Fig. 6 shows the enhanced thermal conductivity as a function of
nanoparticle size of Cu(0.1 wt%). As can be seen from the data, the
thermal conductivity of 25 nm CueH2O nanofluids is larger than
that of 50 nm CueH2O nanofluids. There are two possible explanations
to this phenomenon. On the one hand, in the same mass
fraction, the quantity of small particle is more than that of large
particle. It lead to the former has a larger interfacial area between
liquid and particles. Hence, the heat transfer is turned to more
quick and high efficiency. On the other hand, due to the small size
effect of nanoparticles, the nanoparticles in suspension will random
walk.