The purpose of this study is to investigate the effects of wavy and hydrophobic surfaces and using water/nanofluid on the heat transfer and pressure drop. The volume fraction of the nanofluid is assumed to be 1, 3 and 5% while the slip length is considered 2.5, 5 and 10 percent of microchannel height. The heights of the three-dimensional microchannels (2H) are also assumed as 60, 100, 150 and 200 ?m for Reynolds numbers of 150-700. Thermal boundary condition is a constant surface temperature on upper and lower walls. The length of microchannel is considered as 2120 ?m where the first and last 400 ?m are considered as input and output regions. The remaining 1320 ?m is corrugated using sinusoidal wave for the upper and lower walls. The wavelength of each microchannel and the wave amplitude are 220 ?m and 20 % of microchannel height, respectively. In this study, the properties of fluid are constant and the fluid flow is single phase and incompressible. All simulations are performed using the Ansys fluent (V16.2). The results show that the surface waviness increases the Nusselt number. In addition, using slip velocity and temperature jump decreases the pressure drop. The results also show in a three-dimensional microchannel (2H=200 ?m, Re=150), the pressure drop is increased 11% in compared with two-dimensional one, while the average Nusselt number has no meaningful changes. In a 2H=200 ?m and Re=400 microchannel and the slip boundary (10%) decreases the pressure drop about 25% and the Nusselt number increases 3.7% in comparison to case with no-slip condition. The maximum performance coefficient with the value of 1.3 is obtained for a microchannel with 2H=200 ?m, at Re=700 and a slip length of 20 ?m. Furthermore the pressure drop and the Nusselt number increase when nanofluid is employed. Overall, to analyze the performance of a thermal system, the combined effect of pressure drop and heat transfer rate should be considered. In all cases employed, this coefficient is greater than one, which show suitability of using nanofluid, although the value of increase in the performance coefficient rate are not the same for the different volume fractions. In design of a thermal system, using the nanofluid instead of water-based fluid is recommended, if the increase in heat transfer is more important than increasing the pressure drop. Although, using hydrophobic surface with nanofluid can reduce the pressure dro sharply. Keywords: numerical simulation, three-dimensional wavy microchannel, hydrophobic surfaces, nanofluid, thermal performance coefficien
