Fig. 3 shows a fleld emission scanning electron microscope (FESEM, S-4800, Hitachi, Japan) photograph of r-Al2O3 nanoparticles (Al-13P, Yong-Zhen Technomaterial, Taiwan). The Al2O3 nanoparticles appear aggregated, and the primary particle size is approximately 20–30 nm. According to the data sheet of the dealer and literature, the density of Al2O3 nanoparticles is approximately 3880 kg/m3. The main reason of the use of Al2O3 nanoparticles as nanofluid material is easily dispersed in water and good
suspension performance based on past experience.