Varying cross-sectional area of regenerator for improving efficiency of 4 K pulse tube refrigerator

Stirling-type pulse tube refrigerators (SPTRs) working at the liquid-helium temperatures are attractive in space applications. However, the refrigeration efficiency is as low as 0.5% of the Carnot efficiency or less. A method for improving the performance of the 4 K refrigerator by increasing the cross-sectional area of the regenerator at the colder part is numerically analyzed in this paper, and it is verified through experiments for the first time. The no-load temperature of the variable cross-sectional refrigerator reaches 3.87 K, which is 0.14 K lower than that of the uniform one, and the coefficient of performance (COP) is increased by 66% at 4.2 K. The reason for reducing the heat-associated enthalpy flow is the increment both in the heat transfer area and the total heat capacity of the matrix at the colder part. It is found that the lower the refrigeration temperature, the more significant the improvement is. Meanwhile, this method brings about some side effects that hinder the positive effects. The improvement of the COP tends to vanish at about 6 K. This new method provides a reference for further improving the performance of the SPTRs and other types of regenerative refrigerators working at liquid-helium temperatures.