An improved method for measuring the compactness factor in a porous medium

The motivation for the research was to determine if reducing the thickness of the wire screens in a stacked-screen regenerator, thereby reducing the dead volume, could be accomplished without adversely affecting the compactness factor (j_H/f). During the course of this research an improved method for determining the heat transfer and pressure drop characteristics of a porous medium regenerator was developed. The focus of this paper is to describe this improved approach. The approach integrates experimental data/data reduction with a numerical model to study the flow of helium through a series of stacked, wire-screen regenerators of different geometries and a range of Reynolds numbers typically found in the operation of Stirling cycle cryocoolers. The experimental component is based on the classical transient, step-change temperature technique. The data reduction employs MATLAB to filter, parameterize, and assemble a data file for use with a FORTAN program. The numerical model is an explicit, finite-difference scheme for incompressible flow in a one-dimensional porous medium. The model includes: (1) the measured inlet temperature trace rather than an idealized one, (2) the important effect of energy exchange between the gas and the tube surrounding the regenerator matrix, and (3) an algorithm for choosing the heat transfer coefficient based on the “sponge effect delay time”