Formation and dispersion of ropes in pneumatic conveying

In this study, the solid flow nonuniformities which develop in lean phase u pward 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% les s than 75 mum) for two different 90 degrees circular elbows having pipe ben d 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 me asurements of time-average local particle velocities, concentrations, and m ass 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 s tructure until it disintegrates into large discontinuous clusters at downst ream locations. Comparisons of the results of CFD simulations of turbulent gas-particle flow and time-average experimental data were used to explain r ope formation and dispersion. The CFD simulations, based on the Lagrangian particle-source-in-cell method, predict a denser particle rope as the nondi mensional radius of curvature (R/D) is increased, agreeing with trends in e xperimental data. The individual effects of secondary flows and turbulence on axial dispersion of the rope were studied computationally and the result s show both mechanisms are important. (C) 2001 Elsevier Science S.A. All ri ghts reserved.