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  • 1.
    Teng, Penghua
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Hydraulic Engineering.
    CFD MODELLING OF TWO-PHASE FLOWS AT SPILLWAY AERATORS2017Licentiate 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.

  • 2.
    Teng, Penghua
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Hydraulic Engineering.
    Yang, James
    CFD modeling of two-phase flow of a spillway chute aerator of large width2016In: Journal of Applied Water Engineering and Research, ISSN 2324-9676, E-ISSN 2324-9676, Vol. 4, no 2, p. 163-177Article in journal (Refereed)
    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, computation fluid dynamics 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. 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.

  • 3.
    Teng, Penghua
    et al.
    KTH, School of Architecture and the Built Environment (ABE).
    Yang, James
    KTH, School of Architecture and the Built Environment (ABE), Sustainable development, Environmental science and Engineering, Resources, Energy and Infrastructure.
    CFD modeling of two-phase flow of a spillwaychute aerator of large width2016In: Journal of Applied Water Engineering and Research, ISSN 2324-9676, E-ISSN 2324-9676, Vol. 4, no 2, p. 163-177Article in journal (Refereed)
    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, computation fluid dynamics simulations areused to determine water–air flow behaviors.With Bergeforsen’s 35 m wide aerator, numerical modeling has been performedto 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 agreesreasonably with experimental data. Due to the larger chute width, empirical formulas do not reasonably predict the airdemand. To provide the air required by the aerator, its distribution in the cavity must be guaranteed. We thus looked into theair supply system and the air flux in the cavity to improve the aerator function. Larger vent openings in the middle of thechute are preferable for large-width aerators.

  • 4.
    Teng, Penghua
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Hydraulic Engineering.
    Yang, James
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Hydraulic Engineering.
    Flows over flip-bucket aerators, physical and CFD modeling with prototype testsManuscript (preprint) (Other academic)
    Abstract [en]

    The paper deals with a unique spillway structure, in which an aerator is incorporated in each flip bucket to aerate the flow jet and avoid sub-pressure in the air cavity below. In terms of jet breakup and stability, the physical models and the prototype lead to contradicting conclusions. CFD is performed to help seek the reason for the discrepancy. With sealed aerators, the model studies exhibit intact flow jets featuring negative cavity air pressure and oscillations, suggesting the need for jet aeration. Both the field observations and CFD indicate that the jets break up, allowing air to penetrate into the air cavities. The resulting cavity air pressure drops are small. The discrepancy is due to the effect of surface tension in the physical models leading to the formation of the enclosed air cavities with negative air pressure, which together with air entrainment in the tail-water gives rise to the jet oscillations. It is suggested, for similar flow phenomena, that compound modelling be performed so as to make amends for physical model test results. 

  • 5.
    Teng, Penghua
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Sustainable development, Environmental science and Engineering, Resources, Energy and Infrastructure.
    Yang, James
    KTH, School of Architecture and the Built Environment (ABE), Sustainable development, Environmental science and Engineering, Resources, Energy and Infrastructure.
    Modeling and Prototype Testing of Flows over Flip-Bucket Aerators2018In: Journal of Hydraulic Engineering, ISSN 0733-9429, E-ISSN 1943-7900, Vol. 144, no 12, article id 04018069Article in journal (Refereed)
    Abstract [en]

    The paper deals with a unique spillway which incorporates an aerator in each flip bucket with the intention to aerate the flow and avoid subatmospheric air cavities enclosed by the jets. In terms of jet breakup and stability, the physical models and the prototype lead to contradicting conclusions. With sealed aerators, the models exhibit intact air cavities featuring negative air pressure, suggesting the aeration need. Computational fluid dynamics (CFD) is performed to determine the reason for the discrepancy. Both the prototype observations and CFD indicate that the jets break up as a result of air entrainment; the resulting cavity air-pressure drops are insignificantly small. The discrepancy is due to the small model scale, in which the threshold flow velocity for air entrainment is not met and the prerequisite for jet breakup does not exist. To correctly reproduce similar water-air flow phenomena, the model should be large enough to meet the air-entrainment criterion. When questioning a small-scale model with air-cavity formation, CFD simulations should be performed to check the model results and make corrections, if needed.

  • 6.
    Teng, Penghua
    et al.
    KTH, School of Architecture and the Built Environment (ABE).
    Yang, James
    KTH, School of Architecture and the Built Environment (ABE), Sustainable development, Environmental science and Engineering, Resources, Energy and Infrastructure.
    Modeling and Prototype Testing of Flowsover Flip-Bucket Aerators2018In: Journal of Hydraulic Engineering, ISSN 0733-9429, E-ISSN 1943-7900, Vol. 144, no 12Article in journal (Refereed)
    Abstract [en]

    The paper deals with a unique spillway which incorporates an aerator in each flip bucket with the intention to aerate the flow andavoid subatmospheric air cavities enclosed by the jets. In terms of jet breakup and stability, the physical models and the prototype lead tocontradicting conclusions. With sealed aerators, the models exhibit intact air cavities featuring negative air pressure, suggesting the aerationneed. Computational fluid dynamics (CFD) is performed to determine the reason for the discrepancy. Both the prototype observations andCFD indicate that the jets break up as a result of air entrainment; the resulting cavity air-pressure drops are insignificantly small. The discrepancyis due to the small model scale, in which the threshold flow velocity for air entrainment is not met and the prerequisite for jet breakupdoes not exist. To correctly reproduce similar water–air flow phenomena, the model should be large enough to meet the air-entrainmentcriterion. When questioning a small-scale model with air-cavity formation, CFD simulations should be performed to check the model resultsand make corrections, if needed.

  • 7.
    Teng, Penghua
    et al.
    KTH, School of Architecture and the Built Environment (ABE).
    Yang, James
    KTH, School of Architecture and the Built Environment (ABE), Sustainable development, Environmental science and Engineering, Resources, Energy and Infrastructure.
    Pfister, Michael
    Ecole Polytechnique Federale de Lausanne (EPFL).
    Studies of Two-Phase Flow at a Chute Aeratorwith Experiments and CFD Modelling2016In: Modelling and Simulation in Engineering, ISSN 1687-5591, E-ISSN 1687-5605, article id 4729128Article in journal (Refereed)
    Abstract [en]

    The chute aerator of a spillway is a structure in such a sense that air is, in the intense emulsification, entrained into the highvelocitywater flow. Correctly predicting the air entrainment and two-phase flow pattern at the aerator would contribute to reliablespillway operation. Based on experimental data, 2D numerical simulations are preformed to predict streamwise air concentrationsin the aerated flow, in which a two-fluid model is used. Depending on the air bubble size, relatively good agreement is seen withthe experiments in the air cavity zone. The simulations give rise to higher air concentration downstream of the cavity, which ispresumably due to underestimation of the interfacial forces in the two-fluid model.

  • 8.
    Yang, James
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Sustainable development, Environmental science and Engineering, Resources, Energy and Infrastructure. Vattenfall AB Res & Dev, Alvkarleby Lab, SE-81426 Alvkarleby, Sweden.
    Andreasson, Patrik
    Vattenfall AB Res & Dev, Alvkarleby Lab, SE-81426 Alvkarleby, Sweden.;Lulea Univ Technol, Div Fluid & Expt Mech, SE-97187 Lulea, Sweden..
    Högström, Carl-Maikel
    Vattenfall AB Res & Dev, Alvkarleby Lab, SE-81426 Alvkarleby, Sweden..
    Teng, Penghua
    KTH, School of Architecture and the Built Environment (ABE), Sustainable development, Environmental science and Engineering, Resources, Energy and Infrastructure.
    The Tale of an Intake Vortex and Its Mitigation Countermeasure: A Case Study from Akkats Hydropower Station2018In: Water, ISSN 2073-4441, E-ISSN 2073-4441, Vol. 10, no 7, article id 881Article in journal (Refereed)
    Abstract [en]

    The upgrade of Akkats power station in Sweden included a new, separate waterway for the addition of a 75 MW generating unit. The vertical intake of its headrace was formed by means of lake tapping. A physical model was used to help understand the blasting process involving fragmented rock, water, air, and gas. Upon commissioning of the unit, swirling flows occurred unexpectedly at the intake, which gave rise to negative consequences including limitations in power output. Echo-sounding showed that the blasted piercing resulted in an irregular intake. A hydraulic model, as part of the design process, was built to examine potential countermeasures for vortex suppression. The final solution was a segmented barrier between the intake and the dam. It effectively suppressed the intake flow circulations; only minor intermittent vortices were left. The fabricated steel segments were anchored into the bedrock, stretching to 1.0 m below the lowest legal reservoir level. The local intake headloss was also reduced. The implemented solution was tested under full turbine loading and the result was satisfactory. Even during winter seasons with ice cover above the wall, the power station ran normally. The case study is expected to provide guidance for solving similar problems with vortex formation.

  • 9.
    Yang, James
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Sustainable development, Environmental science and Engineering, Resources, Energy and Infrastructure.
    Andreasson, Patrik
    Teng, Penghua
    KTH, School of Architecture and the Built Environment (ABE), Sustainable development, Environmental science and Engineering, Resources, Energy and Infrastructure.
    Xie, Qiancheng
    The Past and Present of Discharge Capacity Modeling for Spillways-A Swedish Perspective2019In: FLUIDS, ISSN 2311-5521, Vol. 4, no 1, article id 10Article, review/survey (Refereed)
    Abstract [en]

    Most of the hydropower dams in Sweden were built before 1980. The present dam-safety guidelines have resulted in higher design floods than their spillway discharge capacity and the need for structural upgrades. This has led to renewed laboratory model tests. For some dams, even computational fluid dynamics (CFD) simulations are performed. This provides the possibility to compare the spillway discharge data between the model tests performed a few decades apart. The paper presents the hydropower development, the needs for the ongoing dam rehabilitations and the history of physical hydraulic modeling in Sweden. More than 20 spillways, both surface and bottom types, are analyzed to evaluate their discharge modeling accuracy. The past and present model tests are compared with each other and with the CFD results if available. Discrepancies do exist in the discharges between the model tests made a few decades apart. The differences fall within the range -8.3%-+11.2%. The reasons for the discrepancies are sought from several aspects. The primary source of the errors is seemingly the model construction quality and flow measurement method. The machine milling technique and 3D printing reduce the source of construction errors and improve the model quality. Results of the CFD simulations differ, at the maximum, by 3.8% from the physical tests. They are conducted without knowledge of the physical model results in advance. Following the best practice guidelines, CFD should generate results of decent accuracy for discharge prediction.

  • 10.
    Yang, James
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Sustainable development, Environmental science and Engineering, Resources, Energy and Infrastructure.
    Lin, C.
    Kao, M. -J
    Teng, Penghua
    KTH, School of Architecture and the Built Environment (ABE), Sustainable development, Environmental science and Engineering, Resources, Energy and Infrastructure.
    Raikar, R. V.
    Application of SIM, HSPIV, BTM, and BIV techniques for evaluations of a two-phase air-water chute aerator flow2018In: Water, ISSN 2073-4441, E-ISSN 2073-4441, Vol. 10, no 11, article id 1590Article in journal (Refereed)
    Abstract [en]

    Four image-based techniques-i.e., shadowgraphic image method (SIM), high-speed particle image velocimetry (HSPIV), bubble tracking method (BTM), and bubble image velocimetry (BIV)-are employed to investigate an aerator flow on a chute with a 17° inclination angle. The study focuses on their applications to the following issues: (1) to explore the characteristic positions of three water-air interfaces; (2) to interpret the evolution process of air bubbles shed from the wedged tip of the air cavity; (3) to identify the probabilistic means for characteristic positions near the fluctuating free surface; (4) to explore the probability distribution of intermittent appearance of air bubbles in the flow; (5) to obtain the mean streamwise and transverse velocity distributions of the water stream; (6) to acquire velocity fields, both instantaneous and mean, of air bubbles; (7) to construct a two-phase mean velocity field of both water flow and air-bubbles; and (8) to correlate the relationship among the probability distribution of air bubbles, the mean streamwise and transverse velocity profiles of air bubbles, and water stream. The combination of these techniques contributes to a better understanding of two-phase flow characteristics of the chute aerator.

  • 11.
    Yang, James
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Hydraulic Engineering. R&D Alvkarleby Lab, S-81426 Alvkarleby, Sweden.
    Liu, Ting
    Dai, Wenhong
    Teng, Penghua
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Hydraulic Engineering.
    Transient Air-Water Flow and Air Demand following an Opening Outlet Gate2018In: Modelling and Simulation in Engineering, ISSN 1687-5591, E-ISSN 1687-5605, article id 3194935Article in journal (Refereed)
    Abstract [en]

    In Sweden, the dam-safety guidelines call for an overhaul of many existing bottom outlets. During the opening of an outlet gate, understanding the transient air-water flowis essential for its safe operation, especially under submerged tailwater conditions. Three-dimensional CFD simulations are undertaken to examine air-water flow behaviors at both free and submerged outflows. The gate, hoisted by wire ropes and powered by AC, opens at a constant speed. A mesh is adapted to follow the gate movement. At the free outflow, the CFD simulations and model tests agree well in terms of outlet discharge capacity. Larger air vents lead to more air supply; the increment becomes, however, limited if the vent area is larger than 10 m(2). At the submerged outflow, a hydraulic jump builds up in the conduit when the gate reaches approximately 45% of its full opening. The discharge is affected by the tailwater and slightly by the flow with the hydraulic jump. The flow features strong turbulent mixing of air and water, with build-up and break-up of air pockets and collisions of defragmented water bodies. The air demand rate is several times as much as required by steady-state hydraulic jump with free surface.

  • 12.
    Yang, James
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Sustainable development, Environmental science and Engineering, Resources, Energy and Infrastructure.
    Teng, Penghua
    KTH, School of Architecture and the Built Environment (ABE), Sustainable development, Environmental science and Engineering, Resources, Energy and Infrastructure.
    Lin, Chang
    Air-vent layouts and water-air flow behaviors of a wide spillway aerator2019In: Theoretical and Applied Mechanics Letters, ISSN 2095-0349, Vol. 9, no 2, p. 130-143Article in journal (Refereed)
    Abstract [en]

    A spillway aerator should guarantee favorable flow conditions in the coupled water-air system even if the aerator is unconventionally wide. Eight air-vent configurations are devised and incorporated into a 35-m wide chute aerator for a generalized study. Computational fluid dynamics (CFD) simulations are performed to explore their effects on water-jet and air-cavity features. The Re-normalisation group (RNG) k - epsilon turbulence model and the two-fluid model are combined to predict the two-phase flow field. The results demonstrate appreciable influences of the vent layouts on the water-air flow. The air vents stir the air motion and re-distribute the cavity air pressure. Once the vent layout is modified, reciprocal adjustments exist between the jet behavior and air-pressure field in the cavity, thus leading to considerable differences in air-flow rate, jet-trajectory length, vent air-flow distribution across the chute, etc. The large width plays a discernable role in affecting the aerated flow. Telling differences exist between the near-wall region and the central part of the chute. To improve the duct pressure propagation, a gradual augment of the vent area should be assigned towards the chute center. Relative to single-slot vents across the flow, the layouts with segregated vents gain by comparison. A designer should see to it that a vented aerator operates satisfactorily for a given range of flow discharges.

  • 13.
    Yang, James
    et al.
    KTH, School of Architecture and the Built Environment (ABE).
    Teng, Penghua
    KTH, School of Architecture and the Built Environment (ABE).
    Xie, Qiancheng
    Luleå University of Technology (LTU).
    MODELLING OF AIR DEMAND OF A SPILLWAY AERATOR WITH TWO-PHASE FLOW MODELS2018In: E-proceedings of the 2nd International Symposium onHydraulic Modelling and Measuring Technology Congress, 2018Conference paper (Refereed)
    Abstract [en]

    Air demand is an issue of concern for a spillway aerator. To numerically map its air-water flow behavior has a bearing on its design. The recently completed spillway at Bergeforsen features a 35-m wide chute aerator with 13 air vents. With this in background, CFD modelling is performed with three commonly used two-phase flow models, i.e. the Volume-of-Fluid (VOF) Model, Two-Fluid Model and Mixture Model. The purpose is to evaluate these models in terms of water-air flow properties. The simulations have shown that the VOF Model generates the lowest air demand, while the Two-Fluid Model points to a 34% higher value, which is attributable to the differences in the two-phase flow formulations. The resulting air pressure in the air cavity including the air groove leads also to considerable discrepancy in the vent air-flow distribution across the chute and spatial air concentration. Evaluations of two-phase models are necessary, so that a reliable model is adopted for engineering design.

  • 14.
    Yang, James
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Sustainable development, Environmental science and Engineering, Resources, Energy and Infrastructure. Vattenfall AB, R&D Älvkarleby Lab, Älvkarleby, Sweden.
    Teng, Penghua
    KTH, School of Architecture and the Built Environment (ABE), Sustainable development, Environmental science and Engineering, Resources, Energy and Infrastructure.
    Zhang, Hongwei
    Inst Water Resources & Hydropower Res IWHR, Dept Hydraul, Beijing, Peoples R China..
    Experiments and CFD modeling of high-velocity two-phase flows in a large chute aerator facility2019In: Engineering Applications of Computational Fluid Mechanics, ISSN 1994-2060, E-ISSN 1997-003X, Vol. 13, no 1, p. 48-66Article, review/survey (Refereed)
    Abstract [en]

    Mathematical formulations of two-phase flows at an aerator remain a challenging issue for spillway design. Due to their complexities in terms of water-air interactions subjected to high flow velocities, experiments play an essential role in evaluations of numerical models. The paper focuses on the underlying influence of the air-water momentum exchange in the two-phase Two-Fluid Model. It is modified to better represent the drag force acting on a group of air bubbles and the wall lubrication force accounting for near-wall phase interactions. Based on data from a large aerator rig with an approach velocity of 14.3 m/s, the models are evaluated for calculations of entrained air characteristics of a flow mixture. The air bubble diameter used in the modeling ranges from 0.5 to 4 mm as suggested by the experiments. In terms of air cavity configurations and aerator air demand, smaller air bubbles lead to better agreement with the test results. As far as air concentrations are concerned, the modified model gains by comparison. In the air cavity zone, smaller bubble sizes also provide better matches with the experiments. However, the near-base air concentration remains overestimated downstream from the impact area. The fact that the program user must pre-define a single air bubble size in simulations presumably limits the correct reproduction of near-base air concentrations and of their decay.

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