Air transport in chute flows |
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| Institution: | 1. Lahmeyer International GmbH (LI), Bad Vilbel, Germany (formerly VAW);2. Laboratory of Hydraulics, Hydrology and Glaciology (VAW), Swiss Federal Institute of Technology, ETH, Zurich, Switzerland;1. Department of Mechanical Engineering, University of Puerto Rico, Mayagüez Campus, Mayagüez, PR 00681, USA;2. Department of Engineering Science and Materials, University of Puerto Rico, Mayagüez Campus and Institute for Functional Nanomaterials, University of Puerto Rico, San Juan, PR 00931, USA;1. Univ Lyon, CNRS, INSA-Lyon, Université Claude Bernard Lyon 1, CETHIL UMR5008, F-69621 Villeurbanne, France;2. Wroclaw University of Science and Technology, Faculty of Mechanical and Power Engineering, Wroclaw 50-370, Poland;1. School of Software and Communication Engineering, Jiangxi University of Finance and Economics, Nanchang 330013, China;2. School of Computer Science, The University of Nottingham Ningbo China, Ningbo 315100, China;3. School of Computer Science and Telecommunication Engineering, Jiangsu University, Zhenjiang 212013, China;4. Faculty of Science, Engineering and Technology, Swinburne University of Technology, Hawthorn, Victoria 3122, Australia;1. Department of Mechanical Eng., University of Tehran, Tehran, Iran;2. Member of Center of Excellence in Design and Optimization of Energy Systems (CEDOES), Department of Mechanical Eng., University of Tehran, Tehran, Iran;1. School of Engineering Science, University of Science and Technology of China, Hefei 230026, PR China;2. Kavli Institute for Theoretical Physics China at the Chinese Academy of Sciences, Beijing 100190, PR China;1. Laboratory of Hydraulic Constructions (LCH), Ecole Polytechnique Fédérale de Lausanne (EPFL), Station 18, 1015 Lausanne, Switzerland;2. Laboratory for Hydraulic Machines (LMH), Ecole Polytechnique Fédérale de Lausanne (EPFL), Av. de Cour 33 Bis, 1007 Lausanne, Switzerland;3. Stucky LTD, Rue du Lac 33, 1020 Renens VD, Switzerland |
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| Abstract: | The air bubble rise velocity in still water depends mainly on the bubble size and is basically influenced by buoyancy, viscosity and surface tension. In high-speed flows the number of forces acting on air bubbles increases with turbulence, non-hydrostatic pressure gradient, shear forces, bubble clouds and free-surface entrainment. Air bubbles in these flows are used for cavitation protection of hydraulic structures such as chutes, spillways and bottom outlets. Here, air is normally added by means of aerators upstream of regions where the cavitation number falls below a critical value mainly to reduce the sonic velocity of the fluid and cushion the cavitation bubble collapse process. The distance between successive aerators depends basically on the bubble rise velocity. Until today, the bubble rise velocity in high-speed flows was not thoroughly investigated because of limited laboratory instrumentation. The present project focused on the streamwise development of air concentrations in high-speed flows along a 14 m long model chute. The bubble rise velocity was indirectly derived from the air detrainment gradient of the air concentration contour lines downstream of an aeration device. It accounts for the main hydraulic parameters chute slope, Froude number and air concentration. It is demonstrated that the bubble rise velocity in high-speed flow and stagnant water differ significantly due to fracturing processes, turbulence, and the ambient air concentration. |
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