Rotor-Stator Interaction in Low-Specific Speed Francis Turbines
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Several breakdowns in hydropower plants with low specific speed Francis runners have been reported. These breakdowns are understood to be related to pressure pulsations, vibration modulus and the combination of these. One of the main excitation forces in such runners is the pressure fluctuations originating from the rotor-stator interaction. The primary objective for the thesis is to improve the understanding of the rotor-stator interaction to lower the risks of failure in future designs. A setup for the investigation of the pressure pulsations related to the rotor-stator interaction is presented with the use of flush mounted pressure sensors, both onboard the runner and in the vaneless space. In addition, a position sensor is utilized to analyze the pressure data relative to the angular position of the runner. The results from the onboard pressure measurements find that the phase of the guide vane passing pressure seen by the onboard pressure sensors are independent of the guide vane opening. Hence, the potential flow interaction is found to be the dominant effect and no evidence from the viscous wake effect is found on the onboard pressure. A clear resonance peak in the pressure field excited by the second harmonic of the guide vane passing frequency is found in the onboard measurements. The measured pressure is divided into the hydraulic effects and the effects from the Fluid Structure Interaction, named the convective and acoustic pressure. The eigenfrequency and damping are estimated from the measurements. The convective pressure field seems to diminish almost linearly from inlet to outlet of the runner, while the acoustic pressure field has the highest amplitudes in the middle of the runner channel. At resonance, the acoustic pressure clearly dominates over the convective pressure. In the vaneless space the pressure is known to be a combination of two effects; the rotating runner pressure and the throttling of the guide vane channels. The measured pressure is fitted to a theoretical pressure model to separate the two effects for two different runners. The main findings show that the pressure fluctuations in the guide vane cascade are controlled by throttling for low blade loading and the rotating runner pressure for higher blade loading. The results are considered as a good reference for computational fluid dynamics validation and enables researchers to verify their codes and increase the accuracy in the calculation of new designs to reduce the risk of breakdowns in the future.
Has partsPaper 1: Agnalt, Einar; Solemslie, Bjørn Winther; Dahlhaug, Ole Gunnar. Onboard measurements of pressure pulsations in a high head Francis model runner. IOP Conference Series: Earth and Environmental Science (EES) 2019 ;Volum 240.(2) s. 1-10 - Content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence (CC BY 3.0) . Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. https://doi.org/10.1088/1755-1315/240/2/022040
Paper 2: Agnalt, Einar; Solemslie, Bjørn Winther; Dahlhaug, Ole Gunnar. The rotor-stator interaction onboard a low specific speed Francis turbine
Paper 3: Agnalt, Einar; Iliev, Igor; Solemslie, Bjørn Winther; Dahlhaug, Ole Gunnar. On the Rotor Stator Interaction Effects of Low Specific Speed Francis Turbines. International Journal of Rotating Machinery 2019 ;Volum 2019, Article ID 5375149, - This is an open access article distributed under the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. https://doi.org/10.1155/2019/5375149
Paper 4: Agnalt, Einar; Østby, Petter Thorvald Krogh; Solemslie, Bjørn Winther; Dahlhaug, Ole Gunnar. Experimental Study of a Low-Specific Speed Francis Model Runner during Resonance. Shock and Vibration 2018 ;Volum 2018, Article ID 5796875 This is an open access article distributed under the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. https://doi.org/10.1155/2018/5796875