氦气作为增压气体排出贮罐内液氢过程的CFD分析

CFD study on discharging process of liquid hydrogen in tank with helium as pressurized gas

基于计算流体力学方法,数值模拟了用常温氦气作为增压气体压出贮罐内液氢过程的流动和传热传质特性。构建了基于二维轴对称的VOF多相流以及包含氢气和氦气组分流动的气相多组分数值模型,液氢界面相变传质基于Hertz-Knudsen方程计算。分析了排出过程贮罐内压力、温度、液位及液氢相变率随时间的变化,重点考察气相出现在贮罐出口时间,以及此时气相中氦气含量。发现刚开始增压时,高温氦气和低温氢气传热只发生在氦气进口附近,贮罐内压力增加较慢,液氢界面不存在蒸发现象。随着进入氦气增加,贮罐内气相温度逐渐形成分层,在一定时刻,液面上气体温度开始上升,触发沸腾蒸发,导致压力快速增加。由于贮罐出口液体外流导致的减压效应远小于气相空间的压力增速,贮罐压力急剧增加并超过氦气入口,部分低温气体混合物从入口倒流出贮罐,同时使氦气入口处温度降低。由于贮罐内压力增加,底部液氢出口流量随时间呈线性增加。计算结果揭示了液氢贮罐增压流出过程复杂的流动和传热传质特性,对低温液体的储运有实际工程指导意义。

The flow, heat and mass transfer characteristics of liquid hydrogen in a storage tank were calculated numerically in the discharging process with room-temperature helium gas as the pressurized gas based on the computational fluid dynamic method. A two-dimensional axisymmetric numerical model was constructed based on VOF muhiphase flow formulation, in consideration of the helium and hydrogen transfer in the gas phase. The mass transfer due to phase change in the gas-liquid interface was determined by the Hertz-Knudsen equation. The variances of pressure, temperature and liquid level with time were analyzed, and the moment of gas phase fraction as well as the helium gas fraction at the outlet were specially moni-toted. It is found that at the beginning, the pressure in the tank increases slowly due to the slow heat trans- fer between the gases, and evaporation of liquid hydrogen is not triggered. As time elapses, the temperature over the phase interface begins to rise, which triggers the boiling evaporation and leads to a rapid increase in pressure. The pressure becomes even larger than the inlet pressure, as a result, the low- temperature gas mixture flows out of the tank, leading to a decreases in temperature there. The results reveal the complex flow, heat and mass transfer characteristics during the liquid hydrogen discharging process.