Thermodynamic Modelling of Superfluid Stirling Cryocoolers

Abstract

The aim of this paper is to describe and thermodynamically model cryogenic Stirling refrigerators, using Helium in its different forms as the working medium. Helium has unique properties at cryogenic temperatures forming a superfluid. The cryogenic Stirling refrigerators with Helium at such low temperatures make use of the properties of this superfluid nature of Helium, thus they are referred to as Superfluid Stirling Refrigerators (SSR). To make use of these remarkable properties of superfluid helium a different version of Stirling refrigerator is used where superleaks are introduced in the pistons in order to let the superfluid part flow freely but constrain the normal fluid. This cooling procedure has an upper temperature limit as it is based on the superfluidity of helium, hence all the workings of this cycle must be well below the Lambda line. In addition, different models are needed and are used for the different isotopes of helium as their atomic spin nature is different and therefore their behavior at temperatures near absolute zero. In this study of SSR cryocoolers great care is being given towards the thermodynamic behavior of the entire system and working media, as well as different designs of the apparatus.