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An experimental investigation on the effects of cylindrical rods in a draft tube at part load operation in down-scale 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
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
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. article id 012007
Series
IOP Conference Series: Earth and Environmental Science, ISSN 1755-1307, E-ISSN 1755-1315 ; 1079
Keywords [en]
Hydraulic turbine, stationary rod protrusion, rotating vortex rope, draft tube, pressure pulsation
National Category
Fluid Mechanics and Acoustics
Research subject
Fluid Mechanics
Identifiers
URN: urn:nbn:se:ltu:diva-94953DOI: 10.1088/1755-1315/1079/1/012007Scopus ID: 2-s2.0-85141797888OAI: oai:DiVA.org:ltu-94953DiVA, id: diva2:1721168
Conference
31st Symposium on Hydraulic Machinery and Systems (IAHR 2022), Trondheim, Norway, June 26 - July 1, 2022
Funder
EU, Horizon 2020, 814958Available from: 2022-12-21 Created: 2022-12-21 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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