Two-phase critical flow

Critical flow, or flow choking, occurs in a very wide range of engineering applications. The phenomenon is of central importance in water cooled nuclear reactor power plant accident analysis. The mass discharged through the postulated break in a primary coolant system is critical flow limited throughout most of an accident transient. Therefore the primary coolant inventory, and the ability to maintain adequate core cooling, depends strongly upon the critical mass flux. In turn the critical mass flux depends upon the upstream stagnation state and the geometry of the flow channel between the upstream stagnation state and the point of choking. Mechanical and thermal nonequilibrium introduce additional dimensions to the state description for the two-phase fluid and complicate both the calculation of the flow evolution and the relationship between the critical mass flux and the fluid state at the point of choking. Many models have been advanced for two-phase critical flow, however there is no universally satisfactory model for reactor safety evaluations. This paper discusses some of the model approaches, experimental data, and their relationship to applications in which the upstream stagnation state is in the subcooled liquid or low quality regions and the flow evolution is strongly governed by wall friction. Nozzle flow is discussed also for comparison