An experimental investigation of turbulent thermal convection in water-based alumina nanofluid

Abstract

We report heat transfer and flow dynamics measurements of alumina nanofluid in turbulent convective flow. Under the condition of fixed temperature at the top plate and fixed input heat flux at the bottom plate, it has been found that the convective heat transfer coefficient, h, Nusselt number, Nu, and Rayleigh number, Ra, all decrease with the increasing volume fraction ϕ of the nanoparticle. In contrast, the velocity of the convective flow showed no significant change within experimental uncertainty and over the range of nanoparticle concentration of the measurement (from 0% to 1.08%). Under the condition of constant nanoparticle concentration (ϕ=1.08%), a second set of measurements of the heat transport and flow properties have been made over a broad range of Ra (from 2.6×108 to 7.7×109). For heat transport, a transition near Rac≃2.5×109 has been found. For Ra>Rac, the measured Nu of the nanofluid is roughly the same as that of water in terms of both its magnitude and its scaling relation with Ra, which suggests that the nanofluid can be treated as a single phase fluid in this parameter range. For Ra<Rac, Nu becomes smaller than that of the water and the deviation becomes larger with decreasing Ra. In the parameter range of Ra<Rac, the measured instantaneous Nu(t) shows strong and quasiperiodic fluctuations, which is absent when Ra>Rac. This suggests that the significant decrease of the nanofluid Nu comparing to that of water may be caused by the mass diffusion of nanoparticles. Furthermore, measurements of the flow velocity of the bulk nanofluid showed no significant difference from that of water for Ra either above or below Rac. From estimated thermal boundary layer thickness, we found that the deviations of the nanofluid Nu from that of water for Ra<Rac corresponds to the thickening of the thermal boundary layer at both the top and bottom plates. This thickening of the boundary layer at low input heat flux (or low driving strength of the convective flow) cannot be attributed to possible sedimentation of the nanoparticles.