Experimental investigation of the particle size effect on heat transfer coefficient of Al₂O₃ nanofluid in a cylindrical microchannel heat sink

Having the ability to remove high heat flux from the environment, the microchannel heat sinks cooled by nanofluids have been the subject of many researches on different aspects of nanofluid characteristics and microchannel geometry. In this work, convective heat transfer coefficient was investigated using water-based Al₂O₃ nanofluid as the working fluid in a cylindrical microchannel heat sink. The experiments performed at different particle sizes (20, 50, 80 and 135 nm), different flow rates and constant heat flux. This microchannel consisted of 48 parallel channels with a rectangular cross section, and each one had a width of 524 μm, a height of 800 μm, a length of 52 mm and a hydraulic diameter of 632 μm. Ultrasonic irradiation was used to provide the stability of the nanofluid followed by zeta potential measurements. Experimental results showed that increasing the particle size decreased the convective heat transfer coefficient. At constant Reynolds number, the convective heat transfer coefficient increased at all the particle sizes except 135 nm, at which the Nusselt number decreased by 8.5%. An enhancement of 21.9%, 21.1% and 18.7% in Nu was observed for 20, 50 and 80 nm, respectively. Moreover, a correlation considering the effect of nanoparticle size on the Nu number was proposed which could predict the experimental data with an average error of 5.26%.