Journal of AL-Qadisiyia for Engineering sciences, Vol. 9, No.4, 2016 1 Study of microchannel heat sink performance with expanded microchannels and nanofluids Mushtaq Ismael Hasan Mechanical Engineering Department, College of Engineering, Thi-Qar University mushtaq76h@ gmail.com ABSTRACT: In this paper a microchannel heat sink with expanded microchannels and nanofluids is numerically investigated. The object of this paper is to study and improve the cooling performance of microchannel heat sink. Both the geometrical parameters and working fluids were studied and a comparison was made between them. Expanded microchannels (sudden expanded and diverging) were used instead of straight microchannels, also micro pin fins with square and triangular shapes were used for heat transfer enhancement. Sudden expanded microchannels were studied with different expansion ratios and expansion lengths. Three types of nanofluids (Cu-water, Al 2 O 3 -water and Diamond-water) with volume concentration (1 – 5) % were studied as working fluids and their effects on overall performance of heat sink were compared with pure water. The results obtained shows that the overall performance of microchannel heat sink increased with increasing the expansion ratio or decreasing the expansion length. For the same expansion ratio the sudden expanded microchannels gives higher modification compared with diverging microchannels. Also using of nanofluids lead to enhance the heat transfer and the improvement got by geometric parameters such as using of expanded microchannels or fins is much larger than that obtained by using nanofluids for the same heat sink. Key words: Sudden expansion microchannel; Microchannel heat sink; numerical investigation; diverging microchannel; Nanofluid. Nomenclature: Symbol Description SI Unit Symbol Description SI Unit A Cross-sectional area m 2 w Fluid z-component velocity m/s c Nanofluid concentration % W Channel width m C p Specific heat J/(kg K) x Axial coordinate m D h Hydraulic diameter m y Vertical coordinate m H Channel height m z Horizontal coordinate m