CFD simulation of flashing flows for nuclear safety analysis: possibilities and challenges

Due to its relevance for the safety analysis of pressurized water reactors, many research activities on flashing flows in pipes and nozzles arose from the mid of last century. Most of them have focused on the mass flow rate and pressure or temperature fluctuation by means of experiments and system codes. With the increase in computer speed, computational fluid dynamics is used more and more in the investigation of flashing flows, which has the advantage of providing further insights regarding the internal flow structure as well as its evolution. Various mixture or two-fluid models have been proposed in the literature. However, knowledge on the non-equilibrium effects, interphase transfer as well as interfacial area under different flashing conditions is still insufficient, and a general and precise definition of the problem remains a challenge. In this work, the two-fluid model is adopted to simulate various nuclear flashing scenarios (pipe blowdown, nozzle flashing flow, steam-generator leakage, flashing-induced instability, pressure release). It is shown that the thermal phase-change model is superior to pressure phase-change, relaxation and equilibrium models. Nevertheless, efforts are required to improve the interphase heat-transfer model. Furthermore, since flashing is often accompanied with high void fraction and broad bubble size range, a poly-disperse two-fluid model is recommended, and further research is needed to account for the effect of phase change on bubble coalescence and breakup. In addition, during flashing the flow pattern may change from single phase to bubbly flow, churn flow, annular flow, and even mist flow. The rapid change of interfacial topology as well as its influence on the applicability of closure models needs to be considered.