Effect of confinement on heat transfer in aqueous nanofluids from a heated sphere

Abstract

A systematic numerical investigation on heat transfer from a heated sphere settling in aqueous nanofluids has been carried out to elucidate the effect of confinement and the other pertinent dimensionless numbers on heat transfer. In this work, four types of nanoparticles (NPs) have been considered, namely, Al2O3, CuO, SiO2 and ZnO, each of two different diameters of dnp=20nmand 80 nm. In order to account for the variation of properties with temperature, the temperature-dependent equations for the viscosity and thermal conductivity of nanofluids are employed here. Extensive results on streamline and isotherm contours, wake characteristics (length), drag coefficient and local and average Nusselt Number have been obtained and discussed to elucidate the effect of Peclet number (Pe), nanoparticle volume fraction (ϕ) and blockage ratio (λ) over the following ranges of conditions: 5≤Pe≤600, 0≤ϕ≤0.05 and 0.1≤λ≤0.9 where λ is the sphere-to-tube diameter ratio. Overall, the recirculation length shows a positive dependence on both Peclet number (Pe) and particle size (dnp) while it exhibits inverse trends with respect to ϕ and λ. A significant enhancement (20–30%) in the rate of heat transfer over the conventional Newtonian fluids is observed and it is seen to increase with the increasing values of the Peclet number. Finally, a simple predictive correlation for the average Nusselt number is developed which includes the conduction limit and, it enables the estimation of the Nusselt number for the intermediate values of the parameters reported in a new application.