DISCHARGE AND TRANSPORT OF NEARLY BUOYANT GRANULAR SOLIDS IN LIQUIDS - PART I - TOMOGRAPHIC STUDY OF THE INTERSTITIAL VOIDAGE EFFECTS GOVERNING FLOW REGIMES

The interstitial voidage profiles prevailing in static and flowing bed s of nearly buoyant granular materials in aqueous solutions are measur ed directly by scanning with the use of gamma-ray tomography, the cont ents of a mass-flow hopper and vertical stand-pipe system. In a series of 'start-stop' flow and 'steady-state' flow experiments, horizontal line profiles, and radial profiles of interstitial voidage are produce d at different heights within the conical hopper and vertical stand-pi pe sections. The voidage profiles obtained within the static beds prio r to the onset of discharge are compared with the profiles measured at the same heights during batch discharge of the hopper contents. Furth ermore, the plane mean values of the flowing bed voidage are calculate d at different heights using the cross-sectional profiles of voidage o btained under the steady discharge condition. The resulting vertical p rofiles of plane-mean voidage obtained with three different food analo gues are found to reveal hitherto unavailable and highly significant n ew information about the transitions accompanying flow between the pac ked-bed and settling-suspension states as a function of the mixture di scharge rates and the single particle properties, such as particle siz e, shape, and particle density. The experimental results presented her e are subsequently incorporated into simple mean-field models (ignorin g interparticle and wall frictional effects) which are used to calcula te slip velocities of the particle phase, interstitial pore pressures due to the fluid phase, as well as the observed variation of the disch arged solids concentration with the mixture discharge rate. The model predictions are compared with experimental measurements in Faderani et al. (1998, Chem. Engng Sci. 53, 575). (C) 1997 Elsevier Science Ltd.