Simulation aux grandes échelles et identification acoustique des turbines à gaz en régime partiellement prémélangé

Dans les foyers aéronautiques, une des stratégies adoptée pour réduire l'émission de polluants et faciliter la stabilisation de la flamme, est d'injecter séparément les réactifs dans la chambre de combustion. L'objectif de cette thèse est de développer et de valider l'approche LES (Large Eddy Simulation) de la combustion turbulente partiellement prémélangée dans les turbines à gaz. La conception des nouvelles chambres de combustion moins polluantes, passe par la compréhension de phénomènes instationnaires, tels que les instabilités de combustion. Dans ce contexte, des méthodes pour identifier la réponse de la chambre aux perturbations acoustiques, responsables des instabilités, sont étudiées et améliorées. Dans un premier temps, une étude bibliographique rappelle les caractéristiques essentielles de trois types de combustion rencontrés dans les turbines à gaz (prémélange, diffusion, prémélange partiel). Un état de l'art sur la réduction des schémas cinétiques est proposé à l'issu duquel des schémas cinétiques réduits à deux et quatre étapes sont présentés. Ces schémas ont été développés et validés dans cette thèse. Le modèle de flamme épaissie dynamiquement a été spécialement étendue dans cette thèse aux schémas cinétiques multi-réactions (DTF_MS) en prenant en compte les termes sources de toutes les espèces. Il est intégré dans le code LES AVBP. Ces modèles de combustion doivent prendre en compte les spécificités de la réaction et de l'écoulement: zones de mélange, prémélange à richesse variable, interactions flamme/turbulence, émission de polluants. La deuxième partie décrit les méthodes de mesure des fonctions de transfert. Deux méthodes (FTF et ITM), couramment utilisées numériquement et expérimentalement pour identifier la réponse de la flamme, sont comparées. La formulation FTF est améliorée pour être pleinement compatible avec les approches ITM dans le cadre de l'acoustique linéaire 1D. La troisième partie présente l'étude d'un brûleur propane/air, installé au laboratoire EM2C(Ecole Centrale, Paris). Cette partie représente un premier cas de validation de la LES réactive dans une configuration académique non prémélangée. Les nombreuses comparaisons avec l'expérience permettent de vérifier la qualité de la LES et de mettre en évidence les points à améliorer, en particulier la modélisation de la cinétique. L'évaluation de la réponse instationnaire de la flamme soumise à des perturbations acoustiques est réalisée en comparant les moyennes de phase LES/expérience et en déterminant les fonctions de transfert. Les résultats justifient l'utilisation des méthodes FTF étendues. Enfin, les simulations d'un prototype de brûleur industriel (installé au DLR Institut Technologique de Propulsion à Cologne) ont permis de valider différents points. Tout d'abord, elles montrent que les modèles de combustion sont applicables à une géométrie aussi complexe. Ensuite, elles permettent une première validation des modèles développés dans cette thèse. Les champs moyens obtenus avec deux schémas cinétiques (deux et quatre étapes) sont comparés. Enfin, elles rendent compte de phénomènes instationnaires complexes, tels que les PVC ("Precessing Vortex Core"), les couplages acoustique/hydrodynamique et les instabilités de combustion. ABSTRACT : To reduce pollutant emissions and to better stabilize the flame in modern aeroengine combustors, a common strategy is to inject fuel and oxidizer separately. The aim of this thesis is first to develop and to validate the large eddy simulation (LES) of turbulent partially premixed combustion in gaz turbines. The design of new combustion chambers with low level emissions requires the understanding of unsteady phenomena, like combustion instabilities. In this study, two methods for identifying the chamber response to acoustic perturbations are investigated. In the first part, the three types of combustion (premixed, diffusion partially premixed), which are found in combustors are briefly described. Then a state of the art in kinetic scheme reduction techniques is given, followed by the description of a 2-step and a 4-step reduced kinetic schemes, that have been developed and validated for this study. Then, a dynamically thickened flame model (DTF_MS) has been specially developed in the thesis for multi-step reaction schemes. It has been integrated in the LES code AVBP. These models take into account the features of the flow and of the reaction: mixing zones, variable equivalence ratios, flame/turbulence interactions, pollutant emissions. In the second part, methods for measuring flame transfer functions are detailed. Two methods (FTF and ITM) are usually used numerically and experimentally, to identify the unsteady flame response. The FTF formulation is extended to be fully compatible with the approach ITM in the 1D linear acoustic analysis. In the third part, a propane/air burner set up at EM2C laboratory (Ecole Centrale, Paris) is presented. This part is dedicated to the first validation of reactive LES in an academic non premixed configuration. The many comparisons with experiments are used to check the accuracy of LES and to evidence results that need to be improved. The evaluation of the response of the flame under acoustic forcing is realised comparing simulation and experiments in terms of phase average fields and flame transfer functions. Results show that extended FTF methods are needed. Finally, simulations of the prototype of an industrial burner (installed in DLR Institute of Propulsion Technology) are used to validate some points. First, the combustion models can be applied in a complex geometry. Second, this part provides a first validation of the combustion models developed in this thesis. The mean fields obtained with two kinetic schemes (2 and 4 steps) are compared. Third, complex unsteady phenomena, like PVC ("precessing vortex core"), acoustics/hydrodynamics coupling and combustion instabilities, can be captured by LE

Synchronization and optimization of Large Eddy Simulation using an online Ensemble Kalman Filter

An online Data Assimilation strategy based on the Ensemble Kalman Filter (EnKF) is used to improve the predictive capabilities of Large Eddy Simulation (LES) for the analysis of the turbulent flow in a plane channel, $Re_\tau \approx 550$. The algorithm sequentially combines the LES prediction with high-fidelity, sparse instantaneous data obtained from a Direct Numerical Simulation (DNS). It is shown that the procedure provides an augmented state which exhibits higher accuracy than the LES model and it synchronizes with the time evolution of the high-fidelity DNS data if the hyperparameters governing the EnKF are properly chosen. In addition, the data-driven algorithm is able to improve the accuracy of the subgrid-scale model included in the LES, the Smagorinsky model, via the optimization of a free coefficient. However, while the online EnKF strategy is able to reduce the global error of the LES prediction, a discrepancy with the reference DNS data is still observed because of structural flaws of the subgrid-scale model used

Large-eddy simulation analysis of knock in a direct injection spark ignition engine

International audienceDownsized spark ignition (SI) engines running under high loads have become more and more attractive for car manufacturers because of their increased thermal efficiency and lower CO2 emissions. However, the occurrence of abnormal combustions promoted by the thermodynamic conditions encountered in such engines limits their practical operating range, especially in high efficiency and low fuel consumption regions. One of the main abnormal combustion is knock, which corresponds to an autoignition of end gases during the flame propagation initiated by the spark plug. Knock generates pressure waves which can have long term damages on the engine, that is why the aim for car manufacturers is to better understand and predict knock appearance. However an experimental study of such recurrent but non-cyclic phenomena is very complex, and these difficulties motivate the use of CFD for better understanding them. In the present paper, Large-Eddy Simulation (LES) is used as it is able to represent the instantaneous engine behavior and thus to quantitatively capture cyclic variability and knock. The proposed study focuses on the LES analysis of knock for a direct injection SI engine. A spark timing sweep available in the experimental database is simulated, and 15 LES cycles were performed for each spark timing. Wall temperatures, which are a first order parameter for knock prediction, are obtained using a conjugate heat transfer study. Present work points out that LES is able to describe the in-cylinder pressure envelope whatever the spark timing, even if the sample of LES cycles is limited compared to the 500 cycles recorded in the engine test bench. The influence of direct injection and equivalence ratio stratifications on combustion is also analyzed. Finally, focusing on knock, a MAPO (Maximum Amplitude Pressure Oscillation) analysis is conducted for both experimental and numerical pressure traces pointing out that LES well reproduces experimental knock tendencies

Development and application of bivariate 2D-EMD for the analysis of instantaneous flow structures and cycle-to-cycle variations of in-cylinder flow

International audienceThe bivariate two dimensional empirical mode decomposition (Bivariate 2D-EMD) is extended to estimate the turbulent fluctuations and to identify cycle-to-cycle variations (CCV) of in-cylinder flow. The Bivariate 2D-EMD is an adaptive approach that is not restricted by statistical convergence criterion, hence it can be used for analyzing the nonlinear and non-stationary phenomena. The methodology is applied to a high-speed PIV dataset that measures the velocity field within the tumble symmetry plane of an optically accessible engine. The instantaneous velocity field is decomposed into a finite number of 2D spatial modes. Based on energy considerations, the in-cylinder flow large-scale organized motion is separated from turbulent fluctuations. This study is focused on the second half of the compression stroke. For most of the cycles, the maximum of turbulent fluctuations is located between 50 and 30 crank angle degrees before top dead center (TDC). In regards to the phase-averaged velocity field, the contribution of CCV to the fluctuating kinetic energy is approximately 55% near TDC

Uncertainty and Sensitivity Analysis in Turbulent Pipe Flow Simulation

In this study, we would like to evaluate and improve the performance of Wall-Modeled Large-Eddy Simulation (WMLES) on the modeling of a pipe flow for which Direct Numerical Simulation (DNS) data is available [1] and considered as a reference for further comparisons. Models used in WMLES may raise problems of accuracy which come from the uncertain values of model parameters and model simplifications. In this study, we focus firstly on the impact of the model parameter uncertainties on the simulation results, and then on the reduction of these uncertainties via data calibration. These studies using sampling-based approaches can be unaffordable when coupled with a high-fidelity simulation that requires several CPU hours for a single execution. To reduce the computational cost while maintaining a target accuracy, we propose to build surrogate models based on Gaussian Processes for simulations outputs, and replace the simulator for evaluating the large size sampled sets. For this study, a CFD-UQ methodology is developed which couples our internal UQ tool and a CFD solver. It has been applied on a turbulent pipe flow case that allows us to validate its implementation

Large eddy simulation of a growing turbulent premixed flame kernel using a dynamic flame surface density model

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Direct Numerical Simulations of high Karlovitz number premixed flames for the analysis and modeling of the displacement speed

International audienceAn a priori model for premixed turbulent flame combustion in the thin reaction zone (TRZ) regime is presented. This a priori model is deduced from the analysis of data from a series of direct numerical simulations (DNS) of stoichiometric (φ = 1.0) premixed turbulent iso-octane flames, with Karlovitz number ranging from 2.9 to 46.2. For each case two flames are considered: one with unity Lewis numbers to isolate the effect of turbulence on the flame, and one with non-unity Lewis numbers to study the influence of differential diffusion. First the reaction zone is shown to remain thin for each flame, leading to focus this study on a specific iso-surface in the reaction zone and how it is affected by turbulence. Second, the displacement speed S d on this iso-surface shows a differentiate dependency on tangential strain rate and curvature. This dependency is modeled through an expression of S d formally similar to the ones used in laminar flame theories, but using two effective turbulent Markstein lengths in place of the laminar ones. These lengths are shown to depend on the Lewis number and to decrease when the Karlovitz number increases, in agreement with previous studies showing a reduction of the effective Lewis number with the Karlovitz. From these DNS, an extension of the coherent flame model (CFM) to the TRZ regime is proposed, using a fine-grained flame surface density (FSD) located in the reaction zone. Models for the displacement speed, the tangential strain rate, and the stretch due to curvature are proposed. The a priori evaluation of these closures shows a significant improvement compared to the flamelet formulations

Modeling the effect of flame-wall interaction on the wall heat flux

International audienceFor new generation highly downsized spark ignition engines, the proportion of fuel which is consumed near the wall increases significantly. It is therefore necessary to model accurately the flame / wall interactions. While current models take into account the effect of the wall on the flame, the effect of the turbulent flame-wall interaction on the wall heat flux is still lacking. A model, based on the flame surface density concept, is proposed in the present study in order to take into account this effect. It attempts to evaluate the proportion of the flame in the computational cell that interacts with the wall as the quenching distance is reached. A first validation of the new wall model is presented using DNS data of a 2D V-flame

On the use of DNS to analyze heat and mass transfers in a droplet-laden turbulent jet.

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ECFM-LES modeling with AMR for the CCV prediction and analysis in lean-burn engines

A Large-Eddy Simulation (LES) modeling framework, dedicated to ultra-lean spark-ignition engines, is proposed and validated in the present work. A direct injection research engine is retained as benchmark configuration. The LES model is initially validated using the cold gas-exchange conditions by comparing numerical results with PIV (Particle Imaging Velocimetry) experimental data. Then, the fired configuration is investigated, combining ECFM (Extended Coherent Flame Model) turbulent combustion model with Adaptive Mesh Refinement (AMR). The capability of the model to reproduce experimental pressure envelope and cycle-to-cycle variability is assessed. Within the major scope of the work, a particular focus on the Combustion Cyclic Variability (CCV) is made correlating them with the variability encountered in the in-cylinder aerodynamic variations. R3P4. Finally two post-processing tools, Empirical Mode Decomposition (EMD) and Γ3p function, are proposed and combined to analyse for the first time the aerodynamic tumble-based in-cylinder velocity field. Both tools make it possible to get deeply into the insight and visualization of the flow field and to understand the links between its cyclic variability and the combustion cyclic variability