A numerical investigation of laminar flow of a water/alumina nanofluid

The present paper reports the results of a numerical investigation of a tubular developing laminar flow of a water/alumina nanofluid under constant heat flux conditions. The nanofluid is modelled as a water/alumina nanoparticles mixture. Brownian and thermophoretic diffusion effects have been considered through the energy and nanoparticle concentration field equations. Though these modes of diffusion have been suggested extensively in the literature, their effect on momentum and energy transport has not yet been numerically analyzed. In addition, the model includes temperature and concentration effects over the transport properties of the nanofluid. A new CFD solver has been developed for the solution of the present set of governing equations. Model solutions have been sought for conditions corresponding to two experimental investigations carried out elsewhere. Base fluid (water) solutions have been used to asses the accuracy of the numerical model, the results being reasonably comparable to the experimental ones and adequately correlated by the Churchill and Ozoe correlation. Nanofluid solutions have been obtained for nanoparticles volumetric concentrations of up to 6%. A concentration boundary layer has been observed to develop along the wall of the pipe, a region which is progressively depleted of nanoparticles by the action of thermophoretic diffusion. A slight heat transfer enhancement has been found especially at the higher nanoparticle concentrations, with the maximum enhancement being of the order of 5%. The dimensionless numerical results, based on cross section (local) properties, fall within 25% with respect to the experimental ones. The Churchill and Ozoe correlation seems to adequately correlate numerical results for the range of Graetz numbers of the present investigation and for the lower range of nanoparticles concentration. Heat flux effects seem to play a potential role in nanofluid heat transfer at the tube wall though their significance has not been consistently investigated under the present investigation.