Abstract
Crack appearance in the blade is the most common type of fatigue damage in Francis turbines. However, it is sometimes difficult to detect cracks in time using the current monitoring system, even when they are very large. To better monitor cracks, it is imperative to research the effect of a crack on the dynamic behavior of a Francis turbine. In this paper, the dynamic behavior of a Francis turbine runner model with a crack has been researched numerically. The intact numerical model was first validated by the experimental data available. Then, a crack was created at the intersection line between one blade and the crown. The change in dynamic behavior with increasing crack length has been investigated. Crack-induced vibration localization theory has been used to explain the dynamic behavior changes due to the crack. Modal analysis showed that the adopted theory could basically explain the modal behavior change due to the crack. The FFT results of the modal shapes and the localization factors (LF) has been used to explain the forced response changes due to the crack. Based on the above analysis, the challenge of crack monitoring has been analyzed. This research provides some references for more advanced monitoring technologies.
Highlights
The Francis turbine is one type of widely used hydraulic turbines
A large crack is usually originated from a very small crack or flaw, which is usually undetectable by the current monitoring system, and it will continuously grow under hydraulic dynamic force
This failure case indicates the challenge of crack monitoring in Francis turbines
Summary
The Francis turbine is one type of widely used hydraulic turbines. Due to the use of higher heads [1], more frequent extreme off-design operations, and a reduced ratio of thickness/weight in runners, as well as occasional small material flaws, many cases of Francis turbine failure have been reported in the literature [2,3,4,5]. What of concern is of the effect of the disk, some researchers discretized the continuous disk and simplified both whether it can cause the natural frequencies to decrease drastically so that it could be detected by the the disk sectors and blades into lump masses [12] This procedure demonstrated that the disk-dominated monitoring system. It is easy for the disk-dominated modes or blade-dominated modes with high deformations on the disk to have high coupling stiffness between neighboring sectors and it is difficult for strong localization to occur, just as with those shown in [17] This may still depend on the geometry, and unlike the centrifugal impeller, the band of the turbine shown in Figure 2 is more like a thin ring, and the crown usually has low deformation. Potential technologies to monitor the crack will be introduced
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