DC Gas Flows in Stirling and Pulse Tube Cryocoolers

Abstract

Whenever a closed-loop flow path exists in a Stirling or pulse-tube cryocooler, there is a potential for a DC gas flow — where the positive and negative flows over each half cycle do not quite cancel completely. A closed loop may be formed by a displacer seal in a Stirling cooler, which connects the compression and expansion spaces in parallel with the regenerator, or by a bypass orifice in a double-inlet pulse-tube cooler, which connects the warm end of the compliance tube (pulse tube) back to the compression space.

DC gas flows were discovered in simulation models of double-inlet pulse-tube cryocoolers run with the Sage computer program. At the root of the problem are in-phase density and velocity fluctuations in the flow-resistive elements of the closed loop which tend to produce DC pressure gradients. The issue is whether or not these DC pressure gradients completely cancel each other. If they do not, then DC flow is the consequence and designers must contend with new thermal losses associated with components like regenerators and compliance tubes. A DC flow of less than one percent of the AC flow amplitude may be enough to cause significant heat lift degradation