Investigation of Rotating Vortex Rope formation during load variation in a Francis turbine draft tube

Document identifier: oai:DiVA.org:ltu-76950
Access full text here:10.1016/j.renene.2019.11.014
Keyword: Engineering and Technology, Mechanical Engineering, Fluid Mechanics and Acoustics, Teknik och teknologier, Maskinteknik, Strömningsmekanik och akustik, Hydraulic turbine, Rotating Vortex Rope, Load variation, Swirling flow, Flow instability, Strömningslära, Fluid Mechanics
Publication year: 2020
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Abstract:

Rotating Vortex Rope (RVR) has been a matter of focus for years due to the major effects on hydraulic turbine’s efficiency. The exact procedure of RVR formation is still vague. The present research focuses on the dynamics of the RVR formation during the load variation employing transient numerical simulations. Two different geometries including the full geometry and the reduced one, which consists of one stay vane, two guide vanes, one runner blade, one splitter blade and full draft tube, are considered. In order to capture the transient swirling flow features inside the draft tube, the Shear Stress Transport-Scale Adaptive Simulation (SST-SAS) model is utilized to approximate the turbulent stresses. The pressure results inside the draft tube agree well with the experimental measurements. Moreover, the velocity results show the central low-axial-velocity and high-tangential-velocity region in the draft tube properly. The flow structure is visualized using λ2 criterion. The dynamic of RVR and the physics behind the RVR formation are investigated during the load variation. The results indicate four flow regimes with different characteristics during RVR formation. The first flow regime is a stable swirling structure occurring at Best Efficiency Point (BEP). The second flow regime occurs at the beginning of the load variation where signs of flow instabilities appear. These instabilities are temporary and washed down by the upstream flow. Expanding the instabilities and creating the vortical structures in the draft tube are the important flow features in the third flow regime. The fourth flow regime is the presence of a developed rotating rope occurring at the Part Load (PL) condition. The flow regimes differ according to the size and shape of the stalled region during load rejection inside the draft tube cone. They also reveal that despite some shortcomings, the reduced model is reliable to simulate the RVR transient formation. The full geometry simulations could be also applicable for practical problems provided that the modified time step is slightly greater than the main blade rotational angle is used.

Authors

Nahale Sotoudeh

Faculty of Mechanical Engineering, Tarbiat Modares University, Tehran, Iran
Other publications >>

Reza Maddahian

Faculty of Mechanical Engineering, Tarbiat Modares University, Tehran, Iran
Other publications >>

Michel Cervantes

Luleå tekniska universitet; Strömningslära och experimentell mekanik
Other publications >>

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    header: 
        identifier: oai:DiVA.org:ltu-76950
        datestamp: 2021-04-19T12:50:16Z
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                version: 3.7
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                recordContentSource: ltu
                recordCreationDate: 2019-11-29
            identifier: 
                http://urn.kb.se/resolve?urn=urn:nbn:se:ltu:diva-76950
                10.1016/j.renene.2019.11.014
                2-s2.0-85075328751
            titleInfo: 
                @attributes: 
                    lang: eng
                title: Investigation of Rotating Vortex Rope formation during load variation in a Francis turbine draft tube
            abstract: 
Rotating Vortex Rope (RVR) has been a matter of focus for years due to the major effects on hydraulic turbine’s efficiency. The exact procedure of RVR formation is still vague. The present research focuses on the dynamics of the RVR formation during the load variation employing transient numerical simulations. Two different geometries including the full geometry and the reduced one which consists of one stay vane two guide vanes one runner blade one splitter blade and full draft tube are considered. In order to capture the transient swirling flow features inside the draft tube the Shear Stress Transport-Scale Adaptive Simulation (SST-SAS) model is utilized to approximate the turbulent stresses. The pressure results inside the draft tube agree well with the experimental measurements. Moreover the velocity results show the central low-axial-velocity and high-tangential-velocity region in the draft tube properly. The flow structure is visualized using λ2 criterion. The dynamic of RVR and the physics behind the RVR formation are investigated during the load variation. The results indicate four flow regimes with different characteristics during RVR formation. The first flow regime is a stable swirling structure occurring at Best Efficiency Point (BEP). The second flow regime occurs at the beginning of the load variation where signs of flow instabilities appear. These instabilities are temporary and washed down by the upstream flow. Expanding the instabilities and creating the vortical structures in the draft tube are the important flow features in the third flow regime. The fourth flow regime is the presence of a developed rotating rope occurring at the Part Load (PL) condition. The flow regimes differ according to the size and shape of the stalled region during load rejection inside the draft tube cone. They also reveal that despite some shortcomings the reduced model is reliable to simulate the RVR transient formation. The full geometry simulations could be also applicable for practical problems provided that the modified time step is slightly greater than the main blade rotational angle is used.

            subject: 
                    @attributes: 
                        lang: eng
                        authority: uka.se
                    topic: 
                        Engineering and Technology
                        Mechanical Engineering
                        Fluid Mechanics and Acoustics
                    @attributes: 
                        lang: swe
                        authority: uka.se
                    topic: 
                        Teknik och teknologier
                        Maskinteknik
                        Strömningsmekanik och akustik
                    @attributes: 
                        lang: eng
                    topic: Hydraulic turbine
                    @attributes: 
                        lang: eng
                    topic: Rotating Vortex Rope
                    @attributes: 
                        lang: eng
                    topic: Load variation
                    @attributes: 
                        lang: eng
                    topic: Swirling flow
                    @attributes: 
                        lang: eng
                    topic: Flow instability
                    @attributes: 
                        lang: swe
                        authority: ltu
                    topic: Strömningslära
                    genre: Research subject
                    @attributes: 
                        lang: eng
                        authority: ltu
                    topic: Fluid Mechanics
                    genre: Research subject
            language: 
                languageTerm: eng
            genre: 
                publication/journal-article
                ref
            note: 
                Published
                3
                
Validerad;2020;Nivå 2;2020-03-31 (johcin)

            name: 
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                        type: personal
                    namePart: 
                        Sotoudeh
                        Nahale
                    role: 
                        roleTerm: aut
                    affiliation: Faculty of Mechanical Engineering Tarbiat Modares University Tehran Iran
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                        type: personal
                    namePart: 
                        Maddahian
                        Reza
                    role: 
                        roleTerm: aut
                    affiliation: Faculty of Mechanical Engineering Tarbiat Modares University Tehran Iran
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                        authority: ltu
                    namePart: 
                        Cervantes
                        Michel
                    role: 
                        roleTerm: aut
                    affiliation: 
                        Luleå tekniska universitet
                        Strömningslära och experimentell mekanik
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                        cervante
                        0000-0001-7599-0895
            originInfo: 
                dateIssued: 2020
                publisher: Elsevier
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                    type: host
                titleInfo: 
                    title: Renewable energy
                identifier: 
                    0960-1481
                    1879-0682
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                    detail: 
                        @attributes: 
                            type: volume
                        number: 151
                    extent: 
                        start: 238
                        end: 254
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