Hydrodynamic modeling of downward gas-solid flow. Part II: Co-current flow

The one-dimensional model of accelerating turbulent downward co-current gas-solid flow of coarse particles was formulated and experimentally verified by measuring the pressure distribution along the transport tube. The continuity and momentum equations were used in the model formulation and variational model was used for the prediction of the fluid-particle interphase drag coefficient. The experiments were performed by transporting spherical glass particles 1.94 mm in diameter in a 16 mm i.d. acrylic tube at constant solid mass flux of 392.8 kg/m(2) s. Tube Reynolds number ranged from 880 to 11,300 and the slip Reynolds number from 32 to 670. At these conditions, the loading ratio G(p)/G(f) was in the range from 395 to 31. Experimental data for the static fluid pressure distribution along the transport tube agree quite well with the model predictions. The results measured at a distance of 1.51 m from the transport tube inlet show that the particle velocity and the mean voidage increase with the increase in superficial gas velocity. The slip velocity changes from negative values at low gas superficial velocities to positive values at high gas superficial velocities. The same trend was observed for the change of the pressure gradient in the system. The values of the pressure gradient, porosity, particle velocity and slip velocity along the tube were calculated according to the formulated model. The distance from the transport tube inlet at which the slip velocity changes its sign from positive to negative is the function of the gas superficial velocity. At positive slip velocity both gravity and drag contribute to particle acceleration. At negative slip velocity the drag force acts in upward direction resisting the particle acceleration. In downward co-current gas-solid flow acceleration length is relatively long, about two times longer compared to the upward co-current gas-solid flow