ReD-dependence of dynamic thermal conductivities of nanofluids

The dynamic thermal conductivity of nanofluids is examined under fully developed laminar flow conditions (440 < ReD < 818) in a circular tube (1.753-mm ID) subjected to a uniform heat flux. The experiment has been conducted using nanofluids containing Al2O3 nanoparticles of 45-nm nominal diameter dispersed in DI water at two volume concentrations of 2% and 4%. At the small Reynolds numbers, the measured dynamic thermal conductivity is lower by as much as 16% in comparison with the corresponding static thermal conductivity at ReD = 0. The dynamic thermal conductivity gradually increases with increasing ReD, but never exceeds the static conductivity level up to the highest tested Reynolds number of 818. The hot wall-to-fluid temperature gradient drives thermophoretic depletion of nanoparticles from the wall to the tube center. The lower dynamic thermal conductivity at the small Reynolds number is believed to be attributed to the reduced effective conduction near the wall where the nanoparticle concentration is relatively lower than in the core of the tube. In contrast, examination of the heat transfer between the suspended nanoparticles and the surrounding water molecules shows that the nanoparticle thermophoretic velocities increase with increasing Reynolds number. The increased thermophoretic velocities inside the nanofluid tend to compensate for the aforementioned reduced thermal conductivity near the wall, and this is conjectured to account for the recovery of the dynamic thermal conductivity to the static level at the higher Reynolds number.