CFD simulation of flashing scenarios 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. Owing to the increase in computer speed and progress in numerical algorithm, CFD 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 local non-equilibrium effects, interphase transfer as well as interfacial area under different flashing conditions is still insufficient; a general and precise definition of the problem remains a challenge. In this lecture, two-fluid modelling of various nuclear flashing scenarios such as pipe blowdown, nozzle flow, steam-generator leakage, flashing-induced instability and pressure release will be presented. The performance of different mass transfer models including thermal phase-change, pressure phase-change, relaxation and equilibrium models will be compared. Challenges related to interfacial heat transfer, bubble poly-dispersity as well as flow regime transition will be discussed.