MULTIPHASE RANS SIMULATION OF TURBULENT BUBBLY FLOWS

摘要

The subject of this paper is the CFD simulation of adiabatic bubbly air-water flows. The aim is to contribute to the ongoing effort to develop more advanced simulation tools and, perhaps even more challenging, support the case for a more confident application of these techniques to reactor safety studies. The focus of this work is mostly multiphase turbulence and our ability to predict it, since it is a major driver in many areas of multiphase flow modelling, in addition to work on population balance approaches for bubble size prediction and boiling at a wall. The models are validated against a large number of pipe flows which were selected as test cases for both their relative simplicity with respect to the more complex flows encountered in practice, and also for the significant number of experimental studies available. Both upward and downward flows are simulated with the STAR-CCM+ code. Starting from an existing formulation, an optimized bubble induced turbulence model is proposed and compared with other models available from the literature. The model is then included in a Reynolds stress multiphase formulation, which is assessed against experiments and the k-ε model. In this context, the availability of a validated Reynolds stress multiphase formulation would be a significant step forward for the simulation of more complex flow conditions given the known shortcomings of eddy viscosity-based turbulence models. Finally, the performance of a drag model that accounts for the effect of bubble aspect ratio is evaluated because of its ability to improve velocity predictions near the wall.