Experimental investigation on effect of stratification of bimodal settling slurry on slurry flow friction in pipe

Pipe flows of bimodal aqueous slurries of two fractions of sand (coarse sand of particle mass-median size of 1.56 mm and fine-to-medium sand of mass-median size of 0.22 mm) are tested in a laboratory loop (pipe internal diameter of 100 mm) to study the frictional loss reduction due to the presence of the finer fraction in the bimodal slurry. The tested bimodal slurry flows (mean solids volumetric concentrations of 0.23, 0.30, 0.45 respectively) are compared with their coarse slurry counterparts (flows of one-species slurry at solids concentrations of 0.17, 0.24, 0.34 respectively, without the finer fraction) in tests which include measurements of solids distribution in a pipe cross section and measurements of local velocity of solid particles at the bottom of the pipe. The tests reveal a presence of a sliding bed in the bimodal slurry flow and a development of a thin fine-particle layer at the interface between the bottom of the sliding bed and the pipe wall. The measured difference in local velocities of fine particles and coarse particles at the bottom of the pipe indicates that the fine-particle layer effectively separates the sliding bed from the pipe wall hence it is responsible for the reduction of sliding friction at the wall and reduction of the overall frictional loss in bimodal flow compared to its coarse slurry counterpart. The identified loss reducing mechanism can improve modelling of stratified settling slurry flows composed of solids fractions of different sizes interacting with each other in a pipe.