Handbook of Turbomachinery Second Edition Revised - Ventech!

Figure 43 System frequency limitations versus maximum turbine operation time.
Stall Flutter. Stall flutter is similar to the well-known ‘‘stall’’ of an airplane
wing. In steam turbines, it occurs in the last row of LP turbines at low load
and high back pressures when the blades experience a negative angle of
attack at least over the upper portion of their length. This condition is
illustrated in Fig. 44, where the angle of attack approaching the rotating
blade is shown for normal operation (velocity vector labeled V2) and, in
contrast, for off-design or low-load conditions (velocity vector labeled V2
off). A region of stall can form on the trailing side of the blade as shown in
the figure and will result in a form of unstable vibration termed stall flutter.
Stall flutter is a serious potential cause of blade damage as a considerable
number of cycles can accumulate in a very short period of time, leading to
blade failure by high cycle fatigue.
Investigation of stall flutter has a long history including fundamental
work by Sisto in the early 1950s [83]. Flutter is an aeroelastic instability that
occurs when energy is exchanged between the fluid and the structure in a
manner that creates self-excitation. Flutter is limited only by the damping
capability of the blade material and the structural damping. Stable flutter
can occur only when the aerodynamic power flow to the blade is equal to or
greater than the dissipative power of the blade and structure. As it is difficult
to measure or calculate structural damping, a conservative analysis ignores
that component. An extensive study of the potential for flutter was
Copyright © 2003 Marcel Dekker, Inc.