Experimental investigation of transient heat transfer coefficient in natural convection with Al2O3-nanofluids

In this manuscript, the heat transfer coefficient of de-ionized water based nanofluid's containing Al2O3 nanoparticles under natural convection is experimentally investigated. The thermophysical properties of the prepared nanofluid samples which have great roles in the heat transfer process such as the thermal conductivity and viscosity are experimentally measured and their effects are discussed. Furthermore, the transient temperature of the hot and cold walls and the other sides of the container enclosure are measured and the variation of heat transfer coefficient with the volume fraction of the nanoparticles is shown. According to our results, the heat transfer coefficient of nanofluids enhanced at low volume fractions while with further increase in the volume fraction of nanoparticles it decreased as a result of the increase in viscosity. The results show that in comparison with de-ionized water the heat transfer coefficient of nanofluids containing 0.1% volume fraction nanoparticles had a 16.0% enhancement but for a 3.0% volume fraction it had a 24.0% decrease.