jet engine basics.pdf

8/13/2019 Jet Engine Basics.pdf

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Jet Engine BasicsA presentation by M. Ali Shahzad


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What is it?O A jet engine is defined as a machine

designed for the purpose of creating large

volumes of exhaust gases at high velocityO Thus is required for creating thrust needed

to overcome the drag of an airplane


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Power and Energy typesO The jet engine also produces the following

(through Auxiliary Power Unit)

O Electrical powerO Hydraulic power

O Pneumatic power for pressurization

O Hot air for anti-icing protection in components


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Auxiliary Power UnitO The function of APU is to provide power for

starting the main engine of an aircraft

O Smaller engines can be started from electricmotors alone

O APU itself is first started by batter or a

hydraulic accumulator

O They are used for running other electricloads during shutdown of engine


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APU


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Operation CycleO Operation cycle of jet engine is similar to the

automobile engine cycle

O Following processes take placeO Air intake and compression

O Addition of fuel and ignition

O Exhaust gas spins turbine

O Turbine powers the compressorO Gas is allowed to exit at the tailpipe


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StructureO Original design by Sir Frank Whittle in

1930s.


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Single-spool TurbojetO The term single-spool means that there is

only one shaft that connects the turbine

section to the compressor section


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Axial Flow EnginesO Axial flow means the compression is done in

axial axis of the engine

O Primitive designs were based on centrifugalcompression of mixture

O All new jet engines are based on axial design


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Thermodynamic CycleO It is based on the 4-stroke engine as shown

schematically


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Twin Spool ConceptO Introduction of hollow shaft over primary

shaft to create two stages

O Each stage consists of a compressor-turbinepair

O The two stages may run at independent

speeds

O Benefit of accurate matching of turbine andcompressor with airflow characteristics


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N1 and N2 RotorsO First stage of compressor consists low

speed rotor that rotates with second stage

turbine rotor (N1

rotor)O Second stage of compressor consists of high

speed rotor that rotates with first stage

turbine rotor (N2rotor)


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Twin Spool Engine


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TurbofanO A portion of thrust is developed by a fan,

which is multi-bladed propeller attached to

N1

turbineO Much of the air accelerated by the fan does

not pass through the engine

O The fan accelerates a large amount of air at

small acceleration (rather than a smallamount of air at large acceleration


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Bypass RatioO The bypass ratio is defined as the ratio of air

which exits the engine (without going though

the core) to the amount of air which goesthrough the engine core.

O Both components contribute to thrust

O Engines are also classified on the basis of

high and low bypass ratios


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Quantitative discussionO Early turbofan engines had low bypass ratio

(approximately ) i.e half of the thrust was

produced by fan stage and half by primaryflow

O Most commercial aircrafts have high bypass

ratio

O

Current bypass ratios are 5:1. for theseengines, the fan stage produces 75-80%

thrust


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Low vs. High Bypass Ratio


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Triple Spool TurbofanO Triple spool turbofans incorporate three

independent rotors

O These are designed to achieve better fueleconomy since the triple spool can better

match the compressor and turbines with

actual airflow characteristics


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Triple Spool Engine


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Propulsive EfficiencyO This is expressed in terms of inlet velocity of

air and outlet velocity of exhaust gases


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(Contd.)O 100% efficiency would be achieved if inlet

velocity equals outlet velocity, but the required

mass flow through the engine would be infinite

O This does not exist in the real world, but this

indicates that large amount of air gives greater

efficiency (at smaller accelerations) than small

amount of air (at greater accelerations)

O Thus large bypass ratio are required to handlethe accelerations of large amounts of air


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Thrust EquationO According to newtons 3rdlaw of motion, the

action of jet engine is the acceleration of

gas and sending it through the tailpipe.

O The reaction is called thrust


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(Contd.)O In context of jet engines, this form of the

equation is popular:


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Terms in thrust equationO Net thrust is the total usable thrust of the engine

O Gross thrust is the thrust produced at theexhaust of the engine

O This is reduced by the second term called ramdrag

O Ram drag is produced due to slowing down offree stream velocity as it enters the jet engine

O Net force is thus a function of mass flow rates ofair and fuel and the exhaust velocity minusintake velocity


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Air density affecting thrustO Air density is a function of temperature,

pressure and altitude

O It has a significant effect on thrust


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Velocity affecting thrustO Airspeed or velocity has a direct effect on

momentum and pressure of air entering atthe intake

O Effect : By increasing airspeed, momentumof incoming air is increased and thrust islowered (because of smaller velocitydifference).

O Effect 2: By increasing airspeed, the air iscompressed easily by ram effect whichincreases density and thus increases thrust


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(Contd.)O Combining the given effects produces the

following result


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Other factors affecting thrustO Bleed air extraction

O Additional power extraction by hydraulic

pumps, electric generators, fuel pumps etc.O Humidity has a negligible effect on thrust so

its not a problem!


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Installed thrustO The installed thrust is always smaller than

test stand thrust because:

O

Test stand engine uses bell mouth air inlethaving different airflow characteristics than

actual airflow

O Test stand engine does not have to supply

secondary power for electric, hydraulic and

pneumatic systems like an installed engine


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Engine stations designationO For referencing the locations in the engines,

different designations are used which are

called engine stations


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Example 2


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Engine Pressure RatioO It is the ratio of total pressure at the exhaust

to total pressure at the front of the fan

O

It is the basic measure of engine thrustO Used as primary thrust setting in most

commercial engines


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N1and %N1O This is the rotation rate (rpm) of low speed

rotor of a two or three spool engine

O

This is expressed as a fraction against somenominal value in the form of percentage %N1

O General electric and CFMI engines use %N1

as the primary thrust setting


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Corrected %N1O Temperature ratio is defined as the ratio of

exhaust temperature to inlet temperature

O Corrected %N1 is defined as

O The value of x is dependent on engine type

O For three spool engine, N2 and N3 represents

the medium and high rotor speeds.O For a two spool engine, N2 simply represents

high rotor speed


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Types of Engine StallO Two types of engine stall:

O Rotational stall

O

Compressor surge (axisymmetric stall)

Rotational stall can lead to compressor surge in

worst case scenario


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Rotational StallO This is caused by local disruption in the

airflow thus reduces efficiency ofcompressor

O Pockets of stagnant air (called stall cells)rotate along the circumference of the bladeat full radius (50-70% speed of compressor)

O Increases structural load on the airfoil

O When radius of stall cell decreases to coverthe full frontal area, it is called compressor

stall


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Compressor SurgeO Total breakdown of compressor is

compressor surge

O

A reciprocating compressor stall may correctitself to normal airflow

O Surge is a self-locking or self reproducing

phenomena in which the pressure

differences increases beyond recovery and

results in destruction of engine


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Actual Compressor Map


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FlameoutO This is a condition in which the combustion

chamber of the engine lose their ignition

O

Caused by fuel exhaustion, oxygenexhaustion, compressor stall, foreign object

or mechanical failure

O Engine recovery is performed by following a

restart procedure such as a windmill restart

O This is prevented by fine tuning the engine

performance using feedback and avionics


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Example


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BleedO This is the process of extracting

compressed, high temp. air from thecompression engine for miscellaneous uses

O The compressed air can be used for airconditioning, cabin pressurization, de-icingand for pneumatic actuation

O The bleed may also be used for rectifying

surgeO Bleed valves are closed inspooled engines

and opened in unspooled engines


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(Contd.)O Bleed air is hot and must be cooled before

use

O

Used extensively in Environmental ControlSystemO This is a dangerous system since

contaminated air from engine is being used

in other systems, so separated electric

driven compressors are preferred


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(Contd.)O Prevention of over pressurization in

compression section (safety feature)


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Exhaust Gas TemperatureO This the temperature at the engine exhaust

O This is a primary measure of engine healthand is connected with EPR

O At full power EPR there will be maximumEGT

O Engine requires maintenance atEGT limit

O EGT below EGT limit is called EGT margin

O EGT margin is greatest when the engine isnew and decreases with usage of engine

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