Compressors Part 1 - Aircraft Gas Turbine Engines #05

Summary

This 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

00:00

🛠️ 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.

05:00

🚀 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.

10:01

⚙️ 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.

Keywords

💡Compressor

A compressor is a device that increases the pressure of a gas by reducing its volume. In the context of the video, compressors are essential components of gas turbine engines, where they compress air before it is mixed with fuel and burned in the combustion chamber. The script discusses two types of compressors used in engines: axial flow and centrifugal flow. The compressor's function is critical for the efficiency and performance of the engine, as it directly affects the amount of thrust generated.

💡Axial Flow Compressor

An axial flow compressor is a type of compressor where air flows parallel to the axis of the compressor. The script highlights that axial flow compressors can intake a greater mass of air and achieve higher compression ratios compared to centrifugal compressors. This makes them more suitable for large modern engines. The video explains that the axial flow compressor consists of multiple stages, each with rotor blades followed by stator vanes, which work together to convert the kinetic energy of the air into pressure.

💡Centrifugal Compressor

A centrifugal compressor is a type of compressor where air is forced to flow radially outward. The video script mentions that centrifugal compressors were the compressor of choice in early gas turbine engines due to their robustness and ease of manufacturing. However, they have limitations, such as lower air intake capacity and compression ratios compared to axial flow compressors. The script describes how air is introduced into the impeller and is forced outward by centrifugal force, increasing both pressure and velocity.

💡Compression Ratio

Compression ratio refers to the ratio of the pressure of a gas at the outlet of the compressor to the pressure at the inlet. A higher compression ratio indicates more efficient compression. The script explains that single-stage centrifugal compressors have a compression ratio of about four to one, while axial flow compressors can achieve much higher ratios, making them more efficient in engines that require more thrust.

💡Impeller

An impeller is a rotating component in a fluid-moving device, such as a compressor or pump. In the context of the video, the impeller in a centrifugal compressor is driven by a turbine and is responsible for increasing the pressure of the air by converting its kinetic energy into potential energy. The script describes how the air is accelerated by the impeller's rotation and how the pressure increases as the air flows outward between the impeller blades.

💡Diffuser

A diffuser is a component in a compressor that slows down the fluid flow and increases its pressure. In the video script, the diffuser section of a centrifugal compressor is described as a system of stationary divergent ducts that convert the kinetic energy of the airstream into pressure. The script notes that approximately 50% of the pressure rise across the compressor occurs in the diffuser, highlighting its importance in the compression process.

💡Turbine

A turbine is a rotary mechanical device that extracts energy from a fluid flow and converts it into mechanical work. In the video, turbines are mentioned as components that drive the compressors in gas turbine engines. The script explains that the turbine assembly converts the pressure, velocity, and heat of the gases passing through it into mechanical energy, which is then used to drive the impeller of the compressor.

💡Spool

A spool in the context of the video refers to a set of compressor and turbine stages that are mounted on a single shaft. The script mentions that modern engines may have compressors arranged in multiple spools to achieve the desired compression ratios. This arrangement allows for more stages of compression, which is necessary for high-performance engines that require higher compression ratios.

💡Air Annulus

The air annulus is the space between the rotor drum and the compressor outer casing where the compressed air flows. The script explains that as air is compressed and its volume decreases, the size of the air annulus must also decrease to maintain a constant axial velocity. This is achieved by tapering either the compressor casing or the rotor drum, or both, which is crucial for the efficient operation of the compressor.

💡Stall

Stall, as described in the script, refers to a partial breakdown of the airflow through the engine, which can occur if the angle of attack over the rotor blade becomes too low due to changes in airflow velocity and engine RPM. The video explains that if not checked, stall can progress to surge, which is a total breakdown of airflow and can even cause a reversal of the airflow direction, potentially damaging the engine.

💡Surge

Surge is a phenomenon in gas turbine engines where there is a total breakdown of the airflow through the engine. The script describes it as a condition that can occur if a stall is not managed properly. Surge can lead to an instantaneous reversal of the airflow direction, which is highly detrimental to the engine's operation and can cause significant damage.

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