Theoretical and experimental investigations on the piston offset characteristics in a four-stage DC linear compressor unit for a 1.8 K hybrid cryocooler

Abstract

A four-stage DC linear compressor (DCLC) unit with high reliability, long life and lightweight is developed for a Joule-Thomson cooler (JTC), which is the final cooling stage of a 1.8 K hybrid cryocooler using a four-stage Stirling-type pulse tube cryocooler (SPTC) as the precooler. A dynamic model is built to analyze the physical mechanism of piston offset, in which the influence of the resistance on the mean piston position is emphasized. An approach to optimizing the piston offset is proposed by changing the resistance characteristics of both suction and discharge flows. Experimental results show that through the optimization the piston offset can decrease up to 65.4%, and the ultimate pressure ratio (UPR) increases from 4.1 to 4.9 while the motor efficiency slightly decreases by 2.7%. The similar variation tendencies of experimental and theoretical results are observed, and the experimental results generally meet the theoretical expectation. The measured suction pressure of the optimized four-stage DCLC unit is reduced to 1.5 kPa, which makes an important contribution to the hybrid cryocooler in achieving a 1.8 K cooling temperature with He-4 as the only working medium.