On the numerical design of a new type of 4 K GM/PT hybrid refrigerators Y.L. Ju a, * , L. Wang b a Cryogenic Laboratory, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, P.O. Box 2711, Beijing 100080, PR China b Collider Accelerator Department, Brookhaven National Laboratory, Upton, NY 11973, USA Received 5 March 2002; accepted 16 May 2002 Abstract In this paper we developed and designed, based on theoretical considerations, a new type of 4 K GM/PT hybrid refrigerators. The upper warm stage of the hybrid refrigerator is a typical GM refrigeration cycle, and the cold stage is a pulse tube refrigerator (PTR), on which is thermodynamically coupled the upper warm stage. Four different types of phase shifting assembly: (1) a cold auxiliary piston that is connected to the displacer of the upper GM refrigerator stage, (2) an orifice with reservoir encircled the cold head of the upper stage, (3) an orifice with double-inlet, and (4) in combination with the cold auxiliary piston, orifice and double-inlet, has been proposed and analyzed for the adjustment of the phase shift between the gas mass flow and pressure in the pulse tube. Nu- merical simulation is performed to understand the unique thermo-physical features, to reveal the time-dependent dynamic pa- rameters and to quantify the overall cooling performance of the hybrid refrigerator. We rely on a one-dimensional, unsteady compressible flow numerical model that is based on a mixed Eulerian–Lagrangian method developed by the present author. The model will be first applied to analyze the cooling performance of the hybrid refrigerator with different types of phase shifting as- sembly. In what following, it is used to simulate the dynamic parameters in the cold stage of the pulse tube cycle. Next, optimization of the structure parameters and geometrical configuration of the new refrigerator will be presented. Finally, the influences of dif- ferent hybrid regenerative materials on the cooling capacity of the new hybrid GM/PTR will be also discussed. Ó 2002 Elsevier Science Ltd. All rights reserved. Keywords: GM/PT hybrid refrigerator; Numerical study; Liquid helium temperature 1. Introduction There are a growing number of superconducting de- vices that require cryogenic cooling at liquid helium temperatures. Due to the introduction and developments of magnetic regenerative materials, the low temperature performance of GM and pulse tube refrigerators (PTRs) has been significantly improved, particularly at 4.2 K- temperature range [1,2]. GM refrigerators are compact and reliable machines and have been used for commer- cial application in the last decade. Up till now, two-stage GM refrigerators, which provided more than 3.6 W cooling power at 4.2 K with a compressor power of 8 kW [3] and a maximum coefficient of performance (COP) of 6  10 4 [4], have been reported. However, the displacer and associated multiple seal rings operated in the low temperature region cause inevitably serious mechanical vibrations, magnetic noise and temperature fluctuations, and possible limitation of lifetime. Fur- thermore the structure of its cold stage is complex. The absence of the displacer in the PTRs gives them many potential advantages over GM refrigerators in many applications. The development and improvement in recent years enables PTRs to be an ideal alternative of GM refrigerators in terms of design fabrication, ef- ficiency, reliability, and lifetime for space applications. Since 1994 Matsubara and Gao first obtained a tem- perature of 3.6 K using a three-stage PTR [5], the lowest temperature of PTRs is now well below the inversion temperature of 3 He and 4 He. By using 3 He as the working fluid the lowest temperature 1.78 K has been achieved [6]. Such temperatures have not yet been real- ized with conventional GM refrigerators. Up till now, the PTR with 600 mW cooling capacity at 4.2 K on the second stage is now commercially available with an Cryogenics 42 (2002) 533–542 www.elsevier.com/locate/cryogenics * Corresponding author. Tel.: +86-10-6262-7302; fax: +86-10-6256- 4049. E-mail address: yonglin@cl.cryo.ac.cn (Y.L. Ju). 0011-2275/02/$ - see front matter Ó 2002 Elsevier Science Ltd. All rights reserved. PII:S0011-2275(02)00072-3