(82b) Experimental Investigation and Force Balance Modeling of Wet Particle Collisions

Particulate processes are characterised by intense contacts between particles and between particles and the apparatus walls. In addition to the particles often liquids are involved in the process as droplets or layers on the surfaces, e.g. as injected liquids in agglomeration and granulation or wetness of spray dried particles. These liquids have a fundamental influence on the collision dynamics as well as on agglomeration mechanisms of the particles. Although these micro-mechanics are crucial for understanding and modelling of wet particulate systems, until now the collision dynamics of wet particles and walls are still not fully understood.

Thus, in this work collision dynamics are investigated experimentally by evaluating high-speed images of particle-wall collisions as well as numerically by solving force balances of such collisions. The force balance model considers contact forces, viscous as well as capillary forces. For comparison of model and experiments the so called coefficient of restitution (CoR) is used, which is defined as ratio of rebound to impact velocities and therefore, describes the total energy dissipated during the collision. The CoR depends strongly on the collision parameters (e.g. collision velocity, initial particle rotation, collision angle), particle properties (e.g. size, roughness, deformation behaviour) as well as on the properties of the liquid (e.g. layer thickness, viscosity).

Normal and oblique collisions are investigated including rotation of the particles. Experimentally, the normal component of the CoR, defined only by normal translational velocities, was found to be independent of collision angle and initial rotation [1, 2]. It is however strongly dependent of normal collision velocity, layer thickness and viscosity [2, 3]. The tangential CoR in comparison appears to be influenced by initial rotation and collision angle [1, 2], but is independent of most liquid properties. Furthermore, tangential CoR and rotation after collision are strongly linked due to the frictional effect in the contact. Particles size features a complex influence on collision dynamics: for larger particles the normal CoR is constant for a constant ratio of particle diameter to layer thickness; however, with decreasing particle size the influence of capillary forces increases, leading to a decrease of CoR with particle size.

Validation of the numerical model based on force balances with the experimental data shows good compliance. Furthermore, the simulation indicates an even more complex relation between tangential CoR, collision angle and rotation than the experiments suggest.

We gratefully acknowledge for financial support: DFG, Germany and NWO, The Netherlands. Project number HE 4526/9-2.

[1] Crüger, B., Heinrich, S., Antonyuk, S., Deen, N.G., Kuipers, J.A.M., Experimental study of oblique impact of particles on wet surfaces, Chemical Engineering Research and Design 110 (2016) 209-219.

[2] Buck, B., Tang, Y., Heinrich, S., Deen, N.G., Kuipers, J.A.M., Collision dynamics of wet solids: Rebound and rotation, Powder Technology 316 (2017) 218-224.

[3] Crüger, B., Salikov, V., Heinrich, S., Antonyuk, S., Sutkar, V., Deen, N.G., Kuipers, J.A.M., Coefficient of restitution for particles impacting on wet surfaces: An improved experimental approach, Particuology 25 (2016) 1-9.