Microchannels containing cooling fluid are among the most widely used equipment in the cooling of microscale devices, such as heat sinks in the electronics industry. In this numerical research, the flow of water/magnesiumoxide nanofluid in a 3D rectangular microchannel is simulated and investigated. The flow field and heat transfer are analyzed for the laminar flow with Reynold number (Re)= 100, 300, 700, and 1000 and nanoparticle volume fraction (φ) =0, 0.02, and 0.04. The rough surfaces include rectangular cubic ribs arranged in three one in each row along the length with 2, 3, 4, and 5 rows. The ribbed surface is under a constant heat flux. The results include examining changes in Nusselt number (Nu), pressure drop, pumping power, friction factor, and total flow entropy generation. Moreover, the contours of the temperature, pressure, and velocity distribution fields will be discussed. The results reveal that the heat transfer and physics of flow are highly dependent on hydrodynamic behavior. Increasing the number of ribs on the hot surfaces increases the pressure drop, pumping power, and heat transfer. Increasing φ also greatly affects the heat transfer rate. In the case of using 5 ribs and with φ=0.04, in Re=1000 and 700, the microchannel has the highest average Nu, pressure drop, and pumping power.
