Two-phase lattice Boltzmann simulation of the effects of base fluid and nanoparticle size on natural convection heat transfer of nanofluid

The effects of two kinds of base fluid (H2O and Ga) and three different nanoparticle radiuses (20 nm, 40 nm and 80 nm) on the natural convection heat transfer of a rectangular enclosure filled with Al2O3-H2O and Al2O3-Ga nanofluid at different Rayleigh numbers (Ra = 103 and Ra = 105) are discussed based on a two-phase lattice Boltzmann method in this paper. It is found that Al2O3-Ga nanofluid shows a better heat transfer enhancement than Al2O3-H2O nanofluid. Al2O3-Ga and Al2O3-H2O nanofluid with the same smallest radius (r = 20 nm) can enhance the heat transfer by 86.0% and 24.5% compared with water respectively. It is also found that Al2O3-Ga nanofluid at low Rayleigh number (Ra = 103) shows the biggest heat transfer enhancement (shows a 4.4% increase for every nanoparticle radius decreasing 20 nm) compared with Al2O3-H2O nanofluid. The enhancement ratio at low Rayleigh number is about 3-10.8% and 23.5-28.8% more than that at high Rayleigh number for Al2O3-water and Al2O3-Ga nanofluid respectively. In addition, the nanoparticle fraction distributions in the enclosure are also investigated.