Page 221 - Steam Turbines Design, Applications, and Rerating

200 Chapter Eleven

Figure 11.9 Mode shape characterization as a surface. (Dresser-
Rand Company, Wellsville, N.Y.)
11.4.3 Fluctuating forces

Circumferentially oriented flow distortion due to stationary vanes
(nozzles) is shown in Fig. 11.12.
The forces experienced by the blade depend on its relative position
with respect to nozzles in one complete revolution. Under the steady-
state condition, the same pattern of the force is repeated in successive
revolutions making it a periodic force. Such a force is shown in Fig.
11.13.
These time-periodic forces may be composed of many harmonics. The
frequencies of these harmonics are integer multiples of the angular
speed of the turbine and also depend on the number of interruptions
around the annulus.