Performance assessment of different porous matrix geometries for active magnetic regenerators

Abstract

The development of efficient active magnetic regenerators (AMR) is highly dependent on the regenerative matrix thermal performance. Matrix geometries should have a high thermal effectiveness and small thermal and viscous losses. In this study, we present a systematic experimental evaluation of three different regenerator geometries: parallel-plate, pin array and packed bed of spheres. All matrices were fabricated with approximately the same porosity (between 0.36 and 0.37). The cross sectional area and length of the regenerator beds are identical, resulting in the same interstitial area. Hence, any difference in performance between the matrices is due to interstitial heat transfer between the solid and the fluid and losses related to thermal, viscous and magnetic effects. As a means to quantify these losses individually, experiments were first conducted using stainless steel matrices without the application of a magnetic field (passive regenerator mode). Later, gadolinium matrices made with the same characteristics as the stainless steel ones were evaluated in an AMR test apparatus for which experimental results of cooling capacity, temperature span between the thermal reservoirs, coefficient of performance and second-law efficiency were generated as a function of utilization for different operating frequencies. Parallel plates had the poorest performance, while the packed bed of spheres presented the highest cooling capacity. On the other hand, the packed bed also had the highest viscous losses. For this reason, the pin array exhibited the highest COP and second-law efficiency.