Experimental investigation of influence of thermal stratification on operation of cryogenic tanks

Abstract

This research investigated the thermodynamic behavior of cryogenic tanks operating at high pressure, which are used as liquefied natural gas (LNG) fuel tanks, pressurized LNG (PLNG) tanks, etc. The experiment was performed with liquid nitrogen under 0.024 W/mK using vacuum insulation characterized by a heat transfer coefficient similar to that of the polyurethane foam that is commonly installed in LNG storage tanks. The experimental conditions were similar to those of a commonly used type-B (membrane type) LNG storage tank, based on calculations of the thermal aspect ratio. In general, cryogenic liquid stored in tank shows various thermodynamic behaviors due to heat ingress from outside. Especially, owing to thermal stratification, the experimental results indicate different behaviors compared with those predicted by the homogeneous model. First, the results showed that thermal stratification is highly correlated with the thermal aspect ratio, and also that the higher thermal aspect ratio results in faster pressure increases. In addition, this research was experimentally confirmed the process of thermal de-stratification when the boil-off gas is discharged to decrease the pressure to the target pressure. The amount of boil-off gas discharged during the decrease of pressure down to the target pressure was less than that predicted by a homogeneous model in that the internal energy to be removed is significantly lower than that in the homogeneous model. This research analyzed that the amount of boil-off gas emitted during the same time is compared between the cases of continuous discharge of boil-off gas and discrete discharge of boil-off gas. Consequently, this research experimentally confirmed that the discrete venting method causes the emission of a smaller amount of boil-off gas. This experimental results provide useful information for the design and operation of the equipment by analyzing various thermodynamic behaviors involved in the formation and removal of the thermal stratification.