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ISBN: 978-0-89448-767-5; International audience; Hydrodynamics of multiphase flows can have a large influence on the overall design and operation of nuclear reactors. Interfacial forces drive bubbles towards or away from the wall, resulting in wall-peaked or core-peaked void fraction profiles. This change has a dramatic effect on the averaged flow quantities such as wall shear stress, velocity profiles and turbulence levels. Subchannel codes lack information of the underlying flow profiles, and rely on closure models to impose the correct averaged flow quantities. In this work, we propose a criterion to classify bubbly flows as wall-peaked or core-peaked, based on the turbulence level, and the ratio of surface tension to buoyancy forces. The criterion has been developed leveraging a large set of adiabatic air-water bubbly flow experiments, and can be used in subchannel codes to identify the underlying void fraction profile and apply the appropriate closure model. The criterion shows low sensitivity when applied to different operating fluids, and thus can be extended into a generalized formulation that can be applied to all adiabatic bubbly flows. An experiment has also been proposed to confirm the classification criterion at reactor conditions, using water with additives at atmospheric pressure to mitigate the high cost of high-pressure experiments. The new criterion proposed in this work, along with the improved understanding from new experiments, will allow us to develop more physically consistent closure models for subchannel codes.

This thesis is dedicated to the study of unstably stratified homogeneous turbulence.This flow is an idealized framework introduced to investigate the turbulence developing at the centerline of a Rayleigh-Taylor mixing zone. This approach focuses on turbulent quantities, when the mean flow acts on the turbulent field through a mean density gradient.A spectral model and direct numerical simulations are used to study this turbulent flow.The validation step reveals the role of stratification terms on the energy transfer dynamic.Then, a first study shows the emergence of scaling laws in the self-similar state, together with the large scale energy distribution impact on the asymptotic state and on the flow anisotropy. In a second study, the turbulent retroaction on the mean density gradient is introduced in order to bring unstably stratified homogeneous turbulence closer to theRayleigh-Taylor turbulence dynamics. This step leads to investigate the consequences of a stratification inversion on the mixing dynamics through a variable acceleration profile., Cette thèse est consacrée à l’étude de la turbulence homogène stratifiée instable, un écoulement idéalisé décrivant l’évolution de la turbulence au sein d’une zone de mélange de type Rayleigh-Taylor. Cette approche se concentre sur l’évolution des quantités fluctuantes ;l’influence de l’écoulement moyen est prise en compte au travers d’un gradient moyen de densité. Un modèle spectral est utilisé pour étudier cette turbulence, conjointement à des simulations numériques directes. En comparaison avec ces simulations, l’étape de validation du modèle met en lumière le rôle des termes de stratification sur la dynamique du transfert d’énergie. Une première étude montre l’établissement, dans l’état autosemblable, de lois d’échelles ainsi que l’influence de la distribution initiale d’énergie sur l’état asymptotique et sur l’anisotropie de l’écoulement. Dans une seconde étude, la rétroaction de la turbulence sur le gradient moyen est introduite, dans un premier temps, afin de rapprocher la dynamique autosemblable de la turbulence homogène stratifiée instable de celle observée en turbulence Rayleigh-Taylor. Dans un second temps, l’influence d’un renversement de la stratification sur la dynamique du mélange est étudiée au travers d’un profil d’accélération variable.