Combined free and forced convection heat transfer of the copper oxide-heat transfer oil (CuO-HTO) nanofluid inside horizontal tubes under constant wall temperature

In this paper, the combined free and forced convection heat transfer of the heat transfer oil-copper oxide (HTO-CuO) nanofluid flow in horizontal tubes is studied experimentally. The flow regime is laminar and the surface temperature is constant. The effect of nanoparticle mass concentration and microfin tube on the heat transfer rate is investigated as Richardson number is between 0.1 and 0.7. The results show that the combined free and forced convection heat transfer rate increase significantly with the increase in nanoparticle concentration from 0% to 1.5%. As shown in the results, three new correlations are presented to predict the mixed heat transfer rate based on increasing the Richardson number from 0.04 to 0.7. The maximum deviation of both correlations is less than 10%. In addition, a new correlation is suggested to assess the Nusselt number based on the Rayleigh number in horizontal tubes from 1656000 to 8200000. The maximum deviation of the correlation is almost 5%.