Cool-Down Dynamics of a Cryostat with a Closed-Cycle Cryogenic Refrigerator

Abstract

A cryostat designed for a large-scale superconducting motor application is to be cooled by a reversed Brayton cycle cryogenic refrigerator. A set of four high-temperature superconducting (HTS) field winding coils reside in the cryostat. A nominal operating temperature of 33 K for these HTS coils of the synchronous motor is maintained by the refrigerator through circulating cold helium gas through a pair of cooling channels in the cryostat. As part of the rotor of the HTS motor, the cryostat will be rotated at 1,800 rpm under normal operations. To reduce the thermal stress developed in the HTS coils and their supporting structure, the cool-down of the refrigerator and cryostat takes place simultaneously after being coupled by a pair of bayonets. Substantial variation of helium properties through the cool-down temperature range attributes to the dynamic behavior in hydrodynamics and heat transfer of helium gas in the cooling circuits. A numerical model is developed to simulate and characterize the cool-down process. Analytical results define optimal engine speeds at various temperature levels for maximum cooling capacity, which are compared to the design constraints of thermal stress level requirements. Potential hydrodynamic instability issues associated with the parallel cooling channels, typically during the cool-down, are also investigated and will be discussed in details.