Improvement of mass and energy transfer terms by phase change in MARS 3D module

In order to enhance the analysis capability and robustness of MARS 2.0 for thermal nonequilibrium two-phase flow, mass and energy transfer terms by phase change in MARS 3D module were improved and verified based on code calculations for various experiments. In the improved MARS, it is assumed that mass entering to a phase is at saturation and mass leaving a phase has its phase enthalpy in case of phase change, while original MARS assumes all mass entering or leaving a phase at saturation. For the verification of improved MARS capability, the code was run for separete effect tests, FLECHT-SEASET 31805, KAERI DVI, UPTF DVI 272 and an integral effect test, LOFT L2-5 and the results were compared with previous results or experimental results. From the verification calculations, it is found that the code predicts nearly the same results for FLECHT-SEAT and LOFT L2-5, where thermal non-equilibrium between phase is not high. For UPTF DVI and KAERI DVI where thermal non-equilibrium between phases plays an important role, the results of improved MARS for UPTF show better agreement with experimental data and those for KAERI DVI result in enhanced code robustness. From this study, we can conclude that the code improvements have been successfully implemented in the MARS 2.0 and that the MARS analysis capability for thermal non-equilibrium two-phase flow is enhanced with these improvements