Abstract
Nanofluid is considered a suitable way for enhancing the heat transfer performance of conventional fluids. In this article photoacoustic technique as a nonintrusive optical method has been used to measure thermophysical properties (diffusivity, effusivity and conductivity) of transparent nanofluids. TiO2 (approximate to 20 nm) and Al2O3 (3-7 nm) nanoparticles have been prepared and utilized to prepare nanofluids at low volume concentration (0.1-1 vol.%). The prepared nanoparticles were characterized by using X-ray diffraction and TEM. The base fluid was dilute solution of dye crystal violet as a light absorber of known extinction coefficient in deionized water. The measured thermal parameters of the base fluid do not significantly differ from that of deionized water. The results show that thermal parameters of both nanofluids increase as the particle concentrations increase and are higher than the values of the base liquid for all concentrations of TiO2 nanoparticles. Also despite thermal conductivity of bulk Al2O3 is larger than that of TiO2, thermal parameters of Al2O3 nanofluids are generally less than those of TiO2 nanofluids. This contradictory can be explained on the bases of particle size effect. Furthermore, using Hashin and Shtrikman lower bound we were able to estimate an order of magnitude reduction of thermal conductivity of Al2O3 nanoparticles relative to its bulk value. (C) 2012 Elsevier Inc. All rights reserved.