Abstract
An experimental study was conducted of the discharge of granular solid-liquid mixtures from a holding vessel comprising a conical hopper and stand-pipe sections. In a series of batch discharge experiments, measurements were made of i) mixture discharge rate and the liquid fraction in the discharge, ii) dynamic interstitial pressure profiles along the walls of the hopper and the stand-pipe. Furthermore, a specially constructed y-ray tomographic scanner rig was used to produce consecutive tomograms of the horizontal planes at different heights of the mixture beds in both the conical hopper and the stand-pipe sections. Experiments were repeated with three food analogues and one model food; uncooked soaked peas. The food analogues chosen were extruded plastic particles representing a range of particle properties such as size, sphericity, modulus of elasticity and surface roughness. At low mixture discharge rates corresponding to low shear strain rates (< 2 s-1), the discharged mixtures were found to be "solids-rich". With increasing mixture discharge rate, transitions were observed from "solids-rich" to "liquid-rich" discharge as a function of the different solid particles used. Using the plane mean values of the flowing voidage measured at different heights, corresponding mean values of the slip velocity were calculated identifying regions of packed-bed, and settling suspension transitions within the flow field. Fluidisation behaviour of the solid materials were also characterised independently by determining their terminal velocity, minimum fluidisation voidage and velocity and the Richardson and Zaki index. Interstitial pressure profiles measured during discharge were found in good agreement with tomographic flowing voidage measurements. Experimental pressure profiles were compared with predictions based on the use of the Ergun equation modified for radial. Interstitial pore pressure gradients measured along the stand-pipe section were compared with predictions based on the immersed weight of the solid particles in a settling suspension. A mean-field model was proposed using Wallis' Drift Flux Model which was based on the mean values of the liquid and solid phase velocities and the slip velocity between them to describe the effects of particle properties on the packed-bed and settling suspension transitions observed within the bulk flow fields. The model was for the co-current downward flow of solids and liquid which show the exact mathematical nature of the relationship between the solid terminal velocity, mixture discharge rate and the corresponding liquid fraction in the discharge with interstitial flowing voidage. This relationship can be used to predict for a specified value of liquid volume fraction in discharge the corresponding flow regime transitions and the accompanying voidage changes within the hopper and stand-pipe system.