Effect of Viscosity Variation on Natural Convection Flow of Water-Alumina Nanofluid in an Annulus with Internal Heat Generation

Heat transfer enhancement in a horizontal annulus using the variable viscosity property of an Al2O3-water nanofluid is investigated. Two different viscosity models are used to evaluate heat transfer enhancement in the annulus. The base case uses the Pak and Cho model and the Brinkman model for viscosity which take into account the dependence of this property on temperature and nanoparticle volume fraction. The inner surface of the annulus is heated uniformly by a constant heat flux q(w) and the outer boundary is kept at a constant temperature T-c. The nanofluid generates heat internally. The governing equations are solved numerically subject to appropriate boundary conditions by a penalty finite-element method. It is observed that for a fixed Prandtl number Pr = 6.2, Rayleigh number Ra = 10(4) and solid volume fraction phi = 10%, the average Nusselt number is enhanced by diminishing the heat generation parameter, mean diameter of nanoparticles, and diameter of the inner circle. The mean temperature for the fluids (nanofluid and base fluid) corresponding to the above mentioned parameters is plotted as well. (C) 2012 Wiley Periodicals, Inc.