Experimental observation of friction factor variation in regenerators at cryogenic temperatures

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. Widespread experiments have been carried out in the temperature range above room temperature on the friction factor which indicates flow resistance generated by the working gas as it passes through the regenerator. However, it remains unclear whether these experimental formulae are still valid in the cryogenic temperature range in which GM cryocoolers operate, where experimental uncertainty can become significant. Therefore, a new system was developed to measure the friction factor at cryogenic temperatures. The developed system was used to measure the friction factor in zinc spheres which are a typical regenerator material. The measurements revealed that the friction factor varied at cryogenic temperatures compared with that at room temperature. This variation was attributed to changes in the porosity within the regenerator. This paper describes the details of the developed system and the results of the experiments. These results provide necessary verification for applying room-temperature correlations to cryogenic regenerators.