CFD Simulation of Boiling Flows for Nuclear Reactor Thermal Hydraulic Applications

Boiling flows allow removal of significant amounts of heat, provided that the critical heat flux limit is not exceeded. In nuclear reactors, this must be accurately predicted to avoid possible overheating and melting of the fuel rods. With this longer-term objective in mind, the predictive capabilities of a computational fluid dynamic two-fluid Eulerian-Eulerian model are tested against boiling flows of refrigerants in vertical channels. The model, solved within the STAR-CCM + code, combines a Reynolds stress turbulence model, the population balance based Sγ model and a wall boiling model. Generally, the model is demonstrated to predict these flows with a satisfactory accuracy, including the void distribution, although such agreement relies on model tuning for the particular cases of interest. In terms of future developments, a better wall function specific to boiling flows, more general and accurate models of bubble break-up and coalescence, and a more mechanistic formulation of the boiling model, are all identified as areas needing further improvements.