Laminar mixed convection of TiO2-water nanofluid in horizontal pipe flow

Nanofluids - suspensions of nanometer-sized particles dispersed in basefluids like water and ethanol - are a new option to increase heat transfer. However, doubts linger as to if these special fluids indeed enhance heat transfer. Here a water-based TiO2 nanofluid is studied in order to evaluate the convective heat transfer under laminar forced and mixed flow conditions. Viscosity and thermal conductivity of the employed nanofluid are nearly identical with the equivalent values of the basefluid water. Under these conditions, any change of heat transfer can be purely attributed to the migration of nanoparticles themselves. This approach goes far beyond the usual characterisation of nanofluids as effective media. Laminar flow experiments in a uniformly heated pipe are carried out over a large range of parameters. As long as free convection is negligible, nanofluid Nusselt number development along the pipe axis is well described by analytical solutions. However, the onset of free convection is delayed. The following development of the Nusselt number departs significantly from the equivalent water values. Moreover, enhancement of heat transfer due to additional free convection is in general much weaker in nanofluid flow than in single-phase flow. The found deterioration is attributed to the movement nanoparticles due to Brownian and thermophoretic diffusion.