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Air-pocket transport in conjunction with bottom-outlet conduits for dams
KTH, School of Architecture and the Built Environment (ABE), Land and Water Resources Engineering, Hydraulic Engineering. (River Engineering)
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 [en]
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: urn:nbn:se:kth:diva-52727ISRN: KTH/LWR/LIC 2062-SEISBN: 978-91-7501-213-1 (print)OAI: oai:DiVA.org:kth-52727DiVA: diva2:467590
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
List of papers
1. CFD Modeling of Air Pocket Transport in Conjunction with Spillway Conduits
Open this publication in new window or tab >>CFD Modeling of Air Pocket Transport in Conjunction with Spillway Conduits
2011 (English)In: 11th International Conferenceon Fluid Control, Measutements and Visualization, Keeling, Taiwan, December 2-9 2011, 2011Conference paper, Published paper (Refereed)
Abstract [en]

This paper focuses on simulations of enclosed air pocket movements in conjunction with bottom outlet operations. The critical velocity of water for air pocket transport in pipe is the minimal flow velocity for the air pocket start to move downstream. A numerical model is developed to simulate the critical velocity of air pocket transport in pipe flow and to discuss the impacts of tunnel slope, size of the air pocket and wall roughness. The computations are performed in FLUENT using Volume of Fraction (VOF) model combined with k-epsilon model. Parallel computing is adopted for high computational performance.

The modeled critical velocity is compared with experimental results and they increase with increasing slopes. However, as the roughness height defined in the model is not big enough to represent the reality and no wall shear stress is applied in the upper wall where air pocket and wall contact, the modeled critical velocity is smaller than the experimental ones. Therefore, wall roughness contributes to keep the air pocket from moving downstream which is important in modeling critical velocity. However, by assuming a constant wall shear stress for the air phase the same as the water phase will overestimate the shear stress on the air pocket.

Two air pocket volumes are simulated at the slope 0.8 degrees which shows the bigger the air pocket is the higher the critical velocity is. Modeling results also show that the critical velocity is non-zero in horizontal pipe and there is a limit for the carrying capacity at all slopes. The simulations of air pockets with different volumes in the bottom tunnel of Letten dam in North of Sweden is shown in this paper as well.

National Category
Water Engineering
Identifiers
urn:nbn:se:kth:diva-52306 (URN)
Note
QC 20111215Available from: 2011-12-15 Created: 2011-12-15 Last updated: 2012-01-10Bibliographically approved
2. Experiments of Air-pocket Movement in an 18.2 degrees downward 240-mm Conduit
Open this publication in new window or tab >>Experiments of Air-pocket Movement in an 18.2 degrees downward 240-mm Conduit
2012 (English)In: 2012 International Conference On Modern Hydraulic Engineering, Elsevier, 2012, 791-795 p.Conference paper, Published paper (Refereed)
Abstract [en]

Experiments are carried out in a test rig, consisting of a Plexiglas pipe with an inner diameter of 240 mm and an inclination of 18.2o, to investigate air-water two-phase flows in conjunction with bottom spillways. Results show that the critical velocity, which is the minimal water velocity to start moving an air pocket, in the rough pipe, is independent of the air-pocket volume; in the smooth pipe it doesn't increase with increasing diameter as much as the previous researchers indicated. Pipe roughness doesn't affect the velocity of the air-pocket when it moves upstream in the downward inclined pipe.

Place, publisher, year, edition, pages
Elsevier, 2012
Series
Procedia Engineering, ISSN 1877-7058 ; 28
Keyword
two-phase flow, air pocket, critical velocity, roughness effect, diameter effect
National Category
Water Engineering
Identifiers
urn:nbn:se:kth:diva-52309 (URN)10.1016/j.proeng.2012.01.811 (DOI)000306529200137 ()2-s2.0-84863119424 (Scopus ID)
Conference
2012 International Conference on Modern Hydraulic Engineering, CMHE 2012; Nanjing, Jiangsu; 9 March 2012 through 11 March 2012
Note

QC 20111215

Available from: 2011-12-15 Created: 2011-12-15 Last updated: 2012-09-14Bibliographically approved
3. Experimental studies of air pocket movement in a pressurized spillway conduit
Open this publication in new window or tab >>Experimental studies of air pocket movement in a pressurized spillway conduit
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
Keyword
Air pocket, bottom outlet, critical velocity, large diameter pipe, roughness effect
National Category
Water Engineering
Identifiers
urn:nbn:se:kth:diva-52308 (URN)10.1080/00221686.2013.777371 (DOI)000320361500004 ()2-s2.0-84879825055 (Scopus ID)
Note

QC 20130806. Updated from submitted to published.

Available from: 2011-12-15 Created: 2011-12-15 Last updated: 2017-12-08Bibliographically approved

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