Representation of Near-Wall Particle Fate in a Eulerian–Lagrangian Approach for Clarifier Unit Operations

Abstract

Eulerian–Lagrangian methods are common for computational fluid dynamics (CFD) modeling of particle fate. Near-wall turbulence flow profiles are frequently modeled with wall function (WF) for computational efficiency and stability. However, WF does not provide the viscous and buffer sublayers solution for Lagrangian particle tracking (LPT). This study examines Reynolds-averaged Navier-Stokes (RANS)-LPT model combinations with near-wall models (two-layer, low-Reynolds number, WF), common LPT boundary conditions (trap, reflect), near-wall grids resolution (Δy+), and LPT solver configurations. Near-wall LPT in an open channel clarifier illuminates embedded numerical errors/artifacts generated by widely-used RANS-WF-LPT model combinations as benchmarked to direct numerical simulations (DNS). Near-wall LPT can be subject to grid resolution irrespective of particle-boundary interactions and LPT boundary conditions applied. Clarifier simulations illustrate LPT solver configurations influence particulate matter (PM) separation. RANS-LPT solutions are a function of particle diameter, maximum particle integration steps, and to a lesser degree on tracked particle number. Low-Reynolds number and two-layer models improve per particle size separation predictions compared to the WF model by up to 50% for 25–100 μm particles based on physical model benchmarking.