Experiments and Numerical Analysis of Rollover Phenomena in LNG Storage Tanks for Rocket Launch Sites

In the storage and supply of cryogenic propellants at rocket launch sites, reducing boil-off gas (BOG) generation and ensuring safe operation are of critical importance. When liquefied natural gas (LNG) for rocket fuel with different densities is stored in the same tank, stratification may occur. As external heat input reduces the density difference between the upper and lower layers, rollover may be induced, accompanied by rapid mixing and the generation of a large amount of BOG. Because rollover can potentially cause LNG leakage and damage to storage tanks, reliable prediction techniques are required. The long-term aim of this research is to establish a numerical analysis framework capable of predicting rollover phenomena. As a fundamental step toward this goal, laboratory-scale experiments were conducted using simulant liquids in a sealable metal tank to investigate the effects of pressure variations under sealed conditions on liquid behavior and the onset time of rollover. Preliminary heating experiments were also performed to approximate the thermal environment of an actual LNG storage tank and were compared with heating experiments initiated from room temperature. The results demonstrated that sealed conditions delayed the onset of rollover compared with atmospheric conditions. In addition, flash evaporation was observed under sealed conditions, and dedicated experiments confirmed its occurrence. Furthermore, to construct and validate a numerical analysis model for predicting rollover onset, computational fluid dynamics analyses were performed by introducing a density calculation model that accounts for volume contraction and a free-surface evaporation model, while considering the LNG composition. The predicted onset time agreed with previously published experimental data within an error of approximately 4%, demonstrating the validity of the proposed method.