Comparative analysis of energy losses in hydrogen and helium turbo-expanders for hydrogen liquefiers

Liquid hydrogen plays an essential role in the large-scale hydrogen energy development. Claude cycle is the baseline process for most industrial hydrogen liquefactions. The hydrogen turbo-expander is the crucial component, determining the system efficiency and reliability. In this paper, a hydrogen turbo-expander is designed by using a mean-line method with loss models which have been validated against experimental data of helium turbo-expanders. Geometrical and velocity field similarity can be achieved between a hydrogen turbo-expander and a helium turbo-expander by selecting the same specific speed and the characteristic ratio. CFD simulations are performed to illustrate the internal flow field and loss mechanisms in helium and hydrogen turbo-expanders. The CFD predictions of passage efficiencies agree well with that by the loss models, demonstrating the mean-line design method and loss models can be used to guide the preliminary design of hydrogen turbo-expanders. The CFD simulation results show that the passage efficiency of hydrogen turbo-expander is 1.88% higher than that of helium expander. Due to a higher sound speed and a lower viscosity, the hydrogen turbo-expander has smaller nozzle loss, incidence loss and impeller passage compared with the helium turbo-expander. The lower viscosity of hydrogen results in a higher tip leakage flow, and the tip leakage accounts for 1.09% and 1.23% of total mass flow in helium and hydrogen turbo-expanders, respectively. Since the cross flow over the blade tips can increase the vortices and secondary flow losses in impeller passages, it is necessary to reduce tip clearance in the design of hydrogen turbo-expanders.