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Contribution à la modélisation de la combustion turbulente des milieux à deux phases

Abstract : This thesis deals with the modelling of the turbulent combustion in two-phase flow that occurs in Diesel engine. Some experiments, especially those presented in this thesis, suggest that when the fuel needed for the combustion is injected in a liquid form, the fluctuation of the equivalence ratio in gas phase, are enhanced. This phenomenon must be taken into account to represent the mixing process and could be due to the presence of fuel droplets. During their vaporisation, the droplets play the role of local source terms randomly distributed in space, which create fluctuation with a characteristic length scale equal to the mean space between the droplets. These added fluctuations are taken into account by introducing a set of terms due to the vaporisation, in the equation for the root mean square of the mixing variable. Considering the local equation of a two-phase flow medium closes these terms. We have shown the importance of a new variable, which describes the state of mixing at the liquid surface when a new fluid particle is evaporated. A closed equation was written for this variable. Both these new characteristics permit to describe the mixing more accurately. In order to describe the combustion, our rst attempt was to use a model based on the assumption of an in nitely fast chemistry. Thus the chemical state of the medium depends only on the vaporisation and of the mixing with out any influence of the chemistry. After that, the MIL model of Gonzalez and Borghi is used to take into account some effects due to the finite chemical rate, by assuming that the chemistry is infinitely sudden. Both models were tested in two different experimental con gurations: the rst one is a jet flame and the second one is a bomb simulating a Diesel combustion room. The levels of fluctuations found in the numerical simulations are quite high and emphasise the importance estimating accurately the scalar dissipation. The results computed with these models are in agreement with the experiments, especially for the Diesel bomb, the Mil model permits to estimate realistically the auto ignition time and the flame length.
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Submitted on : Monday, February 27, 2006 - 11:47:30 AM
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François-Xavier Demoulin. Contribution à la modélisation de la combustion turbulente des milieux à deux phases. Dynamique des Fluides [physics.flu-dyn]. Université de Provence - Aix-Marseille I, 1999. Français. ⟨tel-00011687⟩

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