Investigation of Heat Transfer Enhancement or Deterioration of Variable Properties Al2O3-EG-water Nanofluid in Buoyancy Driven Convection

Abstract

In this study, the natural convection heat transfer of variable properties Al₂O₃-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 <em>Ra</em> in the range of 10³-10⁷ and volume fraction of nanoparticles from 0 to 0.04 on heat transfer characteristics have been studied. The results verified by making overall comparison with some existing experimental results have shown that for<em> Ra</em>=10³, for which conduction heat transfer is dominant, the average Nusselt number increases as volume fraction of nanoparticles increases, but for higher <em>Ra</em> numbers in contradiction with the constant properties cases it decreases. This reduction, which is associated with increased viscosity, is more severe at <em>Ra</em> of 10⁴ compared to higher <em>Ra</em> numbers such that the least deterioration in heat transfer occurs for <em>Ra</em>=10⁷. This is due to the fact that as <em>Ra</em> 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.