Fluidisation and hydraulic transport of carrot pieces

The behaviour of discs and cylinders of carrot in a water-fluidised bed was studied. Pressure drop measurements and terminal (or transportation) velocity relationships were established using cylinders of carrot, PVC and Nylon 6.6. This enabled the effect of particle density to be studied. Comparisons were made with the results of previous work by McKay and McLain (1980) on spheres and cuboids. At minimum fluidisation, large cylinders did not behave as spherical or near-spherical solids and the equation derived by Ergun (1952) for spheres, relating friction factor to Reynolds number and voidage, did not apply. However, a similar relationship has been established, viz: log f_v=1·439+0·53log Re_de (1 - ε₀) which is valid for cylinders having sphericity factors in the range 0·73-0·87. The relationships derived by Wadell (1934) for spheres and discs, having sphericities from 0·125 to 1·00 and largest dimension 5·00 mm, were unsatisfactory for predicting terminal fluidisation velocities of carrot or plastic cylinders. The resistance coefficient was related to the sphericity factor by: logC_τ=0·77+4· logø_s. As the L/D ratio increased, i.e. the sphericity decreased, the drag on the particles was reduced. This is the converse of their settling behaviour and shows that a resistance coefficient measured in settling conditions cannot be used to predict the fluidising behaviour of cylinders having large L/D ratios. Relationships between the slopes of the plots of log U vs log ε and volumetric shape factor, K, were investigated. The slope, n, of these plots was related to K by: log n=0·28-0·226 log K.