  Nanofluids are suspensions of nanoparticles in a base heat-transfer liquid.
They have been widely investigated to boost heat transfer since they were
proposed in the 1990's. We present a statistical correlation analysis of
experimentally measured thermal conductivity of water-based nanofluids
available in the literature. The influences of particle concentration, particle
size, temperature and surfactants are investigated. For specific particle
materials (alumina, titania, copper oxide, copper, silica and silicon carbide),
separate analyses are performed. The conductivity increases with the
concentration in qualitative agreement with Maxwell's theory of homogeneous
media, except for copper nanofluids which perform better. The conductivity also
increases with the temperature (in addition to the improvement due to the
increased conductivity of water). Surprisingly, only silica nanofluids exhibit
a statistically significant effect of the particle size, whereby smaller
particles lead to faster heat transfer. Overall, the large scatter in the
experimental data prevents a compelling, unambiguous assessment of these
effects. Taken together, the results of our analysis suggest that more
comprehensive experimental characterizations of nanofluids are necessary to
estimate their practical potential.
