H. Khorasanizadeh - Department of Thermo-fluids, Faculty of Mechanical Engineering, University of Kashan, Kashan, I.R. Iran
M. M. Fakhari - Department of Thermo-fluids, Faculty of Mechanical Engineering, University of Kashan, Kashan, I.R. Iran
S. P. Ghaffari - Department of Thermo-fluids, Faculty of Mechanical Engineering, University of Kashan, Kashan, I.R. Iran

In this study, the natural convection heat transfer of variable properties Al۲O۳-EG-water nanofluid in a differentially heated rectangular cavity has been investigated numerically. The governing equations, for a Newtonian fluid, have been solved numerically with a finite volume approach. The influences of the pertinent parameters such as Ra in the range of ۱۰۳-۱۰۷ and volume fraction of nanoparticles from ۰ to ۰.۰۴ on heat transfer characteristics have been studied. The results verified by making overall comparison with some existing experimental results have shown that for Ra=۱۰۳, for which conduction heat transfer is dominant, the average Nusselt number increases as volume fraction of nanoparticles increases, but for higher Ra numbers in contradiction with the constant properties cases it decreases. This reduction, which is associated with increased viscosity, is more severe at Ra of ۱۰۴ compared to higher Ra numbers such that the least deterioration in heat transfer occurs for Ra=۱۰۷. This is due to the fact that as Ra increases, the Brownian motion enhances; thus conductivity improves and becomes more important than viscosity increase. An scale analysis, performed to clarify the contradictory reports in the literature on the natural convection heat transfer enhancement or deterioration of nanofluids, showed that different kinds of evaluating the base fluid Rayleigh number has led to such a difference.

Enclosure, Ethylene Glycol, Nanofluid, natural convection, Nusselt number, Variable properties