Study on the Mechanism of Gas-Phase Space in Liquid Hydrogen Cylinders Under Different Filling Rates

To ensure that the necessary gas-phase safety space is retained after the filling of a liquid hydrogen cylinder and to reduce the risk of rapid pressure rise caused by overfilling, a 34 L vehicle-mounted liquid hydrogen cylinder was taken as the research object. A non-isothermal two-dimensional numerical model considering gas–liquid two-phase flow, heat transfer, and phase change was established. The dynamic and thermal characteristics of cylinders with and without a gas phase space were compared under different filling rates. The results show that, during liquid hydrogen filling, the liquid phase first accumulates at the bottom of the main chamber. Then, the liquid level rises and compresses the upper gas phase, and part of the liquid hydrogen enters the gas-phase space in the later stage. The gas-phase space can delay the occupation of the safety gas cushion by liquid hydrogen, allowing a certain volume of compressible gas to remain during overfilling. The pressure variation presents three stages: a rapid increase in the initial stage, a slower increase in the middle stage, and another rapid increase in the final stage. These stages are related to liquid hydrogen flash evaporation, gas-phase cooling and condensation, and compression of the remaining gas, respectively. The tank temperature generally shows a rapid decrease followed by a slower decrease. As the filling rate increases, the liquid level rises faster, the gas–liquid interface disturbance becomes stronger, the liquid hydrogen enters the gas-phase space earlier, the pressure rise rate increases, and the buffering effect weakens. The results indicate that the gas-phase space structure can improve the safety margin in the final stage of liquid hydrogen cylinder filling, but the filling rate should still be reasonably controlled in actual filling processes.