Design of a Cryogenic Turbine for a Hybrid Cryocooler

Abstract

The hybrid pulse tube-reverse Brayton cycle cryocooler has the potential for cooling to temperatures on the order of 10 K. By using the rectifying interface which converts the oscillating pulse tube flow to continuous flow, both vibrations and low temperature regenerator losses are overcome, making the hybrid an ideal candidate for cooling infrared focal plane arrays which demand low temperature and low vibration. However, the turboexpander within the reverse Brayton cycle is complex and its performance is highly dependent on the performance of its subcomponents, thus necessitating a model predicting the turboexpander performance.

A model was developed to predict the performance of the reverse Brayton cycle stage including the recuperative heat exchanger and turboexpander components. The turboexpander was numerically modeled in detail to include the sub-models of rotordynamics, the thermal and leakage performance of the seal, and the turboalternator. Where possible, the models were verified against either an analytical model or experimental data. A parametric analysis was carried out to determine the optimal design and conditions for the turboexpander.