Characterising vertical plug flow pneumatic conveying using CFD–DEM

Dense phase pneumatic conveying involves complex interactions between particles and fluid in a multi-phase flow. This study employs CFD–DEM simulations to characterise the effects of particle and material properties and operating conditions on vertical plug flow pneumatic conveying systems. The analysis reveals that vertical plug flow is governed by dynamic mechanisms involving particle exchange, plug coalescence, and instability, which can lead to undesirable pressure fluctuations and blockages. Plug coalescence, in particular, results in longer downstream plugs, and is a phenomenon that needs to be considered in system design. A two-stage analysis was conducted to examine the factors influencing plug porosity and velocity. In the first stage, inlet air velocity, particle size, and particle–particle sliding and rolling friction coefficients were identified as significant factors. Notably, the influence of rolling friction on plug porosity and velocity differs from that observed in horizontal conveying, highlighting fundamental differences in bulk particle behaviour. The second stage, using a Taguchi design, revealed particle shape as the most significant parameter affecting plug porosity and a key factor influencing plug velocity. Ellipsoidal particles (with aspect ratios differing from one) yielded higher plug velocities than spheres due to reduced energy dissipation, emphasising the importance of incorporating particle shape effects in simulations. Overall, this study demonstrates the distinct mechanisms governing vertical plug flow, highlighting the limitations of using spherical particles to model non-spherical shapes. These findings provide insights for improving the design and modelling of dense phase pneumatic conveying systems, with implications for various industrial applications.