Air Entrainment in a Tall Plunging Flow Dropshaft
Publication: Journal of Hydraulic Engineering
Volume 142, Issue 10
Abstract
A physical model study was conducted to investigate the mechanisms of air entrainment in a tall plunging dropshaft of 7.7 m 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 2 mm and they fell at an average speed of approximately . 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 . 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.
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Acknowledgments
The writers gratefully acknowledge financial support from the China Scholarship Council, the Natural Sciences and Engineering Research Council (NSERC) of Canada, and the Chinese Water Pollution Control Program (Project Number 2011ZX07301-004). The authors also would like to thank Perry Fedun and Yangbo Tang for their technical assistance.
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Published In
Journal of Hydraulic Engineering
Volume 142 • Issue 10 • October 2016
Copyright
© 2016 American Society of Civil Engineers.
History
Received: Mar 31, 2015
Accepted: Mar 22, 2016
Published online: Jun 9, 2016
Published in print: Oct 1, 2016
Discussion open until: Nov 9, 2016
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