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.
