Extension and further validation of a 3D two-phase flow model for flow, transport and mass transfer in sewer systems
Sewer networks are integral infrastructures in a well-planned city. Their vital role lies in efficiently transporting both wastewater to treatment facilities and rainwater from residential areas. The emission of hydrogen sulphide (H2S) combined with the oxygen (O2) present in the system can pose a significant threat to the integrity of concrete channels which is amplified for older sewer systems. Not only can its release lead to the erosion of sewer walls due to concrete corrosion, but it also presents a safety hazard for those working in the sewer environment. Not to mention the accompanying high costs for maintenance to mitigate these problems. The generation of hydrogen sulphide and the concentration of oxygen in sewers are influenced by various factors, including turbulent flow conditions, hydraulic retention time, flow velocities, and pH value. Numerical modeling with Computational Fluid Dynamics (CFD) software provides a valuable tool for studying multiphase flows and understanding the complex interactions between fluid dynamics and chemical processes. This doctoral thesis focuses firstly on validating the air phase flow in the sewer headspace. The solver for simulating mass transfer of H2S developed by Teuber (2020) in the OpenFOAM framework is further validated and then extended and validated to account for O2. Simulations of the high-resolution models were carried out on a high-performance computing cluster. Chapter 1 of this thesis deal with the general introduction to the problems of odour and corrosion in sewer systems, while chapter 2 provides a concise summary of the model's principles and tools used, along with the developed solver extensions. In Chapter 3 the updated and extended solvers, interH2SFoam and interO2Foam respectively, are validated using a quasi-steady state tank. These solvers accurately reproduced concentration profiles and equilibrium concentrations based on Henry's law, including the temperature-dependent behaviour of the Henry coefficient. In Chapter 4, the volume-of-fluid (VOF) approach implemented in OpenFOAM was applied to analyze two-phase flow hydraulics in a lab-scale rectangular duct. The interH2SFoam solver was further validated for turbulent conditions later in the chapter, with results compared to the results of Teuber et al. (2019b) showing good agreement. To validate the capability of the interO2Foam solver for gas transport in sewer headspace, experimental data from Bentzen et al. (2016) was used. Simulated results were compared to experimental data and a 1D analytical solution for transport, yielding acceptable agreement. In Chapter 5, the interO2Foam solver was validated using the field study of Madsen et al. (2006). The interFoam solver achieved a hydraulically steady state and was validated against analytical data. After which the setup was then investigated for grid convergence using different mesh resolutions. Mesh convergence was investigated using different tools and observing residuals for each parameter, providing a better understanding of flow behaviour versus simulation time required. The chosen mesh was based on its accuracy and simulation time. The simulated data for point injection of O2 were compared to the field results of Madsen et al. (2006), showing good agreement. The same setup was then used for the study to investigate the effects of suction on O2 removal. It not only provided insights into system dynamics, but also is of practical benefit for identifying potential hotspots for H2S and designing strategies for H2S removal. Chapter 6 aimed to establish the relationship between mass transfer and turbulence level by varying the stirring rates in a rotating system. Dynamic meshing was employed for the setup, designed to scale with the experiments conducted by Pacheco Fernández et al. (2020). Multiple mesh tests were performed to achieve grid convergence, and the chosen mesh exhibited good results with optimal computation time. Both solvers showed an increase in mass transfer with higher stirring rates which is consistent with previous research. The time series for concentration of simulated data for H2S and O2 aligned well with measured data, providing valuable information on mass transfer operations in highly turbulent systems. The extended CFD model offers a comprehensive approach to better understand and predict the distribution of H2S and O2 in sewer systems and can serve as a decision support tool for odour control in sewers.
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