Improvement of the MARS subcooled boiling model for low-pressure, low-Pe flow conditions

The subcooled boiling model of a thermal-hydraulic system code, MARS, has been assessed against subcooled boiling experiments in vertical pipes under low-pressure and low-Pe (Peclet number) conditions. The results showed that the MARS subcooled boiling model cannot take into account appropriately the effects of inlet liquid velocity and hydraulic diameter on axial void fraction development. To solve the problems, a new correlation for the net vapor generation (NVG) point is suggested and the wall evaporation model, which is one of the sub-models of the subcooled boiling model, has been semi-empirically modified. The modified models were implemented into MARS and validated using experimental data of 77 different thermal-hydraulic conditions with pressures ranging from 1.1 to 9.8 bar, heat fluxes of 97-1186 kW/m2, mass fluxes of 65-635 kg/m2 s, and hydraulic diameters of 5-21 mm. It was shown that the results of the modified model have better agreements with experimental data under low-pressure (P < 9.83 bar) and low-Pe (Pe ≤ 70,000) conditions.