SUMMARY

The regenerator is a critical component of all Stirling and Pulse Tube Cryocoolers (PTCs). It generally consists of a microporous metallic or rare-earth filler material contained within a cylindrical shell. The accurate modeling of the hydrodynamic and thermal behavior of different regenerator materials is crucial to the successful design of cryogenic systems. Previous investigations have used experimental measurements at steady and periodic flow conditions in conjunction with pore-level CFD analysis to determine the pertinent hydrodynamic parameters, namely the Darcy permeability and Forchheimer coefficients. Due to the difficulty associated with experimental measurement at cryogenic temperatures, past investigations where performed at ambient conditions. These results are assumed to be accurate for cryogenic temperatures since, for fully-developed flow, the Darcy permeability should depend only on the geometry of the porous medium, while the Forchheimer coefficient should depend on geometry as well as dimensionless numbers that do not directly depend on temperature. There is, however, a pressing need to examine the validity or otherwise of the foregoing hypothesis and to determine the hydrodynamic parameters of commonly-applied cryocooler regenerator filler materials and more recently utilized materials, including ErPr, under prototypical conditions.

First, the periodic flow of helium at ambient and cryogenic temperatures within the porous media of representative PTC regenerators will be characterized using several widely used cryocooler regenerator filler materials including #325SS mesh, #400SS mesh, and ErPr powder. Next, a two-stage PTC with 20 W cooling power at 20 K will be designed using industry-standard design tools. The sensitivity of the performance parameters of the PTC to the error associated with using room-temperature closure parameters for the regenerator instead of the more realistic cryogenic-temperature parameters will be examined. Using the experimental data, along with a CFD-assisted methodology, the Darcy permeability values and Forchheimer coefficients that lead to agreement between measurements and simulations are iteratively calculated. The sensitivity, or otherwise, of the coefficients to temperature will be quantified and assessed. Empirical correlations suitable for implementation in CFD codes will then be developed for the prediction of Darcy permeability and Forchheimer coefficient.