Rebound characteristics for 50-μm particles impacting a powdery deposit

The formation of powdery deposits on the tubes in a waste heat boiler reduces the heat transfer coefficient by about 25%. Because of these fouling layers, the efficiency with which energy can be recovered from flue gases is reduced. The growth of the layers by the deposition of fly ash particles onto the tubes is strongly dependent on the interaction of the incident particle with the bed of particles. In this study the interaction is modelled as the outcome of a 2-body collision between an incident fly-ash particle and a second particle that represents a developing fouling layer. The mass of this second particle is dependent on the characteristics of the layer. The present paper focuses on the applicability of the 2-body collision model to predict the rebound characteristics of 50-μm particles impacting a powdery deposit. This is studied by impaction experiments of glass particles on different types of glass layers (powdery vs. glued), under various impaction angles (ranging from 0° to 30°) and with different incident velocities (ranging from 0.5 to 1 m/s). The experiments are carried out in a vacuumed column and, using a digital camera system, individual impacts are recorded. With this set-up the normal coefficient of restitution, the Coulomb friction coefficient and the proportionality factor are determined for particles in the 50-μm range. Besides, a 2-body collision model is developed to simulate the rebound characteristics. From a comparison between the experimental results and the model, it is shown that the 2-body approach to model the rebound characteristics of particles impacting a powdery layer is valid and that the proportionality factor is dependent on the binding between the particles in the layer and the relative particle size of the particles in the layer compared to the size of the incident particles.