Design of Experiment Pressure Measurements Inside the Tokke Runner. * Corresponding author

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1 Proceedings of the International Symposium on Current Research in Hydraulic Turbines CRHT VI March 14, 2016, Turbine Testing Lab, Kathmandu University, Dhulikhel, Nepal Paper no. CRHT Design of Experiment Pressure Measurements Inside the Tokke Runner Katarina Kloster 1*, Einar Agnalt 2 and Ole Gunnar Dahlhaug 3 1 Department of Energy and Process Engineering, NTNU, Kolbjørn Herjes vei 1B, Trondheim, Norway 2 Department of Energy and Process Engineering, NTNU, Kolbjørn Herjes vei 1B, Trondheim, Norway 3 Department of Energy and Process Engineering, NTNU, Kolbjørn Herjes vei 1B, Trondheim, Norway * Corresponding author (katarik@stud.ntnu.no) Abstract In this thesis, solutions on how to analyze the pressure measurements from a turbine runner will be investigated. Pressure pulsation measurements on the Tokke model runner at the Waterpower laboratory at the Norwegian University of Science and Technology will be conducted, using sensors installed in the draft tube cone, vaneless space and runner. Calculation and analysis of the natural frequency of the Tokke turbine runner will be completed, as well as a stress analysis in ANSYS. Keywords: Design of experiment, Onboard pressure measurements, Francis-99 workshops, Tokke model runner 1. Introduction Historically, hydropower turbines were designed with the intention of running at the best efficiency point. However, since the energy act was implemented in the 1990s, the operating regime of the Norwegian turbines have changed and many of the turbines are operated with constant load variation. The induced dynamic pressure fluctuations exerts large stresses on the turbine components and ultimately increase the risk of fatigue failure. The desire for higher efficiencies and lower cost also result in low solidity and compact units which contribute to the increased stress in the runners. The recent breakdowns in high head Francis turbines add to the concern related to the reliability of the turbine runners. Thus, gaining a deeper knowledge of the pressure development in the turbine has become an important topic in the hydropower industry. To gain a better understanding of the fluid structure interaction (FSI) causing these issues, model testing and numerical simulations are needed. A model of the Tokke runner has been designed by NTNU in order to facilitate further research on pressure pulsations inside high head francis turbines. The Tokke runner was originally manufactured by Kværner Brug AS in 1961, and is of splitter blade design with runner blades. This paper describes the experimental setup for pressure measurements inside the Tokke model runner in relation to the Francis-99 workshop. 2. Previous work Relevant experiments and studies with onboard measurements are mainly focused on verification of Computational Fluid Dynamics (CFD), pressure pulsations influence on blade loading, scaling of pressure

2 pulsation between model and prototype and fatigue analysis. However, few onboard pressure and strain measurements have been published, and available measurement reports from high head units are rare. Most of the available experiments are done at the Laboratory for Hydraulic Machinery at Ecole Polytechnique Féderal de Lausanne, EPFL-IMHEF, where onboard pressure measurements have been performed in cooperation with Voith Siemens [1] and GE Energy Hydro [2]. The experiments are conducted on Francis model runners to provide basic data for fine tuning and validation of numerical CFD calculations. The experiments are conducted using a special technique developed to measure the pressure on the model runner blades using miniature pressure transducers embedded in the model runner blades. The static and dynamic pressures are measured in two hydraulic channels by mounting sensors on the pressure side of two runner blades and on the suction side of another two blades. The experiments done in collaboration with Voith Hydro also included four transducers mounted in the runner band. Figure 1 and Figure 2 show the instrumented runner blades during instrumentation. The setup used is not evaluated in any of the provided reports. However, the technique is thought to be too elaborate to use for all model test campaigns and is only recommended for large turbines operating under large head ranges and difficult part load conditions. Figure 2: Pressure transducers mounted in the runner blade at EPFL [1] Figure 1: Instrumented runner during its mounting at EPFL [2]

3 Similar measurements were conducted at the Waterpower Laboratory at NTNU, with the desire to develop a scale up possibility of pressure pulsation levels from model to prototype [3] and verify numerical CFD simulations [4]. Two model runners of the Tokke Francis turbine were made available for on-board pressure measurements - one with a splitter blade design and the other with 17 runner blades. The runners were instrumented with miniature strain gauge based pressure sensors, milled down into the blade surface. The sensors were placed in the same relative position in the two model runners, along three streamlines. Figure 4 give a schematic view of the sensor placement in the splitter blade runner, and Figure 3 show the pressure transducers during installation in one of the model runners. Figure 4: Sensor placement along three streamlines on the model runner blades [3] Figure 3: Pressure transducers installed in the runner blades [3] The NTNU setup proved to function well. However, there is concern related to the durability of the onboard pressure transducers due to the sensors vulnerability and exposure to water leakage. Flush mounted miniature pizoresistive pressure sensors are also used to conduct dynamic wall pressure measurements on a reversible pump-turbine model at the EPFL-LMH testing facility [5]. The study is part of the Hydrodyna collaborative research project, which aims to improve the understanding of pressure fluctuations due to RSI in high head pump turbines. The data collected from the wall pressure measurements from both the stationary and rotating frames are used to validate numerical CFX simulations. The setup is rather extensive, and include pressure transducers in two consecutive impeller channels, with sensors mounted on both pressure and suction sides of the impeller blades. The hub and shroud surfaces are also monitored.

4 Most of the experiments found with onboard measurements are utilizing blade mounted sensors. Thus, the available space is the main restriction when selecting sensors for the experiment. An alternative method is to have the sensors mounted in the hub. This approach is mentioned in the Hydrodyna experiment [5], but the setup is not evaluated in the report. Hub mounting does not restrict the sensor size to the same extent as the blade mounting, which allow the use of more robust and accurate sensors. However, hub mounted sensors do not enable separate measurements on the pressure and suction side of the runner blades. Measurements in the vaneless space combined with draft tube monitoring is typically used to validate numerical CFD calculations [6]. By doing this one assumes that the numerical simulations rightfully predict the flow in the runner channels, based on validation points in the static domain. By conducting additional measurements in the runner channels, more validation point are made available. The experiments mentioned show the possibilities and advantages of getting measurement data from the rotating domain. It also illustrates the extensive modifications needed on the runner to conduct such measurements, which makes it difficult to apply to existing equipment. By investigating the correlation between the measurements upstream and downstream the runner with the measurements conducted in the rotating domain, valuable knowledge about the pressure phenomena might be found. If a clear correlation is identified, measurements conducted outside the rotating domain combined with CFD simulations may fully describe the flow field and pressure inside the runner. 3. Objectives The long term objective of the experiment is to provide additional knowledge of the pressure phenomena occurring in high head Francis turbines and facilitate more accurate calculation of runner lifetime. However, four targets have been defined to establish a framework for the design of the experiment. Obtain measurement data for CFD verification Investigate possible correlation between pressure measurements upstream and downstream the runner with the measurements done in the rotating domain. Identify the phase difference in pressure pulses caused by RSI. Measure the speed of sound in the piping system and through the runner. 4. Measurement setup As the experiments conducted by Kobro et al [3] on the Tokke model runner experienced trouble with the durability of the blade mounted sensors, it was decided to have the pressure transducers mounted in the hub for the following experiment. Thus, five pressure transducers have been mounted in the middle of two hydraulic channels in the model runner. To fully capture the propagation of the pressure pulses created by RSI, one pressure sensor was placed close to the inlet and another close to the outlet of the runner. One sensor was also placed just upstream the splitter blade outlet and another just downstream in order to capture the effect of the change of channel cross section on the pressure pulse. The same methodology was used by Kobro when determining the positions of the transducers mounted on the runner blades, which provide the ability to compare results with earlier measurements conducted with blade mounted sensors. A fifth sensor has been mounted in the next equivalent hydraulic channel in order to identify the phase difference in the pressure pulses. Figure 5 show the positions of the various hub mounted sensors.

5 Figure 5: Onboard pressure sensors Pressure transducers have also been placed in the stationary domain of the turbine. Two sensors have been placed in the vaneless space and four in the draft tube cone. See Figure 6. In addition one sensor has been flush mounted to the penstock, inlet of spiral casing and draft tube outlet to allow measurements of the speed of sound, and a possible correlation between the pressure measurements upstram and downstream the runner and the rotating domain. The placement of the sensors is illustrated in Figure 7. Figure 6: Pressure sensors in vaneless space, PT 20-21, and in the draft tube cone, PT Figure 7: Pressure sensors in pipe line, PT 01-03

6 Table 1 summarize the placement and purpose of the sensors used for the onboard measurements. Table 1: Sensors for onboard measurements Sensor Placement Purpose PT 01 Upstream after pressure tank Speed of sound measurement PT 02 Inlet spiral casing Correlation and speed of sound PT 03 Close to draft tube outlet Correlation and speed of sound PT 10 Close to inlet of runner Onboard analysis. Phase difference RSI PT 11 Close to inlet of runner Onboard analysis PT 12 In front of splitter Onboard analysis PT 13 After splitter Onboard analysis PT 14 Close to outlet Onboard analysis PT Vaneless space PT Draft tube cone RSI Swirl Sensors needed for calculation of running point have also been included. Positioning and purpose of the installed sensors are given in Table 2 and Figure 8. Table 2: Sensors for calculation of running points Sensor Placement Purpose FT 40 Upstream Flow measurement PT 40 Inlet spiral casing Head measurement PT 41 ΔP inlet outlet Head loss over turbine WT 40 Generator Torque WT 41 Shaft Friction bearing ST 40 Shaft RPM ZT 40 Guide vane Angle of guide vane

7 5. Pressure transducers The pressure transducers mounted in the runner hub (PT 10-14) are inorganically bonded piezoresistive sensors provided by Kulite. The transducers are small in size and have a natural frequency of 95 khz. The sensors will be flush mounted with the hub surface to facilitate direct measurement of the pressures. The pressure range of the sensors is bar and the The Full Scale Output Best Fit Straight Line (FSO BFSL) is ± 1%. The pressure transducers mounted in the inlet pipe (PT 01-02) and the draft tube outlet (PT 03) are of a similar kind. However the transducers are larger with a greater natural frequency of 400 khz. The sensors have a pressure range of bar and 0.1% FSO BFSL, and will also be flush mounted. 6. Sources of pressure pulsations Figure 8: Sensors for calculation of running points The frequencies expected to be found during the experiments on the Tokke runner are summarized in Table 3. Table 3: Expected frequencies in the Tokke runner Frequency [Hz] Runner frequency f n 5.55 Guide vane frequency f gv = f n z gv Blade passing frequency f bp = f n z rv Rheinegans frequency f R Sampling rate To avoid faulty results, the sampling rate theorem suggests the use of a sampling rate that is at least the double of the highest expected frequency. However the sampling rate is usually set higher for samples of discrete nature. A higher sampling rate provide better data resolution which makes the analysis more accurate. Kobro et al [3] suggested the use of a sampling rate 10 times the highest expected frequency. Thus, the sampling rate is set to 1665Hz for the various running points.

8 8. Measurements The running points for the experiment have been set equal to the running points provided by the Francis- 99 second workshop test case [7]. Due to the Francis 99 workshop, measurement data from these running point will be available for comparison. Table 4: Running points Parameter PL BEP HL Net head [m] Discharge [m 3 /s] n ED Q ED Measurements will be conducted for the three running points with both a closed and open loop, in addition to transient open loop between the various points. Measurements of the speed of sound with no flow in the system will also be done. Running order: 1. Speed of sound with no flow. Pressure pulse in high-pressure tank. 2. Closed loop. PL, BEP, HL. 3. Open loop. PL, BEP, HL. 4. Transient open loop between PL, BEP and HL. References [1] F. Avellan, S. Etter, J.H Gummer, U. Seidel, "Dynamic Pressure Measurements on a model turbine runner and their use in preventing runner fatigue," [2] B. Nennemann, T. Vu, M. Farhat, "CFD prediction of unsteady wicket gate runner interavtion in Francis turbines: A new standard hydraulic design procedure," [3] E. Kobro, "Measurements of pressure pulsations in Francis turbines.," NTNU, [4] Chirag Trivedi, Michel J. Cervantes, B. K. Ghandi, Ole G. Dahlhaug, "Experimental and numerical studies for a high head Francis turbine at several operating points," [5] V. Hasmatuchi, "Hydrodynamics of a pump-turbine operating at off-design conditions in generating mode," [6] S. Liu, J. Liu, Y. Sun, Z. Zuo and Y. Wu, "Distribution of pressure fluctuations in a prototype pump turbine at pump mode," [7] [Online]. Available: