On the structure of flow in an inclined settling column
Although there have been useful developments in the theory of solid-liquid separation in inclined tubes, many assumptions and concepts have not been tested. In the present study the principal objective was to gain a clearer understanding of the settling mechanism through observation of the flow fields within a particle-free fluid layer which exists at the upper face of the bulk suspension and within the bulk suspension itself. Experimental investigations were carried out using glass beads in an aqueous glycerol solution. Preliminary tests in a vertical column showed that the relationship between the settling velocity Vc, and the solids concentration C could be described by Vc=A(1-C)n when n=4.66 In the subsequent work in an inclined column the flow field in the clear fluid channel and within the suspension core was studied for suspension under the concentration conditions Co = 0.1 and Co 0.2. Characteristics features (width, velocity and average flow) of the clear-fluid layer were measured and compared with existing theory. Generally it was shown that there is a fairly good agreement between theory and experiments especially at early stages of settling and away from the top interface. Particular attention was paid to mode of infiltration from the bulk suspension into the clear-fluid channel. Measurements of the concentration distribution along the settling column were carried out using an electro-conduct ivity technique from suspension. Fluid velocity was measured by a dye-tracer. From the data, it was seen that the fluid velocities within the suspension were exceedingly high, being roughly 10 times the buck settling velocity of the suspension. All the characteristics of the flow field within the clear-fluid channel indicate a change in behaviour between the upper and lower parts of the settling column and take place as if there was a form of rotation above and below a horizontal axis at the middle of the tube. Existing theory (PNK) was modified to take account of the height of the sediment layer at the bottom of the tube. This provided goodagreement with experimental data over the range of conditions studied, apart from the final stages of settlement.