Air–water interaction in a partially filled circular pipe

This study investigates the combined effects of water drag forces and air pressure gradients on air movement within the headspace of a circular pipe. Under free ventilation conditions, where air enters the system solely through natural suction induced by water flow, positive pressure gradients were observed. The air pressure gradients increased with both the filling ratio and water flow rate. It was also found that air near the water surface flowed downstream due to water drag force, while air near the pipe crown moved upstream. Forced ventilation was introduced through air extractors positioned at the upstream and downstream vents, with adjustable power to create varying airflow rates. Results showed that beyond certain thresholds, increased air extractor power did not further enhance ventilation. Under forced ventilation, air velocity profiles in the non-boundary zone exhibited a more uniform distribution due to higher ventilation power. The effects of water drag, air pressure gradients, and pipe wall friction were analyzed. The critical pressure gradients with negligible water drag under forced ventilation were obtained. The drag coefficients of water on the airflow were calculated, ranging from 0.018 to 0.081 for free ventilation and from 0.002 to 0.038 for forced ventilation. A semi-empirical equation for calculating the drag coefficients under free ventilation was proposed. This study provides valuable insight into the dynamics of air movement in sewer systems, emphasizing the critical role of proper ventilation design in mitigating odor and corrosion issues.