Numerical Study on Convective Heat Transfer Enhancement in Horizontal Rectangle Enclosures Filled with Ag-Ga Nanofluid

The natural convection heat transfer of horizontal rectangle enclosures with different aspect ratios (A = 2:1 and A = 4:1) filled with Ag-Ga nanofluid (different nanoparticle volume fractions phi = 0.01, phi = 0.03, phi = 0.05 and radiuses r = 20 nm, r = 40 nm, r = 80 nm) at different Rayleigh numbers (Ra = 1x10(3) and Ra = 1 x 10(5)) is investigated by a two-phase lattice Boltzmann model. It is found that the Nusselt number enhancement ratios of two enclosures (A = 2:1 and A = 4:1) filled with Ag-Ga nanofluid (r = 20 nm) are the same compared with those of the water at the corresponding aspect ratio enclosure. The more flat horizontal rectangular enclosure (A = 4:1) has the higher Nusselt number than the less flat horizontal rectangular enclosure (A = 2:1). It is also found that Nusselt number increases with the decreasing nanoparticle radius. Nusselt number enhancement ratios for every nanoparticle radius reducing by half at high Rayleigh number are higher than those at low Rayleigh number in most cases. The interaction forces between particles are also investigated in this paper. It is found that the Brownian force FB is about two magnitudes greater than that of drag force FD, and the value of driving force FS in A = 4:1 enclosure is about twice the value of driving force FS in A = 2:1 enclosure while other forces are almost the same.