Aqueous nanofluids composed of alumina nanoparticles with different sizes at a concentration from 0.1vol% to 0.5vol% were prepared by a two-step method. The suspension and dispersion characteristics were experimentally examined by zeta potential, average size and absorption spectrum. The thermophysical properties such as the viscosity, surface tension, thermal conductivity, saturation vapor pressure and latent heat of vaporization were measured. The influences of the particle size, particle volume concentration and temperature on the thermophysical property were investigated. It was found that the viscosity and thermal conductivity increased with decreasing nanoparticle size. In contrast, the surface tension, saturation vapor pressure and latent heat of vaporization decrease with decreasing nanoparticle size. The viscosity, thermal conductivity and saturation vapor pressure have an increasing tendency with increasing volume concentration. However, surface tension and latent heat of vaporization showed a reverse tendency. Furthermore, the temperature also showed had obvious influence on the nanofluids viscosity, thermal conductivity and surface tension.

This paper presents an experimental investigation of the heat transfer characteristics of the heat pipe with CuO-water nanofluid. For this purpose, CuO nanoparticles of 30 nm size were dispersed in distilled water to form stable suspension containing 0.1% ~ 2.0% mass concentrations of nanoparticles, and then the heat pipe was produced after CuO-water nanofluid was added in it as the working fluid. Experimental results show that the use of CuO-water nanofluid hold a lower start-up temperature and shorter start-up time for the evaporation section of the heat pipe compared to distilled water. Their heat transfer performance of the evaporation section and condenser section has been improved than that of distilled water. The heat transfer coefficient of nanofluid is higher than that of the base liquid and found to increase by 29.4% and 125.0% for the mass concentration of 0.5% compared with the heat pipe using distilled water while the input power ranging from 15W to 45W. By examining the thermal resistance, it was found that the thermal resistance has been significantly decreased compared with the heat pipe with distilled water. The thermal resistance of heat pipe using CuO-water nanofluid at a mass concentration of 0.5% is 0.36K/W when the input power is 45W, while the thermal resistance of heat pipe using distilled water is 0.80K/W. Further analysis indicates that the heat pipe using CuO-water nanofluid at 1.0% mass concentrations has the best heat transfer performance.

Thermal conductivities of two kinds of nanofluids (SiO2-water and SiO2-ethylene glycol) were measured by transient hot-wire method at different volume fraction and temperature. Influences of volume fraction of particles and temperature on thermal conductivities of nanofluids were analyzed. The Experimental results show that thermal conductivities of nanofluids are higher than those of base fluids, and increase with the increase of volume fraction and temperature. When approximately 0.5% volume fraction of SiO2nanoparticles are added into water and ethylene glycol at the temperature 50°C, the thermal conductivities are enhanced 46.2% and 62.8% respectively.

Nanofluid is a new class of heat transfer fluid which contains suspended metallic or nonmetallic nanoparticles in traditional fluid. In this paper, nanofluids containing low volume concentrations (0.1-0.5vol.%) of Al2O3 and SiO2 nanoparticles were produced using a two-step method with ultrasonication and without any surfactant. Suspension stability of nanofluid was characterized and analyzed using zeta potential, average particle size and absorbancy method. The KD2-pro thermal property meter was used to measure the thermal conductivities of Al2O3(40nm)-water and SiO2(30nm)-water nanofluids at different volume concentrations (0.1-0.5vol.%) and temperature(25-50°C). The results show that adding nanoparticles into base fluid can enhance the thermal conductivity greatly. Furthermore, it is also show that the thermal conductivities increase nearly linearly with the nanoparticle volume concentration increasing, and increase significantly with the temperature increasing. A model of thermal conductivity of nanofluid, which has a much better precision, was proposed. But the values of the model are lower than the measured thermal conductivities.