Handbook of Turbomachinery Second Edition Revised - Ventech!

forcing and damping, and hence the blade stress levels at these crossing-over
points. Given that a component fatigue cycle life span is inversely related to
alternating stress levels, high cycle fatigue has to be an important blading
design aspect, if blades are to be designed to last for a required life span to
give an optimum economic benefit (e.g., in terms of manufacture as well as
maintenance costs) and reliability.
The aerodynamic sources of excitation (forcing) are those nonuniformities
due to blade wakes, pressure waves, and inlet/exit distortions. On the
aerodynamic side, there are two essential parts required for predicting a
blade stress level in a forced response problem:
1. Unsteady aerodynamic forces on blades generated by relative
motions of adjacent blade rows and/or circumferential distortions
2. Aerodynamic damping, effectively the unsteady aerodynamic
forces induced by vibration of a blade once excited
Predictions of unsteady forcing/excitation as well as blade aerodynamic
damping with adequate accuracies are challenging tasks. In the past they
were largely avoided by adopting empirical design rules based on
experiences. But this situation with regard to blade forced response is
about to change. Currently, there are active developments in using advanced
unsteady flow methods to improve the capability for forcing and damping
prediction, as required for current and future high-load blading designs.
There are some basic aeromechanical considerations in designs. The
main objective here is, of course, to reduce or sustain unsteady forcing,
when steady aero loading is increased. It has been shown that the main
contributor to unsteady forces in compressors is the incidence variation,
while the velocity variation in wakes is mainly responsible for forcing in
turbines [10]. The 3D blading with radially leaned blades is widely used in
designs. Although the leaned blading may well be chosen to reduce
aerodynamic losses from a steady flow point of view, it does have a lower
unsteady loading compared to a radially straight one, since a wake shed
from an upstream blade with radial staking will not hit different radial
sections of a downstream blade at the same time (a simple example of 3D
relief of unsteady loading). Given that 3D blading designs are currently
exploited for benefits in aerodynamic performances, it is equally (if not
more) important that such 3D aerodynamic designs are aero-mechanically
sound.
Copyright © 2003 Marcel Dekker, Inc.