempf, A.M. and Beyrau, F.
    Optics Express 20 (2012)
    This paper presents an optical diagnostic technique based on seeded thermographic phosphor particles, which allows the simultaneous two-dimensional measurement of gas temperature, velocity and mixture fraction in turbulent flows. The particle Mie scattering signal is recorded to determine the velocity using a conventional PIV approach and the phosphorescence emission is detected to determine the tracer temperature using a two-color method. Theoretical models presented in this work show that the temperature of small tracer particles matches the gas temperature. In addition, by seeding phosphorescent particles to one stream and nonluminescent particles to the other stream, the mixture fraction can also be determined using the phosphorescence emission intensity after conditioning for temperature. The experimental technique is described in detail and a suitable phosphor is identified based on spectroscopic investigations. The joint diagnostics are demonstrated by simultaneously measuring temperature, velocity and mixture fraction in a turbulent jet heated up to 700 K. Correlated single shots are presented with a precision of 2 to 5% and an accuracy of 2%. © 2012 Optical Society of America.
    view abstract10.1364/OE.20.022118
  • Compressible and incompressible Large Eddy Simulation of a premixed dump combustor
    Ma, T. and Kempf, A.M.
    Proceedings of the ASME Turbo Expo 2 (2011)
    Premixed combustion in the ORACLES dump combustor is investigated by Large-Eddy Simulation. The results are compared with experimental measurements of mean and fluctuating velocities at various points inside the combustor. The LES is performed with the in-house PsiPhi code, which has been modified to account for compressibility so that flame-acoustic interactions can be studied. The modifications include the use of proper boundary conditions that are based on the Navier-Stokes Characteristic Boundary Conditions (NSCBC) [1]. A fixed velocity and temperature inlet as well as a partially reflecting outlet are selected. The reaction rate is modelled using algebraic expressions for the generalised flame surface density (FSD) Σgen. A selection of FSD models [2] were previously tested using the incompressible version of PsiPhi and this work examines three additional models. Previous incompressible works [2, 3] on this setup emulated the effect of acoustic oscillations by introducing sinusoidal pulsations at the inlet with a frequency of 50Hz. We apply the same technique for the simulations and match the results with those from the modified compressible version, albeit for a compact domain which cannot be expected to capture the lowest acoustic frequencies. Apart from assessing performance, we also make comparisons of the simulation cost and stability to gain a better perspective of whether new FSD models and the compressible description are favorable Copyright © 2011 by ASME.
    view abstract10.1115/GT2011-45304
  • Computational error-minimization for LES of non-premixed turbulent combustion
    Geurts, B.J. and Kempf, A.
    ERCOFTAC Series 16 (2011)
    We apply the error-landscape analysis to turbulent non-premixed combustion in a bluff-body flame and investigate the error-reduction that can be achieved by adopting the SIPI algorithm (successive inverse polynomial interpolation) for direct optimization of the combined effect of discretization and modeling errors. Small scale turbulent flow aspects are modeled using the Smagorinsky model and a flamelet formulation is adopted for the combustion process. A systematic study of numerical predictions at various resolutions and different levels of subgrid dissipation is conducted, providing an overview of partial error-cancellation. The general structure of the error-landscape is similar to that found for single phase homogeneous isotropic flow - the application of SIPI results in a considerable reduction of the total error (15-50 % improvement in relative error) after a small number of iterations. The SIPI approach provides an impression of the sensitivity of predictions on numerical and modeling parameters. © Springer Science+Business Media B.V. 2011.
    view abstract10.1007/978-94-007-0231-8_32
  • Computed Tomography of Chemiluminescence (CTC): High resolution and instantaneous 3-D measurements of a Matrix burner
    Floyd, J. and Kempf, A.M.
    Proceedings of the Combustion Institute 33 (2011)
    Computed Tomography of Chemiluminescence (CTC) is a diagnostic technique that provides instantaneous 3-D information on flame geometry and excited species concentrations. The technique reconstructs the 3-D chemiluminescence intensity field using Computed Tomography (CT) from integral measurements of chemiluminescence in the form of camera images. The CTC sensor has been demonstrated for a methane-oxygen Matrix burner consisting of 21 laminar diffusion jet flames using affordable commodity cameras. High resolution 3-D reconstructions obtained from 48 views show good agreement with the observed flame shape and resolve wavelengths of approximately 220 μm. This is sufficient to capture the multiple flame fronts, showing the suitability of CTC for wrinkled turbulent flames. Instantaneous 3-D reconstructions using 10 simultaneous camera measurements also show good agreement with the observed flame shape and are seen to be tolerant of error in the camera location. Measurements with exposure times as short as 62 μs were found to achieve more than sufficient signal-to-noise ratios for successful tomographic reconstructions. Obvious applications for CTC are the measurement of (instantaneous) flame-surface density, wrinkling factor, flame normal direction, and possibly heat release, as well as the observation of transient developments. © 2010 Published by Elsevier Inc. on behalf of The Combustion Institute. All rights reserved.
    view abstract10.1016/j.proci.2010.06.015
  • Computed Tomography of Chemiluminescence (CTC): Instantaneous 3D measurements and Phantom studies of a turbulent opposed jet flame
    Floyd, J. and Geipel, P. and Kempf, A.M.
    Combustion and Flame 158 (2011)
    Time resolved 3D measurements are required to further the understanding of turbulent combustion and to support the development of advanced simulation techniques such as LES. The Computed Tomography of Chemiluminescence (CTC) technique reconstructs the 3D chemiluminescence field of a turbulent flame from a series of integral measurements (camera images). The resulting data can be analysed to obtain the flame surface density, wrinkling factor, flame normal direction and possibly heat release rate, and also to study transient phenomena. High resolution CTC requires measurements from many viewing angles, and the capabilities of recent machine vision cameras make this affordable. The present paper investigates CTC using such commodity cameras. CTC is implemented using five PicSight P32M cameras and mirrors to provide 10 simultaneous views of a premixed turbulent opposed jet (TOJ) flame. The reconstructions are then performed using a 3D Algebraic Reconstruction Technique (ART) algorithm. For the flame investigated, camera exposure times of only 250. μs were found to provide more than sufficient signal-to-noise ratios for ART reconstruction with still shorter exposures times possible. All reconstructions capture the main features of the TOJ flame and were found to provide a useful spatial resolution, even with just 10 views. Detailed Phantom studies were performed to assess the resolution available from ART. The resolution was found to be object dependent but a good working estimate was obtained from a relation by Frieder and Herman (1971) [64]. Reconstructions of realistic LES Phantom data have shown that high resolution reconstructions, which resolve wavelengths of 0.035 object diameters, can be a achieved from only 20 views, with each view costing less than $1000. © 2010 The Combustion Institute.
    view abstract10.1016/j.combustflame.2010.09.006
  • Error analysis of large-eddy simulation of the turbulent non-premixed sydney bluff-body flame
    Kempf, A.M. and Geurts, B.J. and Oefelein, J.C.
    Combustion and Flame 158 (2011)
    A computational error analysis is applied to the large-eddy simulation of the turbulent non-premixed Sydney bluff-body flame, where the error is defined with respect to experimental data. The error-landscape approach is extended to heterogeneous compressible turbulence, which is coupled to combustion as described by a flamelet model. The Smagorinsky model formulation is used to model the unknown turbulent stresses. We introduce several measures to quantify the total simulation error and observe a striking 'valley-structure' in the error that arises as function of the spatial resolution and the Smagorinsky length parameter. The optimal refinement strategy that can be extracted from this error-landscape is reminiscent of that for non-reacting turbulent flow. © 2011 The Combustion Institute.
    view abstract10.1016/j.combustflame.2011.04.012
  • Highly-resolved les and PIV analysis of isothermal turbulent opposed jets for combustion applications
    Stein, O.T. and Böhm, B. and Dreizler, A. and Kempf, A.M.
    Flow, Turbulence and Combustion 87 (2011)
    Turbulent opposed jet (TOJ) burners are an interesting test case for fundamental combustion research and a good benchmark for the available modelling approaches. However, these opposed jet flames strongly depend on the turbulence generation inside the nozzle, which is usually achieved through a perforated plate upstream of the nozzle exit. The present work investigates the flow from these perforated plates and the subsequent turbulence generation in great detail. We present results from highly-resolved large eddy simulations (LES) of the in-nozzle flow in turbulent opposed jets alongside state-of-the-art particle image velocimetry (PIV) at standard and high repetition rates taken inside a glass nozzle. The in-nozzle PIV data provides the LES inflow conditions with unprecedented detail, which are used to follow the initial jet development behaviour known from PIV, before jet coalescence, turbulence production and decay further downstream in the nozzles are successfully predicted. In regions where the PIV experiment suffers from inherent limitations like reflections and the velocity bias, the LES data is available to still obtain a detailed picture of the flow. The sensitivity of the simulations to various physical and numerical parameters is discussed in detail. Results from LES and PIV are compared qualitatively and quantitatively in terms of first and second moments of velocity, temporal autocorrelations, and energy density spectra. Significant deviations are found in the frequency (20%) and strength of vortex shedding from the inlet plane only, whereas the qualitative and quantitative agreement between simulation and experiment is otherwise excellent throughout, implying that a successful large eddy simulation of a turbulent opposed jet can be performed in a domain that includes the perforated plates. © 2010 Springer Science+Business Media B.V.
    view abstract10.1007/s10494-010-9310-3
  • Large eddy simulation of non-reacting gas flow in a 40 MW pulverised coal combustor
    Stein, O.T. and Kempf, A.M. and Ma, T. and Olbricht, C. and Duncan, A. and Lewis, G.D.
    Progress in Computational Fluid Dynamics 11 (2011)
    The Large Eddy Simulation (LES) of non-reacting flow in a full-scale single coal burner test facility is presented. The LES was run with the in-house code PsiPhi of Imperial College, using immersed boundary conditions and the Smagorinsky model. The burner quarl and upstream furnace were discretised with 45 million uniform cubic cells. The LES reveals highly unsteady flow and identifies major recirculation zones crucial for coal flame stabilisation. LES results show a good accordance with available Reynolds-Averaged Navier-Stokes (RANS) data from FLUENT. The cost of the LES is reasonable (2 weeks CPU time, 4 nodes) given the wealth of time-resolved data it provides. © 2011 Inderscience Enterprises Ltd.
    view abstract10.1504/PCFD.2011.042849
  • Large-Eddy Simulation and experiments on non-premixed highly turbulent opposed jet flows
    Pettit, M.W.A. and Coriton, B. and Gomez, A. and Kempf, A.M.
    Proceedings of the Combustion Institute 33 (2011)
    An experimental and computational study is presented on highly turbulent non-premixed counterflows under both isothermal and reactive conditions. Experimentally, Hot Wire Anemometry (HWA), two-dimensional Particle Image Velocimetry (PIV) and OH Planar Laser Induced Fluorescence (PLIF) were applied. Computationally, Large-Eddy Simulations (LES) with a steady flamelet model were used to simulate the flow inside the nozzles and in the opposed flow region, using three different grid resolutions between 1.0 and 0.2 mm (0.5-70 million cells). The combined experimental and computational approach enabled the cross-validation of the simulation, and provided additional insight into the flow field. Both isothermal and burning conditions were examined with turbulent Reynolds numbers reaching a value of 900, demonstrating the system capability of reaching conditions of relevance to practical systems. Importantly, the simplicity of a compact, bench-top experiment is retained. The extension of the computational domain to a region within the nozzles with no optical access reveals the mechanism by which a specially designed turbulence generating plate (TGP) and burner housing yield turbulence intensities well exceeding 20%. The simulated and measured data were found to be in good agreement for first and second velocity moments, for the axial velocity autocorrelation function and for the normalised mean OH fluorescence. Similarity of OH-based flame morphology between experiments and computations also confirms that the LES successfully captures key features of the flow. © 2010 Published by Elsevier Inc. on behalf of The Combustion Institute. All rights reserved.
    view abstract10.1016/j.proci.2010.06.140
  • Phosphorescent thermal history sensors
    Rabhiou, A. and Feist, J. and Kempf, A. and Skinner, S. and Heyes, A.
    Sensors and Actuators, A: Physical 169 (2011)
    The operating temperatures of surfaces in the hot sections of gas turbines are of great practical importance, but are often very hard to measure. Thermal indicating paints offer one possible and practical way, but they have many disadvantages. A novel concept for the utilisation of phosphorescent coatings as thermal history sensors was proposed by Feist et al. [1] in 2007. These phosphor coatings undergo irreversible changes when exposed to high temperatures that affect their photoluminescent properties and are a function of both the temperature and duration of exposure. If care is taken to ensure steady state conditions during exposure, subsequent off-line analysis of emission in the laboratory can reveal the temperature experienced by the coating. In this paper, an investigation of the amorphous-to-crystalline change of Y 2SiO5:Tb is reported and used to provide a proof of concept for a phosphorescent thermal history sensor. Phosphor powder was calcined at different temperatures and for different periods, and characterised using photoluminescence spectroscopy. A calibration curve was generated and shows that this phosphor is suitable for temperature measurements over a temperature range from 600 ° C to at least 1000 °C. With more advanced signal processing routines it is anticipated that the dynamic range might be extended to 1400 °C. Such routines and other materials/physical processes are the subject of on-going research in the area at Imperial College and Southside Thermal Sciences. © 2011 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.sna.2011.04.022
  • Quality issues in combustion les
    Kempf, A.M. and Geurts, B.J. and Ma, T. and Pettit, M.W.A. and Stein, O.T.
    Journal of Scientific Computing 49 (2011)
    Combustion LES requires modelling of physics beyond the flow-field only. These additional models lead to further quality issues and an even stronger need to quantify simulation and modelling errors. We illustrate stability problems, the need for consistent modelling in premixed and non-premixed combustion, and show how RANS models that have frequently been applied in an LES context can lead to strong conceptual errors. We outline the application of the error landscape approach to a complex non-premixed flame, and investigate several error indicators that have been developed for situations where no experimental reference data is available. © 2011 Springer Science+Business Media, LLC.
    view abstract10.1007/s10915-011-9481-7
  • Quality issues of combustion LES
    Kempf, A.M. and Pettit, M.
    ERCOFTAC Series 16 (2011)
    Combustion LES requires additional modelling of physics beyond the flow-field only. These additional models lead to further quality issues and an even stronger need to quantify the errors. The present paper illustrates stability problems, the need for consistent modelling in premixed and non-premixed combustion, and shows how RANS models that have frequently been applied to LES can lead to strong conceptual errors. The paper then outlines the application of Meyer’s error landscape approach to a complex non-premixed flame, and mentions several error-indicators that have been developed for situations where no experimental reference data is available. © Springer Science+Business Media B.V. 2011.
    view abstract10.1007/978-94-007-0231-8_3
  • Concept for a phosphorescent thermal history sensor
    Rabhiou, A. and Feist, J. and Kempf, A. and Skinner, S. and Heyes, A.
    Proceedings of the ASME Turbo Expo 3 (2010)
    The thermal history of hot surfaces is of great practical importance, but very hard to measure. Thermal indicating paints offer one possible and practical way, but they have many disadvantages. A novel concept for the utilisation of phosphorescent coatings as thermal history sensors has been proposed by Feist et al. [1] in 2007. These phosphor coatings undergo irreversible changes when exposed to high temperatures that affect their light emission properties. A subsequent off-line analysis of the emission at room temperature can reveal the temperature history of the coating. In this paper, an investigation of the amorphous-tocrystalline change of Y2SiO5 : Tb is reported and used to provide a proof of concept for a phosphorescent thermal history sensor. The phosphor powder was calcined at different temperatures, and characterised using photoluminescence spectroscopy. A calibration curve was generated from the measurements and is presented and discussed. Copyright © 2010 by ASME.
    view abstract10.1115/GT2010-23517
  • In-nozzle measurements of a turbulent opposed jet using PIV
    Böhm, B. and Stein, O. and Kempf, A. and Dreizler, A.
    Flow, Turbulence and Combustion 85 (2010)
    Turbulent opposed jet burners are an excellent test case for combustion research and model development due to the burners' compactness, relative simplicity, and the good optical access they provide. The flow-field in the flame region depends strongly on the turbulence generation inside the nozzles, so that realistic flow simulations can only be achieved if the flow inside the nozzles is represented correctly, which must be verified by comparison to suitable experimental data. This paper presents detailed particle image velocimetry (PIV) measurements of the flow issuing from the turbulence generating plates (TGP) inside a glass nozzle. The resulting data is analyzed in terms of first and second moments, time-series, frequency spectra and phase averages. The measurements show how individual high velocity jets emerging from the TGP interact and recirculation zones are formed behind the solid parts of the TGP. Vortex shedding is observed in the jet's shear layer were high levels of turbulent kinetic energy are generated. Time series measurements revealed periodic pulsations of the individual jets and implied a coupling between adjacent jets. The peak frequencies were found to be a function of the Reynolds-number. © 2010 Springer Science+Business Media B.V.
    view abstract10.1007/s10494-010-9257-4
  • Les as a prediction tool for lifted flames in a model gas turbine combustor
    Olbricht, C. and Janicka, J. and Kempf, A.
    Proceedings of the ASME Turbo Expo 2 (2010)
    A progress variable approach based on premixed generated manifolds (PFGM) is applied to the LES of a model gas turbine combustor that features a lifted partially premixed flame in a complex flow field. The simulations were performed using two codes with different numerical bases from Imperial College (PsiPhi) and Darmstadt (FASTEST-ECL). Based on the same combustion model, the results from both codes show excellent agreement with each other, and good agreement with the experiments. The lifted flame dynamics, mixing, and product species composition including carbon monoxide concentration are all captured, underlining that both codes can be used to successfully simulate partially premixed model gas turbine combustors. Copyright © 2010 by ASME.
    view abstract10.1115/GT2010-22525
  • chemical mechanisms

  • flame surface densities

  • fluid dynamics

  • large eddy simulation

  • multiscale modeling

  • nanoparticles

  • numerical methods

  • simulation of reactive flows

  • synthesis

  • turbulence

« back