CFD calculations of flow pattern maps and LES of multiphase flows

Abstract

Boiling crisis and flows occurring in a steam generator or a heat exchanger remain a major limiting phenomenon for the analysis of operation and safety of both nuclear reactors and conventional thermal power systems. Firstly, the choice is made to investigate a hybrid modelling of the flow, considering the gas phase as two separated fields, each one being modelled with different closure laws. In so doing, the small and spherical bubbles are modelled through a dispersed approach within the two-fluid model, and the distorted or large bubbles are simulated with an interface locating method.This kind of approach is requiring a set of mass transfer terms between the continuous and the dispersed fields of the same physicochemical phase.The main outcome is the simulation of three field cases with a complete set of coupling terms between the two gas fields. Firstly, validation results are presented in the Castillejos test case where air is injected at the bottom of a cylindrical bubble column. Next, METERO experiments are simulated with this multifield approach. METERO is an experimental rig dedicated to the study of turbulent mixing of air and water in horizontal flows. The different regimes encountered in the METERO experiments, i.e. slug flows, plug flows and stratified dispersed bubbly flows are simulated and presented in the paper. Quantitative results are also presented for the first time in the paper. Finally, MAXI2 experiment (liquid/vapour flow in oblique tubes bundle geometry) is simulated: the three-field approach gives a reasonable agreement with experimental data.The important topic of turbulence modelling for two-phase flows is also addressed in the paper. The key point is that the ADM model could constitute an interesting improvement in further studies.