Particle Size Effects on Collision and Agglomeration in Turbulent Channel Flows

Large eddy simulation is employed to investigate the effect of particle size on particle collision and agglomeration in a fully developed turbulent channel flow. The work adopts an Eulerian-Lagrangian approach which ensures that individual particle behaviour is accounted for by solving the particle equation of motion for each individual particle in a four-way coupled system. The hard-sphere collision model is used to detect particle-particle interactions and, on detection of a collision, particle agglomeration is tested based on the pre-collision kinetic energy, restitution coefficient and the van der Waals interactions. Particle collision and agglomeration are considered in a shear Reynolds number 300 flow of water containing spherical particles of 75 and 250 μm diameter at a volume fraction of O(10- 3). For the particle sizes considered, the overall number of collisions and agglomerations increases with time, although not all collisions result in agglomeration. The rate of collision and agglomeration is considerably higher for the lower particle size, leading to the formation of much larger agglomerates. Particle deposition due to gravitational effects leads to a distribution of collisions and agglomerates over the channel height, with larger values towards the lower regions of the flow. This is more pronounced for the larger particles, with their increased migration into regions near the lower wall with high levels of turbulence leading to collision and agglomeration numbers comparable for the two particle sizes.