Rotating Stall - Turbomachinery Aerodynamics - Lecture Slides, Slides of Turbomachinery

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In this lecture...

• Instability in axial compressors
  • Rotating stall
  • Surge

Instability in axial compressors

• The importance of this range lies in the

fact that engine operation at off-design speeds may occur closer to surge or choke depending upon its operating condition.

• The ability to efficiently operate an engine

at all operating conditions depends upon the matching of all components of an engine within the confines of the stable operating range.

Instability in axial compressors

01

02 P

P

δ

m  θ

Definition of Instability

• There are two areas of compressor

performance that relate to stability:

• Operational stability
• Aerodynamic stability
• Operational stability: matching of

performance characteristics of the compressor with downstream components.

• Aerodynamic stability: limitation of steady

state operation due to stall and surge.

Operational Stability

• In this form of stability analysis, the complete compressor system including inlet ducting, guide vanes, rotors, stators and any pressure recovery exhaust system are considered.
• The operational stability depends upon the rate of change of pressure rise or pressure drop as mass flow rate varies.
• System stability is established when the rate of change of compressor pressure rise with mass flow is algebraically less than the rate of change of throttle pressure drop with mass flow.

Aerodynamic Stability

• Aerodynamic stability is the ability of the entire compressor system to maintain or increase the delivery pressure when the compressor operation has been perturbed to a lower flow.
• The part of the compressor characteristic with a positive slope is a region containing subsystem stall or complete instability resulting in surge.
• In this region, operational stability may be theoretically possible, but not aerodynamic stability.

Aerodynamic Stability

Flow

Pressure ratio

Progressive throttling characteristics

Rotating Stall

• Rotating stall is the most common type of

stall.

• Rotating stall: progression around the

blade annulus of a stall pattern, in which one or more adjacent blade passages are instantaneously stalled, then are cleared for unstalled flow as the stall cell progresses.

• Rotating stall causes alternate loading and

unloading of the blades: fatigue failure.

Rotating Stall

Propagation of rotating stall

Rotating Stall

• Rotating stall often precedes surge.
• The stall patterns move in a direction

opposite to that of the rotor revolution.

• The stall frequency can be as high as

50% of the rotor frequency.

Rotating Stall

• Rotating stall may be initiated due to a

variety of reasons: off-design operation, inflow distortion, blade stagger/profile mismatch etc.

• If allowed to propagate, rotating stall may

lead to surge of the compressor.

• The number of stall cells can be as high as

9 or more or as low as one.

• The number of stall cells is associated with

the type of stall.

Rotating Stall

Flow

Pressure rise

Stall

Flow

Pressure rise

Stall

Progressive or modal stall Abrupt or spike stall

Rotating Stall

Flow

Pressure rise

Variation of stall pattern during progressive stall

Tip stall Part-span stall

Full-span stall

Deep stall