Effect of Regenerator Material Configuration on 4K-GM Cryocooler Performance

This paper describes experimental results of a regenerator study made on a 4K-GM cryocooler. The configuration of the regenerator material is one of the important parameters for the improvement of a 4K-GM refrigerator. This study investigated the effect of the configuration of the regenerator material, specifically its diameter and porosity, on the performance of the cooler. Firstly, the cooling capacity around 6 K was measured for various diameters of lead (Pb) spheres from 0.06 mm to 1.18 mm. Because of the larger heat-transfer area and higher coefficient of heat transfer associated with smaller spheres, they show a higher NTU, which improves the efficiency of the regenerator. But smaller spheres also lead to higher pressure drop in the regenerator, and that reduces the expansion work at the cold end. The study confirmed that an optimal diameter exists when the porosity of the Pb-sphere packed bed is kept constant. However, the noted improvement of cooling capacity with change in diameter was not particularly remarkable. Secondly, the effect of the bed’s porosity on the cooling capacity was measured by mixing two different diameters of Pb spheres. The lower porosity regenerators showed an improvement of cooling capacity in some conditions. Lastly, based on the experimental results with Pb regenerators, the cooling capacity at 4 K was further investigated using HoCu2 as the regenerator material. The porosity of the packed bed of HoCu2 spheres was also changed by mixing different diameters of spheres. The cooling capacity increased about 14 % when the porosity was decreased from 34 % to 27 %