An increase of thermal conductivity up to 21.76% was
obtained with Cu nanoparticles. Besides, it was possible to observe
that thermal conductivity of diathermic oil based nanofluids is
inversely proportional to particle size. Heat transfer coefficient of
nanofluids has been also analyzed [55,56]. Hwang et al. [57] studied
the convective heat transfer coefficient of water–Al2O3
nanofluids flowing in a stainless steel tube. The nanoparticles,
used in this investigation, had an average diameter of 30 nm and
they obtained an increase up to 8.0% at a concentration of solid
phase of 0.3%. Heyhat et al. [58] measured heat transfer coefficient
of the water based nanofluids with Al2O3 nanoparticles with an
average diameter of 40.0 nm and a volume fraction from 0.1 to
2.0% in a circular tube, with constant wall temperature, under
turbulent flow conditions. Results showed an enhancement up to
23.0%, compared to base fluid.