Prediction of entrainment ratio and exergy efficiency for ejectors in a hydrogen liquefaction system

Boil-off gas (BOG) generated during liquid hydrogen storage causes hydrogen loss in liquefaction systems. Liquid hydrogen ejectors can recover BOG by entraining low-pressure tank vapor with high-pressure liquid hydrogen for re-liquefaction. In operation, tank pressure, pipeline heat ingress, and valve regulation change BOG pressure, superheat, and back pressure, causing off-design behavior. Studies often treat entrainment ratio and exergy efficiency as fixed. This study develops an off-design prediction model by combining one-dimensional thermodynamic design with two-dimensional CFD simulations. The 1-D model determines geometry, while CFD quantifies the effects of operating variables on two-phase flow and performance. Empirical correlations for entrainment ratio and exergy efficiency are derived and applied to a hydrogen liquefaction case. Increasing initial BOG pressure from 300 to 600 kPa extends recovery time from 36 to 48 h; at 600 kPa, increasing superheat from 0 to 10 K extends it from 30 to 48 h and reduces exergy efficiency.