Compressors Part 2 - Aircraft Gas Turbine Engines #06

Summary

This 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

00:00

🛫 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.

05:01

🚀 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.

10:04

🔧 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.

Keywords

💡Angle of Attack

The angle of attack refers to the angle between the oncoming airflow and a reference line on a moving object, such as an aircraft wing or a compressor blade. In the context of the video, the angle of attack of a compressor blade is determined by the axial velocity of the air and the rotational speed of the blade. It is crucial for maintaining the efficiency of the compressor, as an imbalance can lead to a stall. The video explains that the forces of the air's axial velocity and the engine's RPM combine to form a vector, which is used to calculate the actual angle of attack.

💡Compressor Stall

A compressor stall is a condition where the airflow through a compressor is disrupted, leading to a loss of efficiency and potential engine failure. The video script discusses how this can occur due to an imbalance between the rotational speed of the compressor blades and the axial velocity of the air passing across them. Causes can include excessive fuel flow, engine operation outside of design parameters, or disrupted airflow to the engine intake. The script also mentions that a compressor stall can be progressive, starting at one blade and potentially affecting the entire engine if not addressed.

💡Axial Velocity

Axial velocity is the component of fluid velocity that is parallel to the axis of a system, such as the airflow through an engine. The script explains that the axial velocity of air is a critical factor in determining the angle of attack for compressor blades. It can be affected by various factors, including engine RPM, throttle settings, and the condition of the engine's components. A change in axial velocity can lead to a compressor stall if not properly managed.

💡Engine RPM

Engine RPM (revolutions per minute) is a measure of how fast an engine's crankshaft is turning. The video script discusses how the RPM of an engine directly affects the rotational speed of the compressor blades. Operating the engine at RPMs that are too high or too low relative to the design parameters can lead to an imbalance with the axial velocity of air, potentially causing a compressor stall.

💡Back Pressure

Back pressure is the pressure exerted in the opposite direction of the flow. In the context of the video, back pressure in the combustion chamber can rise due to abrupt throttle opening, which can reduce the axial velocity of air through the compressor. This can lead to a compressor stall as the relationship between the blade's rotational speed and the air's axial velocity becomes imbalanced.

💡Variable Inlet Guide Vanes (VIVs)

Variable Inlet Guide Vanes are adjustable airfoils located at the entrance to a compressor. The script explains that VIVs can be automatically pivoted to vary the path of the airflow entering the compressor, helping to maintain the proper relationship between the compressor's rotational speed and the velocity of the airflow. This is crucial for preventing compressor stalls, especially during changes in engine speed or during acceleration.

💡Compressor Bleeds

Compressor bleeds are mechanisms that allow excess air to escape from intermediate stages of the compressor. The script describes how bleed valves can be opened to reduce the volume of air in the compressor, which can help prevent choking and maintain optimal airflow velocities. This is particularly useful during low RPM operation or during engine acceleration, where the relationship between RPM and airflow velocity might not otherwise be maintained.

💡Multi-Spool Compressors

Multi-spool compressors are a design feature where the compressor is split into multiple sections, each driven by its own turbine. The video script explains that this design allows for greater operational flexibility and a wider range of RPMs where the compressor can operate efficiently. Each spool (or section) rotates at a speed proportional to its pressure status, which helps maintain the optimal angle of attack for the compressor blades across a broader range of engine operation.

💡Exhaust Gas Temperature (EGT)

Exhaust Gas Temperature is a measure of the temperature of the exhaust gases leaving the engine. The script mentions that an increase in EGT can be an indicator of a compressor stall, as less air is available to cool the combustion process. This can lead to a rise in EGT and a loss of engine thrust, which are critical parameters for pilots to monitor to prevent engine stalls or surges.

💡Surge

Surge is a severe condition in a gas turbine engine where the airflow through the compressor is reversed. The video script describes surge as a potential outcome of a compressor stall if not addressed, where the deterioration of airflow can lead to a complete breakdown of airflow and even an instantaneous reversal of airflow. Surge can cause significant damage to the engine and is indicated by a loud noise, a large rise in EGT, and a loss of thrust.

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