Microconvective Thermal Conductivity in Disperse Two-Phase Mixtures as Observed in a Low Velocity Couette Flow Experiment

Eddy transport associated with microscopic flow fields in shearing two-phase flows was investigated. Although such microconvective effects are expected to be present in all disperse two-phase flows, usually they are masked by other collateral mechanisms and could not be studied critically. In the present study, effective thermal conductivities of neutrally buoyant solid-fluid mixtures were measured in a rotating Couette flow apparatus. Low Reynolds numbers were used to avoid the effects of turbulence. Significant enhancement in effective thermal conductivity was observed when the Pe_d were high. Here Pe_d = ed²/α_f where e is the local mean shear rate, d is the particle diameter, and α_f is the thermal diffusivity of the fluid. Volume fractions employed were φ = 0.15 and 0.30 for polyethylene beads (2.9 mm in diameter) in a mixture of silicone oil and kerosene, and φ = 0.15 for polystyrene particles (0.3 mm in diameter) in a mixture of silicone oil and Freon-113. Single-phase liquid mixtures were also measured in various shear rates to show that the thermal conductivity was independent of shear rate and hence the observed phenomenon was not an instrumental artifact. The dependence of conductivity on particle Peclet number appeared to approach a power law relationship k_e ∝ Pe_d^1/2 for high Peclet numbers (300 < Pe_d < 2000).