Abastract Heat transfer of DI- water and silica nano fluid was investigated in this study experimentally. Experiments were conducted in copper circular microchannels with inner diameter of 0.79 mm and bend radius of 2.5, 5, and 10 cm with the lengths of 8, 16 and 32 cm, respectively in the range of constant heat flux from 9.4 to 30 kW / m 2 . The results showed that due to the creation of Dean Vortexes and secondary flow increases the microchannel heat transfer coefficient with a bend radius of 10 cm to 45% and for microchannel with the bend radius of 5 cm to 63% in compare with the straight channels. This increase in heat transfer coefficient decreases with decreasing the bend radius and microchannel length. The results also showed that the use of nanofluid increased the heat transfer coefficient up to 7% for microchannel with the bend radius of 2.5 cm and up to 17% for microcanal with the bend radius of 5 cm and up to 15% the bend radius of 10 cm. Which can be due to factors such as the Brownian movement of nanoparticles, the molecular surface at the interface between nanoparticles and the fluid, the clustering of nanoparticles, and the heat transfer between nano particles. The curved microchannel friction factor in compare with the straight microchannel increases up to 38%, 32%, and up to 27% of the bend radius of 10, 5, and 2.5 cm, respectively. As a matter of fact, increasing the bend radius increments the friction factor of the fluid which can be neglected according to the high growth of heat transfer coefficient. Keywords: heat transfer- dean vortexes- secondary flow- bend radius- microchannel- friction factor- nano fluid
