Abstract

In this paper, the mixed natural-forced convection is experimentally investigated for the heat-transfer oil-copper oxide (HTO-CuO) nanofluid flows upward in a vertical tube. The flow regime is laminar and the temperature of the tube surface is constant. The effect of the nanoparticles concentration on the heat transfer rate and the pressure dropare studied as Richardson number varies between 0.1 and 0.7. It is observed that the mixed convective heat transfer rate increases with both the nanoparticles concentration and Richardson number. New correlations are proposed to predict the Nusselt number of the nanofluid flow with the reasonable accuracy. In addition, Darcy friction factor of the nanofluid flow is investigated and a new correlation is presented to evaluate the friction factor of HTO-CuO nanofluid flow in vertical tubes. As the heat transfer enhancement methods usually accompany with increment in the pressrure drop, the performance index is evaluated experimentally. As such, the maximum perfomance index of 1.27 is achieved using the 1.5% concentration of the nanoparticles at Richardson number of 0.7. This study provides a platform to design next generation of low flow rate nanofluid-based heat exchangers and may improve the accuracy of predicting the mixed convective characteristics of nanofluid flows.