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21.8 Mo

Thèse présentée à

Normandie Université - Rouen

Spécialité Mécanique des Fluides et Energétique

Par

Léa VOIVENEL

Soutenue le 22 juin 2016

Influences des paramètres hydrodynamiques sur le mélange turbulent de fluides hétérogènes.
Etude expérimentale et analytique
Jury

Examinateur
M. Fabien, ANSELMET Professeur, IRPHE, Marseille
Mme Patricia ERN, Directrice de Recherche, CNRS, IMFT, Toulouse Rapporteur
M. Mickaël BOURGOIN, Directeur de Recherche, CNRS, ENS Lyon
M. William GEORGE, Professeur Émérite, Imperial College, Londres

Membre du Jury
M. Michel CAZALENS, "Executive Expert SAFRAN Group", Paris
M. Emilien VAREA, Maitre de Conférences, CORIA, Rouen

Directrice de thèse
Mme Luminita DANAILA, Professeur, CORIA, Rouen
Co-Directeur de thèse
M. Bruno RENOU, Professeur, CORIA, Rouen

Influences des paramètres hydrodynamiques sur le mélange turbulent de fluides hétérogènes.
Etude expérimentale et analytique

Keywords : turbulence ; variable-viscosity ; jets ; entrainment ; mixing

Understanding the phenomenology associated with heterogeneous mixtures of gases is one of the most persistent challenges in turbulent mixing. Contrary to what has been thought, it was previously found that viscosity variations have a non-negligible impact on mixing and should be taken into account in flow simulation or modeling.

In this study, we carry out a comparison between Constant Viscosity Flows (CVF) and Variable Viscosity Flows (VVF), in a round jet, on the basis of the same initial jet momentum therefore the same injection velocity. The Reynolds number of the jet was in the range Re=8,000-24,000 based on the jet diameter and the propane viscosity.
The considered cases were the following :

  • CVF, which is the baseline case. A nitrogen jet issues in a coflow of nitrogen. The viscosity ratio of the two fluids is Rν = 1.
  • VVF, a propane jet issues in a nitrogen coflow. The latter is 3.5 times more viscous than the propane, so that Rν = 3.5. The density ratio is very nearly equal to 1.
    Due to the scalar active nature, simultaneous velocity and concentration measurements have been performed (using stereo-PIV and PLIF respectively).

The disparity in the birth and growth of turbulence was highlighted using traditional and conditional statistics. The computation of the entrainment ratio allowed us to conclude that, while a classical entrainment phenomenon, is observed for the CVF configuration, the VVF case exhibits a detrainment process. Increasing the Reynolds number leads to a delayed apparition (i.e. far downstream of the injection) of the detrainment process and of the viscous effects.
From the analytical viewpoint, we have established the two point kinetic energy budget equations. The latter physically highlights how energy is transferred from large to small scales, by quantifying the phenomena into play at each scale : turbulent advection, molecular diffusion, turbulent-pressure diffusion, but also –specific to this study- terms reflecting viscosity gradients, as well as turbulence production and spatial decay.

In conclusion, this study demonstrates the influence of the viscosity on the mixing process. Thus, a better understanding of the heterogeneous fluid mixing phenomenology is necessary to improve the efficiency of existing combustion systems, or to orientate the conception of future models used in numerical simulations.

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