Notesale: Turn your study into money

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Whenever a diffusing/decelerating flow is found along a physical surface, the possibility exists
for the flow to retard so severely that the fluid no longer follows the surface
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Streamlines adjacent to wall leave the wall and a reverse flow
pockets develop from that point along the surface
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Flow detachment takes the form of small eddies and is also referred to as flow reversal, in
literature
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But, as the flow moves past the thickest part of the aero
foil it suffers deceleration and increased pressure gradient is experienced
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This instability initiated the flow separation at that point
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This condition is termed as “stall”
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The viscous shear
stresses and adverse pressure gradient overpower the streamlines to cause stall
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Compressors these days are operated very close to stall point to maximize the
compression efficiency
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A compressor stall is a local disruption of the airflow
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There are two types of compressor stall:
Rotating Stall
A local disruption of airflow in a compressor is caused when a small number of compressor
blades experience a stall condition that produces small pockets of relatively stagnant air (called
stall cells) this does not cause any destabilization in compressor operation
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5-0
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These stall cells effect the
blades they interact along the way
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Rotational stalls are momentary, they reduce the efficiency and increase the structural loads
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The low velocity of
high pressure ration stages facilitated the growth and eventually the stall engulfs the entire blade
profile
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The compressor either experiences conditions which
exceed the limit of its pressure rise capabilities or is highly loaded such that it does not have the
capacity to absorb a momentary disturbance, creating a rotational stall which can propagate in
less than a second to include the entire compressor
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If, however, the conditions that
induced the stall remain, the return of stable airflow will reproduce the conditions at the time of
surge and the process will repeat
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The reheating of the same mass of gas during each surge cycle causes a
large temperature increase
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In the
interim, the increase in vibration, thrust, and temperature can cause extensive damage to the
compressor
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Repeated
surges will cause a significant accumulated loss in efficiency
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The flow reversals
during surge can produce a booming noise loud enough to make surge a memorable experience
for personnel in the vicinity of the compressor
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