Computational fluid dynamics (CFD) modeling is possibly the best available technique in designing and predicting the performance of cryocoolers. Pulse tube cryocoolers function with micro porous material housed within their regenerator and heat exchanger components; however, the thermal and hydrodynamic transport phenomena associated with these micro structures are not fully understood. Complete analysis of the fluid-solid interaction through this media can be obtained only by direct pore level simulation1, a method which is time consuming and impractical for system level examinations.
Navier-Stokes and energy equations can be volume averaged, leading to conservation equations which capture the macroscopic fluid behavior in porous media without solving for the detailed fluid motion at the microscopic scale. These porous media equations require empirical closure relations. Constitutive relationships, such as the Darcy permeability and Forchheimer’s inertial coefficient, are needed for the closure of these macroscopic volume-averaged conservation equations. Generally, porous media of interest for cryocoolers are morphologically anisotropic therefore the parameters which characterize them depend on the filler type as well as flow direction. In this work, steady flow hydrodynamic parameters are determined using an experimentally measured relationship between fluid flow rate and the pressure drop across the porous media representing several regenerator fillers. Simulating the experimental test sections in CFD, we can iteratively adjust the viscous and inertial flow resistances until agreement is reached between simulated and experimental results.
This work reports on a study of the effect of average fluid pressure on the hydrodynamic parameters associated with steady state axial flow through four regenerator fillers. A previous investigation3 indicated that the values of these parameters might be sensitive to the average fluid pressure in a porous sample. The previous results were not conclusive, however, as large variations in average pressure were allowed in the tests. The four types of porous material investigated here include a 325 mesh stainless steel wire cloth, a 400 mesh stainless steel wire cloth, a 400 mesh stainless steel sintered wire cloth and a stainless steel metal foam. All test samples are common fillers of cryocooler regenerators. In the following sections, the experimental and computational methodologies employed in solving for steady axial flow hydrodynamic resistances are first discussed. The experimental data for each filler sample at three distinct operating pressures is then presented and discussed.
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