Investigation of thermophysical properties of nanofluids

Nanofluids are liquid suspensions of particles with at least one of their dimensions smaller than 100 nanometer (nm). Nanofluid technology becomes a new challenge for the heat transfer fluid since it has been reported that the thermal conductivity of nanofluid is anomalously enhanced at a very low volume fraction of nanoparticles.In this study, a new application of a hot wire sensor for measurement of thermal conductivity of (nano)fluids, based on a hot wire thermal probe with ac excitation and 3 omega lock-in detection, were presented. Due to modulated heat flow in cylindrical geometry with a radius comparable to the thermal diffusion length, the necessary sample quantity is kept very low, typically 25 microliter. The thermal conductivities of de-ionized water based TiO2, SiO2, Al2O3 nanofluids and ethylene glycol based Al2O3 nanofluids were measured and their dependence of particle volume fraction and temperature were investigated. Our results show that thermal conductivity values are inside the limits of (moderately lower than) Hamilton-Crosser model. Our experiments at different temperatures show that relative thermal conductivity of nanofluids is not related with the temperature of the fluid.For industrial applications of nanofluids, one should also know the viscosity characteristics, since for heat transfer applications pump costs are important. We investigated the temperature dependent viscosity of nanofluids for different particle concentrations by a Sine-wave Vibro Viscometer. Viscosity of our nanofluids increase dramatically with the increase in particle concentration, Einstein model is found to be unable to predict this increase.