Effect of nano-fluid variable properties on mixed convection in a square cavity

The problem of mixed convection fluid flow and heat transfer of Al2O3-water nano-fluid with temperature and nano-particles concentration dependent thermal conductivity and effective viscosity inside a square cavity has been investigated numerically. The geometry of the present work was a square cavity with a heat source on the bottom wall, insulated top wall and moving downward cold side walls. The effects of increase in shear force while the buoyancy force was constant and effects of increase in buoyancy force when the shear force was kept constant were investigated. When the heat source was located in the middle of bottom wall, when the Rayleigh number was kept constant, the effect of addition of nano-particles on enhancement of heat transfer increased with increase in Reynolds number. For a constant Reynolds number and for high Rayleigh numbers, the rate of heat transfer decreased with increase in nano-particle volume fraction. Moreover it was found that the rate of this decrease increased with increase in Rayleigh number. Also the obtained results showed that when the heat source moved toward the side wall, the rate of heat transfer increased. The results obtained using variable thermal conductivity and variable viscosity models were compared to the results obtained by the Maxwell- Garnett model and the Brinkman model. The results showed that significant differences existed between the calculated overall heat transfers for the two different combinations of formulas. Moreover the difference increased with increase in nano-particles volume fraction. (authors)