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Such a design procedure may not always be followed， for the designer may choose to design the stage to operate closer to the positive stalling limit or closer to the negative stalling (choking) limit at design operating conditions to obtain more flexibility at off-design conditions.
Compressor Stall
There are three distinct stall phenomena. Rotating stall and individual blade stall are aerodynamic phenomena. Stall flutter is an aeroelastic phenomenon.
Rotating Stall
Rotating stall (propagating stall) consists of large stall zones covering several blade passages and propagates in the direction of the rotor and at some fraction of rotor speed. The number of stall zones and the propagating rates vary considerably. Rotating stall is the most prevalent type of stall phenomenon.
The propagation mechanism can be described by considering the blade row to be a cascade of blades as shown in Figure 7-32. A flow perturbation causes blade 2 to reach a stalled condition before the other blades. This stalled blade does not produce a sufficient pressure rise to maintain the flowaroundit， and an effective flow blockage or a zone of reduced flow develops. This retarded flow diverts the flow around it so that the angle of attack increases on blade 3 and decreases on blade 1. The stall propagates down-ward relative to the blade row at a rate about half the block speed; the diverted flow stalls the blades below the retarded-flow zone and unstalls the blades above it. The retarded flow or stall zone moves from the pressure side to the suction side of each blade in the opposite direction of rotor rotation. The stall zone may cover several blade passages. The relative speed of propagation has been observed from compressor tests to be less than therotor speed. Observed from an absolute frame of reference， the stall zones appear to be moving in the direction of rotor rotation. The radial extent of the stall zone may vary from just the tip to the whole blade length. Table 7-1 shows the characteristics of rotating stall for single and multistage axial-flow compressors.
Figure 7-.2. .ropagating stall in a cascade.
.10 Gas Turbine Engineering Handbook
Table 7-1Summar叩 of Rotating Stall Data
Single-Stage Compressors
.ropagationWeight-flow T叩pe of.ub-tip Number Rate， StallFluctuationRadial Velocit叩 Radius Of Stall Speed， absl duringstall， Extent of T叩pe Diagram Ratio Zones Rotor Speed IpVlpV Stall Zoneof Stall
Symmetrical 0.50 3 0.420 1.3. Partial Progressive
4 0.475 2.14
↓↓
5 0.523 1.66 0..0 1 0.305 1.2 Total Abrupt
0.0 0.7 0.76 Partial Progressive
1 0.36 1.30 Total Abrupt
0.76 7 0.25 2.14 Partial Progressive
0.25 1.10
5 0.25 1.10
3 0.23 2.02
l-
l-
4 0.4 1.47 Total
3 0.4 2.02 2 0.4. 1.71
l-
Free vortex Solid body Vortex transonic 0.720.60 0.60 0.50 0.50 0.40 6， 1 2 1 1 1 3 2 1 2 0.245 0.4 0.36 0.10 0.45 0.12 0..16 0.634 0.565 . 0.71二1.33 0.60 0.60 0.6 0.60 0.65 . . . . Total Partial Partial Total Partial Total Partial Total Total Partial Progressive Progressive Progressive Abrupt Progressive Abrupt Progressive Progressive Abrupt Progressive
Multistage Compressors
Number of Stall Zones .ropagationRate， StallSpeed， absl Rotor Speed Radial Extent of Stall Zone .eriodicit叩 T叩pe of Stall
3 0.57 Partial Steady Progressive
4 5 6 7
l-
l-
l-
l-
4 0.55 Partial Intermittent Progressive
5 6
l-
l-
l-
l-
Axial-Flow Compressors .11
1 0.4 Partial Steady Progressive
1 0.57 Partial Steady Progressive
2 3 4
1 0.57 Partial Intermittent Progressive
2 3 4 5
1 0.47 Total Steady Abrupt
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