Jet pump development for emergency draining of Kopswerk II. By Thomas Staubli and Raphael Walpen, HTA Lucerne, Switzerland

Transcription

1 Jet um develoment for emergency draining of Koswerk II By Thomas Staubli and Rahael Walen, HTA Lucerne, Switzerland Abstract The lowest oint of the underground hydroelectric ower station Koswerk II of the Vorarlberger Illwerke, Austria, lies 60m below the tailwater level. For emergency draining even in the case of comlete ower failure a water jet um fed by the headwater was designed. The challenge of this jet um develoment lies in the high feeding ressure of 83 bar. Jet ums with such high driving ressure tend to suffer from severe cavitation. Focus during the develoment rocess was therefore on the revention of cavitation. On one hand the jet um was designed following exerimental exerience from several sources. On the other hand numerical flow simulation was erformed. The wall contour of the mixing chamber was changed iteratively in order to avoid low ressure zones. Efficiency of the um could be otimized by adjusting the nozzle osition of the ejector. The global um data obtained by numerical flow comutation agreed well with the exerimental data ublished in literature. Introduction During the last decade the deregulation in the Euroean electricity market has resulted in raidly changing conditions on the market. Due to the growing demand for balancing ower and frequency control an investment in increased umed storage caacity is economically feasible. In 004 the Vorarlberger Illwerke, Austria, started construction for the new umed storage ower lant, Koswerk II, with an installed minimum caacity of 40 MW (3 x 10 MW). The first hydroelectric set is suosed to start oeration at the end of 007 and the last of the three units during the first half of the year 008 [1]. For Koswerk II a concet with three Pelton turbines and searate storage ums was selected as shown in Figure 1. The Pelton turbines enable otimal control ranging between zero load and full load without any major efficiency losses. Figure 1. Cross section of the underground ower house cavern [1] 1

2 The lowest oint of this underground hydroelectric ower house cavern lies 60m below the tailwater level. This underground station should be drainable even in the emergency case of water inleakage coincidental with electric ower outage - utilizing a water jet um fed by the headwater. Jet um In oeration, the jet um has two inflows. Firstly, the rimary stream of the high ressure water flows through a nozzle and the subsequent free jet with high velocities entrains the surrounding fluid. This entrainment causes the secondary inflow at the suction side of the um. Due to the high jet velocity very large shear stresses occur in the mixing zone with the surrounding water. The fluids are comletely mixed after covering a distance corresonding to eight diameters of the mixing ie. Due to the intense momentum exchange during mixing, ressure increases considerably. In the following diffuser the high kinetic energy of the mixed flow is reduced, further increasing ressure, as schematically shown in Figure. The delivery stream is the sum of the two inflow streams. Fluid being umed Q S, Diffuser Mixing ie Discharge Q m, Primary stream Q e, 1 Nozzle Figure. Schematic of the main elements of a jet um The HTA Lucerne was commissioned by the Vorarlberger Illwerke to design an otimized jet um for Koswerk II. Secial attention should be aid to the minimization of cavitation. The levels given by the lant are dislayed in Figure 3. Figure 3 Plant layout and levels

3 A first design of the jet-um was reared in accordance with design guidelines ublished in [], [3], [4], []. The guidelines founded on rather dated, but carefully erformed series of measurement. Boundary conditions are given by the heads in the reservoirs defining a ressure ratio: π = 1 where ressures are as defined in Figure. For desired um flow rate Q s an exerimentally determined mass flow ratio µ is to be satisfied: ρq µ = ρq S Pum efficiency is defined as: P η = P s ρq = ρq S 1 µ = π 1 The results of a numerical simulation of the jet um showed good agreement with the design guidelines of Schulz [4], as demonstrated in Figure 4, where exerimentally determined mass flow ratios and efficiencies (full lines) are comared with the simulation (square and triangle). These agreements indicated that the ublished design criteria are still valid and also that the numerical flow simulation can be trusted. On the basis of this validation the simulation was used as a tool for otimizing local shae forming Mass Flow Ratio [-] Efficiency [-] Pressure Ratio [-] Mass Flow Ratio Mass Flow Ratio Simulation Efficiency Efficiency Simulation Figure 4 Comarison of mass flow ratio and efficiency according to [4] with the result of a numerical simulation. 3

4 After the otimization rocess the following oerational data of the jet um resulted from the numerical flow simulations: Pum level: m Level um outlet: m Max. head water level: 1809 m Flow rate in the suction section: Q S = 0.6 m3/s Flow rate of the rimary jet: Q 0.19 m3/s Flow rate in the delivery section: Q d = 0.7 m3/s Max. ower of riming stream: P = 1.4 MW Power transmitted to the suction stream: P s = 0.3 MW Power transmitted to the delivery stream: P d = 0.46 MW Suction ressure number: π =13.64 Design The jet um was designed utilizing the Unigrahics NX3 CAD-software in accordance with ISO standards. After the creation of the 3D-design model, detailed D-drawings were extracted for every art of the um. Also an assembly drawing was created including information on the forces acting on the concrete. The jet um are being manufactured by the Vorarlberger Illwerke comletely in stainless steel. Figure shows a section through the inlet section. Pressure tas at various ositions will allow detailed verification of the ressure built u once the um has been commissioned. Figure Jet um assembly with enlarged view of the nozzle region 4

5 Numerical modeling The comutations were erformed with CFX.7, a commercial finite volume CFD code [6]. The turbulence model used for the first set of comutations was the SST model (shear stress transort model), develoed by Menter [7]. The model works by imlementing a turbulence/frequency-based model (k-ω) near the walls and the k-ε model in the free flow. The meshes used for the comutations where generated with ICEM-CFD Hexa, allowing for high quality meshes (min. angle = 49.7 deg.) and good resolution of the boundary layers y + is between 8 and 60 on the outer contour and between 0. and 100 inside the injector. Due to the rotational symmetry, only a slice of degrees needed to be simulated. The shear layer of the jet was resolved with an esecially fine mesh. A total of nodes were needed for mesh indeendent results. Figure 6 Mesh resolution within the shear layer of the jet in the nozzle Figure 7 Boundary conditions alied for the simulations

6 Otimization Stewise the outer contour of the injector was modified to reduce low ressure zones at the walls. Static ressure distributions dislayed in Figure 8 show unaccetable low ressure zones for the first design (left), while for the final design low ressure zones could be widely eliminated. Since modification of the contour had considerable influence on the jet um s efficiency, the osition of the nozzle within the injector was also otimized to regain maximum efficiency. Figure 8 Static ressure distributions in the injector (gage ressure) Cavitation in the shear layer In many technical flows shear stresses often lay a secondary role. In the case of the jet s shear layers, however, shear stresses dominate the local flow field. These shear stresses are resonsible for the momentum exchange driving the jet um. These high shear stresses cause a significant roduction of turbulent kinetic energy in the shear layer. Unfortunately this roduction leads to a local anisotroy of turbulence, which is not accounted for when with the SST-model is emloyed. Static ressures are defined by the code as: ' = Stat + ρ k 3 Since the turbulent kinetic energy is very high in the shear layer, no negative ressures are redicted with the SST-model in the shear layers, as observed in Figure 8. However, it is known from jet exeriments that cavitation is to be exected in the shear layer. To investigate local ressures within the shear layer in more detail, the SAS-model (Scale- Adative Simulation) of CFX.10 [8] was used for additional simulation. This model takes inhomogeneous shear into account. The SAS concet is based on the introduction of the von Karman length-scale into the turbulence scale equation. The information rovided by the von Karman length-scale allows SAS models to dynamically adjust to resolved structures in a URANS simulation (unsteady Reynolds Averaged Navier-Stokes), resulting in an LES-like (large eddy simulation) behavior in unsteady regions of the flow field. At the same time, the model rovides standard RANS caabilities in stable flow regions. 6

7 For the SAS-simulations a simlified 3-dimensional model of the exiting jet was generated. The comutational effort for these unsteady simulations was enormous. A Linux Cluster with 18x AMD Oteron 64bit CPU s and 0 Gbyte RAM was used for comutation. Figure 10 shows static ressure distributions for four frames selected out of a time series. Colored zones with low ressures may be exected to cavitate. The cavitating zones will not be in contact with the surfaces of the jet um and accordingly no cavitation damages are to be exected. However, cavitation will cause considerable noise roduction. Also lower frequency ressure fluctuations will occur due to jet instabilities, which are known to be inherent to such jets and which were also observed in the SAS-simulations. Figure 10 Time deendent static ressures in the shear layer redicted with the SAS model Conclusion In the course of develoing the jet um for Koswerk II, knowledge for the otimization of such ums could be gained and consolidated. Flow simulations with the SST- and the SASmodel rovided detailed insight into local flow effects. Cavitation on the injector walls could be avoided by modifying the injector geometry. Further otimization was attained by modifying the nozzle osition. The full 3-D CAD-model and extracted drawings allow the Vorarlberger Illwerke to manufacture the jet um using CNC-machining. Commissioning of the jet um is lanned at the end of 007. On-site measurements will then allow verification of all global um data as well as of the redicted local wall ressures. References [1] Puerer E., Goekler G., The ower lant Kos II Basic conditions, design and details of construction, Hydro00, Villach, Austria, 00 [] Schulz F., "Modellversuche für Wasserstrahl- Wasserumen", Habil. TU Wien, 19 [3] Raabe J., "Hydraulische Maschinen und Anlagen" VDI Verlag, 1989 [4] Schulz H., "Die Pumen", Sringer Verlag, 1977 [] Flügel G., "Berechnung von Strahlaaraten", VDI- Forschungsheft 39, 191 [6] ANSYS CFX.7 User Manual. [7] Menter, F. R., Two-equation eddy-viscosity turbulence models for engineering alications, AIAA Journal 3 (8), [8] Menter F.R., Egorov Y., A Scale-Adative Simulation Model using Two-Equation Models, ANSYS CFX Germany, 00 7

8 Authors T. Staubli graduated in Mechanical Engineering from the Swiss Federal Institute of Technology (ETH) in Zürich. After two years of ost-doctoral research in the field of flow induced vibration at Lehigh University, Pennsylvania, he worked in exerimental fluid mechanics at Sulzer Hydro (now VATECH HYDRO) in Zürich. He then headed the Turbomachinery Laboratory at the ETH Zürich. During this eriod he directed research rojects in the field of hydraulic machinery. Since 1996 he is Professor in Fluid Dynamics and Hydromachines at the HTA Lucerne. He has long time exerience with field testing of hydromachines. R. Walen earned his degree in Mechanical Engineering at the University of Alied Sciences, Lucerne School of Engineering + Architecture (HTA Lucerne) and is currently working as research assistant in the Fluid Dynamics grou. 8