CFD CALCULATIONS OF FLOW PATTERN MAPS AND LES OF MULTIPHASE FLOWS
Résumé
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 modeling of the flow, considering the gas phase as two separated fields, each one being modeled with different closure laws. In so doing, the small and spherical bubbles are modeled 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. stratified flows, slug flows, plug flows, stratified dispersed bubbly flows and dispersed bubbly flows are simulated and presented 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 whereas the standard two-field approach (dedicated to dispersed bubbly flow) fails. The important topic of turbulence modelling for two-phase flows is also addressed in the paper. The key point is that standard LES models may fails in the simulation of large interfaces. But the ADM model could constitute an interesting alternative in further studies.
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