Study of Single Phase Heat Transfer in Various Microchannels Geometries for Waste Heat Recovery Applications and Condensation of R-134a in Rectangular Microchannels at Low Mass Fluxes /

Abstract

Utilization of waste heat from the thermal system has a considerable importance in increasing its performance due to rising trend in cost of fuel and consumption as well. Waste heat before vented into the atmosphere is captured from the exhaust of thermal system to increase the system capability by transfer to refrigerant circulating inside heat exchanger to increase the system performance. Microchannel technology provides new approach to remove larger heat flux in electronic and thermal system. Microchannels in comparison to macrochannels often higher performance due to higher surface area per unit volume ratio they provide. In heat and mass transfer applications, they provide higher heat transfer coefficient. The purpose of introducing microchannels in compact heat exchanger for the waste heat energy is to make system efficient, save cost inventory, less consumption of fuel and less harmful effect of refrigerant on environment in term of global warming potential and ozone depletion potential. The refrigerant is used inside the compact microchannel to recover the waste heat from thermal system. So, the selection and use of environment friendly refrigerants inside microchannels plays a vital role in transferring of thermal energy from refrigerant to surrounding medium. In this research work, different shapes of microchannels including circular, rectangular, square and triangular one is designed and modelled on Engineering Equation Solver program and comparison is done in between them on the basis of heat transfer as well as pressure drop during refrigerant flow at different conditions of temperatures and mass fluxes respectively. It has been found that circular channel among all other channels have higher thermal performance and effectiveness due to maximum contact area of fluids with the channel surface area. Whereas rectangular channel gives better performance followed by the circular channel and triangular channel having least value of heat transfer due to less contact area with surface and larger pressure drop because of lowest channel hydraulic diameter. The better thermal performance is achieved by dividing the total volume of system into smaller channels to increases the surface area for higher heat transfer and system should work under lower Reynold number to limit the pressure drop occurred in the system.