Development and application of a measurement system for effective thermal conductivity in cryocooler regenerators

Abstract

In regenerative cryocoolers, the regenerator is composed of stacked layers of the regenerator material in the form of wire meshes or spheres, and the voids of the regenerator are filled with a working fluid, such as helium gas. Thermal conduction loss occurs in the regenerator because of the large temperature difference between its hot and cold sides. It is important to develop a high-efficiency cryocooler to estimate accurately thermal conduction loss. However, it is difficult to estimate the effective thermal conductivity of these complex structures. Therefore, in this study, we developed a new effective thermal conductivity measurement system that can emulate the operating environment of a cryocooler. The soundness of the developed system was confirmed by measuring the thermal conductivity of SUS304. We also measured the thermal conductivities of stacked lead alloy spheres and zinc spheres, and the voids of the stacks were filled with helium gas, which is a typical regenerator for cryocoolers. Measurements were performed in the temperature range of 5–50 K and the pressure range of 0.5–2.5 MPa. A regression analysis revealed that effective thermal conductivity of regenerator was only dependent on the thermal conductivity of the helium gas.