Air demand in plunging dropshafts of medium height

Falling water in dropshafts can induce a significant amount of airflow into sewer systems. In this study, the air demand in plunging-type dropshafts with medium drops of 3.38 and 1.88 m were investigated. The downward movement of air in the dropshaft is influenced by two components of falling water: the annular flow attached to the shaft wall and the bounced flow (in the forms of mainstream or water droplets). A model for predicting air pressure gradients was developed and experimentally validated. The air demand increased with the air inlet area, the drop height, the water flow rate, and the ratio of the bounced jet to the total water flow rate. A prediction model for the air demand in dropshafts was developed, thus it is possible to assess the air demand of dropshafts across a range of drop heights, diameters, and inflow conditions. When the air inlet to the shaft is restricted, a large negative pressure is generated inside the shaft, increasing the height of the water-air mixture generated by the bottom jet impact, which reduces the impact pressure at the bottom.