CFD Simulation of Multi-Dimensional Effects in Inertance Tube Pulse Tube Cryocoolers
Cha, Jeesung Jeff
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Inertance Tube Pulse Tube Cryocoolers (ITPTC) are a class of rugged and high-endurance refrigeration systems that operate without a moving part at their low temperature end, and are capable of reaching 4 K or lower. ITPTCs are suitable for application in space vehicles, and attempts are underway worldwide to improve their performance and miniaturize their size. The thermo-fluidic processes in ITPTC are complicated, however, and the details of the mechanisms underlying their performance are not well understood. Elucidation of these underlying processes is the objective of this investigation. In this study, the commercial computational fluid dynamic (CFD) package Fluent䵠was utilized for modeling the entire large ITPTC system that includes a compressor, an after cooler, a regenerator that is represented as a porous medium, a pulse tube, cold and warm heat exchangers, an inertance tube, and a reservoir. The simulations represent a fully-coupled system operating in steady periodic mode, without any arbitrary assumptions. The objective was to examine the extent of multi-dimensional flow effects in an inertance tube pulse tube cryocoolers, and their impact on the performance of these cryocoolers. Computer simulations were performed for two complete ITPTC systems that were geometrically similar except for the length-to-diameter ratios of their regenerators and pulse tubes. For each ITPTC system three separate simulations were performed, one with an adiabatic cold-end heat exchanger (CHX), one with a known cooling heat load, and one with a pre-specified CHX temperature. Each simulation would start with an assumed uniform system temperature, and continue until steady periodic conditions were achieved. The results indicate that CFD simulations are capable of elucidating the complex periodic processes in PTCs very well. The simulation results also show that a one-dimensional modeling of PTCs is appropriate only when all the components of the PTC have very large aspect ratios (i.e., L/D >>1). Significant multi-dimensional flow effects occur at the vicinity of component-to-component junctions, and secondary-flow recirculation patterns develop, when one or more components of the PTC system have small aspect ratios. The simulation results, although limited in scope, also suggest that ITPTCs will have a better overall performance if they are made of components with large aspect ratios.