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CFD modelling of two-phase flow of a spillway chute aerator of large width
KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Hydraulic Engineering.
KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Hydraulic Engineering.
2016 (English)In: Journal of Applied Water Engineering and Research, ISSN 2324-9676Article in journal (Refereed) In press
Abstract [en]

An aerator is frequently used to prevent cavitation damages in high-velocity spillways. To understand its characteristics, one often resorts to physical model tests. To complement physical model tests, CFD simulations are used to determine water-air flow behaviors. With Bergeforsen’s 35 m wide aerator, numerical modeling has been performed to evaluate its performance and improve its configuration. The parameters of interest include spillway discharge capacity, air entrainment rate, duct subpressure and air concentration in the aerated flow. The simulated discharge capacity agrees reasonably with experimental data. Due to the larger chute width, empirical formulas do not reasonably predict the air demand. To provide the air required by the aerator, its distribution in the cavity must be guaranteed. This is challenging for wide spillways. We thus looked into the air supply system and the air flux in the cavity to improve the aerator function. Larger vent openings in the middle of the chute are preferable for large-width aerators.

Place, publisher, year, edition, pages
Taylor & Francis Group, 2016.
Keyword [en]
spillway, chute aerator, air demand, air-water flow, CFD, subpressure
National Category
Civil Engineering
Identifiers
URN: urn:nbn:se:kth:diva-179746OAI: oai:DiVA.org:kth-179746DiVA: diva2:889214
Note

QC 20170224

Available from: 2015-12-22 Created: 2015-12-22 Last updated: 2017-02-24Bibliographically approved
In thesis
1. CFD MODELLING OF TWO-PHASE FLOWS AT SPILLWAY AERATORS
Open this publication in new window or tab >>CFD MODELLING OF TWO-PHASE FLOWS AT SPILLWAY AERATORS
2017 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

Due to the high-speed flow in a chute spillway, cavitation damages often occur. This undesired phenomenon threatens the safety of the structure. For the purpose of eliminating the damages, an aerator is often installed in the spillway. To understand its characteristics, physical model tests are a popular method. To complement the model tests, computation fluid dynamics (CFD) simulations are used to study aerator flows. To represent the two-phase flows, multiphase models should be employed. This thesis examines two of them, namely, the Volume-Of-Fluid model (VOF) and Two-Fluid model.

Based on the background of the Bergeforsen dam, the aerator flow is modelled by means of the VOF model. The simulated spillway discharge capacity is in accordance with the experimental data. Compared with the results, empirical formulas fail to evaluate the air supply capacity of aerator as it is wider than the conventional width. A hypothetical vent modification is proposed. For the original and proposed layouts, the study illustrates the difference in the air-flow conditions. The results show that a larger vent area is, for a large-width aerator, preferable in the middle of the chute.

To study the flip bucket-shaped aerators in the Gallejaur dam, physical model tests and prototype observations are conducted. The results lead to contradicting conclusions in terms of jet breakup and air entrainment. A CFD model is, as an option, employed to explain the reason of the discrepancy. The numerical results coincide with the prototype observations. The jet breakup and air entrainment are evaluated from air cavity profiles; the air-pressure drops are small in the cavity. The discrepancy is due to overestimation of the surface-tension effect in the physical model tests.

Based on the experimental data of an aerator rig at the Laboratory of Hydraulics, Hydrology and Glaciology (VAW), ETH Zurich, the Two-Fluid model is used to predict air concentration distributions in the aerated flow. The model includes relevant forces governing the motion of bubbles and considers the effects of air bubble size. The numerical results are conformable to the experiments in the air cavity zone. Downstream of the cavity, the air concentration near the chute bottom is higher, which is presumably caused by the fact that the interfacial forces in the Two-Fluid model are underestimated.

Place, publisher, year, edition, pages
Stockholm: KTH, 2017. 44 p.
Series
TRITA-HYD, 2017:02
Keyword
spillway; cavitation; aerator; air entrainment; air–water flow; VOF model; Two-Fluid model
National Category
Other Civil Engineering
Research subject
Civil and Architectural Engineering
Identifiers
urn:nbn:se:kth:diva-202392 (URN)978-91-7729-304-0 (ISBN)
Presentation
2017-03-20, B24, Brinellvägen 23, Stockholm, 10:00 (English)
Opponent
Supervisors
Note

QC 20170224

Available from: 2017-02-24 Created: 2017-02-23 Last updated: 2017-02-24Bibliographically approved

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