Air Entrainment in a Tall Plunging Flow Dropshaft

A physical model study was conducted to investigate the mechanisms of air entrainment in a tall plunging dropshaft of 7.7m in height. The water flow in the dropshaft was observed to break up into small drops of approximately a few millimeters diameter at a drop height of 5 m. The dominant water drops were found to be approximately 2mm and they fell at an average speed of approximately 6m/s. An analytical model was developed to predict the amount of air drag and air flow needed on the basis of the momentum transfer from the water drops. The drag force induced a vertical air pressure gradient, and the model prediction compared well with the measurements. The entrained air flow rate increased with the water flow rate, but the ratio of the air to water flow rate decreased from approximately 20 to 5 times when the water flow rate increased from 3.9 to 47.7L/s. The air pressure was negative at the top of the dropshaft to allow the ambient air to be entrained, and it increased to the atmospheric pressure at the bottom at which point the outlet was open to the atmosphere. Additional experiments also were conducted with a weir of two different heights at the outlet pipe. At a large water flow rate, the weir would cause a significant blockage in air passage, resulting in a reduced air entrainment.