Experimental and numerical investigation of nanofluids heat transfer characteristics for application in solar heat exchangers

One of the innovative methods of improving heat transfer characteristics of heat exchangers in solar systems is applying nanofluids as the heat transfer media. In this study, laminar convective heat transfer of water-based TiO2 nanofluid flowing through a uniformly heated tube has been investigated via experiments and numerical modeling. The thermal conductivity and dynamic viscosity of the prepared nanofluids have also been measured and modeled at different temperatures and nanoparticle concentrations. Based on the results, a maximum enhancement of 21% in average heat transfer coefficient has been obtained using TiO2/water nanofluids. For the numerical section, the single-phase model was compared with the common two-phase numerical approaches. The numerical investigation indicated that the predicted heat transfer coefficients using single-phase and common two-phase approaches, even based on experimental thermophysical properties of nanofluids, underestimate and overestimate the experimental data, respectively. Therefore, some modifications are implemented to the common two-phase model in order to obtain more accurate predictions of the heat transfer characteristics of nanofluids. This modified model investigated the effects of particle concentration, particle diameter, and particle and basefluid material on the heat transfer rate at different Reynolds numbers. The results indicated that the convective heat transfer coefficient increases with an increase in nanoparticle concentration and flow Reynolds number, while particle size has an inverse effect. The obtained results can be very useful to the investigation of the potential application of nanofluid-based solar collectors. (C) 2015 Elsevier Ltd. All rights reserved.