A horizontal stratified gas–liquid two-phase flow model for the two-fluid model in the WCOBRA/TRAC-TF2 PWR safety analysis code

Abstract

For the two-phase flow in a horizontal pipe, individual phases may separate because of gravity. This horizontal stratification significantly impacts interfacial drag, interfacial heat transfer and wall drag of the two-phase flow. Due to the low interfacial drag and the low interfacial heat transfer, the horizontal stratification in reactor coolant systems is a highly important phenomenon during a postulated loss-of-coolant accident in PWR. The horizontal stratification also impacts other SBLOCA phenomena such as the offtake phenomenon and the cold leg direct contact condensation phenomenon.For the two-fluid model in the WCOBRA/TRAC-TF2 LOCA safety analysis computer code, a horizontal stratification criterion was developed following the inviscid Kelvin–Helmholtz neutral stability analysis. The horizontal stratification criterion combines the Taitel–Dukler model and the Wallis–Dobson model and captures the test data better. The stratification criterion was assessed against various experimental data with a wide range of pressures and pipe sizes. The stratification criterion also matches well with the transition boundaries predicted by the viscous Kelvin–Helmholtz model.The adequacy of the horizontal stratification model is confirmed by examining the predicted flow regime in a horizontal pipe with the measured data of the high pressure TPTF two-phase flow experiments. The void fractions for the horizontal flow are predicted with a good accuracy, which indicates that the interfacial drag and the wall drag in the two-fluid model are properly modeled and the two-fluid momentum equations with the hydraulic slope term reasonably capture the influence of inlet and outlet boundary conditions expected in the PWR LOCA simulation.