Buoyancy-induced natural convective heat transfer along a vertical cylinder immersed in Water–Al2O3 nanofluids for various concentrations (0, 0.05, 0.1, 0.2, 0.4, 0.6 vol %) under constant heat flux condition was investigated experimentally and presented. Thermal stratification was observed outside the boundary layer in the ambient fluid after steady-state condition is achieved as the fluid temperature goes on increasing along the axial direction. Temperature variations of the cylinder along the axial direction and temperature variations of fluid in radial direction are shown graphically. It is observed that the temperatures of the cylinder and the fluid increases along the axial direction and the fluid temperature decreases in the radial direction. Experiments were conducted for various heat inputs (30 W, 40 W, 45 W, and 50 W) and volume concentrations and observed that the addition of alumina nanoparticles up to 0.1 vol % enhances the thermal performance and then the further addition of nanoparticles leads to deterioration. The maximum enhancement in the natural convection heat transfer performance is observed as 13.8%, i.e., heat transfer coefficient is increased from 382 W/m2 K to 435 W/m2 K at 0.1 vol % particle loading.

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