A three-dimensional two-phase model for flow, transport and mass transfer processes in sewers
Sewer networks are one major pillar of modern cities’ infrastructure. Their functionality ensures the transport of wastewater to the sewage treatment plant and the transport of rainwater from residential areas. Damages to sewers cause infiltration and exfiltration and at the same time high costs for rehabilitation. The formation of hydrogen sulphide (H2S) represents a risk factor for the conditions of concrete channels. Its emission cannot only cause the destruction of sewer walls by concrete corrosion, but can also represent a safety risk for sewer workers. Within the last decades, the characteristics of H2S emissions were intensively investigated and various models for predicting odour and corrosion were developed. The current state of the art are one-dimensional model approaches. At the same time, some predominant processes, e.g. the flow velocities in the air phase, are three-dimensional, and H2S emissions are very relevant on locations with high turbulence and complex flow fields (e.g. drops). This work continues at this point. It investigates and extends a three-dimensional twophase model with regard to different aspects. For this purpose the two-phase solver interFoam of the software OpenFOAM is used. Initially, the hydrodynamic properties for different models in closed conduits are investigated by analysing hydrodynamic properties for different models in closed cross sections. The analysis begins with the simulation of a simple single-phase water flow over a ground sill and is then extended to a highly complex sewer geometry. The complex sewer network geometry is compared with results of a 1:20 scale model and existing CFD simulations for an open geometry. The results show a good agreement. Extensions are based on the description of mass transfer using the Henry coefficient. Furthermore, adjustments are made to improve the specifics of H2S emissions in sewers. These include the description of the temperature dependency of the Henry coefficient, the equilibrium between H2S and the bisulphide ion (HS-) in the water phase and the influence of the pH value on this equilibrium. An additional extension describes the concentration of H2S in the air phase as partial pressure. The extensions and adaptations are validated using different analytical examples and the advantages of using a three-dimensional model over a one-dimensional approach are demonstrated using the example of the complex sewer geometry. Finally, the extended solver is coupled with a solver for dynamic geometries to validate the simulated mass transfer under turbulent conditions. The comparison of simulation results for mass transfer in a stirring tank with different stirring rates leads to a good agreement with experimental results from laboratory experiments. This work results in two new solvers, the difference of which lies in the geometry to be described. The first solver can be applied to static meshes, while the second solver can describe dynamic meshes, such as rotating geometries.
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