Improving the performance of room temperature magnetic regenerators using Al2O3-water nanofluid

The present study examines the use of Al2O3-water nanofluid as the heat transfer fluid in a magnetic regenerator and evaluates the improved cooling capabilities of the device. Room-temperature magnetic refrigeration exploits a special property of some materials, the “magnetocaloric effect”, that is the ability of the material to change its temperature when exposed to rapid changes in an external magnetic field. As adiabatic magnetic field variations of 1 to 1.5 T produce a small temperature change of a few degrees, active magnetic regeneration is often employed to achieve larger temperature spans for practical cooling applications. The effectiveness of this process greatly depends on the properties of the heat transfer fluid, which is usually a water-based mixture. High values of the fluid thermal conductivity enhance heat transfer allowing for higher cooling capacity. This work presents the outcome of a comprehensive simulation study evaluating the improved performance of a gadolinium-based regenerator using a water-based mixture with added Al2O3 nanoparticles as the heat transfer fluid. In this analysis, both the volume concentration of nanofluid and the particle size were varied. The combined effect of heat transfer enhancement and pressure drop increase was also investigated through the second law efficiency. On average, a more than 15 % increase in cooling capacity was observed when using Al2O3 (6 % vol, 20 nm particles), and the COP can be preserved or increased with a properly tuned regulation of the cycle time and the utilization factor of the regenerator.