Gas Turbine Handbook : Principles and Practices

Gas Turbine Systems Theory 57
Figure 4-7. Compressor performance curve.
Flutter—A self-excited vibration usually initiated by the airfoil approaching
stall.
Rotating Stall—As each blade row approaches its stall limit, it does
not stall instantly or completely, but rather stalled cells are
formed (Figure 4-8). These tend to rotate around the flow annulus
at about half the rotor speed. Operation in this region is
relatively short and usually proceeds into complete stall. Rotating
stall can excite the natural frequency of the blades.
The best way to illustrate airflow through a compressor stage is
by constructing velocity triangles (Figure 4-9). Air leaves the stator
vanes at an absolute velocity of C 1
and direction θ 1
. The velocity of
this air relative to the rotating blade is W 1
at the direction β 1
. Air
leaves the rotating stage with an absolute velocity C 2
and direction
θ 2
, and a relative velocity W 2
and direction β 2
. Air leaving the second
stator stage has the same velocity triangle as the air leaving the first
stator stage. The projection of the velocities in the axial direction are
identified as Cx, and the tangential components are Cu. The flow
velocity is represented by the length of the vector. Velocity triangles
will differ at the blade hub, mid-span, and tip just as the tangential
velocities differ.
Pressure rise across each stage is a function of the air density,