Experimental investigation of heat transfer of nanofluids flowing inside a circular mini/microchannels

In this study, the thermal performance and the pressure drops of Al2O3, TiO2 and ZnO nanofluids flowing through a horizontal circular microchannel under constant surface temperature boundary condition have been experimentally investigated. Al2O3 (13nm), TiO2 (10-25nm) and ZnO (18nm) nanoparticles were added to deionized water used as base fluid in order to prepare nanofluids with 0.5%, 0.7% and 1.0% volume concentrations. The prepared nanofluids were mixed in Sodium Dodecyl Sulfate (SDS) as surfactant to extend the stability time. An experimental setup has been made for the experimental study. For this purpose, a 20 cm length of microchannels made by both the different materials (Stainless steel, Poly Ether Ether Ketone (PEEK)) and the different inner diameter (400, 750, 1000 μm) have been used under the different surface temperatures (15, 25, 40°C). And also, the nanofluids had the different inlet temperature, the volume flow rates (20, 35, 50 mL/min) and the concentrations have been used as a work fluid. Heat transfer, convection heat transfer coefficient, Nusselt number, pressure drop and friction factor results have been calculated by temperature, flow rate and pressure measurements. The thermal conductivity and viscosity values needed for the calculations have been measured separately. The optimum conditions have been determined from the calculated results being used Taguchi approach. Under those optimum conditions, the experiments have been conducted with deionized water, too. The thermal performance and the pressure drops of the nanofluid and the deionized water obtained under the same optimum conditions have been compared. This study has been showed that what parameters are effective or negligible on the thermal performance and the pressure drop. Furthermore, it has been explained how much those parameters affect the results as percent. For high thermal performance and low pressure drop at heat transfer applications with nanofluids, it has been highlighted what parameters it is necessary to be focused to. Consequently, the best result was obtained for Al2O3 nanofluid. When Al2O3 nanofluid is compared with deionized water, it is obtained that 15.3%, 21.7%, 11.1% increase and 39.2% decrease for heat transfer, convection heat transfer coefficient, Nusselt number and friction pressure drop, respectively.