Recent successful CFD models of cryocooler systems have shown that such models can provide very useful performance predictions for cryocoolers. For miniature cryocoolers, CFD modeling is likely the best technique available as models developed for larger systems may not accurately represent phenomena which become important as the device scale is reduced. Accurate CFD modeling of Stirling and pulse tube refrigerators requires realistic closure relations, particularly with respect to the hydrodynamic and thermal transport processes for the porous media which make up their heat exchangers and regenerators. Generally, these porous media are morphologically anisotropic, and thus the parameters which characterize them are anisotropic as well. Measurement of the hydrodynamic parameters in at least two dimensions is therefore preferred.
Miniature regenerative cryocoolers will require porous regenerator and heat exchanger fillers with considerably smaller characteristic pore sizes than those commonly used in larger scale devices. This paper describes measurements of the hydrodynamic parameters of stacked discs of 635 mesh stainless steel and 325 mesh phosphor bronze using a CFD-assisted methodology. These materials are among the finest commercially available structures and can be suitable for use as miniature regenerator and heat exchanger fillers. Measurements were made in the axial and radial directions for both steady and oscillatory flow. Higher frequency operation is preferred for miniature cryocoolers; therefore a frequency range between 50 and 200 Hz was investigated for the oscillatory flow cases. The test setups for steady flow incorporated static pressure transducers and a mass flow rate meter; for oscillatory flow, the apparatus included dynamic pressure transducers and hot wire probes for CFD model verification. These test setups were each modeled using the Fluent CFD code. The directional Darcy permeability and Forchheimer’s inertial coefficients were obtained based on iterative comparisons between experimental measurements and CFD simulation results.
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