Water/ZnO nanofluid. Thermal conductivity measurements
of ZnO suspensions have been carried out with the same analyzer as for SiO2 suspensions. However, first results with rod-like nanoparticles
were not reproducible. It seems that such behaviour was
due to the electrostatic behaviour of ZnO rods and special care was
taken in applying the hot wire method. As shown by Gautam [11],
the two opposite ends of rod-shaped ZnO crystals can present
opposite electrostatic charges which lead to aggregates. This can
facilitate wire fouling. With the original hot wire, ZnO nanoparticle
deposition was observed on the wire surface. It is the reason why
the hot wire was electrically insulated to limit polarisation and
consequently no wire fouling was detected.
Results are presented in Fig. 3(b) as a function of temperature. It
is observed that the thermal conductivity of the nanofluid is
slightly greater than the water conductivity. The particles being
crystallised no porosity effect occurs as for SiO2(Ag). Thermal
conductivity increases more slowly than that of water with
temperature. This could be due to the thermal conductivity of ZnO
which decreases with temperature [4] and compensates for water
conductivity augmentation. However, the enhancement is lower
than that predicted by the HamiltoneCrosser relationship due
probably to interface effects. It is also lower than that observed with
ZnO/ethyleneglycol suspensions