Free convection heat transfer of non Newtonian nanofluids under constant heat flux condition

Two different kinds of non-Newtonian nanofluids were prepared by dispersion of Al2O3 and TiO2 nanoparticles in a 0.5 wt.% aqueous solution of carboxymethyl cellulose (CMC). Natural convection heat transfer of non-Newtonian nanofluids in a vertical cylinder uniformly heated from below and cooled from top was investigated experimentally. Results show that the heat transfer performance of nanofluids is significantly enhanced at low particle concentrations. Increasing nanoparticle concentration has a contrary effect on the heat transfer of nanofluids, so at concentrations greater than 1 vol.% of nanoparticles the heat transfer coefficient of nanofluids is less than that of the base fluid. Indeed it seems that for both nanofluids there exists an optimum nanoparticle concentration that heat transfer coefficient passes through a maximum. The optimum concentrations of Al2O3 and TiO2 nanofluids are about 0.2 and 0.1 vol.%, respectively. It is also observed that the heat transfer enhancement of TiO2 nanofluids is higher than that of the Al2O3 nanofluids. The effect of enclosure aspect ratio was also investigated. As expected, the heat transfer coefficient of nanofluids as well as the base fluid increases by increasing the aspect ratio.