Analysis of the effect of collisions on the gravitational motion of large particles in a vertical duct

Experimental and numerical investigations of the gravitational motion of large particles in a vertical duct have been carried out in order to improve the knowledge of particle-to-particle collision effects in gas–solid confined flows. An experimental facility, consisting of a collisional flow of large spherical glass particles (3 mm in diameter) in a vertical convergent channel has been set up. Particle flow characteristics were investigated by means of a photographic technique of visualization combined with image analysis. Experiments have been performed with two particle mass flow rates corresponding to solid volume fractions of 6.5×10−4 and 1.9×10−3. A significant effect of the collisions between particles upon the evolution of some of the characteristics of the flow has been experimentally shown. Particle–wall collision properties have also been measured in order to provide coefficients of the collision model used in a three-dimensional Monte Carlo type Lagrangian simulation code. Particle-to-particle collisions have been taken into account by means of the probabilistic collision model. The comparison of measurements with predictions of the Lagrangian simulation code showed the suitability of the probabilistic collision model. However, the necessity for a better knowledge of particle-to-particle collision frequency, as also for an accurate knowledge of particle–wall collision properties, is pointed out. Non-negligible effects of spin-induced forces on large particles have also been observed.