Flow through Stirling engine regenerators with and without a gap between the matrix and the container wall: Part 2, experiments

The Stirling engine regenerator is an energy storage device that exchanges thermal energy with the working gas as the gas oscillates between the hot and cold sides of the engine. In the life of the engine, an annular gap may develop between the regenerator material and the wall of the casing that holds it. This gap may be due to thermal growth of the casing wall or compression of the regenerator matrix. In this paper, a stacked-screen porous medium with a high void volume is used to physically model a regenerator. A matrix of uniform porosity fills the entire casing; or, a uniform porosity matrix is separated from the casing wall over half its axial length by an annular gap. Two gap sizes are considered. In the experiments, the flow is steady. Flow directions are such that the gap is either in the upstream half of the regenerator or in the downstream half. This experimental paper has a companion paper in which the flow is modeled numerically. Total pressure drop measurements and exit-plane velocity measurements in steady flow characterize the change in the flowfield with the addition of a gap. Darcy velocities are up to 3.5 m/s. Velocities measured at the porous medium exit plane are presented for cases of Reynolds numbers based upon hydraulic diameter and Darcy velocity ranging from 45 to 195. The regenerators with gaps have lower pressure drops than those without a gap. Furthermore, the regenerators with upstream gaps have lower pressure drops than regenerators with downstream gaps. The velocity leaving the downstream gap is between 1.34 and 3 times the velocity leaving the matrix at the same radial location in the no-gap regenerator. This depends on the gap size and Reynolds number. When the gap is upstream, the exit plane velocities downstream of the gap are lower than the exit plane velocities at the same radial location in the case of no gap, suggesting that the upstream disturbance to the flow due to the gap persists in the porous medium even after 21 hydraulic diameters of travel. An integral of the near-wall velocities to determine the flow rates over the radial distance influenced by the gap shows that the half-length gap creates a significantly different flow in one direction versus the other. This asymmetry of flow behavior would have important effects on oscillatory flow in the Stirling regenerator.