  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 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. The
conductivity also increases with the temperature (in addition to the
improvement due to the increased conductivity with water). Surprisingly, only
silica nanofluids exhibit a statistically significant effect of 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.
