Reversible Cycle Piston Pulse Tube Cryocooler

Abstract

The replacement of the orifice and reservoir elements of a fixed orifice pulse tube cryogenic refrigerator with an expander piston results in a reversible cycle pulse tube machine. At the 7th International Cryocooler Conference, Ishizaki and Ishizaki presented the experimental performance of a machine where the phase relationship between the compressor and expander pistons was maintained by a kinematic drive. An alternate implementation can be based on flexure bearing supported reciprocating linear drive engines wherein the phase shift between the compressor and the expander pistons is maintained by either appropriate plumbing of several dual piston engines across a like number of regenerator/pulse tube elements, or by separate compressor and expander engines with appropriate phase control electronics.

A generalized lumped parameter model of thermally regenerated engines has been developed and programmed to predict the comparative thermodynamic performance of Alpha type Stirling cycle, fixed and modulating orifice pulse tube, and piston-pulse tube machines. Subject to the modeling approximations, the analysis predicted the following: Alpha Stirling Cycle — 35%; Piston-Pulse Tube — 29%; Modulating Orifice Pulse Tube — 19.5%; Fixed Orifice Pulse Tube — 16.5% of Carnot for a 5 to 10 watt at 60K cryocooler with a 300K heat rejection temperature.

The basis of the analytical model and computer program, the results of the computer analysis, a preliminary design of the flexure supported dual piston integrated compressor-displacer engine, and a schematic of the fluid circuit required for phase shift control will be presented. The prime cryocooler application of this technology is in multistage applications.

A major attraction of piston-pulse tube machines is that all moving parts are operating at effectively the heat rejection temperature, which simplifies the design and fabrication of cryogenic refrigerators, and which allows increased temperature ratios (with resulting higher Carnot limits) for power producing integrated alternator-heat engines.