Numerical Assessment of Gas–Solid Flow in a U-Bend

Abstract

The incompressible, single-phase and multiphase, isothermal and steady-state flows in a U-bend are studied numerically using FLUENT 6.0. A basic understanding of gas–solid flow and pressure behavior in a U-bend is frequently required to obtain better U-bend design, for example, in pneumatic conveying dryers. Fyhr and Rasmuson found that U-bends significantly influence the drying behavior in such a dryer since they enhance velocity differences (slip) between suspended material and the drying medium. For single-phase flow, influences of the gas inlet velocity and the bend radius ratio on the velocity and pressure profiles in a U-bend are studied. The numerical calculations were validated against experimental data from the literature. The gas inlet velocity varies from 5 up to 25 m s−1 while the bend radius ratio varies from 2 up to 8 m m−1, The gas inlet velocity has a strong influence on the total pressure drop of the system and the bend radius ratio strongly affects the velocity and pressure profiles in a U-bend. For multiphase flow, the bend radius ratio was varied from 3 up to 8 with constant gas velocity of 15 m s−1. There are three important effects of the U-bend: increase in slip velocity; an accumulation of particles along the outer walls of the U-bend and the beginning of downstream pipe; and an increase in pressure drop. A higher slip velocity and a better dispersion of particles are desired for a pneumatic conveying drying system while an increase in pressure drop is not favorable. Choice of the appropriate velocity and bend shape can, therefore, be utilized in the overall optimization of the operating conditions of pneumatic conveying systems.