Handbook of Turbomachinery Second Edition Revised - Ventech!

Figure 44 Stall flutter of low-pressure rotating blades during low-load and highexhaust
pressure operation.
conducted by Omprakash et al. [84] for last-stage blading in a 3,600 rpm
machine. The model developed is described along with the results of a
particular analysis. They concluded that the bladed disk under analysis was
not susceptible to flutter at loads of 50, 75, or 100% off the design condenser
back pressure of 5.08 kPa (1.5-in. Hg). However, at a higher back pressure
of 10.16 kPa (3.0-in. Hg), the first tangential mode became aerodynamically
unstable. A further increase in back pressure would make all the modes
aerodynamically unstable.
Flutter stability can be determined by the net power flow to the bladed
disk at various amplitudes of vibration. Normal modes of vibration are
calculated by a bladed disk modal analysis at the operating speed.
Aerodynamic forces can be calculated from the pressure distribution
derived from a separate two-dimensional cascade analysis using compressible
inviscid flow for a range of flow angles corresponding to different
operating conditions.
Considerable effort has been spent analyzing and developing design
methods to avoid stall flutter. Common testing methods include cascade,
model, and/or full-scale testing. An alternative approach is to make the
blade strong enough to resist the dynamic stresses resulting from stall
flutter.
Copyright © 2003 Marcel Dekker, Inc.