Analysis of the Repeat Collision Effect in Simulated Particle-Laden Flows With and Without Agglomeration

Direct numerical simulation, facilitated by the spectral element method, has been used to study collisions and agglomeration in particle-laden fluid flows through a channel at shear Reynolds number Re = 300. The particulate phase is simulated using Lagrangian particle tracking and a stochastic technique. Curiously, the implementation of agglomeration in the deterministic approach caused a major reduction in the particle collision rate. To determine the cause of this effect, analysis of the collision rate across the channel for different particle properties was performed. For systems without agglomeration, collisions between particles tended to be located on consistent streamlines, occurring between single pairs of particles. A numerical analysis of this effect confirmed the result, where inter-particle collisions across the channel mostly took place between non-unique particle pairs, i.e. particle pairs which collide more than once within a short timeframe. This repeat collision effect was weakest towards the channel walls, and a dependency on the size of the turbulent eddies within the channel was observed. Repeat collisions are shown to be almost eliminated with the addition of the agglomeration mechanism. The impact of this effect on the accuracy of the stochastic technique is discussed, and modifications are suggested to account for the repeat collisions. Even with these the collision rate was exaggerated compared to the deterministic approach.