Individual Blade Stall
This type of stall occurs when all the blades around the compressor annulus stall simultaneously without the occurrence of a stall propagation mechanism. The circumstances under which individual blade stall is estab-lished are unknown at present. It appears that the stalling of a blade row generally manifests itself in some type of propagating stall and that indi-vidual blade stall is an exception.
Stall Flutter
This phenomenon is caused by self-excitation of the blade and is aero-elastic. It must be distinguished from classic flutter， since classic flutter is a coupled torsional-flexural vibration that occurs when the freestream velocityover a wing or airfoil section reaches a certain critical velocity. Stall flutter，on the other hand， is a phenomenon that occurs due to the stalling of the flow around a blade.
Blade stall causes .arman vortices in the airfoil wake. Whenever the frequency of these vortices coincides with the natural frequency of the air-foil， flutter will occur. Stall flutter is a major cause of compressor blade failure.
.erformance Characteristics of an Axial-Flow Compressor
The calculation of the performance of an axial-flow compressor at both design and off-design conditions requires the knowledge of the various types of losses encountered in an axial-flow compressor.
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.12 Gas Turbine Engineering Handbook
The accurate calculation and proper evaluation of the losses within the axial-flow compressor are as important as the calculation of the blade-loading parameter， since unless the proper parameters are controlled， the efficiency drops. The evaluation of the various losses is a combination of experimental results and theory. The losses are divided into two groups:
(1) losses encountered in therotor， and (2) losses encountered in the stator. The losses are usually expressed as a loss of heat and enthalpy.
A convenient way to express the losses is in a nondimensional manner with reference to the blade speed. The theoretical total head available (.tot)is equal to the head available from the energy equation (.th二 .tot) plus thehead， which is lost from disc friction.
.tot二 .th + .df (7-2.)
The adiabatic head that is actually available at the rotor discharge is equal tothe theoretical head minus the head losses from the shock in therotor， theincidenceloss， the blade loadings and profile losses， the clearance betweenthe rotor and theshroud， and the secondary losses encountered in the flow passage
.ia二 .th -.in -.sh -.bl -.c -.sf (7-30)
Therefore， the adiabatic efficiency in the impeller is
. imp二 .ia (7-31).tot
The calculation of the overall stage efficiency must also include the lossesencountered in the stator.Thus， the overall actual adiabatic head attained would be the actual adiabatic head of the impeller minus the head lossesencountered in the stator from wake caused by the impellerblade， the loss ofpart of the kinetic head at the exit of the stator， and the loss of head from the frictional forces encountered in the stator
.oa二 .ia -.w -.ex -.osf (7-32)
Therefore， the adiabatic efficiency in the stage
.stage二 .oa (7-33).tot
The losses as mentioned earlier can be further described:
1.
Di，句1ri句tionlo，， . This loss is from skin friction on the discs that house the blades of the compressors. This loss varies with different types of discs.
2.
ln句iden句elo，， . This loss is caused by the angle of the air and the blade angle not being coincident. The loss is minimum to about an angle of .4o， after which the loss increases rapidly.
3. Blade
loadin于 and ro1ilelo，， . This loss is due to the negative velocitygradients in the boundary layer， which gives rise to flow separation.
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