Experimental investigations of the unsteady flow in a Francis turbine draft tube cone

Transcription

1 IOP Conference Series: Earth and Environmental Science Experimental investigations of the unsteady flow in a Francis turbine draft tube cone To cite this article: A Baya et al 2 IOP Conf. Ser.: Earth Environ. Sci View the article online for updates and enhancements. Related content - Unsteady pressure measurements and numerical investigation of the jet control method in a conical diffuser with swirling flow A I Bosioc, C Tanasa, S Muntean et al. - Experimental investigations of the swirling flow in the conical diffuser using flowfeedback control technique with additional energy source C Tnas, A I Bosioc, R F Susan-Resiga et al. - Unsteady pressure measurements of decelerated swirling flow in a discharge cone at lower runner speeds A I Bosioc, S Muntean, C Tanasa et al. Recent citations - Extension of Operating Range in Pump- Turbines. Influence of Head and Load Carme Valero et al - The effect of runner cone design on pressure oscillation characteristics in a Francis hydraulic turbine Z D Qian et al This content was downloaded from IP address on 26/8/28 at :24

2 IOP Conf. Series: Earth and Environmental Science 2 (2) 27 doi:.88/755-35/2//27 Experimental investigations of the unsteady flow in a Francis turbine draft tube cone. Introduction A Baya, S Muntean 2, V C Câmpian 3, A Cuzmoş 3, M Diaconescu 4 and G Bălan 4 Department of Hydraulic Machinery, Politehnica University of Timişoara Bv. Mihai Viteazu, RO-3222, Timişoara, Romania Tel/Fax: (+4) Centre of Advanced Research in Engineering Sciences, Romanian Academy Timişoara Branch Bv. Mihai Viteazu 24, RO-3223, Timişoara, Romania 3 Research Center in Hydraulics, Automation and Heat Transfer, Eftimie Murgu University of Reşiţa P-ţa. Traian Vuia -4, RO-3285, Reşiţa, Romania 4 Râmnicu Vâlcea Subsidiary, S.C. Hidroelectrica S.A. Str. Decebal, RO-24255, Râmnicu Vâlcea, Romania abaya@mh.mec.upt.ro Abstract. Operating Francis turbines at partial discharge is often hindered by the development of the helical vortex (so-called vortex rope) downstream the runner, in the draft tube cone. The unsteady pressure field induced by precessing vortex rope leads to pressure fluctuations. The paper presents the experimental investigations of the unsteady pressure field generated by precessing vortex rope and its associated pressure fluctuations into a draft tube of the Francis turbine operating at partial discharge. In situ measurements are performed in order to evaluate the pressure fluctuations and vortex rope frequency at partial load operation. Three pressure taps are installed on the cone wall of the draft tube in order to record the unsteady pressure. As a result, the Fourier spectra are obtained in order to evaluate the amplitude of pressure fluctuations and vortex rope frequency. Moreover, the wall pressure recovery along to the draft tube cone is acquired. Finally, conclusions are drawn in order to present the vortex rope effects. The variable demand on the energy market, as well as the limited energy storage capabilities, requires a great flexibility in operating hydraulic turbines. As a result, turbines tend to be operated over an extended range of regimes quite far from the best efficiency point. In particular, Francis turbines operated at off-design operating conditions have a high level of residual swirl at the draft tube inlet as a result of the mismatch between the swirl generated by the guide vanes and the angular momentum extracted by the turbine runner []. Further downstream, the decelerated swirling flow in the draft tube cone often results in vortex breakdown, which is recognized now as the main cause of severe flow instabilities and pressure fluctuations experienced by hydraulic turbines operated at part load. These phenomena generate strong vibrations and noise [2] that may produce failures on the mechanical elements of the machine [3, 4]. Several methods have been proposed to mitigate the vortex and the vibrations [5]. The most effective and applied method is air admission into the draft tube [6]. However, Susan-Resiga et al. [7, 8] have introduced a novel method to decrease the vortex rope by using an axial jet issued from the crow tip. This new method is alternatively to the previous solutions in order to remove the vortex rope and its associated pressure fluctuation [9] as well as to improve the pressure recovery and the draft tube efficiency []. The paper presents our experimental investigations into a real Francis turbine with medium specific speed in order to evaluate the problems generated by vortex rope at part load operation []. 2. Test case: Francis turbine Brădişor HPP is located in south of Romania on the Lotru River. The underground powerhouse is equipped with two vertical units. Each unit consist a Francis turbine rated at power of 57.5 MW each under 28.5 m rated nominal head, see Table. The distributor consists of 6 stay vanes and 6 guide vanes whilst the runner has 4 c 2 Ltd

3 -,34, A +,6 +,4 5-3,6-3,76 5 -,6 Arbore hidrogenerator +5, ,95 +, ,62 Ø 25; z=48; M=3 Ø 2275 Ø 6 Ø 9;z=4 Ø 4 H7 Ø 9 f7 Ø 57 Ø 8; z=4 Ø 86; z=4 Ø 24; z=6 Ø 2 45 Ø 2275 Ø ,54 +,45 -,73 -, Ø 29; z=48; M42 +,87 Ø 3; z=36; M48 Ø 34 Ø 267; z=48; M ,5 -,85-3, ,7-6,45 A 25th IAHR Symposium on Hydraulic Machinery and Systems IOP Conf. Series: Earth and Environmental Science 2 (2) 27 doi:.88/755-35/2//27 blades with the reference radius R 2e =.375 m. Each unit is supplied by individual power conduits including intake and penstock. The units share a common tailrace tunnel (i.e. free surface flow) with 3 km length. Figure (left) shows the hydropower plant cross view while the Francis turbine cross section is presented in Figure (right) with parameters from Table 2. Table Parameters of the Francis turbine with medium specific speed Parameter maximum head H max nominal head H n minimum head H min nominal power P n nominal speed n Value m 28.5 m. m 57.5 MW 375 rpm Ø 35 Fig. Cross section through the hydropower plant (left) and cross section through the Francis turbine (right) Table 2 Parameters of the Francis turbine with medium specific speed Parameter Value equations according to IEC [2] characteristic speed n q n = nq H discharge coefficient φ.28 ( ) q 3 ϕ = Q πωr2e ψ = 2E ωr2e 3 5 λ = 2EQ πω R2e. 5. ν = ϕ ψ energy coefficient ψ.264 ( ) 2 hydraulic power coefficient λ.354 ( ) dimensionless characteristic speed ν The following problems were detected during to the part load operation conditions: i) the vibrations are generated and propagated to the hydraulic turbine elements; ii) the runner cone (ogive) is removed, see Fig. 2(right); iii) the runner band seal is worn; iv) the cracks are initiated at the junction between crown and runner blades on the trailing edge and leads to failure of the runner blades []. The air admission through the choke 2

4 IOP Conf. Series: Earth and Environmental Science 2 (2) 27 doi:.88/755-35/2//27 valve situated to the end of hollow shaft is performed in the hydropower plant if the pressure level goes down to the atmospheric pressure, Fig. 2(left). Fig. 2 The runner cone was removed due to the vortex rope generated at part load conditions. The runner without cone part (left) and the runner cone (ogive) recovered from draft tube (right) 3. Experimental investigations The experimental investigations are performed in 3 operating points (9 situated at part load conditions, one operating point near to the best efficiency point (BEP) and 3 operating points at full load conditions). Figure 3 presents the hill chart of the Francis turbine with regimes computed based on last ten years of operation (red points) and the operating points investigated experimentally at constant head H=8 m (black circles). The experimental investigations at part load operating conditions were realized with air admission through the hollow shaft of the turbine. Fig. 3 Hill chart of the Francis turbine with regimes computed based on last ten years of the operation (marke d with red points) and the operating points investigated experimentally at constant head H=8 m (marked with black circles) The draft tube cone is performed from three parts with total height three times reference radius (h=3r 2e ) at runner outlet and the semi angle of cone 4º. Three pressure taps were flush mounted on the wall of the draft tube cone in order to evaluate the pressure fluctuations and the wall pressure recovery for all operating range, see Figure 4(left). The pressure taps were mounted along to the element of the cone with the tap number 3 (denoted 3

5 IOP Conf. Series: Earth and Environmental Science 2 (2) 27 doi:.88/755-35/2//27 PT3) situated at.7 m (.65R 2e ) downstream from runner outlet and the pressure tap number (PT) at.45 m (.275R 2e ) with respect to the P3 near to the elbow of the draft tube, Figure 4(right). As a result, the unsteady pressure in the pressure taps was recorded for each operating regime. The mean value (P) and fluctuant component (p ) are obtained from unsteady pressure signal recorded in the pressure taps using the following equation p P + = p' () Fig. 4 The pressure taps flush mounted on the wall along to the element of the cone: the photo (left) and the sketch (right) Consequently, the wall pressure recovery is computed using eq. (2) based on averaged values between taps, see Table 3. The wall pressure recovery on the first part of draft tube cone is denoted χ 23 while the wall pressure recovery between PT and PT3 marked χ 3 is obtained along to the draft tube cone. The reference section is no. 3 displaced just downstream to the runner. P wall P + gz wall + gz ρ ρ χ i 3 i3 2 2 Q A 3 2A3 Ai where i =,2 corresponds to the pressure taps (PT) installed on the wall cone, Fig. 4 (right). (2) Table 3 Wall pressure recovery along to the draft tube cone for variable operating points Q/Q bep χ 23 χ 3 Operating regimes Part load near to BEP Overload

6 IOP Conf. Series: Earth and Environmental Science 2 (2) 27 doi:.88/755-35/2//27 Figure 5 shows the distribution of the wall pressure recovery (χ 23 and χ 3 ) for all operating points investigated. The wall pressure recovery on the cone χ 3 (marked with black circle in Fig. 5) seems to be quite similar with the values measured on the first part of the cone χ 23 at full load operation and near to best efficiency point. Contrary, at part load operation, the wall pressure recovery on the first part of the cone χ 23 (plotted with red square in Fig. 5) is smaller than the wall pressure recovery along to the cone χ 3 due to the strong influence of the residual swirling level downstream to the runner. The maximum value χ 23 =.75 of wall pressure recovery coefficient on the first part of draft tube cone is reached at 85% from best efficiency point discharge (Q bep ) while the maximum value χ 3 =.86 of wall pressure recovery coefficient on the draft tube cone at 75% from Q bep. That situation is obtained due to the runner is designed with free swirling flow at best efficiency point in conjunction with high draft tube cone (h=3r 2e ) and small semi angle of cone. However, one should bear in mind that the measurements at part load operation are performed with air admission just downstream to the runner through the hollow shaft. Consequently, the wall pressure recovery at part load operation includes the influence of the air admission..5 X23, X3 [ ] X3 X Q/Qbep [ ] Fig. 5 The wall pressure recovery along to the draft tube cone for all operating range: on the first part of cone (χ 23 ) and along to the element of cone (χ 3 ). The pressure fluctuations (p ) are analyzed based on Fourier spectra in order to evaluate the amplitude and frequency. Figure 6 presents the Fourier spectra in the pressure taps (PT, PT2 and PT3 in Fig. 4) flush mounted on the cone wall at four part load operating conditions. The fundamental harmonic (st corresponds to the vortex rope and associated frequency is around 2-25% from runner frequency. The maximum amplitude (.3% from head) is obtained for pressure transducer PT3 situated just downstream to the runner at part load operation (around 6% see Fig. 6). The maximum amplitude decreases with more than 5% if the turbine operates at regime above 8%. The maxima of dimensionless amplitude and associated frequency for each operating point as well as each wall pressure tap are presented in Tab. 4. The maximum amplitude measured in pressure transducer PT situated near to the elbow is two times smaller than the maximum amplitude from PT3, see Fig. 6. Moreover, at part load operating regimes (Q.6Q bep ) the maximum amplitude corresponds to, Table 4. The air admission is connected with the air volume absorbed [6] through the hollow shaft. In the experimental investigations performed into the hydropower plant, the air is absorbed through the choke valve only if the pressure goes down to the atmospheric pressure, see Figure 2. 5

7 IOP Conf. Series: Earth and Environmental Science 2 (2) 27 doi:.88/755-35/2//27 p /(rho*e) [%].5.5 st harmonic frequency PT Q/Qbep= st harmonic frequency PT2 Q/Qbep= st harmonic frequency PT3 Q/Qbep=.589 p /(rho*e) [%] st harmonic frequency PT Q/Qbep= st harmonic frequency PT2 Q/Qbep= st harmonic frequency PT3 Q/Qbep=.696 p /(rho*e) [%] st harmonic frequency PT Q/Qbep= st harmonic frequency PT2 Q/Qbep= st harmonic frequency PT3 Q/Qbep=.795 p /(rho*e) [%] st harmonic frequency PT Q/Qbep= st harmonic frequency PT2 Q/Qbep= st harmonic frequency PT3 Q/Qbep= Fig. 6 Fourier spectra for unsteady pressure measured on cone wall in all three taps (see Fig. 4): PT left column, PT2 center column and PT3 right column. The data are presented at different dimensionless discharge values at part load conditions Q/Q bep :.589 (first row),.696 (second row),.795 (third row) and.875 (fourth row). 6

8 IOP Conf. Series: Earth and Environmental Science 2 (2) 27 doi:.88/755-35/2//27 Table 4 Maximum amplitude and associated frequency at part load operating regimes measured on the draft tube cone wall in the hydropower plant with air admission Pressure tap no. (PT) Pressure tap no. 2 (PT2) Pressure tap no. 3 (PT3) Q/Q bep [-] A/(ρE) f/n A/(ρE) f/n A/(ρE) f/n [%] [-] [%] [-] [%] [-] (2 nd.7356 ( st.344 ( st (2 nd.649 ( st.344 ( st (2 nd.572 ( st.73 ( st ( st.7 ( st.2462 ( st ( st.444 ( st.638 ( st ( st.277 ( st.94 ( st ( st.2337 ( st.99 ( st Figure 7 presents the waterfall diagrams for pressure transducers PT (left) displaced near to the elbow and PT3 (right) located downstream to the Francis runner. In these plots, the overload regimes (Q/Q bep > ) are marked with red color while the operating points from part load (Q/Q bep < ) are presented with blue color. Fig. 7 Waterfall diagrams for pressure transducers flush mounted on the wall of the draft tube cone: P displaced near to the elbow and P3 located near to the Francis runner. The overload regimes are plotted with red color while the part load regimes are presented with blue color. The measurements into the HPP are performed with air admission through the hollow shaft. 4. Conclusion The paper presents our ongoing efforts in order to solve the problems generated by vortex rope at part loa d operation in a real Francis turbine with medium specific speed. As a result, the following problems are identified: i) vibrations are generated and propagated to the hydraulic turbine elements; ii) the runner cone (ogive) is removed; iii) the runner band seal is wore; iv) the cracks are initiated at the junction between crown and runner blades on the trailing edge and lead to failure of the runner blades. Experimental investigations are performed in order to evaluate the wall pressure recovery along to the cone of the draft tube as well as pressure fluctuations due to the vortex rope at part load conditions. First, three 7

9 IOP Conf. Series: Earth and Environmental Science 2 (2) 27 doi:.88/755-35/2//27 pressure taps was flush mounted on the wall along to the element of the cone. Second, the unsteady pressure in the pressure taps is recorded in thirteen operating point at constant head (nine operating points at part load and four operating points at full load). The wall pressure recovery on the draft tube cone and the pressure fluctuations generated by the vortex rope are computed based on experimental data. Consequently, the fundamental harmonic (st corresponds to the vortex rope and associated frequency is around 2-25% from runner frequency. The maximum amplitude measured with air admission (.3% from turbine head) reveals in pressure transducer PT3 situated just downstream to the runner at part load operation (Q.6Q bep ). The maximum amplitude decreases with more than 5% if the turbine operates at regimes above.8q bep. The maximum amplitude is two times smaller in pressure transducer PT situated near to the elbow than the maximum pressure from PT3. Moreover, at part load regimes with Q.6Q bep the maximum amplitude corresponds to. Acknowledgments Dr. Sebastian Muntean was supported by the Romanian Academy program Hydrodynamics Optimization and Flow Control of the Hydraulic Turbomachinery in order to Improve the Energetic and Cavitational Performances. Experimental investigations in hydropower plant have been performed by team from Research Center in Hydraulics, Automation and Heat Transfer from the Eftimie Murgu University of Resita. Nomenclature A S L Q H f E=gH Pressure amplitude [Pa] Cross-section area [m 2 ] Length [m] Discharge [m 3 /s] Head [m] Frequency [Hz] Specific energy [J/kg] p p P χ ρ g Unsteady static pressure [Pa] Fluctuation of static pressure [Pa] Mean static pressure [Pa] Wall pressure recovery [Pa] Fluid density Gravitational acceleration [m 2 /s] Subscript and superscript bep best efficiency point p Unsteady static pressure [Pa] References [] Susan-Resiga R, Ciocan G D, Anton I and Avellan F 26 Analysis of the Swirling Flow Downstream a Francis Turbine Runner ASME J. of Fluids Engineering [2] Grein H 98 Vibration Phenomena in Francis Turbines: Their Causes and Prevention Proc. of the th IAHR Symp. in Hydraulic Machinery Equipments and Cavitation (Tokyo, Japan) [3] Casanova F 29 Failure analysis of the draft tube connecting bolts of a Francis-type hydroelectric power plant Engineering Failure Analysis [4] Saeed R A and Galybin A N 29 Simplified model of the turbine runner blade Engineering Failure Analysis [5] Thicke R H 98 Practical solutions for draft tube instability Water Power & Dam Construction 33(2) 3-37 [6] Papillon B, Kirejczyk J and Sabourin M 2 Atmospheric air admission in hydro turbines HydroVision (Charlotte North Carolina, USA) p 3C [7] Susan-Resiga R, Vu T C, Muntean S, Ciocan G D and Nennemann B 26 Jet Control of the Draft Tube Vortex Rope in Francis Turbines at Partial Discharge Proc. of the 23 rd IAHR Symp. on Hydraulic Machinery and Systems (Yokohama, Japan) p F92 [8] Susan-Resiga R, Muntean S, Hasmatuchi V, Avellan F and Anton I 2 Analysis and Prevention of Vortex Breakdown in the Simplified Discharge Cone of a Francis Turbine ASME J. of Fluids Engineering 32(5) 52 [9] Bosioc A, Tănasă C, Muntean S and Susan-Resiga R 2 Unsteady pressure measurements and numerical investigation of the jet control method in a conical diffuser with swirling flow Proc. of the 25 th IAHR Symp. on Hydraulic Machinery and Systems (Timisoara, Romania) (submitted) [] Bosioc A I, Tănasă C, Muntean S and Susan-Resiga R 2 Pressure recovery improvement in a conical diffuser with swirling flow using water jet injection Proc. of the Romanian Academy, Series A: Mathematics, Physics, Technical Sciences, Information Science (submitted) 8

10 IOP Conf. Series: Earth and Environmental Science 2 (2) 27 doi:.88/755-35/2//27 [] Frunzaverde D, Muntean S, Marginean G, Campian V, Marsavina L, Terzi R and Serban V 2 Failure analysis of a Francis turbine runner Proc. of the 25 th IAHR Symp. on Hydraulic Machinery and Systems (Timisoara, Romania) (submitted) [2] International Electrotechnique Commission 999 IEC 693 Standard Hydraulic Turbines, Storage Pumps and Pump-Turbines-Model Acceptance Tests International Electrotechnical Commission (Geneva, Switzerland) 9