Experimental investigation on cavity flow natural convection of Al2O3–water nanofluids

Thermo-physical properties of nanofluids have attracted the attention of researchers more than the heat transfer characteristic of nanofluids. On the other hand, contradictory results were reported on the thermal-fluid behaviour of nanofluids numerically and experimentally in the open literature. In addition to that, experimental natural convection has been investigated less than others. In this paper, characteristic and stability of Al2O3–water nanofluid (d = 30 nm) has been analyzed by using Malvern Zetasizer, Zeta potential, and UV–visible spectroscopy. The natural convection of Al2O3–water nanofluids (formulated with single-step method) was experimentally studied in detail for volume fractions of 0, 0.05, 0.1, 0.2, 0.4 and 0.6% in a rectangular cavity, heated differentially on two opposite vertical walls for Rayleigh number (Ra) range 3.49 × 108 to 1.05 × 109. The viscosity of the Al2O3–water nanofluids are also measured experimentally in a temperature range between 15 °C and 50 °C and effect of temperature and volume fraction on viscosity have investigated. Detailed study on the influence of nanoparticle concentration on natural convection heat transfer coefficient was performed. It was found that increasing concentration of nanoparticles improves heat transfer coefficient up to an optimum value of 15% enhancement, at 0.1% volume fraction, then further increasing of concentration of the nanoparticles deteriorates natural convection heat transfer coefficient. This research also supports the idea of “for nanofluids with thermal conductivity more than the base fluids, there may exist an optimum concentration which maximizes the heat transfer in an exact condition as natural convection, laminar forced convection or turbulence forced convection”.