Numerical study on flow and mixing characteristics of ejectors in hydrogen liquefaction systems

In hydrogen liquefaction systems, ejectors can replace throttle valves to recover boil-off gas (BOG) from liquid hydrogen storage tanks and enable its re-liquefaction. However, most research focuses on system analysis, with limited attention to the internal mixing layer characteristics of ejectors. The mechanisms affecting the thickness and energy distribution of the mixing layer remain unclear, and the impact of operating conditions is difficult to quantify. In particular, variations in the release pressure of storage tank and heat leakage from the tank to the ejector, resulting in the secondary fluid superheating, can alter the mixing layer and influence the ejector's performance in BOG recovery and re-liquefaction. In this study, the one-dimensional preliminary design model is firstly used to determine the key ejector’s geometries, and the detailed CFD simulations are then performed to resolve the internal flow and mixing processes. The results show that the mixing layer forms at the interface between the primary and the secondary streams, and thickens downstream. For subsonic secondary fluid, the mixing layer develops more uniformly, and the entrainment ratio reaches a maximum at the secondary inlet pressure of 500 kPa. When the secondary fluid is superheated, the secondary inlet superheat has the stronger influence on the performance: the superheat of 10 K yields both the largest final mixing-layer thickness and the highest entrainment ratio. The findings provide theoretical foundations for optimizing the performance and operating conditions of ejectors in hydrogen liquefaction systems, contributing to enhanced BOG recovery and re-liquefaction efficiency.