Hybrid Eulerian-Lagrangian method for soot modelling applied to ethylene-air premixed flames

International audienceSoot formation has become an important issue in the design of gas turbine combustors due to its environmental impact and its contribution to radiative heat transfer in the combustion chamber. However, efficient and accurate prediction of soot particles formation, growth, oxidation and interaction in gas turbine combustors is still an open field in computational fluid dynamics. The present approach proposes to combine a reduced gas-phase chemistry, a sectional model for polycyclic aromatic hydrocarbons, and a Lagrangian description of soot particles dynamics. The Lagrangian description has been chosen for its ability to simulate the evolution of the particle size distribution. A numerical procedure is proposed to minimise its CPU cost. This approach was successfully applied to the simulation of steady laminar premixed ethylene-air flames at three fuel equivalence ratios, which constitutes a prerequisite towards its use in an aeronautical combustion chamber.La formation de particules de suie constitue un enjeu du design des moteurs aéronautiques, du fait des contraintes environnementales croissantes dans l'industrie des transports et de la contribution importante des suies au rayonnement dans les chambres de combustion. Toutefois, en CFD, il n'existe actuellement aucune méthode à la fois précise et efficace pour la prédiction de la formation, de la croissance et de l'oxydation des particules de suie. Ainsi, l'approche présentée ici propose de décrire la chimie en phase gazeuse par un mécanisme réactionnel réduit combiné à un modèle sectionnel pour les hydrocarbures aromatiques polycycliques (PAH), et d'y coupler une description Lagrangienne de la dynamique des particules de suie. La description Lagrangienne est en effet très utile pour simuler l'évolution de la distribution en taille des particules. Une méthode numérique de réduction de population permet également ici de réduire le coût CPU du solveur Lagrangien. L'approche a été appliquée avec succès à la simulation de flammes laminaires stationnaires de prémélange à trois richesses différentes, ce qui constitue une première étape vers l'application à un foyer aéronautique

Influence de l'évaporation de gouttes multicomposant sur la combustion et des effets diphasiques sur l'allumage d'un foyer aéronautique

La conception de nouveaux moteurs impose de respecter des normes de sécurité concernant les performances d'allumage et de ré-allumage en conditions critiques. Des campagnes d'essais étant onéreuses, les industriels cherchent donc à disposer d'outils numériques ables. Afin d'améliorer la simulation des écoulements, le caractère multicomposant du carburant doit être pris en compte. L'objectif de cette thèse est d'étudier l'influence de l'évaporation d'un brouillard de gouttes sur un écoulement réactif. Pour cela, une étude de la propagation d'une flamme laminaire 1D est réalisée à l'aide d'un code de calcul multiphysique (CEDRE). Un train continu de gouttes monodisperse est injecté, les gouttes étant mono ou bicomposant. L'influence de la dynamique d'évaporation sur la combustion est étudiée. Deux cinétiques chimiques réduites multicomposant sont comparées. La composition, le diamètre et la richesse initiale des gouttes ont un impact sur la structure de flamme, la vitesse de flamme et la composition des gaz brûlés. Ensuite, l'effet de l'évaporation est étudié en phase d'allumage pour un brouillard de gouttes polydisperses monocomposant avec un modèle de noyau d'allumage local. L écoulement instationnaire non-réactif dans un secteur de chambre industriel (MERCATO) est calculé avec une approche LES. Le caractère instationnaire, voire périodique, de la phase dispersée est mis en évidence en certains points de l'écoulement. Les résultats, associés au modèle d'allumage et à des critères, sont utilisées pour réaliser une carte de probabilité d'allumage. Des essais de calcul d'allumage complet de la chambre sont réalisés. Les résultats indiquent une surestimation des termes sources liés à l'évaporation de la phase dispersée et à la combustion.The design of new aircraft engines needs in particular to comply with safety standards for the performance of stabilized combustion and ignition or re-ignition under critical conditions. Experimental campaigns are expensive, so numerical tools are needed. To improve the accuracy of the models used to simulate ow, the multicomponent nature of the fuel must be taken into account, whether it is kerosene or alternative fuel. The objective of this thesis is to study the in uence of a droplet mist vaporization on a reactive ow. For this, an academic study of the propagation of a 1D laminar ame is performed using a CFD code {CEDRE). A continuous stream of monodisperse droplets is injected, the droplets being mono or bicomponent. The influence of the dynamics of evaporation on combustion is particularly studied. Two reduced multicomponent chemical kinetics are compared. The composition, the diameter and the initial equivalent ratio of droplets have an impact on the structure of the ame, the flame speed and composition of the burnt gases. A local ignition kernel model is applied to study the in uence ofvaporization on ignition in the case of monocomponent, polydisperse droplets. Experimental data are available for a monosector combustion chamber (MERCATO) so the non-reactive unsteady flow is simulated with a LES approach. The unsteady, sometimes periodic, nature of the dispersed phase is highlighted in some points of the ow. A ignition model is applied to instantaneous ow elds and criteria are analysed to realise an ignition probability map which validates the approach. Finally, ignition of a combustion chamber is tested. The results point out an overestimation of source terms related to the evaporation of the dispersed phase and combustion.TOULOUSE-ISAE (315552318) / SudocSudocFranceF

Methodology for the numerical prediction of pollutant formation in gas turbine combustors and associated validation experiments

International audienceFor aircraft engine manufacturers the formation of pollutants such as NOx or soot particles is an important issue because the regulations on pollutant emissions are becoming increasingly stringent. In order to comply with these regulations, new concepts of gas turbine combustors must be developed with the help of simulation tools. In this paper we present two different strategies, proposed by ONERA and DLR respectively, to simulate soot or NOx formation in combustors. The first one is based on simple chemistry models allowing significant effort to be spent on the LES description of the flow, while the second one is based on more accurate, but also more expensive, models for soot chemistry and physics. Combustion experiments dedicated to the validation of these strategies are described next: The first one, performed at DLR, was operated at a semi-technical scale and aimed at very accurate and comprehensive information on soot formation and oxidation under well-defined experimental conditions; the second one, characterized at ONERA, was aimed at reproducing the severe conditions encountered in realistic gas turbine combustors. In the third part of the paper the results of combustion simulations are compared to those of the validation experiments. It is shown that a fine description of the physics and chemistry involved in the pollutant formation is necessary but not sufficient to obtain quantitative predictions of pollutant formation. An accurate calculation of the turbulent reactive flow interacting with pollutant formation and influencing dilution, oxidation and transport is also required: when the temperature field is correctly reproduced, as is the case of the ONERA simulation of the DLR combustor, the prediction of soot formation is quite satisfactory while difficulty in reproducing the temperature field in the TLC combustor leads to overestimations of NOx and soot concentrations.Pour les constructeurs de moteurs d’avion, la formation de polluants comme les NOx ou les particules de suies est une question importante car la réglementation sur les émissions polluantes est de plus en plus sévère. Pour respecter cette réglementation, de nouveaux concepts de foyers de turbine à gaz doivent être développés avec l’aide d’outils de simulation. Dans cet article, nous présentons deux stratégies différentes proposées par l’ONERA et le DLR pour simuler la formation des suies et des NOx dans les chambres de combustion. La première est basée sur des modèles chimiques simples permettant de faire porter l’effort de calcul sur la description LES de l’écoulement, tandis que la seconde est basée sur des modèles physico-chimiques de formation des suies plus précis mais aussi plus coûteux en temps de calcul. Des expériences de combustion conçues pour la validation de ces stratégies sont ensuite décrites : La première, réalisée au DLR, reproduit la combustion à une échelle semi-industrielle et a pour but de donner une information très précise et complète sur les mécanismes de formation des suies et leur oxydation dans des conditions expérimentales parfaitement maîtrisées ; la seconde, réalisée à l’ONERA, a pour but de reproduire de façon réaliste les conditions sévères rencontrées dans les foyers de turbine à gaz industrielles. Dans la troisième partie du papier, les résultats des simulations de combustion sont comparés à ceux des expériences de validation. Il est démontré que la description précise de la physique et de la chimie intervenant dans la formation des polluants est nécessaire mais non suffisante pour simuler correctement les quantités de polluants formés. Un calcul précis de l’écoulement turbulent réactif interagissant avec les mécanismes de formation, de dilution, d’oxydation et de transport des polluants est également nécessaire : Lorsque le champ de température est correctement reproduit comme c’est le cas pour la simulation ONERA du foyer DLR, la simulation de la formation des suies est assez satisfaisante, alors qu’une difficulté pour reproduire le champ de température dans le foyer TLC conduit à une surestimation des concentrations de NOx et de suies

Experimental and numerical investigation of the response of a swirled flame to flow modulations in a non-adiabatic combustor

Turbulent combustion models for Large Eddy Simulation (LES) aims at predicting the flame dynamics. So far, they have been proven to predict correctly the mean flow and flame properties in a wide range of configurations. A way to challenge these models in unsteady situations is to test their ability to recover turbulent flames submitted to harmonic flow modulations. In this study, the Flame Transfer Function (FTF) of a CH4/H2/air premixed swirled-stabilized flame submitted to harmonic flowrate modulations in a non-adiabatic combustor is compared to the response computed using the Filtered TAbulated Chemistry for LES (F-TACLES) formalism. Phase averaged analysis of the perturbed flow field and flame response reveal that the velocity field determined with Particle Image Velocimetry measurements, the heat release distribution inferred from OH* images and the probability of presence of burnt gases deduced from OH-Planar Laser Induced Fluorescence measure- ments are qualitatively well reproduced by the simulations. However, noticeable differences between experiments and simulations are also observed in a narrow frequency range. A detailed close-up view of the flow field highlight differences in experimental OH* and numerical volumetric heat release rate distributions which are at the origin of the differences observed between the numerical and experimental FTF. These differences mainly originate from the outer shear layer of the swirling jet where a residual reaction layer takes place in the simulations which is absent in the experiments. Consequences for turbulent combustion modeling are suggested by examining the evolution of the perturbed flame brush envelope along the downstream distance of the perturbed flames. It is shown that changing the grid resolution and the flame subgrid scale wrinkling factor in these regions does not alter the numerical results. It is finally concluded that the combined effects of strain rate and enthalpy defect due to heat losses are the main factors leading to small but sizable differences of the flame response to coherent structures synchronized by the acoustic forcing in the outer shear layer of the swirling flow. These small differences in flame response lead in turn to a misprediction of the FTF at specific forcing frequencies

Classical pathway activity C3c, C4 and C1-inhibitor protein reference intervals determination in EDTA plasma

Introduction: Reference intervals (RIs) for complement assays in EDTA plasma samples have not previously been published. The objectives of the present study were to validate and/or determine RIs for classical pathway (CP50) activity and C3c, C4 and C1 inhibitor protein (C1INH) assays and to assess the need for age-specific RIs in EDTA plasma. Materials and methods: We retrospectively evaluated a cohort of 387 patients attending our university hospital and known to be free of complement- modifying diseases. The need for age partitioning was assessed and RIs were calculated according to the CLSI protocol. Results: No need for age partitioning was evidenced for CP50 activity, C3c and C4 concentrations and RIs (90% CI) were calculated from the pooled data: 35.4 (33.1-37.2) to 76.3 (73.7-83.6) U/mL for CP50 activity, 0.80 (0.75-0.87) to 1.64 (1.59-1.72) g/L for C3c, and 0.12 (0.10-0.14) to 0.38 (0.36- 0.40) g/L for C4. Our results highlight a positive association between age and C1INH concentrations. We derived 3 age partitions (6 months to 30 years, 30-50 and > 50 years) and the related RIs: 0.20 (0.18-0.21) to 0.38 (0.36-0.40) g/L, 0.22 (0.20-0.24) to 0.39 (0.36-0.41) g/L and 0.25 (0.22-0.27) to 0.41 (0.40-0.43) g/L, respectively). Conclusions: The newly determined RIs for CP50 activity were higher than those provided by the manufacturer for EDTA plasma samples, whereas those for C3c and C4 RIs were similar to the values provided for serum samples. The C1INH concentration and activity were found to be associated with age and age-specific RIs are mandatory for this analyte

Simulation des grandes échelles en aérothermique sur des maillages non-structurés généraux

PARIS-BIUSJ-Thèses (751052125) / SudocPARIS-BIUSJ-Physique recherche (751052113) / SudocSudocFranceF

Large Eddy simulation of a turbulent spray jet flame using filtered tabulated chemistry

International audienceThis work presents Large Eddy Simulations of the unconfined CORIA Rouen Spray Burner, fed with liquid n-heptane and air. Turbulent combustion modeling is based on the Filtered TAbulated Chemistry model for LES (F-TACLES) formalism, designed to capture the propagation speed of turbulent stratified flames. Initially dedicated to gaseous combustion, the filtered flamelet model is challenged for the first time in a turbulent spray flame configuration. Two meshes are employed. The finest grid, where both flame thickness and wrinkling are resolved, aims to challenge the chemistry tabulation procedure. At the opposite the coarse mesh does not allow full resolution of the flame thickness and exhibits significant unresolved contributions of subgrid scale flame wrinkling. Both LES solutions are extensively compared against experimental data. For both non-reacting and reacting conditions, the flow and spray aerodynamical properties are well captured by the two simulations. More interesting, the LES predicts accurately the flame lift-off height for both fine and coarse grid conditions. It confirms that the modeling methodology is able to capture the filtered turbulent flame propagation speed in a two-phase flow environment and within grid conditions representative of practical applications. Differences, observed for the droplet temperature, seems related to the evaporation model assumptions. Nomenclature (Nomenclature entries should have the units identified) A = Reynolds filter of variable A A = Favre filter of variable A * Research Engineer, MBDA, [email protected] † Professor, CentraleSupélec