Airflow in Gravity Sewers – Determination of Wastewater Drag Coefficient

Abstract

Several experiments have been conducted in order to improve the understanding of the wastewater drag and the wall frictional force acting on the headspace air in gravity sewers. The aim of the study is to improve the data basis for a numerical model of natural sewer ventilation. The results of the study shows that by integrating the top/side wall shear stresses the log-law models for the air velocity distribution along the unwetted perimeter resulted in a good agreement with the friction forces calculated by use of the Colebrook-White formula for hydraulic smooth pipes. Secondly, the water surface drags were found by log-law models of the velocity distribution in turbulent flows to fit velocity profiles measured from the water surface and by integrating the water surface drags along the wetted perimeter, mean water surface drags were found and a measure of the water surface drag coefficient was found.

Using computational fluid dynamics to describe H2S mass transfer across the water–air interface in sewers

“…Figures 6.4 and 6.5 present the resulting phase fraction, velocity and concentration profiles along the height of the domain in the middle of the pipe. The simulated velocity profiles indicate a good agreement with the measured values by Bentzen et al (2016). For a pipe with a length of 15 m and the analysed flow velocities, the concentration profiles show that almost no mass transfer across the water surface into the air phase can be observed.…”

“…In a second setup, mass transfer in a rectangular duct is analysed using two welldocumented examples of water-air pipe flow as they have been described by Bentzen et al (2016) (test cases no. 7 and 21).…”

“…The air phase is only accelerated by the movement of the water surface. Bentzen et al (2016) measured resulting velocity profiles in detail using Laser Doppler Anemometry (LDA) velocity measurements. The flow characteristics of the two test cases analysed are listed in Table 6.1.…”

“…The computational mesh consists of 3,029,223 cells. The setup of boundary conditions is similar to the rectangular pipe of Bentzen et al (2016). The hydrodynamic model has been simulated for 200s, until steady-state conditions were reached.…”

“…• The solver was then applied to a rectangular pipe with a length of 15 m and two different flow velocities. The resulting air flow velocities were compared to experimental results by Bentzen et al (2016). The mass transfer simulations showed a minor importance of the mass transfer simulations in this case due to the strong advective transport in the domain.…”