Sedimentation in nanofluids during a natural convection experiment

Abstract

This study presents an experimental investigation of the thermophysical behavior of γ-Al2O3–deionized (DI) H2O nanofluid under natural convection in the classical Rayleigh–Benard configuration, which consists of a cubic cell with conductive bottom and top plates, insulated sidewalls and optical access. The presence of nanoparticles either in stationary liquids or in flows affects the physical properties of the host fluids as well as the mechanisms and rate of heat and mass transfer. In the present work, measurements of heat transfer performance and thermophysical properties of Al2O3–H2O nanofluids, with nanoparticle concentration within the range of 0.01–0.12vol.%, are compared to those for pure DI water that serves as a benchmark. The natural convective chamber induces thermal instability in the vertical direction in the test medium by heating the medium from below and cooling it from above. Fixed heat flux at the bottom hot plate and constant temperature at the top cold plate are the imposed boundary conditions. The Al2O3–H2O nanofluid is tested under different boundary conditions and various nanoparticle concentrations until steady state conditions are reached. It is found that while the Rayleigh number, Ra, increases with increasing nanoparticle concentration, the convective heat transfer coefficient and Nusselt number, Nu, decrease. This finding implies that the addition of Al2O3 nanoparticles deteriorates the heat transfer performance due to natural convection of the base fluid, mainly due to poor nanofluid stability. Also, as the nanoparticle concentration increases the temperature at the heating plate increases, suggesting fouling at the bottom surface; a stationary thin layer structure of nanoparticles and liquid seems to be formed close to the heating plate that is qualitatively observed to increase in thickness as the nanoparticle concentration increases. This layer structure imposes additional thermal insulation in the system and thus appears to be responsible in a big extend for the reported heat transfer degradation. Also, for relatively high nanoparticle concentrations of 0.06 and 0.12vol.%, as the heating flux increases the rate of heat transfer deterioration increases. Specifically in the case of maximum nanoparticle concentration, 0.12vol.%, when the turbulence intensity increases, by increasing the applied heat flux, the Nusselt number remains constant in comparison with lower nanoparticle concentrations. This behavior can be attributed mainly to the physical properties of the Al2O3 nanopowder used in this study and the resulting interactions between the heating plate and the nanoparticles.