Air flow in sanitary sewer conduits due to wastewater drag: a computational fluid dynamics approach

An accurate calculation of air flow in sanitary sewer conduits is a key input for improved understanding of odorous-compound emissions, efficient design of ventilation systems, and the occurrence of sewer fabric corrosion. In this study, the driving force of wastewater drag is considered and conceptually viewed as a Couette flow. Both turbulent and laminar flow regimes are modeled. In the turbulent flow regime, the Reynolds-averaged-Navier-Stokes equations are closed with an anisotropic turbulence model that consists of two sub-models: a generalized eddy viscosity mixing length model for the turbulent shear stresses and a semi-empirical model for the turbulent normal stresses. Solution of the resulting set of parabolic equations is implemented in a Gelerkin finite element framework. The predictive performances of the models are in agreement with longitudinal velocity measurements reported in the literature. Although the secondary flows computed are within a few percentage of the main flow velocity, the mean flow field is affected considerably by the secondary currents in every case. The present study suggests that models currently in use for estimating ventilation rates in sewer drains generally over predict the turbulent streamwise mean velocity. Key words: air flow, computational fluid dynamics, finite element method, mixing length, sewer conduit, turbulence-driven secondary currents, ventilation model, wastewater drag.