Pressure drops and flow patterns in sand-mixture pipes

Abstract

The paper discusses a relation between the flow friction and the flow pattern in a pipe transporting a sand–water mixture. The study is based on the laboratory experiments that were carried out in a 150-mm pipe for flows of various sand fractions and different flow patterns. Three pipe inclinations (horizontal, vertical and −35° descending pipes) were used to establish different flow patterns. During the tests the flow patterns varied from fully stratified to fully suspended. In the fully stratified flow all particles were transported in a granular bed sliding over the bottom of a pipe or in a shear layer linked to the granular bed (all particles were transported as the contact load). In the fully suspended flow no bed was present in a pipe and all particles were dispersed within a carrying liquid (all particles were transported as the suspended load). In many cases the flow was partially stratified, i.e. a proportion of solid particles occupied a granular bed at the bottom of a pipe and the rest of the particles were dispersed within a carrying liquid above the bed. The tested solids were three narrowly graded fractions of sand ( d 50=0.12, 0.37 and 1.85 mm) and two sorts of mixed sand (blended from two narrowly graded fractions: the 0.37-mm sand+the 0.12-mm sand or the 1.85-mm sand+the 0.12-mm sand). The distribution of solids across the pipe cross-section was measured to identify the flow pattern. Integral flow parameters (mean mixture velocity, delivered concentration of solids, differential pressure) were measured to determine the flow friction for various flow conditions. The paper presents a survey of selected results from a large database collected during a long measuring campaign. It shows the way the measured solids distributions across a pipe cross-section were analyzed to distinguish between the contact load and the suspended load and compares the frictional pressure drops in mixtures of different flow patterns. The measurements confirmed that flows with a higher degree of flow stratification exhibit considerably higher friction than flows with a lower degree of stratification. For the horizontal flow of the mixture of the volumetric concentration 0.12–0.13 at velocities slightly above the deposition-limit threshold (around 3 m/s) the frictional pressure drop in the 0.12-mm sand mixture was approximately two thirds of that in the 0.38-mm sand mixture and less than one half of that in the 1.85-mm sand mixture. The addition of the 0.12-mm sand (the solids concentration 0.13–0.15) to the mixture of the concentration 0.12–0.13 of either the 0.37-mm sand or the 1.85-mm sand reduced the solids effect on the total frictional pressure drop to approximately one half at velocities slightly above the deposition-limit threshold. The vertical flows exhibit slightly less solids friction than the horizontal non-stratified flows. For the mixtures of concentration 0.26 of both the 0.37-mm sand and the mixed sand blended of the 0.37-mm sand and the 0.12-mm sand, the solids effect on the total frictional pressure drop in vertical flow was approximately two thirds of that in a non-stratified horizontal flow. A comparison of the experimental data with the predictive models for the frictional pressure drop suggests that more reliable models are available for stratified flows than for non-stratified flows.