Simulation study on the effect of residual gas on the insulation performance of liquid hydrogen spheres

Large liquid hydrogen (LH₂) storage spheres are commonly insulated using evacuated hollow glass microspheres (HGMs), with residual gas pressure and composition playing a critical role in thermal performance. In this study, the pressure distribution within the insulation layer was estimated using rarefied gas theory and incorporated into a one-dimensional heat transfer model to evaluate the effective thermal conductivity (ETC) and total heat leakage. The influence of bulk density on thermal insulation was first analysed. The ETC variation across a full range of cold vacuum pressure (CVP) was then examined. Lastly, the effect of substituting nitrogen (N₂) with alternative gases for jacket evacuation was analysed, and the underlying heat transfer mechanisms were revealed. The findings indicated that at high CVPs, the heat transfer process was dominated by gaseous conduction, with insulation performance significantly affected by the bulk density. Under low CVPs, radiative heat transfer prevailed in warmer regions, while both radiation and conduction contributed to the lower temperatures. The ETC reached limiting values of 24.88 and 0.64 mW/(m·K) at CVPs above 1000 Pa and below 0.1 Pa, respectively, with minimum heat leakage recorded at approximately 125 W. Moreover, at CVPs exceeding 1 Pa, the residual gas type markedly influenced insulation behaviour, and krypton (Kr) reduced the LH₂ evaporation rate by over 18.43 % compared to N₂.