DISCHARGE AND TRANSPORT OF NEARLY BUOYANT GRANULAR SOLIDS IN LIQUIDS - PART II - AN INVESTIGATION OF THE EFFECTS OF PARTICLE PROPERTIES ON MIXTURE-DISCHARGE RATES AND PORE-PRESSURE PROFILES

Three different solid-liquid food analogues and one model food were us ed in a series of flow experiments in a model conical hopper and verti cal stand-pipe system. The mixture discharge rates, the liquid fractio n in the discharge and the differential pore pressures set up during f low along the hopper and stand-pipe walls were monitored. The liquid f raction in the discharge decreases from an asymptotic limit set at hig h mixture rates to values considerably less than 0.5 as the discharge rate is reduced. Individual solid particle properties affect both the value of the critical discharge rate, below which the discharge has a higher solid content and the limiting value of the discharge rate, abo ve which the liquid content remains unchanged. The results obtained wi th near-spherical particles couple well with the tomographic observati ons of the corresponding flow fields presented in Part I (Faderani et al., 1998, Chem. Engng Sci. 53, 553), which indicate the onset of a se ttling suspension well within the conical hopper at low mixture-discha rge rates but a packed-bed flow extending almost to the plane of the h opper orifice as the mixture rate is increased. With cylindrical parti cles, the packed-bed to settling suspension flow transition is postpon ed to occur within the vertical stand-pipe resulting in a high liquid content in the discharge even at small-mixture discharge rates. The pr ofiles of pore pressure measured along the hopper and stand-pipe walls during steady discharge agree well with the observed flow regime tran sitions and the changes in the liquid content of the discharge. The va lues of pore pressures corresponding to the packed-bed flow regime are compared with the predictions based on a modified form of the Ergun e quation first proposed by Mills Lamptey and Thorpe (1990, Chem. Engng Sci. 46, 2197). The interstitial fluid pressures measured in the suspe nsion regime are also compared with the theoretical predictions corres ponding to the incipient settling condition. Good agreement with exper iment is reported in both regimes when the transition from packed-bed to settling suspension occurs at the vicinity of the hopper orifice. A simple, first-order theoretical treatment based on Wallis' Drift-Flux Model (1969, One-dimensional Two-phase Flow, McGraw-Hill, London) exp lains well the observed relationships between the mixture discharge ra tes, the voidage profiles within the flow field and the liquid content of the discharge. (C) 1997 Elsevier Science Ltd.