Numerical investigation of subcooled two-phase flow and heat transfer characteristics in a liquefied natural gas cargo containment system

Abstract

The increase demand in liquefied natural gas (LNG) leads to an increase in the import and export of LNG cargo using LNG carriers with composite insulation and double-hull structures for membrane-type storage tanks. Therefore, the transient characteristics of cryogenic heat transfer in LNG cargo containment systems have become important for understanding the physics when LNG vaporizes into the gas phase to minimize the cargo loss of boil-off gas (BOG). However, public papers on fundamental research on long-term computational fluid dynamics (CFD) simulations are still lacking. In the present study, the transient flow behaviors of LNG fluids and the heat transfer characteristics were numerically studied by applying the simplified single-layer insulation in an LNG cargo containment system, based on an initial temperature of T-LNG,T-0 = 110.5 K and saturation temperature of T-LNG,T-sat = 111.5 K in the boundary condition of IGC code. The proposed analysis procedure can serve as a reference for relative comparison of predicted BOG difference in LNG cargo holds.Consequently, the heat transfer mode between the two-phase methane and the solid insulation could be typically classified into two different regimes: (1) a subcooled regime before reaching the saturation temperature from the initial temperature, and (2) a boiling regime after reaching the saturation temperature. In the subcooled regime, the subcooled liquid exhibited bulk flow motion induced by natural convection. The loss rate of LNG mass per time interval was extracted from the best-fit correlations as -0.0448 kg/s at 1 d (10(5) s) to cool down the insulation system and -0.0072 kg/s at 5 d (5 x 10(5) s) to reach the LNG saturation temperature. When the predicted value of the loss rate of LNG mass by the CFD simulation was regarded as the generation rate of boil-off gas, it was smaller than the designed value of the boil-off gas of 0.464 kg/s corresponded to boil-off rate (BOR) about 0.2 %/day. In the boiling regime and under sufficiently cooled conditions, the loss rate of LNG mass per time interval did not consistently exceed the BOR value. The liquid flow exhibited a turbulent-like motion despite the relatively low velocity at 10 d (10(6) s). Especially, the increase in the effective thermal conductivity was a good indicator for distinguishing between the boiling and subcooled regimes.