Experimental and numerical study of natural convection in a square enclosure filled with nanofluid

The coefficient of thermal conductivity and viscosity of Al ₂O₃-water nanofluid is measured, and its heat transfer is experimentally investigated in a square enclosure. In addition, a 2D two-phase Lattice Boltzmann model considering interaction forces (gravity and buoyancy force, drag force, interaction potential force and Brownian force) between nanoparticles and base fluid is developed for natural convection of nanofluid, and is applied to simulate the flow and heat transfer of Al₂O ₃-water nanofluid in the square enclosure by coupling the density distribution (D2Q9) and the temperature distribution with 4-speeds. In this paper, the effects of different nanoparticle volume fractions (φ = 0.25%, φ = 0.5%, φ = 0.77%) and different Rayleigh numbers (Ra = 30,855,746 and Ra = 63,943,592 for φ = 0.25%, Ra = 38,801,494 and Ra = 67,175,834 for φ = 0.5% and Ra = 55,888,498 and Ra = 70,513,049 for φ = 0.77%) on heat transfer in the transition region are experimentally and numerically discussed. The numerical results have a good agreement with the experimental results. It is found that the heat transfer of nanofluid is more sensitive to the thermal conductivity than viscosity at low nanoparticle fractions and it is more sensitive to the viscosity than the thermal conductivity at high nanoparticle fractions. In addition, the forces between water and nanoparticles are analyzed, and the nanoparticle volume fraction distribution is investigated. It is found that the temperature difference driving force makes the greatest contribution to the nanoparticle volume fraction distribution, and nanoparticle volume fraction distribution is opposite to that of the water phase density distribution.