Natural convection heat transfer inside vertical circular enclosure filled with water-based Al2O3 nanofluids

Experimental investigation on natural convection heat transfer has been carried out inside vertical circular enclosures filled with Al2O3 nanofluid with different concentrations; 0.0%, 0.85% (0.21%), 1.98 (0.51%) and 2.95% (0.75%) by mass (volume). Two enclosures are used with 0.20 m inside diameter and with two different aspect ratios. The bottom surface of the enclosure is heated using a constant heat flux flexible heater while the upper surface is cooled by an ambient air stream. Various uniform heat fluxes have been used to generate the natural convection heat transfer data. The average Nusselt number is obtained and correlated with the modified Rayleigh number at each concentration ratio of the nanofluid. The average Nusselt number is obtained for each enclosure and correlated with the modified Rayleigh number using the concentration ratio as a parameter. The results show that the heat transfer coefficient increases as the concentration increases up to a specific value of the concentration and then it decreases as the concentration continues to increase compared to the basic fluid of pure water. Furthermore, a general correlation is obtained using the volume fraction and the aspect ratio as parameters.

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► Circular enclosures filled with nanofluid with aspect ratios 0.0635 and 0.127 are used.
► Increasing the nanofluid concentration enhances the heat transfer up to ϕ ≤ 0.51%.
► The percentage of heat transfer coefficient enhancement is aspect ratio dependent.
► General correlations are obtained for the average Nusselt numbers verses the modified Ra∗.
► An overall correlation is obtained for both enclosures using the aspect ratio.