Cracking reason for Francis turbine blades based on transient fluid structure interaction

Abstract: With the rapid development of hydroelectric technology, stability problems of hydropower units is gradually drawing people's attention. Especially the crack problem has always existed in various large hydropower stations both at home and abroad, such as the abroad Otani force power station, the domestic Xiaolangdi power station, Yantan Dam, and all runner blades in power stations mentioned above had crack problem of different degrees after putting into operation for some period, which seriously affected the turbine units' hydraulic performance and stable operation. A certain power plant's Francis turbine had blade crack problem of different degrees on the runner blade over the years, and most of the cracks belonged to perfoliate crack. To analyze the causes of the runner blade cracks, this paper adopted fluid dynamics primarily to conduct the numerical simulation on hydraulic turbine units' comprehensive unsteady turbulent flow under four operating conditions with different outputs at the rated head, and then analyzed and compared the changes of the velocity field, pressure field, as well as pressure pulsation situations inside the runner under various conditions. The paper focused on the channel vortex inside the flow passage and the amplitude and frequency resistance of the pressure fluctuation caused by the channel vortex under low load, and the results showed that under the rated condition, the flow field was well-proportioned and there didn't exist the channel vortex, while under the low load condition (45% of the output), there existed the channel vortex, the pressure pulsation amplitude reached its maximum when the output was 30 MW, which was about 4.3% of the head for the units, and the long-term operation under low load condition tended to cause blade fatigue. Next, this paper adopted structure finite element technique to conduct dynamic stress analysis on the units under the above-mentioned four conditions. And what could be concluded was that the stress mainly was concentrated in four areas, namely the blade leading edge and runner crown junction, the tailing edge and runner crown junction, the leading edge and the runner bottom junction and tailing edge and runner bottom junction; the maximum principal stress occurred on the connection of the runner crown and the tailing edge, and the maximum dynamic stress occurred under low load condition, which could reach 164.3 MPa and was close to the blade fatigue strength i.e. 210 MPa. Finally, this paper conducted single-blade modal analysis and calculation on the runner, and obtained the first ten-order natural frequency of the single-blade in the air and in the water separately. Modal calculation results showed that the natural frequency of the runner blade in the water was far away from the unit's rotating frequency and its low-order frequency was doubled, as well as the blade's passing frequency, and thus natural vibration wouldn't occur on the blade surface during the operation process. In short, This paper analyzed reasons for blade crack and put forward homologous crack control countermeasure by means of CFD (computational fluid dynamics) calculation, and can provide a certain reference for the units' stable operation.