Rheological properties of alumina nanofluids and their implication to the heat transfer enhancement mechanism

Nanofluid is a novel heat transfer fluid prepared by dispersing nanometer-sized solid particles in a traditional heat transfer fluid for heat transfer enhancement. The microstructure investigation of nanofluids by rheological techniques shows that the particles do not exist as individual particles and nanofluids of rodlike alumina nanoparticles have a sol- or weakly flocculated gel-structure depending on particle concentration. The rate of thermal conductivity increase with concentration is faster in the sol state than in the weakly flocculated gel state. When the nanofluid becomes a strongly flocculated gel thermal conductivity remains almost the same as the pure liquid value. It is concluded that the Brownian motion plays a key role in enhancing thermal conductivity. The present study is the first report on the thermal conductivity of nanofluids with the characterized dispersion status.