Measured and Calculated Performance of a High Frequency, 4 K Stage, He-3 Regenerator
Lewis, M. A.
Bradley, P. E.
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Efficiencies of small 4 K cryocoolers are less than 1% of Carnot, whereas 80 K cryocoolers achieve efficiencies of up to almost 20% of Carnot. The primary loss mechanisms in low temperature regenerative cryocoolers are caused by the non-ideal gas properties of the He-4 working fluid and the reduced volumetric heat capacity of the regenerator matrix compared with that of He-4 at these low temperatures. A recently developed model, REGEN3.3, which incorporates the thermodynamic and transport properties of He-3, shows that using He-3 should improve the performance of 4 K regenerators considerably. REGEN3.3 was used to design an optimized test apparatus to measure the performance of a He-3, 4 K regenerator with the warm end precooled by a Gifford- McMahon cryocooler to about 35 K. The test regenerator is designed to operate at 30 Hz and uses a layered matrix of gadolinium oxysulphate (GOS) spheres at the cold end and erbium-praseodymium (ErPr) spheres at the warm end. The experimental test apparatus, testing procedures and the results are presented. In addition, a novel method to improve the phase shifting at the warm end of the small He-3 cold-stage pulse tube is described along with its impact on the cycle performance.