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Contribution à la modélisation de la combustion non-prémélangée turbulente dans les écoulements rapides

Abstract : This work is dedicated to the study of non-premixed turbulent combustion in high-speed and two-phase flows. From a modeling point of view, it is of primary importance to take into consideration the coupling that exists for those configurations between the effects of compressibility, turbulent mixing, and chemistry. In supersonic reactive flows, the conversion of a portion of kinetic energy into heat or enthalpy is to influence significantly the early developments of chemical reactions and thus, lead to an increase of the chemical conversion of reactants into products of reaction, and contribute to the stabilisation process of the flame. Moreover, the value of the time scale representative of the turbulent mixing and the value of the chemical time scale are expected to be comparable, so that the fast chemistry assumption can be questionned. In this study, an approach based on the estimation of the joint PDF of two scalars, the first quantity describing the local composition of the mixture, and the second characterizing the progress of the reaction, is retained. The model relies on the sudden chemistry assumption, thus permitting a strong but clearly stated functional dependence between the two scalars introduced. As a result, the joint PDF of the two scalars can be simply expressed from the knowledge of the marginal mixture fraction PDF. This approach is extended to the description of supersonic combustion by considering the variations of total enthalpy. Finally, the fluctuations of composition induced by the evaporation of one of the two reactants are also taken into account. The full model is implemented into Computational Fluid Dynamics code, solving the three-dimensional compressible and reactive Navier-Stokes equations. In this work, the representation of the mixing layers and discontinuities is improved thanks to the use of a mesh adaptation strategy, coupled to the CFD code. The numerical and modeling approach is then validated against various configurations : coflowing H2-vitiated air supersonic jets, highly underexpanded jets, and a Scramjet-like combustion chamber. Finally, the modeling framework is also extended to the simulation of the Mascotte configuration (ONERA) with sub-critical liquid oxygen. The results obtained on those configurations are in satisfactory agreement with both underlying physics and experimental data when available, and the numerical code and modeling strategy are shown to be viable tools for further developments and investigations within U-RANS or LES approaches, and for its extension to more extreme conditions (super-critical oxygen).
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Jean-François Izard. Contribution à la modélisation de la combustion non-prémélangée turbulente dans les écoulements rapides. Sciences de l'ingénieur [physics]. ISAE-ENSMA Ecole Nationale Supérieure de Mécanique et d'Aérotechique - Poitiers, 2009. Français. ⟨tel-00523520⟩

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