Non-Newtonian nanofluid in a micro planar sudden expansion considering variable properties

The heat transfer and flow pattern of Al2O3-water nanofluid in a micro planar sudden expansion with constant heat flux boundary conditions are numerically studied. The Al2O3-water nanofluid is treated as a single phase non-Newtonian fluid with variable physical properties and the power-law rheology is adopted to describe the characteristics of the flow, in which the flow behavior index depend on the nanoparticle volume fraction. A systematic study of the Al2O3-water nanofluid for a wide range of generalized Reynolds number, 50 ≤ Regen ≤ 500, and nanoparticle volume fraction 0% ≤ φ ≤ 3% is presented. The critical generalized Reynolds number at which flow bifurcation occurs is carefully studied and the heat transfer enhancement due to the non-Newtonian rheology is reported. It is shown that the flow bifurcation is delayed when the nanoparticle volume fraction increases and is advanced as the boundary heat transfer rate increases. The heat transfer deterioration brought by the recirculation area is reduced under a higher nanoparticle volume fraction and the reduction ratio increases with the generalized Reynolds number. The non-Newtonian model results are also compared with the results using a Newtonian model, which indicates that there will be a huge overestimate on the system pressure drop if Newtonian model is taken.