Compressors Part 2 - Aircraft Gas Turbine Engines #06
Summary
TLDRThis script delves into the mechanics of compressor stalls in engines, explaining how the interplay between blade rotation speed and air's axial velocity determines the angle of attack. It outlines causes like fuel flow imbalances, engine speed variances, and disrupted airflow, leading to stall indications like RPM fluctuations and increased EGT. The narrative progresses to discuss preventative systems like variable inlet guide vanes and compressor bleeds, and the evolution of multi-spool compressors to enhance operational flexibility and reduce stall risks.
Takeaways
- 🔍 The angle of attack of a compressor blade is influenced by the axial velocity of air and the rotational speed of the blade.
- 🚫 A compressor stall can occur due to an imbalance between the rotational speed of the blade and the axial velocity of air, caused by factors like excessive fuel flow or engine operation outside design parameters.
- 🛠️ To demonstrate the effect of imbalance, the script suggests using buttons to adjust engine RPM and observe changes in blade angle of attack.
- ⚙️ Engine overspeed or underspeed can alter the rotational speed of compressor blades, affecting the angle of attack and potentially causing stall.
- 🌀 Turbulent air entering the engine or damaged compressor components can disrupt the airflow and lead to a stall by changing the axial velocity.
- 📈 Compressor stall indicators include fluctuations in engine RPM, increased engine vibration, and a rise in exhaust gas temperature (EGT).
- 🔁 Compressor stall is a progressive phenomenon that can start at a single blade and spread if unaddressed, potentially leading to a surge.
- 🛑 In case of a surge, the throttle of the affected engine must be closed slowly, and the cause investigated to prevent catastrophic failure.
- 🛠️ Systems like variable inlet guide vanes, variable stator vanes, compressor bleeds, and multi-spool compressors are designed to prevent compressor stalls and surges.
- ⚙️ Variable inlet guide vanes (VIVs) and variable stator vanes (VSVs) help maintain the correct angle of attack by adjusting the airflow path into the compressor.
- 💨 Compressor bleed systems allow excess air to escape from intermediate stages to maintain optimal airflow and prevent choking, thus reducing the risk of stall.
Q & A
What is the angle of attack of a compressor blade influenced by?
-The angle of attack of a compressor blade is influenced by the axial velocity of the air passing across it and the rotational speed of the blade.
What can cause an imbalance leading to compressor stall?
-An imbalance leading to compressor stall can be caused by an imbalance between the rotational speed of the blade and the axial velocity of the air passing across it, which can occur due to various reasons such as excessive fuel flow, engine operation outside design parameters, or turbulent airflow to the engine intake.
How can excessive fuel flow lead to compressor stall?
-Excessive fuel flow, possibly caused by abrupt throttle opening, can generate back pressure in the combustion chamber, leading to a reduction in the axial velocity of air through the compressor, potentially causing a stall.
What is the effect of engine operation outside its design RPM parameters?
-Operating the engine above or below its design RPM parameters can alter the rotational speed of the compressor blades, which may either increase or decrease the angle of attack to a point where blade efficiency is compromised, leading to a stall.
What are the indications of a compressor stall?
-Indications of a compressor stall can include fluctuations in engine RPM, increased engine vibration, audible noise in the cockpit, and an increase in exhaust gas temperature (EGT).
How does a compressor stall progress?
-Compressor stall is a progressive phenomenon that can initially occur at just one blade and, if unaddressed, can worsen to affect the whole engine, potentially leading to a complete breakdown of airflow called a surge.
What is the significance of a surge in an engine?
-A surge signifies a complete breakdown of airflow through the engine, which can cause an instantaneous reversal of airflow, loud bangs, and potentially catastrophic damage to the compressor rotor blades if not managed properly.
What are some systems designed to prevent compressor stall or surge?
-Systems designed to prevent compressor stall or surge include variable inlet guide vanes (VIVs), variable stator vanes, compressor bleeds, and multi-spool compressors.
How do variable inlet guide vanes help in preventing compressor stall?
-Variable inlet guide vanes can be automatically pivoted to vary the path of the airflow into the compressor, maintaining the proper relationship between the compressor rotational speed and the velocity of the airflow, thus preventing stall.
What is the purpose of compressor bleeds in an engine?
-Compressor bleeds allow excess volume of air to escape from intermediate stages of the compressor, reducing the choking effect and maintaining the axial velocity of the air closer to the optimum value, thereby reducing the likelihood of compressor stall.
Why are multi-spool compressors used in engines?
-Multi-spool compressors, where each compressor section is driven by its own turbine, allow for greater operational flexibility over a wider RPM range, reducing the chance of compressor stall by maintaining the optimum angle of attack over a broader range of conditions.
Outlines
🛫 Compressor Stall Phenomenon
This paragraph delves into the mechanics of a compressor stall, explaining how the angle of attack of a compressor blade is determined by the interplay between the axial velocity of air and the rotational speed of the blade. It identifies factors that can cause a compressor stall, such as imbalances in rotational speed and axial velocity, excessive fuel flow, engine operation outside design parameters, and issues with engine intake or compressor components. The paragraph also discusses the signs of a compressor stall, including fluctuations in engine RPM, increased vibration and noise, and a rise in exhaust gas temperature. It concludes by highlighting the progressive nature of compressor stalls and the potential for a surge if left unchecked, emphasizing the importance of prompt corrective action.
🚀 Preventing Stall and Surge in Aircraft Engines
The second paragraph focuses on the importance of pilot awareness in preventing compressor stalls and surges. It stresses the need for smooth throttle operation and adherence to RPM and ambient density restrictions to ensure engine reliability. The paragraph then describes various systems designed to prevent these phenomena, including variable inlet guide vanes (VIVs), variable stator vanes (VSVs), compressor bleeds, and multi-spool compressors. Each system is explained in terms of its function and how it helps maintain optimal airflow and compressor performance. The discussion also touches on the disadvantages of these systems, such as decreased mass airflow leading to reduced thrust and increased exhaust gas temperature.
🔧 Evolution of Axial-Flow Engine Design
The final paragraph discusses the evolution of axial-flow engine design, starting with the challenges of early engines that had many compressor stages on a single shaft, leading to operational inflexibility and a tendency for compressor stalls at lower RPMs. To address these issues, designers introduced multi-spool compressors, dividing the compressor into high-pressure (HP), low-pressure (LP), and sometimes intermediate-pressure (IP) sections, each driven by its own turbine. This design allows for a broader RPM range where the compressor can operate efficiently, reducing the risk of stalls. The paragraph also explains the concept of 'spools' and how the speed of each spool is denoted, with the low-pressure spool referred to as 'N1' and subsequent spools as 'N2' and 'N3' in engines with three spools.
Mindmap
Keywords
💡Angle of Attack
💡Compressor Stall
💡Axial Velocity
💡Engine RPM
💡Back Pressure
💡Variable Inlet Guide Vanes (VIVs)
💡Compressor Bleeds
💡Multi-Spool Compressors
💡Exhaust Gas Temperature (EGT)
💡Surge
Highlights
Compressor stall results from an imbalance between the rotational speed of the blade and the axial velocity of air.
Excessive fuel flow due to abrupt throttle opening can reduce axial velocity and cause compressor stall.
Operating the engine above or below design RPM can destroy compressor blade efficiency by altering the angle of attack.
Turbulent airflow at the engine intake can reduce axial velocity and cause compressor stall.
Damage or contamination of compressor components like rotor blades or stator vanes reduces compressor efficiency.
A damaged turbine cannot generate sufficient power to drive the compressor, affecting engine performance.
A lean fuel-air mixture caused by throttle retardation can increase axial velocity through the compressor.
Compressor stall progresses and can lead to a complete breakdown of airflow, resulting in a surge.
Surge is a severe condition caused by fuel system malfunction or engine control mishandling.
Variable inlet guide vanes (VIGVs) help prevent compressor stall by adjusting airflow at low rpm.
Variable stator vanes (VSVs) also assist in maintaining proper airflow during engine acceleration and deceleration.
Early axial-flow engines had stalling issues, leading to the development of multi-section compressors.
Splitting compressors into high-pressure (HP) and low-pressure (LP) sections improved operational flexibility.
Later designs added an intermediate-pressure (IP) compressor for even greater stall prevention and efficiency.
Maintaining optimal airflow over a wide range of RPM is key to preventing compressor stalls in modern engines.
Transcripts
let's examine the phenomenon called
stall more closely we've said that the
angle of attack of a compressor blade is
the result of the axial velocity of the
air passing across it and the rotational
speed of the blade
we said that the forces of the airs
axial velocity and the engine rpm
combined to form a vector which allows
us to find the actual angle of attack of
the airflow over the blade
so the compressor stall can be caused by
an imbalance between the rotational
speed of the blade and the axial
velocity of the air passing across it
which can occur for various reasons some
of which will now examine use the
buttons to increase or decrease the
engine rpm and see the effect that has
on the blade angle of attack
excessive fuel flow which may be caused
by abrupt throttle opening during an
attempt to gain rapid engine
acceleration the back pressure generated
in the combustion chamber may rise to
the extent that it will cause a
reduction in the axial velocity of the
air passing through the compressor
engine operation either above or below
the engine design rpm parameters
engine over speed or under speed will
increase or decrease the rotational
speed of the compressor blades
situations which may either increase or
decrease the angle of attack to the
point where the efficiency of the blade
is destroyed and thus the axial velocity
of the air flow is reduced
turbulent or disrupted effort to the
engine intake this will reduce the axial
velocity of the airflow through the hole
of the compressor
contaminated or damaged compressor
components rotor blades or Stata veins
which will reduce the efficiency of the
compressor as a whole this will cause an
increase in the axial velocity of the
airflow through the compressor because
of the decreased compression ratio
a contaminated or damaged turbine will
not be capable of generating the power
required to drive the compressor at the
correct speed this will make the
compressor incapable of generating a
sufficiently high compression ratio
which in turn will mean that the axial
velocity through the compressor will
increase
excessively lean fuel air mixture which
could be caused by abrupt throttle
retardation this will cause the axial
velocity of the airflow through the
compressor to be increased by the
decreasing combustion chamber back
pressure
any of the conditions just mentioned can
cause a compressor stall to commence and
as soon as it does there is a partial
breakdown of airflow through the engine
the indications of compressor stall can
be any or all of the following
fluctuations in the engine rpm
an increase in the vibration level of
the engine which can generate noise
which may become audible in the cockpit
depending on whether the engines are
wing mounted or rear fuselage-mounted
and an increase in the exhaust gas
temperature otherwise known as egt
this lanta affect the increase in
exhaust gas temperature is caused by the
fact that there is less air going to the
combustion chambers hence there is less
air to call the products of combustion
the exhaust gasses
remember that compressor stall is a
progressive phenomenon it could in
theory initially occur at just one blade
subsequently worsening to encompass the
whole of one stage and then if nothing
is done to prevent it it can affect the
whole engine
the progressive deterioration of the
speed of the airflow through the engine
caused by the extol phenomenon will
eventually cause a complete breakdown of
airflow through the engine which we said
was called a surge
in severe cases a surge could cause an
instantaneous reversal of the airflow
through the engine with air being
expelled through the engine intake with
a loud bang
if search does occur the throttle of the
affected engine must be closed slowly
and the calls investigated
Serg is most commonly caused by either
fuel system malfunction or engine
control mishandling
in extreme cases the surge could inflict
such large bending stresses on the
compressor rotor blades that they
contact the stator blades with
potentially catastrophic results
apart from the loud noise that usually
accompanies a surge there is the large
rise in the egt and the resulting loss
of thrust may cause the aircraft to your
the pilot must always be conscious of
the causes of stall or surge if he is to
prevent either from occurring smooth
operation of the throttles both when
advancing and retarding them will ensure
reliable and prompt response from the
engine the pilot must also be very aware
of the restrictions which rpm and the
ambient density place upon the
powerplant and amend engine handling
accordingly
operation of the engine outside the
optimum rpm and axial velocity range is
inevitable
although design criteria are after all
aimed at producing the greatest engine
efficiency near maximum rpm engine
operation at power settings below that
point has to occur if we are to be able
to throttle the engine back from full
power
so engine operation below the maximum
power level means that we are committed
to altering the rotational speed of the
compressor and also the axial velocity
of the air as it passes through the
engine unfortunately by doing either of
these we are encouraging the onset of
stall and surge
systems that ensure that Serge install
do not happen have to be fitted to the
engine here are some of those systems
variable in it guide vanes all the ITV's
variable state of veins
compressor bleeds
Multi spool compressors
variable in that guidelines or VI TVs
are fitted to engines which have a
particular problem with inherent
compressor stall at low rpm or during
engine acceleration or deceleration
variable Inlet guide vanes are fitted
just in front of the first rotor stage
variable in that guide range can be
automatically pivoted around their own
axis to vary the path of the airflow
going into the compressor so maintaining
the proper relationship between the
compressor rotational speed and the
velocity of the airflow through the
front compressor stages
at low compressor speeds the variable
Inlet guide vanes are angled to impart
the greatest amount of swirl to the air
thereby correcting the relative airflow
to obtain the optimum angle of attack
over the rotor blades maintaining this
optimum angle of attack allows a smooth
and rapid engine acceleration
compressor speeds the variable Inlet
guide vanes reduce the swirl imparted to
the airflow thereby maintaining the
correct angle of attack of the air
flowing over the rotor blades
after the first rotor stage has been
successfully negotiated the airflow may
still have problems further down the
compressor when the engine is operating
at other than its optimum conditions to
minimize those problems some engines are
fitted with variable state of vanes
variable state of veins can be pivoted
automatically so that as the compressor
speed is reduced from the optimum design
value they are progressively closed to
maintain the airflow onto the following
rotor blades at an acceptable angle of
attack
in some engines at low compressor rpm
the relationship between rpm and airflow
axial velocity may not be maintained to
give the rotor blades the optimum angle
of attack unless some of the excess
volume of air is allowed to escape from
the intermediate stages of the
compressor
if a compress a breed valve like the one
shown here is fitted to the compressor
casing at a position adjacent to the
intermediate stages of the compressor
it can be opened at low rpm and during
engine acceleration to allow some of the
excess volume of air to escape
this will have the effect of bringing
the axial velocity of the air in the
earlier stages of the compressor closer
to the optimum value and also of
reducing the choking effect in the rear
of the compressor
this diagram shows a pneumatically
operated compressor bleed fitted to the
intermediate compressor section of a
bypass engine
when the engine is operating at
reasonably high power settings the high
pressure or HP compressor output will be
high enough to lift at the diaphragm in
the actuator valve
allowing high-pressure air to maintain
the pre valve closed
conversely when engine rpm drops the
high-pressure compressor output pressure
also drops eventually the dropping
pressure will allow the actuator valve
diaphragm to fall and it's attached
valve to close off the supply of HP air
to the bleed valve
consequently with its supply of HP air
cut off the breed valve opens allowing a
reduction in the volume of air through
the compressor which would otherwise
tend to choke the flow of air through
the engine
this combination of optimum airflow
velocity and reduced choking will ensure
that compressor stall is less likely to
occur during the time the pleads are
open but there are disadvantages to the
use of the system
opening any compressor bleed whether
it's a bleed used as a store
preventative measure or alternatively a
bleed used to supply air for aircraft
services decreases the mass airflow
through the engine
a decrease in mass airflow through the
engine will cause a drop in thrust for a
given throttle position which raises the
engine's specific fuel consumption or
SFC
a decrease in mass airflow also raises
the engines exhaust gas temperature
because the amount of cooling air
available in the combustion chambers
will have decreased
the design of early axial-flow engines
was developed by adding more compressor
stages on one shaft to obtain higher and
higher compression ratios
unfortunately having many compressor
stages on one shaft makes it
increasingly difficult to retain the
engine's operational flexibility in
terms of being able to operate it over a
reasonable RPM range
compressor blade angles are arranged to
give peak engine performance around
maximum rpm when the values of the axial
velocity of the air flow and the
rotational speed of the blade combine to
produce a vector which is the optimum
angle of attack of the airflow over the
blade
any reduction of engine rpm changes the
symmetry of the vector diagram relating
the RPM to the axial velocity
thus the angle of attack no longer
retains its optimum value because of
this compressor stall becomes an
ever-present problem at lower engine
speeds
so to overcome the tendency that early
axial flow engines had of the compressor
stalling at low rpm designers split the
compressor initially into two separate
sections the two sections were called
respectively the high pressure or HP
compressor
and the low-pressure or LP compressor
in later more powerful engines designers
split the compressor into three sections
by adding an intermediate pressure or IP
compressor
each compressor section is driven
through a shaft by its own turbine
at any given power setting the speed of
rotation of the compressors increases in
proportion to its pressure status
thus the intermediate pressure
compressor rotates faster than the
low-pressure compressor
and the high-pressure compressor rotates
faster than the intermediate pressure
compressor
we said earlier that together the
compressor the turbine and the shaft
upon which they are both mounted form a
spool
by designing the engine so that upon
closing the throttle the speed of the
low-pressure spool falls off more
rapidly than the intermediate pressure
and high-pressure spools it can be
arranged that the optimum shape of the
vector diagram relating to compressor
blade angle of attack can be maintained
over a much greater engine rpm range
thus greatly reducing the chance of
compressor stall
when referring to the speed of rotation
of the spools its usual to call the
speed of rotation of the low-pressure
spool and one
and the speed of rotation of the next
spool in the engine and to
if the engine has three spools then the
speed of rotation of what would be the
high-pressure spool would be called n3
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