09. Heat and Mass Transfer

The transition to reduce greenhouse gas emissions has spotlighted liquid hydrogen (LH2) as a promising alternative fuel. LH2 is created by cooling hydrogen gas to 20K, but due to stratification in LH2 tanks due to temperature differences, it slowly evaporates, producing boil-off gas (BOG) and causing self-pressurization in LH2 storage tanks. This necessitates venting to release excess pressure, resulting in hydrogen loss. Factors such as insulation quality and surface-to-volume ratio significantly impact BOG formation and heat leakage. This paper investigates two crucial aspects for the industrial application of liquid hydrogen (LH2) tanks: heat leakage from the environment and self-pressure estimation. Heat leakage to a 2.5-ton LH2 tank was computed using a computational fluid dynamics (CFD) model with ANSYS-CFX, modelling the inner/outer vessels, vacuum, and insulation domains for accurate prediction. Simulations were performed with insulation thermal conductivities ranging from 0.02 mW/mK to 0.05 mW/mK. The heat transfer data was then used for self-pressurization estimation using a time-dependent thermodynamic model, validated against NASA’s experimental data. Analysis revealed that as the thermal conductivity of the multilayer insulation (MLI) increases from 0.02 mW/mK to 0.5 mW/mK, heat leakage to the tank increases by approximately 29.4%. Self-pressurization of the LH2 tank could occur within 7 to 10.5 days, depending on insulation quality. The study also highlighted the impact of insulation quality and surface-to-volume ratio on boil-off gas (BOG) formation. Boil-off mass for a 2.5-ton LH2 tank was computed to be 0.03% per day using insulation with a thermal conductivity of 0.05 mW/mK. These findings and the thermodynamic model provide critical insights for the safe transportation of LH2 via cryogenic storage tanks.