Nanoparticles for convective heat transfer enhancement: heat transfer coefficient and the effects of particle size and zeta potential

This work presents a study of using the Wilson Plot method to determine the convective heat transfer coefficient (CHTC) of the following nanoparticles in water as the base fluid: SiO2, TiO2, and Al2O3. The experiments were carried out in a double layer concentric glass tube in which the hot fluid and nanofluids exchange heat in a counter current fashion without direct contact. Attention was also given to the volumetric concentration, flow rate, and the size of nanoparticles to investigate their effects on CHTC. From the experiments, it was found that by adding nanoparticles, the CHTC of water can generally be enhanced and a 45% increase has been achieved with a 0.5 vol% concentration of Al2O3 nanoparticles at an intermediate Reynolds number around 4100. Moreover, simply reducing nanoparticle size and increasing the nanofluid flow rate do not necessarily lead to the CHTC enhancement, rather, they have adverse effects. It is concluded that the enhancement depends on the stability of the dispersed nanoparticles that can be characterized by their overall mean size and zeta potential as useful measures.