By decreasing the size of electronic devices, the generated heat per volume unit increases, and as electronic devices became smaller, heat removal of them became a critical factor for reliable operation. Micro?channel heat sink (MCHS) systems are one of the most suitable apparatus to remove high heat load from electronic devices. A micro channel heat sink is a passive device contains some parallel micro?channels with large thermal capacity and with a large surface area relative to its volume. Geometry of the channels and type of coolant are two effective factors on the thermal performance of the heat sink systems. Therefore, in this work, the effects of these parameters on the thermal performance of heat sink are investigated. The hydrodynamics and heat transfer parameters of nanofluids are investigated numerically using CFD analysis. Laminar convective heat transfer of pure water and 1% and 2% volume fraction of Alumina-water nanofluids in a straight micro?channel heat sink under constant wall heat flux condition is studied. This work is performed in two parts. In the first part, the single-phase and two-phase approaches have been used for modeling of heat transfer of pure water and Alumina-water nanofluids in a straight micro channel. The results of simulation are compared with the experimental data. The results showed that CFD predictions via two-phase model shows better agreement with the experimental measurements. In nanofluid with 1% concentration, the average relative error between the experimental data and CFD results based on two-phase model is 11.39% while for nanofluid with 2% concentration is 2%. In the second part of this work, the effect of ribs through the microchannel is investigated. The effects of the geometry and dimensions of the ribs on the Nusselt number and friction factor in the microchannel are studied. Results showed that channel with 5 ribs with aspect ratio of 2.857 has the best thermal performance. Keywords: micro?channel heat sink, nanofluid, two-phase mixture model, hydrodynamic parameter, heat transfer, rib-microchannel.
