Experimental analysis of natural convection in vertical annuli filled with AlN and TiO2/mineral oil-based nanofluids

The thermal performance of mineral oil based nanofluids is experimentally investigated when they are used as a cooling fluid in a system that operates under natural convection. This system consists of a closed annular vertical cavity that is filled with two types of nanofluids made of different nanoparticles and concentrations. The inner cylinder works as a constant heat source while the outer cylinder removes the thermal energy with a constant temperature condition. High Prandtl number nanofluids (70-300) produced by dispersing AlN and TiO2 nanoparticles in mineral oil (0.01, 0.1 and 0.5 wt%) were previously characterized and then tested in this system to evaluate the effect in the free convective heat transfer coefficient under high Rayleigh numbers (>106). Additionally, oleic acid treated nanoparticles were also used to disperse nanofluids and compared to those without any treatment. Correlations for Nusselt number in terms of Rayleigh number are obtained for different nanoparticle concentrations. Under certain conditions, enhancements in heat transfer coefficients were found when using nanofluids while mainly deteriorations occurred when compared to base fluid.