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Incipient motion of solitary air pockets in a rectangular pipe
KTH, School of Architecture and the Built Environment (ABE), Land and Water Resources Engineering (moved 20130630), Hydraulic Engineering (moved 20130630).
KTH, School of Architecture and the Built Environment (ABE), Land and Water Resources Engineering (moved 20130630), Hydraulic Engineering (moved 20130630).
2013 (English)In: Journal of Applied Water Engineering and Research, ISSN 2324-9676, E-ISSN 2324-9676, Vol. 1, no 1, 58-68 p.Article in journal (Refereed) Published
Abstract [en]

The operation of bottom-outlet gates often gives rise to entrained air in the form of air pockets in the conduit under full-flow conditions. If unexpectedly released, it would cause problems for both personnel security and operational function. The present study addresses, through experimentation, the incipient movement of solitary air pockets in a rectangular pipe. A horizontal pipe and a 9.6° downward-inclined pipe are examined. The cross-section of the pipe measures 200 mm (width) by 250 mm (height). As distinct from a circular pipe, an air pocket in the rectangular pipe exhibits, at its incipient motion, a shape that depends mainly on factors such as the sloping angle of the pipe, cross-sectional location of the air pocket and its volume. These factors also determine the critical velocity of the air pocket. The experiments have shown that only small air pockets can exist under the roof. The corner is a cross-sectionally equilibrium position for larger air pockets. The air pocket in the corner position takes the shape of an elongated rectangular prism in the horizontal pipe and a triangular prism in the sloping one. When compared with a circular pipe, the critical velocity of air pockets in the rectangular pipe is lower if the pipe is horizontal and higher if it has a downward inclination.

Place, publisher, year, edition, pages
Taylor & Francis, 2013. Vol. 1, no 1, 58-68 p.
Keyword [en]
bottom outlet, rectangular conduit, air entrainment, incipient motion, critical velocity, experiments
National Category
Water Engineering
Identifiers
URN: urn:nbn:se:kth:diva-141191OAI: oai:DiVA.org:kth-141191DiVA: diva2:695567
Note

QC 20140211

Available from: 2014-02-11 Created: 2014-02-11 Last updated: 2017-12-06Bibliographically approved
In thesis
1. Modelling air―water flows in bottom outlets of dams
Open this publication in new window or tab >>Modelling air―water flows in bottom outlets of dams
2014 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

If air is entrained in a bottom outlet of a dam in an uncontrolled way, the resulting air pockets may cause problems such as blowback, blowout and loss of discharge capacity. In order to provide guidance for bottom outlet design and operation, this study examines how governing parameters affect air entrainment, air-pocket transport and de-aeration and the surrounding flow structure in pipe flows. Both experimental and numerical approaches are used.

Air can be entrained into the bottom outlet conduit due to vortex formation at the intake if the intake submergence is not sufficient. The influent of the intake entrance profiles and channel width on the critical submergence were studied in the experiment.

The experimental study was performed to investigate the incipient motion of air pockets in pipes with rectangular and circular cross sections. The critical velocity is dependent on pipe slope, pipe diameter, pipe roughness and air-pocket volume. If the pipe is horizontal, air removal is generally easier in a rectangular pipe than in a circular pipe. However, if the pipe is downward-inclined, air removal is easier in a circular pipe.

When a bottom outlet gate opens, air can become entrained into the conduit in the gate shaft downstream of the gate. Using FLUENT software, the transient process of air entrainment into a prototype bottom outlet during gate opening is simulated in three dimensions. The simulations show in the flow-pattern changes in the conduit and the amount of air entrainment in the gate shaft. The initial conduit water level affects the degree of air entrainment. A de-aeration chamber is effective in reducing water surface fluctuations at blowout.

High-speed particle image velocimetry (HSPIV) were applied to investigate the characteristics of the flow field around a stationary air pocket in a fully developed horizontal pipe flow. The air pocket generates a horseshoe vortex upstream and a reverse flow downstream. A shear layer forms from the separation point. Flow reattachment is observed for large air pockets. The air―water interface moves with the adjacent flow. A similarity profile is obtained for the mean streamwise velocity in the shear layer beneath the air pocket.

Place, publisher, year, edition, pages
KTH Royal Institute of Technology, 2014. xiv, 32 p.
Series
TRITA-LWR. PHD, ISSN 1650-8602 ; 2014:02
Keyword
Air pocket, Air entrainment, Bottom outlet, Critical velocity, Critical submergence, CFD, Experiment, Vortex, PIV, Two-phase air―water flow
National Category
Civil Engineering
Research subject
Land and Water Resources Engineering; Civil and Architectural Engineering
Identifiers
urn:nbn:se:kth:diva-141182 (URN)978-91-7595-017-4 (ISBN)
Public defence
2014-02-28, Kollegiesalen, Brinellvägen 8, KTH, Stockholm, 10:00 (English)
Opponent
Supervisors
Note

QC 20140211

Available from: 2014-02-11 Created: 2014-02-11 Last updated: 2014-02-11Bibliographically approved

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