Quantitative analyses of factors affecting thermal conductivity of nanofluids using an improved transient hot-wire method apparatus

While there are several factors affecting nanofluid thermal conductivity such as particle size, particle and fluid type, temperature and volume concentration, the effect of each factor on the thermal conductivity of the nanofluid is not clarified despite numerous experimental and theoretical efforts. In this work, four nanofluid samples with different particle size and volume fraction were prepared and the impact of the particle size and volume fraction on the effective thermal conductivity of the ethylene glycol/Al2O3 nanofluid was investigated using advanced transient hot wire system which avoids the capacitance influence and natural convection. The analysis result shows that particle size is more influential than volume fraction at the very low volume fraction less than 0.25% and the impact is more obvious with lesser volume fraction. Other than particle size and volume fraction, particle number density was turned out to be statistically very important factor affecting thermal conductivity of nanofluid when the volume fraction reaches to a certain degree of 0.2%. Under this volume fraction, the thermal conductivity of nanofluid decreases with the number density and this result may be attributed to hydrodynamic interactions induced by large fluid bodies traveling with nanoparticles in Brownian motion. This work provides not only the quantitative analyses of factors affecting the thermal conductivity of nanofluids but also the possible mechanisms of enhanced thermal conductivity. (C) 2015 Elsevier Ltd. All rights reserved.