Optimizing the magnetic heat exchangers of thermomagnetic motors: A heat transfer study

Abstract

This work explored the optimization procedures of heat ex- changers to be used in thermomagnetic motors. The article main focus is the use of CFD simulations to maximize the heat exchange rate between the magnetic materials and the hot and cold fluids. This is done in order to have the maximum possible operational frequency in thermomagnetic motors. This was accomplished by optimizing the geometries of the channels of the heat exchangers. As the focus of this work was to improve the thermo- magnetic heat exchangers geometrically, some important characteristics were observed. First of all, taking into account the fact that the heat transfer rate is highly influenced by the contact area between the fluid and the plate, and that the length of the plate is constant and depends on constructive characteristics of the motor, we have that, in order to improve the heat transfer speed, the perimeter of the channels must be increased. In this work only channels having convex cross section formats were considered, but the use of channels with concave cross section may lead to higher contact areas and therefore improve even more the heat transfer rate. Another very important aspect is that the temperature gradient along the plate needs also to be taken into account, because the magnetic force distribution applied on the plates is perpendicular to its linear displacement, so this gradient can lead to some misalignment of the force on the plates leading to the blockage of the motor; in this sense we showed that the best option to maximize the temperature homogeneity is the use of a non-uniform cross section in the channels. Future works shall be developed considering the real properties of the magneto-caloric materials for the simulations, which will allow the use of the CFD simulations together with magnetic simulations to determine the dynamics of thermomagnetic motors and also their efficiencies.