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Study on the Accuracy of RANS Modelling of the Turbulent Flow Developed in a Kaplan Turbine Operated at BEP. Part 2 - Pressure Fluctuations
Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics. University Politehnica of Bucharest, Romania.ORCID iD: 0000-0002-1252-3680
University Politehnica of Bucharest, Romania.
Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.ORCID iD: 0000-0001-7599-0895
2019 (English)In: Journal of Applied Fluid Mechanics, ISSN 1735-3572, E-ISSN 1735-3645, Vol. 12, no 5, p. 1463-1473Article in journal (Refereed) Published
Abstract [en]

The aim of the paper is to investigate the limitations of unsteady Reynolds-averaged Navier-Stokes (RANS) simulations of the flow in an axial turbine. The study is focused on modelling the pressure pulsations monitored on the runner blades. The scanned blade geometry renders the meshing process more difficult. As the pressure monitor points are defined on the blade surface the simulation relies on the wall functions to capture the flow and the pressure oscillations. In addition to the classical turbulence models, a curvature correction model is evaluated aiming to better capture the rotating flow near curved, concave wall boundaries. Given the limitations of Reynolds-averaged Navier-Stokes models to predict pressure fluctuations, the Scale Adaptive Simulation-Shear Stress Transport (SAS-SST) turbulence model is employed as well. The considered test case is the Porjus U9, a Kaplan turbine model, for which pressure measurements are available in the rotating and stationary frames of reference. The simulations are validated against time-dependent experimental data. Despite the frequencies of the pressure fluctuations recorded on the runner blades being accurately captured, the amplitudes are considerably underestimated. All turbulence models estimate the correct mean wall pressure recovery coefficient in the upper part of the draft tube.

Place, publisher, year, edition, pages
Physics Society of Iran , 2019. Vol. 12, no 5, p. 1463-1473
Keywords [en]
Turbulence modelling, Pressure fluctuation, Pressure recovery, Curvature correction, Scale Adaptive Simulation
National Category
Fluid Mechanics and Acoustics
Research subject
Fluid Mechanics
Identifiers
URN: urn:nbn:se:ltu:diva-73851DOI: 10.29252/jafm.12.05.29705ISI: 000482650000010Scopus ID: 2-s2.0-85071429109OAI: oai:DiVA.org:ltu-73851DiVA, id: diva2:1313735
Note

Validerad;2019;Nivå 2;2019-09-03 (johcin)

Available from: 2019-05-06 Created: 2019-05-06 Last updated: 2022-10-31Bibliographically approved
In thesis
1. Numerical modelling of a Kaplan turbine at different operating conditions: model and prototype
Open this publication in new window or tab >>Numerical modelling of a Kaplan turbine at different operating conditions: model and prototype
2022 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The increasing variability of the energy market has created a less favourable context for hydropower plants. The operating conditions of the hydraulic turbines are impacted by the fast power regulation required to compensate for the implementation of renewable energy resources such as wind, wave, or solarpower. The turbines are frequently working in off-design conditions and therefore, their efficiency and life span are reduced. Under damaging operating regimes such as part load and speed-no-load operation, unsteady flow structures, asymmetric flow and high-pressure fluctuations develop.

Low-head power plants are usually equipped with Kaplan turbines, i.e., double-regulated axial hydraulic machines. The guide vanes and the runner blades can be adjusted separately allowing Kaplan turbines to operate at high efficiency over a wider range of flow rates compared to single-regulated turbines. However, recurrent transient and off-cam operation is accounted for the premature wear, fatigue, and failure of Kaplan turbines.

Experimental and numerical studies are carried out to understand, prevent and mitigate the negative effects of transient operation on hydraulic turbines. Numerical simulations serve as a practical and cost-efficient supplement to model testing and can provide detailed flow information that is difficult to obtain otherwise. The experimental and numerical investigations carried out on small-scale turbine models are convenient and accessible but limited. Studies concerning full-scale large turbines are, on the other hand, challenging considering the production losses, large scales, high Reynolds numbers, and significant computational demands.

This thesis presents a numerical analysis of the flow developed inside a Kaplan turbine model and prototype, working as a propeller turbine, under different operation conditions. The objective was to explore the means of creating numerical models that could be used in the industry to test, diagnose and optimize the exploitation of axial turbines. The test case was the Porjus U9 Kaplan model and prototype. All the numerical simulations were validated against experimental data. Different operating regimes of the turbine model and prototype were modelled.

The model turbine was investigated numerically at the best efficiency point and during the transient operation from the best efficiency point to part load. The influence of different turbulence models and inlet boundary conditions on the accuracy of the numerical simulations was assessed. Additionally, a time step sensitivity analysis showed that the main parameters of the turbine model were reasonably predicted with large time step values, 61° and 121° of the runner rotation, considerably reducing the simulation time and computational costs. The formation of the rotating vortex rope was captured during the guide vane closure. The frequency of the pressure fluctuations monitored on the runner blade was accurately predicted compared to the experimental values.

The operation of the Porjus U9 prototype at the best efficiency point, part load and speed-no-load was investigated numerically. The sensitivity of the numerical models to the runner blade clearance size, the epoxy layer added to the runner blade in the experimental campaign to fix the pressure sensors in their position and the runner blade angle was explored. Similar to the model simulations, the rotating vortex rope was visible at part load in the prototype simulations. The frequency of the pressure pulsations was accurately predicted while the amplitude was poorly estimated regardless of the operating point and scale of the turbine.

Place, publisher, year, edition, pages
Luleå: Luleå University of Technology, 2022
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-93444 (URN)978-91-8048-170-0 (ISBN)978-91-8048-171-7 (ISBN)
Public defence
2022-12-01, E632, Luleå tekniska universitet, Luleå, 09:00 (English)
Opponent
Supervisors
Available from: 2022-10-05 Created: 2022-10-04 Last updated: 2022-11-10Bibliographically approved

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Iovanel, Raluca GabrielaCervantes, Michel

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