Off-design performance of a helium two-phase turboexpander designed through multi-objective optimization

Helium turboexpanders are the core component in the cryomodules of large-scale scientific facilities requiring ultra-low-temperature environments. However, when operating at two-phase region, the thermal characteristics become more complex, it is vital to achieve efficient design and evaluation for helium two-phase turboexpanders. This study proposes an integrated optimization design method for helium two-phase turboexpanders. Subcooling constraint and humidity loss correction were incorporated into the one-dimensional mean-line thermodynamic design. In the aerodynamic design, CFD numerical simulations were verified by experiments, which were combined with a multi-objective genetic algorithm to achieve three-dimensional optimization on both isentropic efficiency and liquid mass fraction. The relationship between performance parameters and key geometric parameters were analyzed using response surface method, consistent with optimization results. Results demonstrate that the optimized turboexpander achieves an isentropic efficiency of 85.24% and a liquid mass fraction of 1.44%, representing improvements of 6.51% and 24.14% over the initial design respectively, which can be further adjusted by characteristic ratio. The flow field distribution was significantly improved after the optimization, confirming the effectiveness of the design method. Under some off-design conditions, the expansion-compression fluctuations were observed in the nozzle, which cause the deviation of performance of turboexpander under design condition. Thus, the inlet temperature and expansion ratio of the turboexpander should be carefully controlled. The condensation compressibility coefficient was introduced to characterize the unstable flow patterns clearly.