Two-phase critical flow

Abstract

Understanding the heat and mass transfer phenomena occurring during critical flow of two-phase mixtures is of primary importance in the safety analyses of pressurized water, boiling water and liquid-metal-cooled nuclear reactors. It has been shown that during a blowdown incident, the critical flow rate of the two-phase mixture may be affected by a variety of parameters such as the fluid stagnation conditions, the configuration of the blowdown vessel, the length and diameter of the exhaust duct, the purity of the liquid and the local and frictional pressure losses in the flow channel. The complexity of the thermodynamic phenomena taking place during the blowdown process resulted in many studies which compare a particular theory with selected sets of experimental data. However, in the absence of an adequate theory which is applicable over the entire range of the parameters encountered in the nuclear industry, there is a tendency to rely on semi-empirical correlation of the existing data. The main objective of this paper is to provide a general review of two-phase critical flow from the viewpoint of the needs of thermal-hydraulic systems codes and to conduct a systematic evaluation of the existing data and theoretical models in order to quantify the validity, under various conditions, of several of the more widely used critical flow models. Ten different critical flow models have been formulated and tested in this study against an extensive set of data from critical flow experiments with water as the test fluid. Results of the present study are expected to enhance the understanding of the predictive capabilities and limitations of the critical flow models currently used in the power industry.