Formation and dispersion of ropes in pneumatic conveying

In this study, the solid flow nonuniformities which develop in lean phase upward flow in a vertical pneumatic conveying line following a horizontal-to-vertical elbow were investigated. Laboratory experiments were conducted in 154 and 203 mm I.D. test sections using pulverized-coal particles (90% less than 75 mum) for two different 90 degrees circular elbows having pipe bend radius to pipe diameter ratios of 1.5 and 3.0. The experiments covered a range of conveying air velocities and solids mass loadings. Experimental measurements of time-average local particle velocities, concentrations, and mass fluxes were obtained using a fiber-optic probe which was traversed over the cross-section of the pipe. The measurements indicate a continuous rope-like structure forms within the elbow. The rope maintains its continuous structure until it disintegrates into large discontinuous clusters at downstream locations. Comparisons of the results of CFD simulations of turbulent gas-particle flow and time-average experimental data were used to explain rope formation and dispersion. The CFD simulations, based on the Lagrangian particle-source-in-cell method, predict a denser particle rope as the nondimensional radius of curvature (R/D) is increased, agreeing with trends in experimental data. The individual effects of secondary flows and turbulence on axial dispersion of the rope were studied computationally and the results show both mechanisms are important. (C) 2001 Elsevier Science S.A. All rights reserved.