A numerical investigation of transient natural convection heat transfer of aqueous nanofluids in a horizontal concentric annulus

A numerical study of transient natural convection heat transfer of aqueous nanofluids in a horizontal annulus between two coaxial cylinders is presented. The effective thermophysical properties of water in the presence of copper oxide nanoparticles with four different volume fractions are predicted using existing models, in which the effects of the Brownian motion of nanoparticles are taken into consideration. The predicted development of convective flow and heat transfer of nanofluids is presented by means of the average Nusselt number over the outer cylinder. The flow development time towards a steady state and the time-averaged Nusselt number are predicted and scaled with Rayleigh number. It is shown that at constant Rayleigh numbers, the time-averaged Nusselt number is gradually lowered as the volume fraction of nanoparticles is increased. In addition, the time-averaged Nusselt number will be overestimated if the Brownian motion effects are not considered.