Liquified hydrogen fuel can only be transported in cryogenic tanks (cryotanks), which maintain temperatures below -253°C – the boiling point of hydrogen. Despite thermal insulation, the liquefied fuel in a cryotank experiences a degree of vaporization. The flow rate of vaporization is measured as boil-off gas (BOG). Too high BOG can result in excess internal pressure inside the tank, leading to cracks and fissures. This makes understanding and controlling BOG a key factor in cryotank design.
To this end, a research team, led by Professor Jong-Chun Park of Pusan National University in South Korea, have investigated how BOG varies with another critical design parameter called tank filling ratio (FR) – the ratio of the mass of liquefied fuel in the tank to the capacity of the tank at 15°C. “In our study, we performed experiments, as well as simulations, to analyze the thermodynamic characteristics of the tank,” said Prof. Park. The study has been published in Volume 255 of the Energy journal.
From their experiments, the researchers found that BOG increases quadratically with FR. They also found that while the temperature within the liquid phase remained constant, the temperature of the vapor phase decreased non-linearly with FR. The researchers then performed multiphase-thermal flow simulations of the tank using computational fluid dynamics. This allowed them to easily visualize the heat transfers, thermal flows, and vaporization within the vacuum-insulated tank. “We adopted the Rohosenow’s phase change model for the simulations, which allowed us to reproduce the vaporization process within the tank. From our simulations, we were finally able to reveal the mechanism of BOG as a result of vaporization,” explained Prof. Park. The researchers validated their simulations using the data from the experiments conducted through a collaboration with Daewoo Shipbuilding & Marine Engineering Co., Ltd.
The multiphase-thermal simulation technique used in this research could accelerate the design of safe and efficient commercial cryotanks for liquefied hydrogen, with applications of this research ranging from automobiles and aerospace to offshore power plants.