Mixed convection of alumina–water nanofluid inside a concentric annulus considering nanoparticle migration

Abstract

A theoretical investigation was conducted of laminar fully developed mixed convection of alumina–water nanofluid through a vertical annulus, to improve its heating/cooling performance. We focused on controlling the nanoparticle migration and studying how it affected the heat transfer rate and pressure drop. Because the nanoparticles have very small dimensions, we only considered Brownian motion and thermophoretic diffusivity as the main causes of nanoparticle migration. Because thermophoresis is very sensitive to temperature gradients, we imposed various temperature gradients using asymmetric heating. Considering hydrodynamically and thermally fully developed flow, the governing equations were reduced to two-point ordinary boundary value differential equations and were solved numerically. The imposed thermal asymmetry changed the direction of nanoparticle migration and distorted the velocity, temperature, and nanoparticle concentration profiles. Moreover, we found optimum values for the radius ratio (ζ) and heat flux ratio (ɛ); with these optimum values, the nanofluid enhanced the efficacy of the system.