Effect of the Aspect Ratio and Tilt Angle on the Free Convection Heat Transfer Coefficient Inside Al₂O₃-Water-Filled...

This experimental study provides a comprehensive investigation of natural convection heat transfer inside shallow square cuboid enclosures filled with aluminum oxide-water nanofluid at four different volume concentrations: 0.0%, 0.2%, 0.4%, and 0.8%. Two square cuboid enclosures were used with sizes 30 × 30 × H cm³, where H is the inside thickness of the enclosures. This led to two different enclosure aspect ratios (κ = H/30 = 0.033 and 0.066). Four inclination angles to the horizontal position of the enclosures were used: 0°, 30°, 60°, and 90°. The crucial thermophysical properties of the synthetic nanofluid were obtained. The thermal conductivity of the nanofluid was measured experimentally at various volume concentrations. Furthermore, the viscosity and density were also measured experimentally at temperatures ranging from 15 to 40 °C as a function of the volume concentration. The heat transfer data were generated by heating the lower surface of the enclosure using a uniform flexible heat flux heater. The opposite surface was cooled using an air fan. The results of the experimental physical parameter measurements show that the percent of maximum deviation in thermal conductivity with those in the literature were 6.61% at a 1.0% volume concentration. The deviation of dynamic viscosity was between 0.21% and 16.36% at 0.1% and 1% volume concentrations, respectively, and for density it was 0.29% at 40 °C and a 1% volume concentration. The results showed up to a 27% enhancement in the Nusselt number at an angle of 60° and a 0.4% volume concentration in the largest aspect ratio (κ = 0.066). However, for the low aspect ratio enclosure (κ = 0.033), there was no noticeable improvement in heat transfer at any combination of volume concentration and inclination angle. The results show that the inclination angle is a significant factor in natural convection only for large aspect ratio enclosures. Furthermore, for large aspect ratio, the Nusselt number increased until the angle approached 60°, then it decreased again.