CFD Simulation of Boiling Flows with an Eulerian-Eulerian Two-Fluid Model

In the nuclear field, computational fluid dynamics (CFD) is being applied increasingly often to improve the prediction of the boiling flows that are to some extent experienced in almost all water-cooled reactors. For reactor-scale flows, although the development of advanced CFD methods is progressing at a fast rate, Eulerian-Eulerian averaged two-fluid models still represent the only practicable option. In these models, boiling at the wall is normally incorporated using the heat flux partitioning approach and, overall, a number of sub-models and closure relations are required, some of which still rely mainly upon empirical arguments. In view of this, further progress is needed in our ability to model and predict boiling flows and, eventually, extend the modelling to other areas such as the prediction of the critical heat flux. The present paper discusses the development of an Eulerian-Eulerian two-fluid model, implemented in the STAR-CCM+ CFD code. The model includes a Reynolds stress multiphase turbulence model and the Sγ population balance model with improved breakup and coalescence sources. Turbulence and average bubble diameter predictions are firstly, and independently, validated against air-water bubbly flows. Finally, the whole boiling model is assessed against a large database of subcooled boiling flows that covers an extended range of flow conditions. Validation is extended to some recent experiments and some sensitivity studies are made on the modelling of interfacial forces and condensation in the bulk of the flow. Overall, many areas of the model reach a satisfactory accuracy, but a number of weaknesses still persists and, consequently, areas of further improvement are identified.