Experimental Study of Plunging-Flow Dropshafts with an Internal Divider for Air Circulation

Abstract

Plunging-flow dropshafts can entrain a large amount of air into sewer systems, pressurize downstream airspace, and cause potential sewer odor issues. In this study, a new design for a plunging-flow dropshaft is investigated by installing an internal divider to form a wet shaft and an air shaft. The air shaft allows the air entrained by the falling water to partially circulate back to the wet shaft, thus significantly reducing the air demand from the outside. Laboratory experiments were conducted using a 7.72-m-tall dropshaft with a divider with different sizes of opening for air circulation. External air demand and air pressure variation in the dropshaft were examined. The effects of inflow impingement and breakup of inflow into small water drops were found to be the main cause of the air pressure gradient along the dropshaft. Air flow circulation was modeled and compared well with the experimental measurements. The divider was found to effectively reduce the external air demand and the air pressurization of the dropshaft.