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Fluid Structure Interaction study in Francis Turbine
 

Fluid Structure Interaction study in Francis Turbine

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Numerical Simulation of Fluid Structure Interaction in Hydro Turbines by Srinivasa Rao, Project Head, QuEST and Katsumasa Shimmei, Deputy General Manager Hitachi Works HITACHI, Ltd., Power Systems.

Numerical Simulation of Fluid Structure Interaction in Hydro Turbines by Srinivasa Rao, Project Head, QuEST and Katsumasa Shimmei, Deputy General Manager Hitachi Works HITACHI, Ltd., Power Systems.

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    Fluid Structure Interaction study in Francis Turbine Fluid Structure Interaction study in Francis Turbine Presentation Transcript

    • Fluid Structure Interaction study in Francis Turbine
    • Introduction About QuEST Global QuEST Global Engineering is a diversified Engineering Services company, employing 2000 professionals across 12 global delivery centers The company helps customers in Aero centers. engine, Aerospace & Defense, Power Generation, Oil & Gas, and other verticals to cut product development and project costs, shorten lead times, extend capacity and maximize engineering resources availability by providing support across the complete product life cycle from design and modeling through analysis prototyping automation data documentation analysis, prototyping, automation, documentation, instrumentation and controls, embedded systems development, manufacturing support, vendor management, and in-house precision machining. We leverage our local presence and global reach to support engineering cost reduction initiatives for our customers. About Hitachi Hitachi, Ltd., (NYSE: HIT / TSE: 6501), headquartered in Tokyo, Japan, is a leading global diversified product & services company with approximately 400,000 employees worldwide. The Th company offers a wide range of systems, products and services i market sectors ff id f t d t d i in k t t including information systems, electronic devices, power and industrial systems, consumer products, materials, logistics and financial services. QuEST Global – Hitachi Ltd. Confidential
    • Typical Hydropower Plant Setup Francis Turbine QuEST Global – Hitachi Ltd. Confidential 3
    • Basic Concepts High head turbines are subjected to dynamic forces. Some significant sources are 1. Interaction between runner and guide 2. Vortex shedding at the trailing edge of vane 3. Cavitation at the trailing edge of the runner vanes g g Testing of hydraulic turbine are expensive http://en.wikipedia.org/wiki /File:Turbine_Francis_Worn.JPG Is there any way to reduce the testing cause…? h d h YES, through CFD… QuEST Global – Hitachi Ltd. Confidential 4
    • Basic Concepts Physical Problems Analytical Solutions Experimental CFD What is CFD…? It is the science (art) to replace the partial differential equations (which in this case describe the flow of fluids) with numbers and simple algebraic expressions, and to solve them through iteration in time and/or space to obtain a final numerical description of the total flow field under consideration. CFD works by dividing the region of interest into a large number of cells or control volumes (the mesh or grid). It solves the following equations for any problem Conservation of Mass Conservation of Momentum Conservation of Energy QuEST Global – Hitachi Ltd. Confidential 5
    • Basic Concepts Advantages 1. Accuracy 2. 2 Ease-of-Use E f U 3. Speed 4. Powerful Visualizations Disadvantages or Limitations 1. CFD solutions and accuracy rely upon physical models (e.g. turbulence, compressibility, chemistry, multiphase flow, etc.). 2. Solving equations on a computer invariably introduces numerical errors. • Round-off error • Truncation error CFD can be used in: 1. Conceptual studies of new designs 2. Detailed product development 3. Troubleshooting 4. Redesign CFD analysis complements testing and experimentation. It reduces the total effort required in the laboratory. QuEST Global – Hitachi Ltd. Confidential 6
    • Overview Hydro turbines are desired to work over a wide range of operating conditions due to load variations. Fatigue: During dynamic loading, structures fails below its yield strength. In some cases, when fluid interacts with a solid structure, exerting pressure that may cause deformation in the structure and, thus, alter the flow of the fluid itself. When the flow induces significant stresses in the solid; however, since the resulting deformation of the solid is small, the flow field is not greatly affected QuEST Global – Hitachi Ltd. Confidential 7
    • Background of the study 1. In the present study, Experimental testing of a Francis Turbine had shown that there is a crack developed at the root of the runner blade after some hours of operation, and this was the probably caused due to the runner blade interaction with the guide vanes vanes. When looked at this problem, the flow induces stress in the turbine and the deformation is very small and will not alter flow. 2. In an other study, Experimental testing of a Francis Turbine stay vane had shown that there was vortex shedding from the vane during operation, and this was the probable cause for the observed structural failure in the vane In this case, the vortex shedding frequency when match the resonance of the vane, the vane vibrates violently and will alter the flow. These two problems has to solved using computational methods with different approaches . Such problems can be solved using Fluid Structure Interaction (FSI) approach. FSI simulations can be broadly categorized as one-way or two-way coupled. One way FSI analysis (Interaction of runner blades with guide vanes) Two ways FSI analysis (Vortex shedding) QuEST Global – Hitachi Ltd. Confidential 8
    • Case study 1. In Vortex shedding was one of the causes proposed for the failure of the original Tacoma Narrows Bridge (Galloping Gertie) in 1940, but was rejected because the frequency of the vortex shedding did not match that of the bridge The bridge actually failed by aero elastic flutter bridge. flutter. 2. A thrill ride "Vertigo" at Cedar Point in Sandusky, Ohio Suffered the fate of vortex shedding during the winter of 2001, one of the three towers collapsed. The ride was closed for the winter at the time. 7th November 1940; 11:00 AM http://www.mae.cornell.edu/images/etc/fluids http://www.cigital.com/justiceleague/wp-content/uploads /structure/tacoma.gif /2007/07/bridge.gif QuEST Global – Hitachi Ltd. Confidential 9
    • Contents 1. Executive Summary a. a Objective b. Approach 2. 2 Model and Assumptions 3. Boundary Conditions 4. Results and Di 4 R lt d Discussions i 5. Conclusions QuEST Global – Hitachi Ltd. Confidential 10
    • Executive Summary Objective of the study One Way FSI Analysis To develop the numerical methodology of Fluid Structure Interaction to find the intensity of stress levels at root of the Francis Runner at operation condition Two Way FSI Analysis To develop the numerical methodology of Fluid Structure Interaction to evaluate the ability of an unsteady numerical simulation to accurately reproduce the vortex shedding frequency on the natural f t l frequency of the stay vane. f th t QuEST Global – Hitachi Ltd. Confidential 11
    • Executive Summary Approach One Way FSI Analysis 1. Perform the unsteady CFD simulation of Francis Turbine and save the result file 2. Map the results (Pressure) on the structure and perform the FEA simulation 3. 3 Check for the maximum stress in the runner Two Way FSI Analysis 1. 2D Flow field Analysis to Determine Required Mesh Density 2. 3D Modal Analysis of the Vane to Determine Natural Frequencies 3. 3D Flow field Analysis of the Vane to Determine Span wise Variation in Shedding Frequency 4. 2D Flow field Analysis of the Vane to Determine Shedding Frequency at Various Flow rates 5. 5 3D FSI Analysis at a velocity where the shedding frequency coincides the natural frequency of the stay vane QuEST Global – Hitachi Ltd. Confidential 12
    • Model and Assumptions One Way FSI A design geometry was used for creating a CFD and FEA model It should be noted that the design model does not include the fillets Cavitation effect are neglected do to higher computational time Draft tube RUNNER Fluid Domain Solid Domain D i Guide vane Solid region Geometry Mesh 13 QuEST Global – Hitachi Ltd. Confidential
    • Model and Assumptions Two Way FSI A design geometry was used for creating a CFD and FEA model It should be noted that the design model does not include the fillets Cavitation effect are neglected do to higher computational time Fluid Fl id Domain Fluid Mesh Solid Solid Domain Mesh QuEST Global – Hitachi Ltd. Confidential 14
    • Boundary Conditions Upper Cavity Working Fluid : Water Runner Inlet Guide Draft D f Lower Vanes Tube Cavity Outlet One Way FSI Turbine Material : Structural Steel QuEST Global – Hitachi Ltd. Confidential 15
    • Boundary Conditions Outlet Fluid Structure Interaction Blade Frictionless Support (Symmetry) Inlet Two Way FSI QuEST Global – Hitachi Ltd. Confidential 16
    • Results and Discussions Observations One Way FSI Analysis y y 1. Form this analysis, it was seen that the stress were maximum at the root of the runner leading edge. 2. Form the stress harmonic levels is seen at the root that the cause due the interaction of Guide Vane and Runner blade blade. 3. This analysis doesn't conclude the failure of the runner at root. Fatigue analysis has to be carried our to confirm. Variation of Nomalized Von Mises Stress 1.02 1 02 1.00 Hub 0.98 σ/σ Max 30 mm M 0.96 0.94 0.92 Probe at which the stress values were monitored 0.90 0.000 0.005 0.010 0.015 0.020 0.025 0.030 0.035 Time (s) QuEST Global – Hitachi Ltd. Confidential 17
    • Results and Discussions One Way FSI Normalized Pressure Fluctuation at Trailing Edge of Guide Vane 1.04 The Peak stresses in the runner were 1.02 1 02 predicted to occur at the LE root near the Hub region. 1.00 Ps/Ps (Max) These unsteady pressure effects are quite 0.98 considerable (~ 4.5% variation) during each ( ) g 0.96 vane passing. This results in huge variations 0.94 in stress levels. 0.92 Normalized Von-Mises Stress 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 1.2 Time (s) Ti () 1.0 Limitations Node to node connectivity of fluid and structural mesh will increases 0.8 ess Normilized Stre 0.6 the accuracy of the stress prediction. 0.4 0.2 0.0 Hub Leading Edge Hub Trailing Edge Shroud Leading Edge Shroud Trailing Edge QuEST Global – Hitachi Ltd. Confidential 18
    • Results and Discussions Observations Two Way FSI Analysis y y 1. Analysis was carried our with 3 Mesh configurations 13,000 cells, 16,000 cells and 3,50,000 cells. It was seen that 16,000 and 3,50,000 cell had a close match of results and for further analysis 16,000 cells mesh were used. First Natural Second Natural Cases Mesh Size Frequency Frequency 1 13,000 91.55 (Hz) 183.1 (Hz) 2 16,000 85.45 (Hz) 170.9 (Hz) 3 , , 3,50,000 85.45 (Hz) ( ) 164.8 (Hz) ( ) 2. It was seen from 3D Modal analysis, the major frequencies (Bending: 73 Hz & Torsional: 155 Hz) QuEST Global – Hitachi Ltd. Confidential 19
    • Results and Discussions Observations Two Way FSI Analysis y y 3. From CFD results, FFT calculations for different points were done and when plotted, it was seen that the frequency had very little effect over the span. This concludes this analysis can be done with 2D to determine the vortex shedding frequency. Frequency Distribution behind the Trailing Edge of Hydrofoil @ different Span 2.5 1 2 Point 1 Point 2 Point 3 3 2.0 4 5 Point 4 Point 5 Point 6 de 6 Amplitud 1.5 15 7 8 1.0 Point 7 Point 8 Point 9 9 0.5 Symmetry 0.0 0 200 400 600 800 1000 1200 1400 Points just behind the Frequency (Hz) hydrofoil at different spans QuEST Global – Hitachi Ltd. Confidential 20
    • Results and Discussions Observations Two Way FSI Analysis y y 4. For 2D CFD analysis with different inlet velocities, a. From the FFT analysis it was seen that at inlet velocity of 12m/s, the vortex shedding frequency is 73 Hz b. This concludes that for inlet velocity of 12m/s, the vortex shedding frequency matches the natural frequency of the model c. So, FSI analysis can be done with a inlet velocity of 12m/s Frequency plot @ R1P1 Maximum Velocity Frequency 2.5 (m/s) (Hz) 2 73 11 65 Am plitude 12 73 1.5 13 78 1 0.5 05 0 0 100 200 300 400 500 600 700 800 900 Frequency (Hz) QuEST Global – Hitachi Ltd. Confidential 21
    • Results and Discussions Observations Two Way FSI Analysis y y 5. The Stress were monitored at the root of the blade. The values were around 10 times less then the yield stress of the material. Displacement of the Hydrofoil at Trailing Edge in Mid Span 5 Von Mises Stress (MPa) 4 3 2 cement (mm) 1 0 Displac -1 -2 -3 -4 -5 0.00 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.10 Time (s) QuEST Global – Hitachi Ltd. Confidential 22
    • Conclusions One Way FSI From this analysis, it was seen that the stresses were developed at the root of the blade which developed as a crack of the turbine due to runner interaction with guide vane. Improvement in the results would have been achieved if the there would have been node to node connectivity and using same geometry (Fluid and Surface in common) Two Way FSI Form this analysis, it was seen that the vortex shedding frequency derived from flow induced vibrations match the resonance (bending mode) of the vane. For such lockup conditions, Karman vortices exhibit a strong instability and substantial increase in vibration level. The Stress at the root of the blade were around 10 times less then the yield stress of the material. So, cycle calculation would help in determining the fatigue characteristics and number of cycles the vane can sustain. Coupled analysis did not show the resonant interaction between fluid and solid that was expected. This is due the oscillations have not reached a constant phase and recommended to run from some more time QuEST Global – Hitachi Ltd. Confidential 23
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