Design and optimization of the four-stage recuperative coiled tube-in-tube heat exchanger for a 1.8 K hybrid cryocooler

Abstract

The recuperative coiled tube-in-tube heat exchanger (RCTTHE) is a key component of the hybrid cryocooler formed by a four-stage Stirling-type pulse tube cryocooler and a JT cooler, and its heat exchanger effectiveness and pressure drop of low-pressure side play an important role in enhancing the overall performance of the hybrid cryocooler. This paper develops a finite difference model to systematically investigate the four-stage RCTTHE. The flow and heat transfer characteristics in the temperature range of 4–300 K under different dimensional and operating parameters are investigated in detail. The heat exchanger effectiveness and pressure drop of the low-pressure side are optimized to meet the requirements of the 1.8 K hybrid cryocooler with He-4 as the only working medium. The experimental investigations of the hybrid cryocooler based on the optimized four-stage RCTTHE are conducted. Satisfactory agreements are observed between the theoretical and experimental data, and thus verify the proposed model. The experimental results show that the pressure drop of the low-pressure side of the RCTTHE reaches a considerably low value of 518 Pa and an effectiveness of higher than 97.1 % is achieved for the four-stage RCTTHE, both of which make a significant contribution to the hybrid cryocooler in achieving 1.8 K.