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Experimental Investigation and Mitigation of Part-load Pressure Pulsations in Hydro Turbines Using Solid-body Protrusion inside the Draft Tube
Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.ORCID iD: 0009-0004-2676-3839
2024 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The global demand for electricity generation has been increasing over the recent decades and is expected to grow steadily. Therefore, activating new energy sources to improve the present capacity of electricity production is inevitable. In addition, the reduction of greenhouse gas emissions is another growing concern that is increasingly promoted. Consequently, the integration of clean sources of energy into the electrical grid is a necessity. Renewable energy sources such as wind and solar appear as practical and easy-to-harvest solutions that are clean and sustainable. Thus, their penetration into the electrical grid is both encouraged and pursued on a global scale. However, these sources are intermittent and have a slow regulation response, and an electric network predominantly comprising such sources faces challenges in adapting to the market’s fluctuating demand. As a result, a rapid-response auxiliary source is required in such networks to balance the grid output and guarantee a stable supply of electricity. Hydropower is an ideal and clean alternative that can adopt this regulation role due to its short response time. The shift in hydropower implementation towards this new role requires a broader range of operations with more frequent transitions between the design and off-design conditions. However, current hydro turbines are designed to operate at a limited range of the highest efficiency, termed the best efficiency point (BEP). When operating away from the BEP, hydro turbines confront adverse flow-induced phenomena such as vortex breakdown that can induce pressure pulsations and periodic loadings. These oscillations can cause power swings and aggravated wear rates on turbine compartments through increased fatigue. Part-load (PL) turbine operation is a condition where the precession of a rotating vortex rope (RVR) in the turbine diffuser induces harmful oscillations. With prolonged turbine PL operations, these machines are expected to face a shortened life span and increased repair cycles. Therefore, the need for practical flow control methods to reduce the pressure pulsations under PL is growing.

The present thesis aims to introduce and investigate the concept of protrusion-based methods to mitigate PL pressure pulsations. The latter is attempted by perturbing the flow in the turbine with the radial insertion of solid bodies into the draft tube. The proposed geometries include cylindrical rods and flat plates. The impact of cylindrical rods has been examined on multiple scales of axial turbines, including a downscaled model turbine, a model turbine, and a prototype. These effects were observed with different measurement tools depending on the investigated turbine. The obtained resultspresented in this work for rod protrusion experiments include turbine operation parameters, timeresolved pressure data, strain data, flow visualization, and efficiency. As an improvement to the rod protrusion concept, the flat plates were tested on the downscaled model turbine. The results from these measurements consist of turbine operation parameters, time-resolved pressure data from the draft tube and vaneless space, and efficiency.

Investigated under different PL conditions, the rods could effectively mitigate the RVR-induced pressure pulsations at upper and lower PL conditions. The obtained mitigation rates under these conditions reached as high as 80%. In addition, this method proved effective in reducing flow induced fatigue at lower PL and even speed-no-load (SNL) conditions. However, rod protrusion entailed mixed results under the PL conditions where the RVR induced the strongest periodic oscillations. Moreover, the rods caused a maximum efficiency drop of approximately 3% of the BEP efficiency. More importantly, for all the investigated scales, under the conditions where the rods appeared effective, an optimum protrusion length was found where the most significant mitigation occurred. On the other hand, the investigation of plates on the downscaled model showed complete mitigation of the RVR-induced pressure pulsations under the entire turbine PL range, and the turbine efficiency was even improved under lower PL conditions.

The investigation of both protrusion-based methods verified the need for an adjustable mitigation technique that can adapt to variable flow conditions under different PL ranges. Protrusion-based flow control systems can be incorporated as a new degree of freedom in the existing turbines and modify their operational maps. Thus, depending on the turbine operating condition, the solid bodies can be protruded at a given length or retracted completely to reduce the flow-induced adverse effects with marginal efficiency penalties. Hence, the turbine can operate at an extended range with fewer consequences, which will be a key requirement for hydropower in the future.

Place, publisher, year, edition, pages
Luleå: Luleå University of Technology, 2024.
Series
Doctoral thesis / Luleå University of Technology 1 jan 1997 → …, ISSN 1402-1544
National Category
Fluid Mechanics and Acoustics
Research subject
Fluid Mechanics
Identifiers
URN: urn:nbn:se:ltu:diva-105417ISBN: 978-91-8048-572-2 (print)ISBN: 978-91-8048-573-9 (electronic)OAI: oai:DiVA.org:ltu-105417DiVA, id: diva2:1856890
Public defence
2024-06-18, E632, Luleå University of Technology, Luleå, 09:00 (English)
Opponent
Supervisors
Available from: 2024-05-08 Created: 2024-05-08 Last updated: 2024-05-28Bibliographically approved
List of papers
1. An experimental investigation on the effects of cylindrical rods in a draft tube at part load operation in down-scale turbine
Open this publication in new window or tab >>An experimental investigation on the effects of cylindrical rods in a draft tube at part load operation in down-scale turbine
2022 (English)In: 31st IAHR Symposium on Hydraulic Machinery and Systems 26/06/2022 - 01/07/2022 Trondheim, Norway, Institute of Physics Publishing (IOPP), 2022, article id 012007Conference paper, Published paper (Refereed)
Abstract [en]

The present work examines the effects of the radial protrusion of four cylindrical rods at different lengths within the flow field of a down-scaled turbine draft tube under part-load operating conditions. Four rods were placed on the same plane 90 degrees apart. The protrusion length was varied from zero to approximately 90 % of the draft tube radius. Time-resolved pressure measurements were performed to quantify the effect of the rod protrusion, using two pressure sensors at the same vertical level 180 degrees apart. Such sensor configuration enabled the decomposition of the signals into rotating and plunging components of the rotating vortex rope (RVR). The results show that different levels of mitigation are achieved for the rotating and plunging components depending on the protrusion length. The effects on the plunging component differ from the ones on the rotating component. The RVR plunging pressure pulsations slightly increase with the initial rod protrusion and then significantly drop after a certain length. On the contrary, the rotating component of the pressure pulsation amplitudes immediately decreases with the onset of rod protrusion. However, an optimum length is obtained in both cases where the highest mitigation occurs before reaching the maximum protrusion. This observation falls in line with the previous investigations conducted for oscillatory rod protrusions, further approving the point that a closed-loop controller should accompany the mitigation technique to achieve optimum mitigation.

Place, publisher, year, edition, pages
Institute of Physics Publishing (IOPP), 2022
Series
IOP Conference Series: Earth and Environmental Science, ISSN 1755-1307, E-ISSN 1755-1315 ; 1079
Keywords
Hydraulic turbine, stationary rod protrusion, rotating vortex rope, draft tube, pressure pulsation
National Category
Fluid Mechanics and Acoustics
Research subject
Fluid Mechanics
Identifiers
urn:nbn:se:ltu:diva-94953 (URN)10.1088/1755-1315/1079/1/012007 (DOI)2-s2.0-85141797888 (Scopus ID)
Conference
31st Symposium on Hydraulic Machinery and Systems (IAHR 2022), Trondheim, Norway, June 26 - July 1, 2022
Funder
EU, Horizon 2020, 814958
Available from: 2022-12-21 Created: 2022-12-21 Last updated: 2024-05-08Bibliographically approved
2. Characterization of The Rotating Vortex Rope Pressure Oscillations in a Kaplan Model Turbine Draft Tube
Open this publication in new window or tab >>Characterization of The Rotating Vortex Rope Pressure Oscillations in a Kaplan Model Turbine Draft Tube
2023 (English)In: International Journal of Fluid Machinery and Systems, ISSN 1882-9554, Vol. 16, no 2, p. 204-218Article in journal (Refereed) Published
Place, publisher, year, edition, pages
Korean Society for Fluid Machinery, 2023
National Category
Fluid Mechanics and Acoustics
Research subject
Fluid Mechanics
Identifiers
urn:nbn:se:ltu:diva-103336 (URN)10.5293/ijfms.2023.16.2.204 (DOI)
Note

Godkänd;2024;Nivå 0;2024-01-01 (hanlid);

Available from: 2023-12-18 Created: 2023-12-18 Last updated: 2024-05-08Bibliographically approved
3. Experimental Investigation of Part Load Vortex Rope Mitigation With Rod Protrusion in an Axial Turbine
Open this publication in new window or tab >>Experimental Investigation of Part Load Vortex Rope Mitigation With Rod Protrusion in an Axial Turbine
2024 (English)In: Journal of Fluids Engineering, ISSN 0098-2202, E-ISSN 1528-901X, Vol. 146, no 8, article id 081205Article in journal (Refereed) Published
Abstract [en]

The present paper investigates the rotating vortex rope (RVR) mitigation on an axial turbine model by the radial protrusion of four cylindrical rods into the draft tube. RVR mitigation is of particular interest due to the unfavorable pressure pulsations it induces in the hydraulic circuit that can affect turbine life and performance. The protrusion lengths, which were the same among the four rods, were varied according to a predefined sequence. The experiments were performed under four part-load regimes ranging from upper part load to deep part load. Time-resolved pressure measurements were conducted at two sections on the draft tube wall along with high-speed videography and efficiency measurement to investigate the effect of the mitigation technique on the RVR characteristics and turbine performance. The recorded pressure data were decomposed and studied through spectral analyses, phase-averaging, and statistical analyses of the RVR frequency and peak-to-peak pressure amplitude distributions. The results showed different levels of pressure amplitude mitigation ranging from approximately 10% to 85% depending on the operating condition, protrusion length, and the method of analysis. The hydraulic efficiency of the turbine decreased by a maximum of 3.5% that of the best efficiency point (BEP) with the implementation of the mitigation technique. The variations in the obtained mitigation levels and efficiencies depending on protrusion length and operating condition indicate the need for the implementation of a feedback-loop controller. Thus, the protrusion length can be actively optimized based on the desired mitigation target. 

Place, publisher, year, edition, pages
ASME Press, 2024
Keywords
rotating vortex rope, hydraulic turbines, swirling flow, part load, pressure measurement, turbine efficiency
National Category
Fluid Mechanics and Acoustics
Research subject
Fluid Mechanics
Identifiers
urn:nbn:se:ltu:diva-104969 (URN)10.1115/1.4064610 (DOI)
Funder
EU, Horizon 2020, (Grant No. 814958; Funder ID: 10.3030/814958)
Note

Validerad;2024;Nivå 2;2024-04-04 (joosat);

Full text: CC BY license

Available from: 2024-04-04 Created: 2024-04-04 Last updated: 2024-05-08Bibliographically approved
4. Extending the operating range of axial turbines with the protrusion of radially adjustable flat plates: An experimental investigation
Open this publication in new window or tab >>Extending the operating range of axial turbines with the protrusion of radially adjustable flat plates: An experimental investigation
2024 (English)In: Renewable energy, ISSN 0960-1481, E-ISSN 1879-0682, Vol. 225, article id 120232Article in journal (Refereed) Published
Abstract [en]

The implementation of hydropower to stabilize electrical grids dictates more frequent off-design operations of these renewable energy resources. Flow instabilities under such conditions reduce the efficiency of hydro turbines. Part-load operation is particularly detrimental since the development of a rotating vortical structure termed rotating vortex rope (RVR) in the draft tube leads to periodic pressure pulsations that jeopardize turbine performance. This paper experimentally explores a novel solution involving the protrusion of flat plates into the turbine draft tube. Three flat plates equally separated by 120° were vertically installed on the draft tube wall. The plates were protruded up to 83% of the draft tube local radius under four different part-load conditions. Their impact was observed through time-resolved pressure measurements in the draft tube and vaneless space, as well as efficiency measurements. The results demonstrated successful RVR mitigation, achieving a maximum 85% reduction in pressure oscillation amplitudes. Protruding flat plates disrupted RVR periodicity and coherence, confining its orbit to the draft tube center. This approach proved particularly effective at lower part-load conditions, enhancing turbine hydraulic efficiency by increasing torque extraction. Reducing the adverse effects under part load, the proposed method appears promising in extending the operational range of hydraulic turbines.

Place, publisher, year, edition, pages
Elsevier, 2024
Keywords
Hydropower, Part-load, Pressure measurement, Rotating vortex rope, Turbine efficiency, Vortex rope mitigation
National Category
Fluid Mechanics and Acoustics Energy Engineering
Research subject
Fluid Mechanics
Identifiers
urn:nbn:se:ltu:diva-104627 (URN)10.1016/j.renene.2024.120232 (DOI)2-s2.0-85187203326 (Scopus ID)
Funder
EU, Horizon 2020, 814958
Note

Validerad;2024;Nivå 2;2024-03-18 (hanlid);

Full text license: CC BY

Available from: 2024-03-18 Created: 2024-03-18 Last updated: 2024-05-08Bibliographically approved
5. Decreasing runner fatigue in an axial turbine with draft tube rod protrusions
Open this publication in new window or tab >>Decreasing runner fatigue in an axial turbine with draft tube rod protrusions
Show others...
(English)Manuscript (preprint) (Other academic)
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:ltu:diva-105413 (URN)
Available from: 2024-05-08 Created: 2024-05-08 Last updated: 2024-05-08
6. Experimental investigation of vortex ropemitigation in a 10 MW axial turbine
Open this publication in new window or tab >>Experimental investigation of vortex ropemitigation in a 10 MW axial turbine
(English)Manuscript (preprint) (Other academic)
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:ltu:diva-105416 (URN)
Available from: 2024-05-08 Created: 2024-05-08 Last updated: 2024-05-08

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