Modelling the impaction of a micron particle with a powdery layer

The interaction of an incoming micron particle with already deposited particles is an important factor in particulate fouling of heat exchangers. A numerical model was developed based on the discrete element method to simulate this interaction. The contact forces between the colliding particles are based on the concept of contact mechanics, which takes plastic deformation of particles into consideration. The numerical model predicts the critical sticking and removal velocities, which are important parameters in determining the fouling rate of heat exchangers. Very detailed information of the bed dynamics can be extracted from the numerical model. It appears that the time required for a particle to be ejected out of a bed of particles due to an incident particle impact is proportional to the interacting particles diameter and to the square root of the number of bed layers. The maximum indentation in an incident particle hitting a bed of particles is proven theoretically and numerically to be directly proportional to the velocity and diameter of the incident particle if plastic deformation occurs. Experiments were carried out in a vacuumed column to validate the numerical model. In the experiments, incident particles dropped onto a bed of particles and the sticking, bouncing and removal behaviour were measured as a function of the incident particle impact speed. Both the numerics and the experiments showed that there are velocity regimes at which the incident particle sticks, bounces off or removes particles from the bed of particles. The regimes overlap due to the impact angle effect. The numerical model predictions regarding the critical sticking and removal velocities are in agreement with the measured values. © 2006 Elsevier B.V. All rights reserved.