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Experimental studies of air pocket movement in a pressurized spillway conduit
KTH, School of Architecture and the Built Environment (ABE), Land and Water Resources Engineering, Hydraulic Engineering.
KTH, School of Architecture and the Built Environment (ABE), Land and Water Resources Engineering, Hydraulic Engineering.
2013 (English)In: Journal of Hydraulic Research, ISSN 0022-1686, E-ISSN 1814-2079, Vol. 51, no 3, 265-272 p.Article in journal (Refereed) Published
Abstract [en]

Undesired air entrainment in a bottom outlet conduit causes pressure transients, leading to conduit vibrations, blowbacks and discharge pulsations and thus endangers operational safety. In this study, the propagation velocity of a solitary air pocket and the characteristics of its critical velocity were examined in experiments conducted using a 240-mm-diameter pipe. Air pocket movement depends on the pipe diameter, slope, roughness and air pocket size. The critical pipe Froude number for initiating downstream movement of an air pocket is smaller in a larger pipe, most likely due to the scale effect and/or to a smaller reduction in the effective cross-sectional area. The critical velocity in rough pipes was found to be independent of the air pocket size. A minimum Froude number was suggested for a rough pipe instead of a critical pipe Froude number because the air removal process was found to involve successive air losses from the air pocket caused by turbulence.

Place, publisher, year, edition, pages
Taylor & Francis, 2013. Vol. 51, no 3, 265-272 p.
Keyword [en]
Air pocket, bottom outlet, critical velocity, large diameter pipe, roughness effect
National Category
Water Engineering
Identifiers
URN: urn:nbn:se:kth:diva-52308DOI: 10.1080/00221686.2013.777371ISI: 000320361500004Scopus ID: 2-s2.0-84879825055OAI: oai:DiVA.org:kth-52308DiVA: diva2:465913
Note

QC 20130806. Updated from submitted to published.

Available from: 2011-12-15 Created: 2011-12-15 Last updated: 2017-12-08Bibliographically approved
In thesis
1. Air-pocket transport in conjunction with bottom-outlet conduits for dams
Open this publication in new window or tab >>Air-pocket transport in conjunction with bottom-outlet conduits for dams
2011 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

Undesired air entrainment in bottom outlet conduits of dams may cause pressure transients, leading to conduit vibrations, blowback, discharge pulsation and even cavitation, and jeopardize the operational safety. Due to design limitations or construction costs, it is impossible to create an air free environment in a pressurized pipe. Therefore, it is essential to understand the air transport in enclosed pipes in order to provide guidance in bottom outlet design and operation. The commonly used criterion of the air-pocket movement in pipe flow is the water flow velocity for starting moving an air pocket, the so-called critical velocity.

In this thesis, the classical Volume of Fluid (VOF) model combined with the k-ε turbulence model is adopted for the computation of the critical velocity of a 150-mm pipe. The computed critical velocities are compared with the experimental results. The governing parameters investigated in this study include pipe slope and diameter, wall shear stress and air-pocket volume. Meanwhile, the carrying capacity (air-pocket velocity/ flow velocity) at all pipe slopes are analyzed. The simulation results of air pockets with different volumes in the bottom outlet conduit of Letten Dam in Sweden are presented in this study.

Moreover, experimental study was conducted to measure the critical velocity for a 240-mm Plexiglas pipe. The results are in agreement with the experiments performed by HR Wallingford (HRW) in 2003 in terms of the effects of pipe slope and air-pocket volume; however, the critical Froude pipe number is slightly smaller in this study. In rough pipes, a larger critical velocity is required compared with that in the smooth pipe. The removal mechanism in the rough pipe involves the successive loss of air caused by turbulence. This explains that the air-pocket size, with the dimensionless air-pocket volume n < 0.015, has little impact on the critical velocity for the rough pipe. In addition, roughness has little impact on the air-pocket velocity when it moves upstream in the downward inclined pipe. The trapped air bubbles most likely remain permanently in the rough pipe.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2011. x, 25 p.
Series
Trita-LWR. LIC, ISSN 1650-8629 ; 2062
Keyword
Air-water two-phase flow, critical velocity, diameter effect, roughness, VOF model, bottom outlet, experiment
National Category
Fluid Mechanics and Acoustics Geotechnical Engineering
Identifiers
urn:nbn:se:kth:diva-52727 (URN)KTH/LWR/LIC 2062-SE (ISRN)978-91-7501-213-1 (ISBN)
Presentation
2012-01-19, V3, Teknikringen 72, KTH, Stockholm, 10:30 (English)
Opponent
Supervisors
Note
QC 20120110Available from: 2012-01-10 Created: 2011-12-19 Last updated: 2012-01-10Bibliographically approved
2. 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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