Opposite ReD-dependencies of nanofluid (Al2O3) thermal conductivities between heating and cooling modes

The dynamic thermal conductivities of nanofluids (Al2O3) in heating or cooling under fully developed laminar flow conditions show opposite dependence on Reynolds numbers, i.e., the dynamic conductivities under the heating conditions increase with increasing Re-D but under the cooling conditions, the dynamic conductivities decrease with increasing Re-D. Furthermore, the dynamic conductivities for cooling are higher than those for heating, and the thermal conductivities of stationary nanofluids with uniform distributions fall between these two values, for the entire tested Re-D range from 300 to 800. We believe that the main reason for this distinction is because of the drastically different cross-sectional nanoparticle concentration distributions that are in turn attributed to the opposite thermophoretic behavior near the tube wall between heating and cooling. The near-wall nanoparticle concentrations for cooling are substantially higher than those for heating; however, the stationary nanofluid with no thermophoresis maintains its uniform concentration in the middle between the two concentrations. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4747329]