In order to study the impact of coolant with variable thermal property on the flow and heat transfer characteristics under different Reynolds numbers in rectangular microchannel heat sink, a three-dimensional conjugate heat transfer in a rectangular microchannel heat sink were analyzed numerically. Firstly, in general, a numerical model was established on the conditions that: 1) the microchannel heat sink was made by silicon, deionized water was applied as coolant fluid; 2) fluid was incompressible laminar steady flow; 3) the values of thermal conductivity coefficient and dynamic viscosity of the fluid varied with temperature, specific heat capacity at constant pressure was given; 4) the effect of radiation and natural convection heat transfer were ignored; and 5) in the control unit, constant heat flux at 70 W/cm2 was applied in the heat sink surface, inlet fluid temperature was specified at 20 ℃, and Reynolds numbers were from16 to 333. Secondly, according to different thermal and physical properties, the average heat transfer coefficient of the channel wall was applied to compare the different heat transfer characteristics, and the surface friction coefficient was used to describe the variation of flow characteristics in microchannel, respectively. Finally, Fluent 6.3.26 was applied based on finite volume method for discrete equations, pressure-velocity coupling method used SIMPLE algorithms, and momentum and energy equations used second-order upwind scheme, respectively. In order to avoid errors caused by entrance effect, a ratio of 1.01 continuous grid was performed along the long side of the flow direction of the z axis. To verify the reliability of the established numerical model, the experimental data of resistance characteristics under intraductal laminar flow conditions performed using Shah and London was used to compare with this paper's numerical calculation results. A good agreement was found. The simulation results indicated that: 1) When Reynolds number was 16, compared with coolant with constant thermal property, coolant with variable thermal property had better average convective heat transfer coefficient and average Nusselt number, which was particularly evident on the side wall of the micro channel. While, with the increase of Reynolds number, the gap between average convective heat transfer coefficient and average Nusselt number of the both coolants was significantly reduced. When Reynolds number increased to 333, the heat transfer characteristics of coolant with variable thermal property and coolant with constant thermal property were approximate. 2) For flow characteristics, when Reynolds number was 16, compared with coolant with constant thermal property, coolant with variable thermal property had significantly lower surface friction coefficient. When Reynolds number increased to 70, the gap between friction coefficients of the both coolants was reduced dramatically. However, coolant with variable thermal property still had obvious advantages. When Reynolds number increased to 333, with the approximate flow characteristics, the friction coefficient of coolant with variable thermal property was only slightly lower than that of the coolant with constant thermal property. 3) At low Reynolds number, the coolant numerical assumption based on the constant thermal property deviated from the actual situation, which explained the reason of the deviation of the experimental data and the simulation results to a certain extent. ©, 2015, Chinese Society of Agricultural Engineering. All right reserved.
