Compressors Part 1 - Aircraft Gas Turbine Engines #05
Summary
TLDRThis script discusses the mechanics of gas turbine engines, focusing on the two primary compressor types: axial flow and centrifugal. It explains how these compressors, driven by a turbine, increase engine efficiency by compressing air before combustion. The axial flow compressor, capable of higher compression ratios and greater air intake, is more suitable for modern engines despite the centrifugal's historical prevalence. The script also delves into the principles of each compressor, detailing the conversion of kinetic energy to pressure and the challenges of maintaining efficiency across varying engine speeds, including the risks of stall and surge.
Takeaways
- 🔧 To enhance the efficiency of gas turbine engines, air must be compressed before fuel is added and combustion occurs.
- ⚙️ There are two main types of compressors used in engines: axial flow and centrifugal flow.
- 🔩 The centrifugal compressor was initially preferred due to its robustness and ease of manufacturing.
- 📉 However, the centrifugal compressor has limitations, such as lower air intake capacity and compression ratios compared to axial flow compressors.
- ⏫ Axial flow compressors can achieve higher compression ratios and are more suitable for large, modern engines.
- 🔄 The centrifugal compressor works by converting the kinetic energy of the air into pressure through an impeller and diffuser.
- 🌀 In a centrifugal compressor, approximately 50% of the pressure rise occurs in the impeller and the other 50% in the diffuser.
- 🚫 The use of more than two centrifugal compressor stages in series is not feasible due to high tip speeds and centrifugal loading.
- 🌐 The axial flow compressor consists of multiple stages with rotor and stator blades, converting air velocity into pressure incrementally.
- 🔁 Modern engines may use a combination of centrifugal and axial compressors, or multiple spools, to achieve high compression ratios.
- 💨 Efficient operation across the entire speed range of a compressor is challenging due to the interdependent relationship between compression ratio and engine RPM.
Q & A
What is the purpose of compressing air before it is fed into a gas turbine engine?
-The purpose of compressing air before it is fed into a gas turbine engine is to increase the efficiency of the engine. The compressed air is mixed with fuel and burned in the combustion chambers, and the subsequent expansion in the turbines drives the engine.
What are the two types of compressors used in gas turbine engines?
-The two types of compressors used in gas turbine engines are the axial flow compressor and the centrifugal compressor.
Why was the centrifugal compressor the compressor of choice in early gas turbine engines?
-The centrifugal compressor was the compressor of choice in early gas turbine engines because it is more robust than the axial flow compressor and is easier and cheaper to manufacture.
What are the disadvantages of the centrifugal compressor compared to the axial flow compressor?
-The centrifugal compressor has disadvantages such as a lower capacity to intake air and a lower achievable compression ratio compared to the axial flow compressor, which makes it less efficient in large modern engines.
How does the axial flow compressor differ from the centrifugal compressor in terms of air intake?
-The axial flow compressor can intake a far greater mass of air than the centrifugal compressor when both have the same frontal cross-sectional area.
What is the role of the turbine assembly in a gas turbine engine?
-The turbine assembly converts the pressure, velocity, and heat of the gases passing through the turbine into mechanical energy, which is used to drive the impeller of the compressor at high speed.
How does the diffuser section in a centrifugal compressor contribute to the compression process?
-The diffuser section in a centrifugal compressor is a system of stationary divergent ducts that convert the kinetic energy of the airstream into potential energy, or pressure.
What is the typical compression ratio for a single stage centrifugal compressor?
-A very efficient single stage centrifugal compressor would have a compression ratio in the region of four to one, meaning the outlet pressure is four times greater than its inlet pressure.
Why is it not feasible to use more than two centrifugal compressor stages in series?
-It is not feasible to use more than two centrifugal compressor stages in series due to excessive impeller tip speeds and extreme centrifugal loading, which prohibit efficient operation of a third stage.
How does the axial flow compressor achieve the conversion of kinetic energy into pressure energy?
-The axial flow compressor achieves the conversion of kinetic energy into pressure energy through a series of stages, each consisting of a row of rotor blades followed by a row of stator vanes, which increase the air pressure by converting kinetic energy into pressure energy.
What is the significance of the air annulus in an axial flow compressor?
-The air annulus is the space between the rotor drum and the compressor outer casing. It is significant because it helps maintain the axial velocity of the air reasonably constant as it passes through the compressor, which is necessary as the air is compressed into a smaller volume and its density increases.
What is the phenomenon called 'stall' in the context of compressors, and what can it lead to?
-Stall is a partial breakdown of the airflow through the engine, caused by a reduction in axial velocity to a point where turbulent airflow occurs. If not checked, it may lead to a condition called 'surge', which is a total breakdown of the airflow through the engine, potentially causing the airflow to instantaneously reverse its direction.
Outlines
🛠️ Compression in Gas Turbine Engines
This paragraph explains that for gas turbine engines to operate efficiently, the air entering the engine must be compressed before fuel is added and combustion occurs. It discusses two types of compressors—axial and centrifugal—both driven by turbines connected by a shaft. The centrifugal compressor, though robust and easy to manufacture, has certain limitations compared to the axial compressor, which can handle more air and achieve higher compression ratios. This results in greater thrust in engines using axial compressors compared to those using centrifugal compressors.
🚀 Centrifugal Compressor Principles
This paragraph dives into the working principles of a centrifugal compressor. It explains how the turbine assembly converts pressure, velocity, and heat into mechanical energy, which drives the compressor impeller at high speeds. Air enters through the center of the impeller, is forced outward, increasing pressure as it moves. The diffuser then converts the kinetic energy into pressure. Centrifugal compressors can achieve a compression ratio of 4:1, but using more than two stages is inefficient due to excessive tip speeds and loading. As a result, high compression ratios above 12:1 are not feasible with centrifugal compressors.
⚙️ Axial Flow Compressor Principles
The paragraph explains the operation of an axial flow compressor, which works similarly to a centrifugal compressor but with different mechanics. Axial compressors use rotor blades to increase air velocity and stator vanes to convert that velocity into pressure. Though the pressure increase per stage is small (1.1 or 1.2:1), many rotor stages can be used in modern engines, with up to three spools driven by separate turbines. This allows advanced engines, like the Rolls-Royce Trent, to achieve compression ratios exceeding 35:1, generating compressor outlet temperatures of up to 600°C.
🔄 Variable Compression in Axial Compressors
This paragraph details how the size of the air annulus (the space between the rotor drum and outer casing) decreases to maintain axial velocity as the air is compressed. The design helps ensure efficiency over the engine’s speed range, but challenges arise as compression ratios increase. The relationship between axial velocity and rotor blade angle affects air pressure zones, and maintaining efficiency across varying engine speeds becomes more difficult.
⚡ Compressor Efficiency at Different Speeds
The final paragraph describes the challenge of maintaining compression ratios when engine speed changes. As compressor speed decreases, the compression ratio falls, increasing the volume of air passing through the engine. Conversely, when speed exceeds the design limit, compression increases, reducing air volume. These changes alter the angle of attack on rotor blades, potentially causing turbulent airflow or stalling. Prolonged stalling may lead to a surge, where airflow through the engine reverses, a dangerous condition for engine performance.
Mindmap
Keywords
💡Compressor
💡Axial Flow Compressor
💡Centrifugal Compressor
💡Compression Ratio
💡Impeller
💡Diffuser
💡Turbine
💡Spool
💡Air Annulus
💡Stall
💡Surge
Highlights
Air compression is essential for gas turbine engine efficiency.
There are two types of compressors: axial flow and centrifugal flow.
Centrifugal compressors are more robust and cheaper to manufacture.
Axial flow compressors can intake a greater mass of air and achieve higher compression ratios.
Axial flow compressors generate more thrust compared to centrifugal flow compressors.
Centrifugal compressors have a disadvantage of limited efficiency at high compression ratios.
The turbine assembly converts gas pressure, velocity, and heat into mechanical energy for the compressor.
Airflow in a centrifugal compressor is directed outwards by centrifugal force.
Diffuser section in centrifugal compressors converts kinetic energy into pressure.
Single stage centrifugal compressors have a compression ratio of about four to one.
Using two centrifugal compressor stages in series is common practice.
Axial flow compressors consist of multiple stages with rotor and stator blades.
Axial flow compressors are more efficient for higher power engines.
Compression ratios over 35 to 1 can be achieved with multi-stage axial flow compressors.
Some engines use a combination of centrifugal and axial flow compressors.
The air annulus space must be reduced as air is compressed to maintain constant axial velocity.
Efficient compressor operation is challenging over the entire speed range due to changing compression ratios.
Stall and surge are potential issues that can disrupt airflow in compressors.
Transcripts
to increase the efficiency of the gas
turbine engine the air being fed into it
must be compressed
before it has fuel added to it and it's
burnt in the combustion chambers
and it's subsequently expanded in the
turbines
there are two types of compressor being
used in engines presently available one
allows axial airflow through the engine
the other creates centrifugal flow
through the engine
in each case the compressors are driven
by a turbine which is coupled to the
compressor by a shaft
you
the centrifugal compressor is much more
robust than the axial flow compressor
that and the fact that it is the easiest
and cheapest of the two types to
manufacture made it the compressor of
choice in early gas turbine engines
the centrifugal compressor does however
have one or two disadvantages which have
relegated it to the second position with
regards to its use in large modern
engines
firstly if we compare two compressors
one centrifugal and the other axial each
having the same frontal cross-sectional
area
we would first of all find that the
axial flow compressor can take in a far
greater mass of air than the centrifugal
compressor
and secondly that much higher
compression ratios can be attained in
the axial flow compressor
since the amount of thrust generated by
a gas turbine engine depends partly upon
the mass of air flowing through it
it can be demonstrated that when
comparing two engines each having the
same frontal cross-sectional area the
engine which has an axial flow
compressor will generate more thrust
than the engine with a centrifugal flow
compressor
will now examine the principles of the
centrifugal compressor
the turbine assembly
shaft converts the pressure velocity and
heat of the gases passing through the
turbine into mechanical energy
used to drive the impeller of the
compressor round at high speed
air is introduced continuously into the
eye the center of the impeller by
rotating guide vanes and centrifugal
force causes the air to flow outwards
across the impeller towards the tip
because of the divergent shaped formed
between the impeller blades the pressure
of the air increases as it flows
outwards between them
turbine is adding mechanical energy into
the equation the heirs velocity also
increases
the air leaves the tip of the impeller
and passes into the diffuser section the
diffuser section is a system of
stationary divergent ducts
sardines to convert the kinetic energy
of the airstream its velocity into
potential energy pressure
as you can see from the graph in
practice approximately 50% of the
pressure rise across the compressor
occurs in the impeller and the other 50%
in the diffuser section
a compression ratio of the very
efficient single stage centrifugal
compressor would be in the region of
four to one this means that the outlet
pressure of the compressor would be four
times greater than its inlet pressure
engine compression ratios using
centrifugal compressors two of them
would have to be used in series with
each other
practice it's not been found feasible to
use more than two centrifugal compressor
stages together
excessive impeller tip speeds and
extreme centrifugal loading prohibit
efficient operation of a third stage
as a result of this engine compression
ratios of much greater than twelve to
one are not considered possible using
centrifugal compressors
we will now examine the principles of
the axial flow compressor which are
basically the same as those of the
centrifugal flow compressor the axial
flow compressor converts the velocity of
the air stream its kinetic energy into
potential energy or pressure
the means which it uses to achieve this
conversion are however different to
those used in the centrifugal compressor
the axial flow compressor an example of
which is shown here consists of a number
of stages
a stage embodies one row of rotor blades
of air for section which are fastened to
a disc
followed by one row of state of veins
also of airfoil section
the state of veins are fastened to the
compressor outer casing
the spaces between the rotor blades in
the state of aims form divergent
passages
a number of disks the number equates to
the number of stages are fastened
together to form an integral rotor drum
which is driven by a turbine
in the rotor blades which are turned
continuously at high speed by the
turbine mechanical energy is added and
converted into both kinetic velocity
energy and potential pressure energy
within the state of veins the air
pressure is increased by the conversion
of the kinetic energy into pressure
energy
essentially then the rotor stages of an
actual flow compressor can be seen as
doing the same job as the impeller in a
centrifugal compressor
while the stator stages of an axial flow
compressor can be compared to the
diffuser in a centrifugal compressor
across each stage is only quite small
the ratio being about 1.1 or 1.2 to 1
this means that in the first stage the
pressure might only increase by about 3
pounds per square inch
as a consequence of this in order to
achieve the compression ratio demanded
by more powerful engines many rotor
stages may be fitted on one shaft which
is driven by its own turbine as shown
here
assuming that the pressure ratio for
each of these ten stages was 1.2 to 1
the output pressure for this compressor
would be in the region of 91 pounds per
square inch
this arrangement where a number of
compressor rotor stages on a single
shaft are driven by a turbine is turned
a spool
or modern engines compressors may
consist of up to three spools
method of compression that in an engine
like the rolls-royce Trent compression
ratios in excess of 35 to 1 can be
attained in this engine the pressure
rise over the last stage may be greater
than 80 pounds per square inch
is generated can result in compressor
outlet temperatures of up to 600 degrees
Celsius
although we have only shown engines
which have just centrifugal or axial
flow compressor x' some lower powered
engines do use a combination of
centrifugal and axial compressors
the space between the rotor drum and the
compressor outer casing is called the
air annulus
to maintain the axial velocity of the
air reasonably constant as it passes
through the compressor as it's being
compressed into a smaller and smaller
volume and its density is being
increased the size of the air annulus
must be reduced
this gradual con
vergence of the annulus is achieved by
either tapering the compressor outer
casing or the rotor drum
or in some cases a combination of both
is used
increasing the compression ratio of a
compressor makes it progressively more
and more difficult to ensure that it
operates efficiently over the whole of
its speed range
this diagram shows the vectorial
relationship between the axial velocity
of the air flowing through a compressor
and the RPM of that compression
that relationship gives us the angle of
attack over the rotor blade
and determines the pressure zones either
side of the blade
if the compression ratio of this
particular compressor is designed to be
twenty-two to one at 100% engine rpm
then this diagram depicts the volume of
a unit of air and a normal compression
reducing as it passes through the
compressor at 100% power
the vectorial relationship between the
engine rpm and the airflow axial
velocity will give this angle of attack
over the rotor blade
and these pressure zones which are the
optimum that would occur at the design
point
the design point is that point in the
engines performance criteria where its
operating at its optimum compression
ratio rpm and air mass flow
a problem which is associated with the
compressor operating efficiently over
its complete speed range is caused by
the fact that the compression ratio of
the engine Falls is the speed of
rotation of the compressor Falls and
vice-versa
therefore when the engine is operating
at low rotational speeds the air is not
being compressed so much as at the
design point and the volume which it
occupies inside the engine becomes
greater and greater
engine has been throttled to 60% of its
full power setting and the compression
ratio is now reduced to 11 to 1
the volume of the same
we're entering the compressor is larger
when compressed only by eleven to one
then when it was compressed at 22 to one
to get through the compressor
in the same amount of time it took when
he was compressed to 22 to 1 the
increased volume of air must be moving
faster
the changed relationship between the
increased airflow actual velocity and
the reduced rpm will give a low angle of
attack over the rotor blade
we'll reduce the size of the pressure
zones as shown here
if on the other hand the engine is
allowed to rotate faster than its design
maximum then its compression ratio will
increase accordingly in this case the
engine is operating at 105 percent of
its optimum figure and the compression
ratio has increased to 24 to 1
the volume of the one unit of air
entering the compressor will reduce
further than it would at 100% rpm
because the compression ratio is now
twenty four to one
to get through the compressor in the
same amount of time it took when it was
compressed at a twenty two to one ratio
the decreased volume of air will be
moving slower
once again the changed relationship
between the airflow axial velocity and
the RPM will change the angle of attack
but this time with decreased air flow
velocity and an increased rpm it will
generate a high angle of attack over the
rotor blade
the reduction in axial velocity happens
throughout the compressor
reduction in axial velocity can reach a
point where turbulent airflow and a
phenomenon called stall may occur
Stahl is a partial breakdown of the
airflow through the engine and is a
progressive condition which if it's not
checked may produce an event called
surge
Serge is a total breakdown of the
airflow through the engine which can in
the worst case cause the airflow through
the engine to instantaneously reverse
its direction of flow
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