Suspension viscosity and bubble rise velocity in liquid-solid fluidized beds

The effective viscosity which characterizes the pseudo-homogeneous property of the liquid-solid suspension in gas-liquid-solid fluidization is examined in light of the velocity of single bubbles rising through the suspension. Experiments conducted in this study cover a wide range of bubble diameters (2–23 mm) under high solids holdup (0.48 – 0.57) conditions. The study reveals that the liquid-solid medium exhibits a homogeneous, Newtonian property at any given solids holdup when the bubble diameters are greater than 12–17 mm. The effective viscosities obtained in this study based on equivalency of the single bubble rise velocity in Newtonian media as well as those reported in the literature are found to follow the Mooney-type relationship for solids holdup dependence. The two parameters underlying this relationship can be correlated as a function of the particle terminal velocity, particle shape and packed solids holdup. When the bubble diameters are smaller than 12–17 mm, the effective viscosity of the liquid-solid medium deviates from the viscosity of the corresponding Newtonian liquid. The deviation which marks the reduction in the bubble rise velocity reflects a significant close-range interaction between particles. In this bubble size range, the liquid-solid medium exhibits a non-Newtonian property characterized by shear-thinning behavior with flow index ≈ 12.

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