Simulation of Offgas Scrubbing by a Combined Eulerian-Lagrangian Model

In an industrial scrubber water droplets are introduced into the dust laden offgas in order to capture the fine dust particles. If the offgas stream is additionally guided through a contraction (e.g. a Venturi), the dust capturing efficiency is further increased because of the larger relative velocities between dust and droplets and the reduced droplet sizes. From a physical point of view the scrubbing process comprises a whole set of phenomena starting from droplet break-up and coalescence to the behaviour of the wall film and the dust capturing mechanism itself. The aim of this study is to address these phenomena by dedicated sub-models and incorporate them into a comprehensive simulation model within the OpenFOAM framework. Thereby, representative droplets are traced in a Lagrangian frame of reference while the fine dust particles are treated as additional passive Eulerian phases being allowed to drift with respect to the gas phase. Dust phases’ diffusivity is computed from its diameter and the local turbulence characteristics. Droplet break-up is considered by the Taylor Analogy Breakup (TAB) model. Dust capturing is triggered by impaction, interception and diffusion mechanism. In a further step the influence of a wall film is considered by a model extension solving the shallow water equations at wall boundaries. It is shown that on principle, modelling droplet deposition to the film is feasible as well as modelling droplet entrainment from the film due to film detachment and wave cresting. For the case without wall film numerical results are in good agreement with measurements found in literature as well as with a one-dimensional calculation approach that has been previously developed by the authors.