Influence of gravity level on the self-pressurization performance of large scale cryogenic storage tank.

The self-pressurization process of cryogenic liquid hydrogen tank considering the phase change, liquid thermal stratum and surface tension is investigated with a computational fluid dynamic (CFD) model. Validated by the experiment data obtained from the flight test, the improved CFD model is selected to research the influences of reduced gravity on tank pressurization process. In order to recognize the tank pressurization and thermal stratification performance clearly, four gravity levels (1g0, 10-1g0, 10-2g0 and 10-3g0) are considered and compared with each other. The results indicate that the gravity level has a great effect on pressure increasing process. Some valuable conclusions are listed as: (1) Comparing with the pressure difference of 1g0, the pressure differences in reduced gravity varying from 10-1g0 to 10-3g0 are increased by 10.7%, 16.7% and 27.6%, respectively. Opposite to the change tendency of pressure differences, evaporation losses are 1.593kg, 0.637kg, 0.332kg and 0.232kg with the gravity level ranging from 1g0 to 10-3g0. In addition, the phase change form at the beginning of the pressurization process is condensation, with the time continuing, evaporation would become the leading phase change form. Heat leakage through tank wall plays a leading role in tank pressure rising, the effects of evaporation quality are not appeared in short time. (2) With the increasing of gravity levels, the temperature distributions in the ullage region become more uniform, and the liquid stratum layers develop faster. Calculated with CFD model and compared with two theoretical models, it turns out that the relative errors of stratum thickness between CFD model and Tellep model are always smaller than that of stratum thickness between CFD model and Reynolds model. However, it is opposite for the comparison of stratum temperature. (3) Different direction stream cycles are forming in the upper part of the ullage. Many large and small vortex or loops are developing between two slosh baffles. The streamline distribution in the liquid stratum like a plume and forms circulate loop between the tank wall and axis. (4) Influenced by surface tension, liquid close to the tank wall moves up along the wall in reduced gravity. The interface in the gravity level of 10-1g0 is still flat as same as that of 1g0, while others are becoming curved with the interface area increasing.