Fluid Flow and Heat Transfer in a Composite Trapezoidal Microchannel

The study considered the analysis of heat transfer in a composite channel of trapezoidal cross-section fabricated by etching a silicon <100> wafer and bonding that with a slab of gadolinium. Gadolinium is a magnetic material that exhibits high temperature rise during adiabatic magnetization around its transition temperature of 295K. Heat was generated in the substrate by the application of magnetic field. The conjugate heat transfer scenario where part of generated heat is directly dissipated to the working fluid from gadolinium whereas part is conducted through the silicon structure and reaches the working fluid was studied. Water, ammonia, and FC-77 were studied as the possible working fluids. This kind of heat exchanger is being developed for a micro-scale refrigeration system that works with magnetic heating and cooling principle. Equations governing the conservation of mass, momentum, and energy were solved in the fluid region. In the solid region, heat conduction equation was solved. The volumetric heat generation rate due to magnetic heating was included in the gadolinium portion of the composite channel. A grid independence study was carried out to choose the optimum number of elements to mesh the channel geometry and surrounding structure. A thorough investigation for velocity and temperature distribution was performed by varying channel aspect ratio, Reynolds number, and the magnetic field. The thickness of gadolinium slab, spacing between channels in the heat exchanger, and fluid flow rate were varied. To check the validity of simulation, the results were compared with existing results for single material channels. It was found that the peripheral average heat transfer coefficient and Nusselt number is larger near the entrance and decreases downstream because of the development of the thermal boundary layer. With the increase in Reynolds number, the outlet temperature decreased and the average heat transfer coefficient increased.