Viscosity data for Al2O3-water nanofluid-hysteresis : Is heat transfer enhancement using nanofluids reliable?

Abstract The effect due to temperature and particle volume concentration on the dynamic viscosity for the water–Al 2 O 3 nanofluid has been experimentally investigated. The viscosity data were collected using a ‘piston-type’ commercial viscometer for temperatures ranging from room condition up to 75 °C. Two different particle sizes, namely 36 and 47 nm, have been considered. It has been found that, in general, nanofluid dynamic viscosity increases considerably with particle volume fraction but clearly decreases with a temperature increase. The viscosity values obtained for 36 and 47 nm particle-sizes are relatively close ones and others, except for high particle fractions. The complete viscosity database is presented. Results have clearly revealed the existence of a critical temperature beyond which the particle suspension properties seem to be drastically altered, which, in turn, has triggered a hysteresis phenomenon. Such a critical temperature has been found to be strongly dependent on both particle concentration and size. The hysteresis phenomenon has raised serious concerns regarding the reliability of using nanofluids for heat transfer enhancement purposes. Data have also shown that the Einstein's formula and some other ones originated from the classical linear fluid theory seem to be limited to nanofluids with low particle fractions.