Turbulence Modelling of High-Pressure Convective Boiling Two-Phase Flows

Abstract

This article is a contribution to the modelling of multidimensional high-pressure convective boiling two-phase flows relative to PWR’s thermal hydraulics conditions. Postulating that the turbulence is one possible physical mechanism for heat removal from the wall towards the two-phase flow core, this work focuses on modelling turbulent transport terms in the momentum and energy balance equations. Using the pioneering work of Sato et al., [1], [2], the momentum and the energy balance equations are derived for a two-phase mixture. Such a system can be expressed as a combination of parameters, which include the local void fraction as well as the fluid velocity profile, the wall shear stress and the eddy diffusivity. By specifying a closure relation for this last parameter, a numerical solution can be obtained. As a preliminary step towards a numerical solution, the turbulent structure of the two-phase flow is expressed as a linear superposition of an inherent liquid turbulence and an additional one due to the bubble agitation. On the basis of this theory, the mixture velocity and temperature profiles can be predicted provided that the local void fraction and the wall shear stress are known. The model is then tested against the experimental data bank DEBORA (Garnier et al., [3]) which is devoted to the study of high pressure boiling flows. The first results are encouraging for the mechanical part but some discrepancies are observed on temperature profiles for boiling tests. This work should be continued in order to (i) improve the model especially for the thermal aspects and (ii) identify the key parameters responsible for the heat flux limitation (DNB).