Understanding the heat and mass transfer phenomena occurring during critical flow of two-phase mixtures is of primary importance for the safety analyses of nuclear reactors. The complexity of the thermodynamic phenomena taking place during flow expansion 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 talk will provide a general review of two-phase critical flow with special emphasis on the knowledge gape in the current thermal-hydraulic systems codes. A systematic evaluation of the existing data and theoretical models is presented 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.