Investigation of helium and nitrogen mixtures in a pulse tube refrigerator

Normally, the working fluid used in regenerative cryocoolers is not condensed at any point in the working cycle. Helium is the popular working medium for such systems, rather than a gas mixture because of its excellent thermodynamic and transport properties. However, our theoretical analyses indicate that pure helium is not the best working fluid for regenerative refrigerators near 80 K, either for the best cooling power, or for the best coefficient of performance (COP). Motivated by the advantages of pulse tube refrigerators, which have no moving parts in the cold end, this paper describes an experimental investigation on a two-component, multi-phase helium and nitrogen gas mixture in a single-stage pulse tube refrigerator. The experimental results show that the COP and the cooling power can be increased to some extent near 80 K with the use of two-component gas mixtures with less than 25% nitrogen. A relatively stable temperature platform at the triple point of nitrogen (63.15K), which is independent of the nitrogen fraction, is obtained when the cooling power is below 7W.