Convective Performance of Nanofluids in a Laminar Thermally Developing Tube Flow

While many of the published papers on nanofluids focus on measuring the increased thermal conductivity of the suspension under static conditions, the convective performance of these fluids has received relatively little attention. The present work measures the effective thermal conductivity of nanofluids under developing convective boundary layer conditions in tubes of diameter 5 mm. The experiments use a hydrodynamically fully developed laminar tube flow in the range 500≤Re≤1600 with constant wall heat flux. The experiments were validated through measurements on pure de-ionized (DI) water, which results in a thermal conductivity value that agrees within 0.4% of handbook value. The increase in effective thermal conductivity for DI-water/Al2O3 nanofluids is 6% for 2% volume concentration of Al2O3, which is consistent with the previously reported conductivity values for this sample. For a suspension of multiwall carbon nanotubes in silicone oil, the thermal conductivity is increased by 10% over that of the base fluid for a concentration of 0.2% by volume. Scanning electron microscopy was utilized to examine the structure of the dry state of the nanotubes and elucidate the performance differences of carbon nanomaterials.