Review of two-phase steam-water critical flow models with emphasis on thermal nonequilibrium

A review of the two-phase critical flow models has been presented with particular attention to the light water reactor (LWR) safety application. Pertinent experimental results have also been reviewed. No experiment has yet been performed in reactor-size pipe diameters (approx. 300 mm) with relatively short pipe lengths (approx. 1000 mm). From the small scale tests, it has been found that the critical flow rate increases rapidly as the pipe length is shortened to zero. This is particularly true if the upstream fluid condition is near saturation or subcooled, as is the case during the early stages of a hypothetical LOCA in a LWR system. In this case, both the homogeneous-equilibrium model (HEM) and the Moody model underpredict the critical flow rate data considerably. The effect of thermal nonequilibrium is believed to be the reason for this discrepancy. Models which include the effect of thermal nonequilibrium have been reviewed in detail. Although a large number of lumped as well as distributed models have been proposed, there is no general correlation for any of these models. In view of the complexity of the problem, the relaxation-type models for the actual rate of vapor generation seem to be the most logical approach at this time