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Extending the operating range of axial turbines with the protrusion of radially adjustable flat plates: An experimental investigation
Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.ORCID iD: 0009-0004-2676-3839
Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.ORCID iD: 0000-0002-3349-601X
Hydraulic Machinery Research Institute, School of Mechanical Engineering, College of Engineering, University of Tehran, Tehran, Iran.
Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.ORCID iD: 0000-0001-7599-0895
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. Vol. 225, article id 120232
Keywords [en]
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: urn:nbn:se:ltu:diva-104627DOI: 10.1016/j.renene.2024.120232Scopus ID: 2-s2.0-85187203326OAI: oai:DiVA.org:ltu-104627DiVA, id: diva2:1845175
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
In thesis
1. Experimental Investigation and Mitigation of Part-load Pressure Pulsations in Hydro Turbines Using Solid-body Protrusion inside the Draft Tube
Open this publication in new window or tab >>Experimental Investigation and Mitigation of Part-load Pressure Pulsations in Hydro Turbines Using Solid-body Protrusion inside the Draft Tube
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:nbn:se:ltu:diva-105417 (URN)978-91-8048-572-2 (ISBN)978-91-8048-573-9 (ISBN)
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

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Shiraghaee, ShahabSundström, JoelCervantes, Michel J.

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