Analysis of Transient Heat Transfer in a Microchannel Heat Exchanger During Magnetic Heating of the Substrate Material

The paper presents a systematic analysis of heat transfer processes during the heat up and cool down phases of a magnetic material when subjected to a magnetic field. As a first step towards the development of a MEMS magnetocaloric refrigerator for hydrogen liquefaction, a computer simulation of fluid flow and transient heat transfer in microchannels was carried out. The study considered microchannels with rectangular and square cross sections with heat generation in the substrate due to imposed magnetic field. The results computed were for gadolinium substrate and water as the working fluid. In order to achieve the liquefaction of a cryogen such as hydrogen, heat need to be removed from the working fluid by taking the advantage of the demagnetization of the material when the magnetic field is removed. The purpose of this study is to explore the transient heat transfer coefficient when the fluid is circulated through the substrate via microchannels. The application of the magnetic field was simulated by using the concept of volumetric heat source distributed uniformly over the entire solid material. Because of the relatively small size of the MEMS device, the magnetic field strength is expected to be uniform throughout the material. The strength of the source was calculated from energy balance during magnetization of the material. From the simulation results, plots of Nusselt number and heat transfer coefficient over the length of the channel as well as locally at different sections were obtained. A thorough investigation for velocity and temperature distributions were performed by varying channel aspect ratio, Reynolds number, and heat generation rate in the channel.