Heat transfer and flow behaviour of aqueous suspensions of titanate nanotubes (nanofluids)

Titanate nanotubes of an aspect ratio of .apprx. 10 are synthesized, characterized, and dispersed in water to form stable nanofluids contg. 0.5, 1.0, and 2.5 wt.% of the nanotubes. Expts. are then carried out to investigate the effective thermal cond., rheol. behavior and forced convective heat transfer of the nanofluids. The results show a small thermal cond. enhancement of .apprx. 3% at 25 Deg and .apprx. 5% at 40 Deg for the 2.5 wt.% nanofluid. The nanofluids are found to be non-Newtonian with obvious shear thinning behavior with the shear viscosity decreasing with increasing shear rate at low shear rates. The shear viscosity approaches const. at a shear rate higher than .apprx. 100-1000 s-1 depending nanoparticle concn. The high shear viscosity is found to be much higher than that predicted by the conventional viscosity models for dil. suspensions. Despite the small thermal conduction enhancement, an excellent enhancement is obsd. on the convective heat-transfer coeff., which is much higher than that of the thermal cond. enhancement. In comparison with nanofluids contg. spherical titania nanoparticles under similar conditions, the enhancement of both thermal cond. and convective heat-transfer coeff. of the titanate nanotube nanofluids is considerably higher indicating the important role of particle shape in the heat-transfer enhancement. Possible mechanisms are also proposed for the obsd. enhancement of the convective heat-transfer coeff. [on SciFinder (R)]