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Experimental Investigation of Part Load Vortex Rope Mitigation With Rod Protrusion in an Axial Turbine
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
Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.ORCID iD: 0000-0001-7599-0895
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. Vol. 146, no 8, article id 081205
Keywords [en]
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: urn:nbn:se:ltu:diva-104969DOI: 10.1115/1.4064610OAI: oai:DiVA.org:ltu-104969DiVA, id: diva2:1848630
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
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, JoelRaisee, MehrdadCervantes, Michel J.

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