This paper reports an experimental study on the natural convection heat transfer of nanofluids in a vertical square enclosure. In the present experiment oil-based titanium dioxide nanofluids of various concentrations from 0 wt.% to 0.8 wt.% are formulated. The averaged particle size of titania is about 21 nm, and oil is turbine oil. The Rayleigh number varies in the range of 4.21*107- 2.18*108. The results with the assessment based on the changes in thermophysical properties of the nanofluid formulated, shows that efficacy of applying the nanofluid for natural convection heat transfer enhancement in enclosure is inferred to be generally infeasible and showing systematic heat transfer degradation over the entire range of the Rayleigh number considered. In the previously published work, different behaviors (enhancement or deterioration) were presented in average Nusselt number as the volume fraction of nanoparticles increases depending on condition. Discussion of the results suggests that changes in the nanofluids’ thermal conductivity and viscosity could not explain the observed decrease in the heat transfer coefficient, and particle–surface interactions may play an important role.

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