混流式水轮机主轴中心孔补水对尾水管性能的影响
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摘要
混流式水轮机在低负荷工况下运行时,尾水管内出现旋转的偏心涡带,会引起强烈的压力脉动和振动,严重威胁厂房的安全。为了使机组稳定运行,该文提出了一种通过从上冠泄水锥引入高压补水的方法来降低尾水管的不稳定性。该文首先采用商业软件CFX16.0,对某电站混流式水轮机在低负荷工况下进行了可靠而准确的全三维非定常数值模拟,结果表明在该工况下尾水管内部存在明显的偏心涡带,并伴随着振幅较大的压力脉动,这与试验结果相吻合。其次,对该工况下不同补水流量进行了数值模拟计算,研究表明:尾水管内补高压水可以有效降低尾水管内部的流动损失,且随着补水量的增加而越小,但过大的补水量会引起叶片正背面压力的降低,影响水轮机的空化性能,故补水量的大小必须综合考虑;主轴中心孔高压补水可以增加转轮出口的轴向速度,从而改变涡带内速度场的分布,可有效消除回流现象,当补水流量过小时,抑制回流作用不明显;当补水量为进口流量1%时,尾水管内部压力脉动振幅变化不大,改善效果不明显;当补水量为进口流量3%时,尾水管内部涡带由双螺旋变成单螺旋,锥管段压力脉动振幅不减反增,不稳定性有所加剧;当补水量为进口流量5%时,尾水管内部压力脉动振幅从18.4%降低至1.63%,同时改变了压力脉动的主频,使其远离转轮主频,避免发生共振,提高了机组的稳定性。
Low load operation of hydropower station refers to an abnormal working condition in which a hydraulic turbine operates under small guide vane opening.If the turbine operates under low load condition for a long time,it will affect not only the performance of the turbine,but also the stability of the whole power station and even the power system.Practice shows that,when operating under low load condition,the rotating eccentric vortex causes strong pressure fluctuation and vibration to appear in the draft tube,and threatens the safety of the plant.Generally,injecting air to draft tube is one of the most widely used methods to improve pressure fluctuation at present,but it may cause some acoustic effects.This paper proposes a method of injecting high pressure water from the runner cone to reduce the instability in the draft tube of a Francis turbine.The method does not need to modify the runner’s geometry,or add any equipment in the draft tube.In order to verify the feasibility of this method,three-dimensional unsteady numerical simulations of a Francis turbine under low load conditions in a power plant were carried out accurately.The results indicated that there obviously existed an eccentric vortex in the draft tube under part load conditions in the investigation,accompanied by large amplitude pressure fluctuation.The CFD(computational fluid dynamics)results on turbine performance in hydraulic efficiency were observed to be in good agreement with the experiment results.According to the above results,the CFD numerical simulation was further applied to verify the technical effect of high pressure water supply in draft tube.The main research results could be summarized as follows.Firstly,the method of injecting high pressure water to draft tube could effectively reduce the energy loss of the flow in the draft tube,and the loss reduced with the increase of the amount of water admission.However,excessive water admission would not only decrease the turbine efficiency due to the increase of the jet efficiency loss,but also cause pressure reduction on runner blades,which can affect the cavitation performance of the Francis turbine.Therefore,the amount of water admission must be considered synthetically.For the current situation,the water admission should not exceed 5%of the inlet flow rate.Secondly,previous studies showed that the pressure fluctuation caused by the spiral vortex was related to the sharp decrease of the axial velocity of the runner.Therefore,injecting high pressure water to draft tube could increase the axial velocity at the outlet of the runner,and the increase of the axial velocity could change the distribution of velocity field in the vortex rope,which could effectively eliminate the backflow phenomenon,but the inhibition of backflow was not obvious when the amount of water admission was too small.Thirdly,when the water admission was 1%of the inlet flow rate(0.488 m3/s)in this condition,there was no obvious improvement because the fluctuation amplitude changed slightly.When the water admission was increased to3%of the inlet flow rate,the vortex shape in the draft tube changed from double-helix to single helix,and the amplitude of pressure fluctuation in the draft tube increased rather than decreased,and thus the instability of the flow in the draft tube increased.When increasing the water admission further to 5%of the inlet flow rate,the amplitude of pressure fluctuation in the draft tube decreased sharply from 18.4%to 1.63%.At the same time,the main frequency of the pressure fluctuation was also changed,which was helpful for avoiding the resonance and improving the stability of the unit.In short,it is feasible and effective to inject high pressure water from the runner cone to the draft tube of the Francis turbine,which can improve the flow field in the draft tube and reduce the instability of the draft tube.
Low load operation of hydropower station refers to an abnormal working condition in which a hydraulic turbine operates under small guide vane opening.If the turbine operates under low load condition for a long time,it will affect not only the performance of the turbine,but also the stability of the whole power station and even the power system.Practice shows that,when operating under low load condition,the rotating eccentric vortex causes strong pressure fluctuation and vibration to appear in the draft tube,and threatens the safety of the plant.Generally,injecting air to draft tube is one of the most widely used methods to improve pressure fluctuation at present,but it may cause some acoustic effects.This paper proposes a method of injecting high pressure water from the runner cone to reduce the instability in the draft tube of a Francis turbine.The method does not need to modify the runner’s geometry,or add any equipment in the draft tube.In order to verify the feasibility of this method,three-dimensional unsteady numerical simulations of a Francis turbine under low load conditions in a power plant were carried out accurately.The results indicated that there obviously existed an eccentric vortex in the draft tube under part load conditions in the investigation,accompanied by large amplitude pressure fluctuation.The CFD(computational fluid dynamics)results on turbine performance in hydraulic efficiency were observed to be in good agreement with the experiment results.According to the above results,the CFD numerical simulation was further applied to verify the technical effect of high pressure water supply in draft tube.The main research results could be summarized as follows.Firstly,the method of injecting high pressure water to draft tube could effectively reduce the energy loss of the flow in the draft tube,and the loss reduced with the increase of the amount of water admission.However,excessive water admission would not only decrease the turbine efficiency due to the increase of the jet efficiency loss,but also cause pressure reduction on runner blades,which can affect the cavitation performance of the Francis turbine.Therefore,the amount of water admission must be considered synthetically.For the current situation,the water admission should not exceed 5%of the inlet flow rate.Secondly,previous studies showed that the pressure fluctuation caused by the spiral vortex was related to the sharp decrease of the axial velocity of the runner.Therefore,injecting high pressure water to draft tube could increase the axial velocity at the outlet of the runner,and the increase of the axial velocity could change the distribution of velocity field in the vortex rope,which could effectively eliminate the backflow phenomenon,but the inhibition of backflow was not obvious when the amount of water admission was too small.Thirdly,when the water admission was 1%of the inlet flow rate(0.488 m3/s)in this condition,there was no obvious improvement because the fluctuation amplitude changed slightly.When the water admission was increased to3%of the inlet flow rate,the vortex shape in the draft tube changed from double-helix to single helix,and the amplitude of pressure fluctuation in the draft tube increased rather than decreased,and thus the instability of the flow in the draft tube increased.When increasing the water admission further to 5%of the inlet flow rate,the amplitude of pressure fluctuation in the draft tube decreased sharply from 18.4%to 1.63%.At the same time,the main frequency of the pressure fluctuation was also changed,which was helpful for avoiding the resonance and improving the stability of the unit.In short,it is feasible and effective to inject high pressure water from the runner cone to the draft tube of the Francis turbine,which can improve the flow field in the draft tube and reduce the instability of the draft tube.
引文
[1]Chen Y N.Karman vortex streets and flow-induced vibrations in tube banks[J].Journal of Engineering for Industry,1973,95(1):410-412.
[2]Wang,Si Ying,Lai Bing.Kinematics and forces of a flexible body in Karman vortex street[J].Science Bulletin,2009,54(4):556-561.
[3]Zuo Zhigang,Liu Shuhong,Liu Demin,et al.Numerical analyses of pressure fluctuations induced by interblade vortices in a model Francis turbine[J].Journal of Hydrodynamics,2015,27(4):513-521.
[4]Zhang P Y,Zhu B S,Zhang L F.Numerical investigation on pressure fluctuations induced by interblade vortices in a runner of Francis turbine[J].Dadianji Jishu/large Electric Machine&Hydraulic Turbine,2009,35(6):35-38.
[5]吴广宽,罗兴锜,冯建军,等.基于瞬态流固耦合的混流式转轮叶片裂纹成因分析[J].农业工程学报,2015,31(8):92-98.Wu Guangkuan,Luo Xingqi,Feng Jianjun,et al.Cracking reason for Francis turbine blades based on transient fluid structure interaction[J].Transactions of the Chinese Society of Agricultural Engineering(Transactions of the CSAE),2015,31(8):92-98.(in Chinese with English abstract)
[6]杜凯堂.五强溪水电站混流式机组不稳定现象的分析和处理[J].大电机技术,2006(4):40-45.Du Kaitang.The instability analysis for Francis turbine of wuqiangxi hydropower station and its Improvement[J].Large Electric Machine and Hydraulic Turbine,2006(4):40-45.(in Chinese with English abstract)
[7]钱勤,张克危,刘伦洪,等.大朝山水电站6号机组运行稳定性与转轮动应力研究[J].大电机技术,2006(3):35-39.Qian Qin,Zhang Kewei,Liu Lunhong,et al.Research on operation stability and runner dynamic stress of dachaoshan hydropower station unit 6[J].Large Electric Machine and Hydraulic Turbine,2006(03):35-39.(in Chinese with English abstract)
[8]张兴栋,黄少洪.天生桥一级水电站水轮机转轮裂纹原因分析[J].水电自动化与大坝监测,2004,28(3):19-21.Zhang Xingdong,Huang Shaohong.Research on runner crack of tianshengqiao hydropower station[J].Hydropower and Pumped Storage,2004,28(3):19-21.(in Chinese with English abstract)
[9]唐培甲.岩滩水电站水轮机振动问题的研究[J].红水河,2000,19(03):59-62.Tang Peijia.Analysis of turbine vibration for yantan hydropower station[J].Hongshui River,2000,19(03):59-62.(in Chinese with English abstract)
[10]汪宝罗,郑源,屈波.大型混流式水轮机组尾水管压力脉动模型试验研究[J].水力发电,2008,34(3):83-87.Wang Baoluo,Zheng Yuan,Qu Bo.Study on model test of vibration of draft tube of large size fransis turbine-generating unit[J].Water Power,2008,34(3):83-87.(in Chinese with English abstract)
[11]Gerlich R.Measurement of unsteady flow in high specific speed model Francis turbine[J].Water Conservancy and Hydropower Express,1994(18):11-16.
[12]Anup K C,Lee Y H,Thapa B.CFD study on prediction of vortex shedding in draft tube of Francis turbine and vortex control techniques[J].Renewable Energy,2016,86(1):1406-1421.
[13]Paik J,Sotiropoulos F,Sale M J.Numerical simulation of swirling flow in complex hydroturbine draft tube using unsteady statistical turbulence models[J].Journal of Hydraulic Engineering,2005,131(131):441-456.
[14]钱忠东,陆杰,郭志伟,等.水泵水轮机在水轮机工况下压力脉动特性[J].排灌机械工程学报,2016,34(08):672-678.Qian Zhongdong,Lu Jie,Guo Zhiwei,et al.Characteristics of pressure fluctuation in pump-turbine under turbine mode[J].Journal of Drainage and Irrigation Machinery Engineering,2016,34(08):672-678.(in Chinese with English abstract)
[15]敏政,李秋桐,李琪飞,等.灯泡贯流式水轮机尾水管流动特性分析[J].中国农村水利水电,2016,01(12):152-157.Min Zheng,Li Qiutong,Li Qifei,et al.The analysis of the flow characteristic of draft tube based on cavitation Flow of bulb turbine[J].China Rural Water and Hydropower,2016,01(12):152-157.(in Chinese with English abstract)
[16]Li W F,Feng J J,Wu H,et al.Numerical Investigation of Pressure fluctuation reducing in draft tube of Francis turbines[J].International Journal of Fluid Machinery&Systems,2015,8(3):202-208.
[17]钱忠东,郑彪,杨建东,等.水轮机补气条件下压力脉动的数值分析[J].水力发电学报,2008,27(4):126-134.Qian Zhongdong,Zheng Biao,Yang Jiandong,et al.Numerical simulation and analysis of pressure fluctuation in Francis hydraulic turbine with air admission[J].Journal of Hydroelectric Engineering,2008,27(4):126-134.(in Chinese with English abstract)
[18]张梁,刘树红,罗先武,等.混流式水轮机的尾水管电磁抑涡装置:中国专利,100864353[P].2006-03-01.
[19]李章超,常近时.轴向射水减弱尾水管低频压力脉动试验[J].农业机械学报,2013,44(5):45-49.Li Zhangchao,Chang Jinshi.Experiment of mitigation of low frequency pressure fluctuation in draft tube by axial water Jet[J].Transactions of the Chinese Society for Agricultural Machinery,2013,44(5):45-49.(in Chinese with English abstract)
[20]Qian Z D,Huai W X,Li W,et al.The effect of runner cone design on pressure oscillation characteristics in a Francis hydraulic turbine[J].Proceedings of the Institution of Mechanical Engineers Part A Journal of Power&Energy,2012,226(1):137-150.
[21]杨锡平,张思青,于凤荣,等.尾水管导流栅的减振性能分析[J].水利水电技术,2015,46(12):65-67.Yang Yiping,Zhang Siqing,Yu Fengrong,et al.Analysis on damping of diversion barrier for draft tube[J].Water Resources and Hydropower Engineering,2015,46(12):65-67.(in Chinese with English abstract)
[22]N Nishi M,Liu S.An outlook on the draft-tube-surge study[J].International Journal of Fluid Machinery&Systems,2013,6(1):33-48.
[23]Lin K,Zhang Y,Di H,et al.Experimental study of underfloor electric heating system with latent thermal storage a air supply[J].Taiyangneng Xuebao/acta Energiae Solaris Sinica,2005,26(6):820-824.
[24]王林.混流式水轮机尾水管压力脉动试验分析[J].水电站机电技术,2016,39(07):5-8.Wang Ling.Measurement of pressure fluctuation in Francis turbine draft tube[J].Mechanical&Electrical Technique of Hydropower Station,2016,39(07):5-8.(in Chinese with English abstract)
[25]王福军.计算流体力学-CFD软件原理与应用[M].北京:清华大学出版社,2004.
[26]陶文铨.数值传热学[M].西安:西安交通大学出版社,2001.
[27]Menter F R.Two-equation eddy viscosity turbulence models for engineering applications[J].AIAA Journal,1994,32(8):1598-1605.
[28]Ming W U,Zhang Y W,Zhang J H.Numerical simulation on cavitation flow field based on SST turbulence model[J].Computer Simulation,2010,27(7):330-334.
[29]Rocha P A C,Rocha H B,Carneiro F O M,et al.k-ω,SST(shear stress transport)turbulence model calibration:A case study on a small scale horizontal axis wind turbine[J].Energy,2014,65(3):412-418.
[30]Kumaran K.A comparison of numerical predictions of the supersonic combustion of hydrogen using S-A and SST k-ωmodels[J].Progress in Computational Fluid Dynamics An International Journal,2009,9(9):475-489.
[2]Wang,Si Ying,Lai Bing.Kinematics and forces of a flexible body in Karman vortex street[J].Science Bulletin,2009,54(4):556-561.
[3]Zuo Zhigang,Liu Shuhong,Liu Demin,et al.Numerical analyses of pressure fluctuations induced by interblade vortices in a model Francis turbine[J].Journal of Hydrodynamics,2015,27(4):513-521.
[4]Zhang P Y,Zhu B S,Zhang L F.Numerical investigation on pressure fluctuations induced by interblade vortices in a runner of Francis turbine[J].Dadianji Jishu/large Electric Machine&Hydraulic Turbine,2009,35(6):35-38.
[5]吴广宽,罗兴锜,冯建军,等.基于瞬态流固耦合的混流式转轮叶片裂纹成因分析[J].农业工程学报,2015,31(8):92-98.Wu Guangkuan,Luo Xingqi,Feng Jianjun,et al.Cracking reason for Francis turbine blades based on transient fluid structure interaction[J].Transactions of the Chinese Society of Agricultural Engineering(Transactions of the CSAE),2015,31(8):92-98.(in Chinese with English abstract)
[6]杜凯堂.五强溪水电站混流式机组不稳定现象的分析和处理[J].大电机技术,2006(4):40-45.Du Kaitang.The instability analysis for Francis turbine of wuqiangxi hydropower station and its Improvement[J].Large Electric Machine and Hydraulic Turbine,2006(4):40-45.(in Chinese with English abstract)
[7]钱勤,张克危,刘伦洪,等.大朝山水电站6号机组运行稳定性与转轮动应力研究[J].大电机技术,2006(3):35-39.Qian Qin,Zhang Kewei,Liu Lunhong,et al.Research on operation stability and runner dynamic stress of dachaoshan hydropower station unit 6[J].Large Electric Machine and Hydraulic Turbine,2006(03):35-39.(in Chinese with English abstract)
[8]张兴栋,黄少洪.天生桥一级水电站水轮机转轮裂纹原因分析[J].水电自动化与大坝监测,2004,28(3):19-21.Zhang Xingdong,Huang Shaohong.Research on runner crack of tianshengqiao hydropower station[J].Hydropower and Pumped Storage,2004,28(3):19-21.(in Chinese with English abstract)
[9]唐培甲.岩滩水电站水轮机振动问题的研究[J].红水河,2000,19(03):59-62.Tang Peijia.Analysis of turbine vibration for yantan hydropower station[J].Hongshui River,2000,19(03):59-62.(in Chinese with English abstract)
[10]汪宝罗,郑源,屈波.大型混流式水轮机组尾水管压力脉动模型试验研究[J].水力发电,2008,34(3):83-87.Wang Baoluo,Zheng Yuan,Qu Bo.Study on model test of vibration of draft tube of large size fransis turbine-generating unit[J].Water Power,2008,34(3):83-87.(in Chinese with English abstract)
[11]Gerlich R.Measurement of unsteady flow in high specific speed model Francis turbine[J].Water Conservancy and Hydropower Express,1994(18):11-16.
[12]Anup K C,Lee Y H,Thapa B.CFD study on prediction of vortex shedding in draft tube of Francis turbine and vortex control techniques[J].Renewable Energy,2016,86(1):1406-1421.
[13]Paik J,Sotiropoulos F,Sale M J.Numerical simulation of swirling flow in complex hydroturbine draft tube using unsteady statistical turbulence models[J].Journal of Hydraulic Engineering,2005,131(131):441-456.
[14]钱忠东,陆杰,郭志伟,等.水泵水轮机在水轮机工况下压力脉动特性[J].排灌机械工程学报,2016,34(08):672-678.Qian Zhongdong,Lu Jie,Guo Zhiwei,et al.Characteristics of pressure fluctuation in pump-turbine under turbine mode[J].Journal of Drainage and Irrigation Machinery Engineering,2016,34(08):672-678.(in Chinese with English abstract)
[15]敏政,李秋桐,李琪飞,等.灯泡贯流式水轮机尾水管流动特性分析[J].中国农村水利水电,2016,01(12):152-157.Min Zheng,Li Qiutong,Li Qifei,et al.The analysis of the flow characteristic of draft tube based on cavitation Flow of bulb turbine[J].China Rural Water and Hydropower,2016,01(12):152-157.(in Chinese with English abstract)
[16]Li W F,Feng J J,Wu H,et al.Numerical Investigation of Pressure fluctuation reducing in draft tube of Francis turbines[J].International Journal of Fluid Machinery&Systems,2015,8(3):202-208.
[17]钱忠东,郑彪,杨建东,等.水轮机补气条件下压力脉动的数值分析[J].水力发电学报,2008,27(4):126-134.Qian Zhongdong,Zheng Biao,Yang Jiandong,et al.Numerical simulation and analysis of pressure fluctuation in Francis hydraulic turbine with air admission[J].Journal of Hydroelectric Engineering,2008,27(4):126-134.(in Chinese with English abstract)
[18]张梁,刘树红,罗先武,等.混流式水轮机的尾水管电磁抑涡装置:中国专利,100864353[P].2006-03-01.
[19]李章超,常近时.轴向射水减弱尾水管低频压力脉动试验[J].农业机械学报,2013,44(5):45-49.Li Zhangchao,Chang Jinshi.Experiment of mitigation of low frequency pressure fluctuation in draft tube by axial water Jet[J].Transactions of the Chinese Society for Agricultural Machinery,2013,44(5):45-49.(in Chinese with English abstract)
[20]Qian Z D,Huai W X,Li W,et al.The effect of runner cone design on pressure oscillation characteristics in a Francis hydraulic turbine[J].Proceedings of the Institution of Mechanical Engineers Part A Journal of Power&Energy,2012,226(1):137-150.
[21]杨锡平,张思青,于凤荣,等.尾水管导流栅的减振性能分析[J].水利水电技术,2015,46(12):65-67.Yang Yiping,Zhang Siqing,Yu Fengrong,et al.Analysis on damping of diversion barrier for draft tube[J].Water Resources and Hydropower Engineering,2015,46(12):65-67.(in Chinese with English abstract)
[22]N Nishi M,Liu S.An outlook on the draft-tube-surge study[J].International Journal of Fluid Machinery&Systems,2013,6(1):33-48.
[23]Lin K,Zhang Y,Di H,et al.Experimental study of underfloor electric heating system with latent thermal storage a air supply[J].Taiyangneng Xuebao/acta Energiae Solaris Sinica,2005,26(6):820-824.
[24]王林.混流式水轮机尾水管压力脉动试验分析[J].水电站机电技术,2016,39(07):5-8.Wang Ling.Measurement of pressure fluctuation in Francis turbine draft tube[J].Mechanical&Electrical Technique of Hydropower Station,2016,39(07):5-8.(in Chinese with English abstract)
[25]王福军.计算流体力学-CFD软件原理与应用[M].北京:清华大学出版社,2004.
[26]陶文铨.数值传热学[M].西安:西安交通大学出版社,2001.
[27]Menter F R.Two-equation eddy viscosity turbulence models for engineering applications[J].AIAA Journal,1994,32(8):1598-1605.
[28]Ming W U,Zhang Y W,Zhang J H.Numerical simulation on cavitation flow field based on SST turbulence model[J].Computer Simulation,2010,27(7):330-334.
[29]Rocha P A C,Rocha H B,Carneiro F O M,et al.k-ω,SST(shear stress transport)turbulence model calibration:A case study on a small scale horizontal axis wind turbine[J].Energy,2014,65(3):412-418.
[30]Kumaran K.A comparison of numerical predictions of the supersonic combustion of hydrogen using S-A and SST k-ωmodels[J].Progress in Computational Fluid Dynamics An International Journal,2009,9(9):475-489.