Simulations of Particle Detachment From a Flat Surface

Understanding particle detachment from surfaces is necessary to better characterize dust generation and entrainment in large-scale industries (such as metallurgical and foundry facilities, clean room settings, semiconductor device fabrication) and in health care (powder inhalers in pharmaceuticals, or the unwanted respiratory exposure to small particles, e.g., asbestos). The present work investigates the aerodynamics of a particle attached to a surface subjected to given fluid flow. The particle is represented as a sphere in a parallel plate channel, and this constitutes a three-dimensional (3D) flow problem. Since many of the challenges and results of the full 3D problem are also manifest in a corresponding two-dimensional (2D) configuration, as a first step, 2D simulations are conducted in a parallel plate channel, with the particle approximated as a cylinder. In both 2D and 3D cases, to model the particle just touching the surface leads to a singularity in grid generation; we have addressed this issue with two concurrent approaches. In the first approach, the particle is located at various finite distances from the surface; results are then extrapolated to zero height (particle just touching the surface). In the second approach, the bottom of the particle is embedded into the surface at different depths; again, results are extrapolated to zero embedding depth. In all cases, the flow is assumed to be steady, incompressible and laminar: (1 < Re_channel < 2000), and is represented by the Navier-Stokes equations. A fully-developed velocity profile is specified at the channel inlet. The computational domain is discretized using structured and hybrid grids, considering the boundary-layer physics. The governing equations are solved using the finite-volume FLUENT code. From the obtained numerical results, the coefficients of lift, drag and moments are computed, and compared with the results available in the published literature. For the particle touching the surface, aerodynamic forces (drag and lift) and moments are obtained by extrapolation for both approaches (particle located at finite height off the wall, and the partially embedded particle). The results of the 2D and 3D simulations show that, for a particle touching the surface, a threshold velocity (with corresponding threshold Re) exists for particle detachment (i.e., positive lift), and the moment plot indicates that the particle will tend to roll as it detaches.