Abstract
Three different solid-liquid food analogues and one model food were used in a series of flow experiments in a model conical hopper and vertical stand-pipe system. The mixture discharge rates, the liquid fraction in the discharge and the differential pore pressures set up during flow along the hopper and stand-pipe walls were monitored. The liquid fraction in the discharge decreases from an asymptotic limit set at high 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, above which the liquid content remains unchanged. The results obtained with near-spherical particles couple well with the tomographic observations of the corresponding flow fields presented in Part I (Faderani
et al., 1998,
Chem. Engng Sci.
53, 553), which indicate the onset of a settling suspension well within the conical hopper at low mixture-discharge rates but a packed-bed flow extending almost to the plane of the hopper orifice as the mixture rate is increased. With cylindrical particles, the packed-bed to settling suspension flow transition is postponed to occur within the vertical stand-pipe resulting in a high liquid content in the discharge even at small-mixture discharge rates. The profiles of pore pressure measured along the hopper and stand-pipe walls during steady discharge agree well with the observed flow regime transitions and the changes in the liquid content of the discharge. The values of pore pressures corresponding to the packed-bed flow regime are compared with the predictions based on a modified form of the Ergun equation first proposed by Mills Lamptey and Thorpe (1990,
Chem. Engng Sci.
46, 2197). The interstitial fluid pressures measured in the suspension regime are also compared with the theoretical predictions corresponding to the incipient settling condition. Good agreement with experiment 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) explains well the observed relationships between the mixture discharge rates, the voidage profiles within the flow field and the liquid content of the discharge.