Study On Key Issues Of Reform For Axial Flow Turbine Runner

A number of axial flow hydro-generating units suffer from serious cavitation damage, worse operation stability and bad overall performance. Study on the key issues of axial flow turbine runner's transformation, in order to improve hydraulic turbine's overall performance, to enhance the use efficiency of hydropower resources with low water head and reliability as well as economy of hydro-generating unit operation, has the very vital significance. In this paper, by means of CFD technology and fluid-structure interaction method, the research of certain key issues in the transformation of axial flow turbine runner were carried out, such as flow simulation, dynamic stress analysis, fluid-structure interaction calculation modification design and the performance estimate.Take the axial flow hydraulic turbine as the object of study; the dynamic stresses on the runner blade were analyzed on the basis of numerical simulation of three dimensional flow in entire hydraulic turbine overflow components. The fluid-structure interaction calculation of axial flow blade was conducted. Then the modification design and the performance estimate to axial flow runner blade of HuBei LuShui power plant were carried on.(1) The numerical simulation of the flow in the whole flow passage of axial flow hydraulic turbine was done. The influence of operation condition on the flow field was analyzed, as well as the pressure fluctuation on flow surface and unsteady flow characteristic.The results of steady flow simulation show that the nonuniformity of stay vanes out angle along the circumferential distribution becomes serious with increase of flow. With increase in the guide vane opening, the flow elements uniformity along the circumferential distribution in guide vane area gradually weakened. The distribution differences of water pressure on various runner blades caused by flow nonuniformity behaved most obviously nearby the hub, and blade flange took the second place. The distribution differences of cross section behaved more obviously on first half part of blade.The results of unsteady flow simulation show that under different operating conditions, the differences of pressure distribution tendency on press face and suction face of the guide vane in the similar moment are quite different; The basic frequency of the pressure fluctuation frequency in guide vane contains the same low frequency component within the draft tube's; The pressure fluctuation frequency of bottom ring and the mid-point includes the close frequency components of rotation frequency. And the low frequency components of the pressure fluctuation frequency in draft tube inlet are one of the main sources in hydraulic turbine pressure fluctuation. Besides low frequency, intermediate frequency components close to multiple rotation frequency are also included in the pressure fluctuation frequency of the blade surface, and the secondary sources in hydraulic turbine pressure fluctuation.(2) Strength analysis of the axial flow runner and blade and dynamic stress analysis of single blade were carried on separately. The stress distribution characteristic and stress concentration zone on runner and blade are discussed under different operating conditions. The relationship between dynamic stress on axial flow blade and pressure fluctuation in flow channel was revealed.The results indicate that the blade have emerged on three areas of stress concentration under hydraulic pressure function; As the blade angle increases, maximum stress value of the overall runner decreases gradually, and the stress gradient on blade surface increases gradually from the wheel hub to the rim. It also reveals that uneven degree of stress distributes on the blade surface increases.The blade dynamic stress analysis results show that maximum stress appears on the junction of blade pressure surface as well as suction surface and the flange near the downstream side, and it changes between them in different time of the different conditions; The high amplitude dynamic stress in the junction of blade and flange is obviously larger than the result of static stress analysis. The stress changes in the central blade more seriously than in wheel hub and rim, and more obviously in outlet of blade than in inlet of blade; The dynamic stress frequency components of blade are basically the same as hydraulic pressure frequency in the flow passage, which indicates that the high amplitude dynamic stress is mainly caused by water pressure fluctuation.(3) The vibration characteristics of the axial flow runner blade were analyzed. The fluid-structure interaction calculation of axial flow blade was conducted. The influence of FSI on flow field of blade zone and stress distribution on blade was studied.According to calculation results, the natural frequency of blade in the water decrease with the characteristics of non-linear; the pressure difference between press side and suction side increases after considering the FSI, to a certain extent, which will worsen cavitations performance of the blade. Meanwhile, stress distribution on the blades does not change significantly. Comparing with the results of unsteady flow simulation, the change of maximum stress value and maximum strain value after considering the FSI differ from time points. The research demonstrates that the FSI not only changes the distribution of the flow field in blade area, but also have a greater impact on the stress of the blades.(4) The modification design and the performance estimate to axial flow runner blade of HuBei LuShui power plant were performed.By optimizing the airfoil of blade geometry to improve the hydraulic performance of the runner in the main operating conditions, the performance prediction of the modified runner was carried out, and the characteristic diagram of curves of fixed-blade propeller, the comprehensive characteristic diagram of curves of propeller blade and the hydraulic moment curve of guide vane under the maximal head were drawn up. The actual operation of the station indicates that each performance index of ZZX30 runner modified meets the requirement. And ZZX30 has high efficiency and output stability, as well as better overall performance than before. The power station has obtained good economic efficiency, and it also shows the effectiveness of the methods of modification design and performance estimate used in this paper.