Numerical calculation of pneumatic conveying in horizontal channels and pipes: Detailed analysis of conveying behaviour

Abstract

The present study deals with pneumatic conveying of spherical particles in horizontal ducts, a 6 m long rectangular cross-section horizontal channel and a circular pipe, from a numerical perspective. The three-dimensional numerical computations were performed by the Euler–Lagrange approach in connection with the k– ε and a Reynolds Stress turbulence model accounting for full two-way coupling. For the calculation of the particle motion all relevant forces (i.e. drag, slip-shear and slip-rotational lift and gravity), inter-particle collisions and particle-rough wall collisions were considered. For all considered cases an average air velocity of 20 m/s was selected. Calculations are carried out for spherical glass beads with a diameter of 130 μm at a mass loading of 1.0 (kg particles/kg air). Additionally, different wall roughnesses are considered. The agreement of the computations with experiments was found to be satisfactory for mean and fluctuating velocities of both phases as well as for the normalised particle mass flux in the case of the channel flow. The main part of this contribution is related to a detailed analysis on the differences between pneumatic conveying properties in the rectangular channel and the circular pipe. For that the influence of wall roughness and the degree of coupling (i.e. two- or four-way) was analysed by visualising the cross-sectional distributions of air and particle properties. The observed focussing effect has a remarkable influence on particle concentration distribution in the pipe cross-section and the wall collision frequency over the circumference. Distinct differences in the velocity profiles of both phases (mean velocities and fluctuating components), as well as the concentration profiles, for channel and pipe flow are identified. The particle fluctuating velocities in the pipe are higher than in the channel for all situations, yielding mostly higher wall collision frequencies. As a consequence, in all the considered cases, the pressure drop in the pipe is larger than in the channel, especially for high wall roughness.