Experimental investigation of mixed convection heat transfer of ferrite-based nanofluids in multiple microchannels

The objective of this study is to experimentally investigate the mixed convection heat transfer characteristics of ferrite-based (Fe2O3.NiO) nanofluids in multiple microchannel heat sinks. Two rectangular cross-sectioned microchannel heat sinks having two different heights of H = 1 mm and 1.8 mm and a width of 300 μm were used. Ferrite-based nanoparticles were suspended into the pure water at two different volumetric ratios of 0.25 and 0.5%, and experiments were performed for both pure water and nanofluids. Constant heat flux was applied to the bottom wall of the microchannels by the cartridge heaters placed in heat sinks. Ferrite-based nanofluids were prepared by the two-step method, and the average size of the particles was below 20 nm. The thermal conductivity and viscosity values of all fluids used in the present study were measured in a temperature range of 20–60 °C. Increasing the channel height from 1 to 1.8 mm caused an increase in the Nusselt number about 9.4–10.7, 9.9–13.9 and 5.8–11.7% for the pure water, the 0.25 vol.% Fe2O3.NiO-water nanofluid and the 0.5 vol.% Fe2O3.NiO-water nanofluid, respectively. The addition of Fe2O3.NiO nanoparticles into the base fluid further increased the natural convection effects compared to pure water. The effects of the natural convection heat transfer in H = 1.8 mm were more dominant than those of H = 1 mm at the same Grashof number values.