Convective and thermodynamic analysis of oscillatory flow in pulse tube cryocoolers

Abstract

Pulse tube cryocooler (PTC) is a technology for cryogenic cooling which has no moving part in its cold-end, unlike other types of cryocoolers such as Stirling cryocooler and Gifford-McMahon (GM) cooler. The inherent simplicity and reliability of PTCs make them particularly attractive for applications where long-life and reliability are critical. This thesis focuses on the thermo-fluid aspects of PTCs, in particular, in the pulse tubes and the regenerators, which are the most important components in a PTC. A pulse tube is an empty tube which invokes a phase lag between mass flow and pressure of the working fluid (often helium) under an imposed oscillating gas flow. It replaces the cold piston in a Stirling cooler or the movement of the displacer in a GM cooler. For best performance, pulse tube cryocoolers are installed vertically with their cold ends pointing downwards with respect to gravity. A major problem for pulse tubes is the occurrence of convective instability under non-ideal, tilted configuration, however, which can result in a dramatic drop in the performance of a PTC. In this investigation, computer simulations and experiments are used to investigate the convective instability in a PTC under static and dynamic tilt conditions. To examine the effect of static tilt, the cooling performance of a commercial pulse tube cryocooler is measured when it is tilted from an ideal vertical orientation. In dynamic tilt experiments, the performance of the cryocooler when it undergoes up-and-down (heave) or side-to-side (roll) periodic motions is examined. A combined system-level analysis method and component-level computational fluid dynamics (CFD) simulation is developed to model the operation of a pulse tube cryocooler under tilt conditions. The influence of minor geometric features is studied parametrically. Regenerators are the crucial components of PTCs as well as other regenerative cryocoolers. Regenerators are also an important cause of losses in PTCs. Regenerators are usually made of solid micro-porous materials, such as metal powders or screens, and are subject to a periodic flow of the cryocooler’s working fluid. Ideally, regenerator filler materials must have good thermal interaction with gas and low flow resistance to minimize irreversibility. Unfortunately, these are conflicting requirements. In this thesis, the entropy generation due to both heat transfer and viscous dissipation in generators were investigated based on detailed pore-level simulations. A semi-analytical mathematical model was developed to estimate the entropy generation in regenerators. Some parameters that assess the relationship between entropy generation in the regenerator and the overall efficiency of a PTC were elucidated. The developed semi-analytical model can be used as a general guideline for the geometric design of regenerator fillers for the minimization of entropy generation.