aerospace propulsion study for shenyang aerospace university by lale420 (1)

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Turbine Engine

© Devinder K Yadav

Gas Turbine Theory 1

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Gas Turbine Cycles

• Closed circuit gas turbine powerplant

• Open circuit gas turbine powerplant

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Closed circuit gas turbine powerplant

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Open circuit gas turbine powerplant

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Basic Gas Turbines Engines

The turbine engine produces thrust by increasing the

velocity of the air flowing through the engine. It

consists of:

• air inlet,

• compressor,

• combustion chambers,

• turbine section,

• exhaust section,

• accessory section.

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7



Turbine engine advantages over a piston

engine:

• less vibration

• increased aircraft performance

• reliability

• ease of operation.

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Piston Engines v Turbine Engines

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How Turbine engine works

Physics applicable to jet engines

•Newton’s Third Law of Motion

•Charles’ First Gas Law

•Charles’ Second Gas Law

•Pascal’s Law

•Bernoulli’s Theorem

•First Law of Thermodynamics

•Second Law of Thermodynamics

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For every action there is an equal and

opposite reaction

• Turbine engines are known as reaction

engine

Newton’s Third Law of Motion

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• When the pressure of a gas remains

constant, the volume of the gas will

increase as it’s temperature is increased

Charles’ First Gas Law

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Charles’ Second Gas Law

• When the volume of a gas is held

constant, the pressure of the gas will

increase as it’s temperature is increased

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Pascal’s Law

• Pressure always acts at right angles to

any confining surface, undiminished

throughout the fluid regardless of shape

and size of the container

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• The sum of all energies in a perfect fluid

must remain constant

• If kinetic energy increases then potential

energy must decrease, ie:- velocity is

inversely proportional to pressure

Bernoulli’s Theorem

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• Energy can neither be created nor

destroyed

The First Law of Thermodynamics

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• Energy will always flow from an area of

higher potential to an area of lower

potential

Second Law of Thermodynamics

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A convergent duct

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A divergent duct

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The Turbine Engine

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The Brayton Cycle

A B C D

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Pressure vs Temperature

Temperature

Pressure

Atmospheric

Pressure

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Enthalpy vs Entropy

Entropy

Usability of heat energy

Enthalpy

Total

energy of

the gas

Atmospheric

Pressure

A

B C

D

A B C D

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Temperature, pressure and velocity

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Force (F) = ma = (weight ÷ gravity) × acceleration

Thrust (T) = ma + (pressure × area)

T =

Where,

Wa - weight of air

V1 – velocity of airplane

V2 – velocity of air at jet nozzle

Wf – weight of fuel

Aj – area of jet nozzle

Pj – static pressure of jet nozzle

Pam – ambient static pressure

The Jet Engine Equation

Pam)Aj(PjVfg

WfV1)(V2

g

Wa

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• A common method of determining engine

thrust

• EPR is the ratio between the total

pressure in the exhaust duct and the total

pressure at the inlet to the engine

Engine Pressure Ratio (EPR)

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A larger EPR = more thrust

Typical EPR values (Boeing 727):

NB. EPR is only useful as a measure of

thrust on those engines with fixed area

exhaust nozzles 37

Thrust versus horsepower

• Recall: power = rate of doing work

• in other words:

– Lift up a one pound weight through 550 feet in one

second and you have 1 horsepower

• Mathematically:

– Power = Force x Distance

Time

• Propeller torque and RPM are used to

calculate horsepower

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Thrust versus horsepower • Power harder to measure in a jet engine (time

and distance elements not always involved)

• Once a jet engine is moving forward then a comparison can be made

• At an airspeed of 375 mph (325 kts), one lb of thrust = 1 HP

• THP = thrust x TAS (kts)

325

• So a B777 engine produces 90,000lbs of thrust – On take off (100kts) = 27690 HP

– During climb (300kts) = 83070 HP (assuming full power)

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Methods of Jet Propulsion

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A Ram Jet Engine

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A Pulse Jet Engine

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A Rocket Engine

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Gas Turbine Engine

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Turbojet:

Turbofan

Turbojet

Turboshaft

Gas Turbine Engine Types

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Turbojet engines Also called as the pure jet.

The compressor section passes inlet air at a high rate of

speed to the combustion chamber.

The combustion chamber contains the fuel inlet and igniters

for combustion. The expanding air drives a turbine, which is

connected by a shaft to the compressor, sustaining engine

operation.

The accelerated exhaust gases from the engine provide

thrust.

Turbojet engines are limited on range and endurance. They

are also slow to respond to throttle applications at slow

compressor speeds.

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Turboprop engines

A turboprop engine is a turbine engine that drives a

propeller through a reduction gear. The exhaust gases

drive a power turbine connected by a shaft that drives the

reduction gear assembly.

Turboprop engines are most efficient at speeds between

250 and 400 m.p.h. and altitudes between 18,000 and

30,000 feet. They also perform well at the slow airspeeds

required for takeoff and landing, and are fuel efficient. The

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Turbofan engines

Turbofan engines are designed to create additional thrust by diverting a

secondary airflow around the combustion chamber.

The turbofan bypass air generates increased thrust, cools the engine, and

aids in exhaust noise suppression. This provides turbojet-type cruise

speed and lower fuel consumption.

The inlet air that passes through a turbofan engine is usually divided into

two separate streams of air. One stream passes through the engine core,

while a second stream bypasses the engine core. It is this bypass stream

of air that is responsible for the term “bypass engine.” A turbofan’s bypass

ratio refers to the ratio of the mass airflow that passes through the fan

divided by the mass airflow that passes through the engine core.

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Turboshaft engines

It delivers power to a shaft that drives something other than

a propeller.

The biggest difference between a turbojet and turboshaft

engine is that on a turboshaft engine, most of the energy

produced by the expanding gases is used to drive a turbine

rather than produce thrust.

Many helicopters use a turboshaft gas turbine engine. In

addition, turboshaft engines are widely used as auxiliary

power units on large aircraft.

Turbojet

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Turbojet

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Turboprop

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Turboprop

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Turbofan

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Turbofan

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Turboshaft engine

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Turboshaft engine

High bypass ratio turbofan

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Low bypass ratio turbofan

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Fan Bypass Ratio

It is the ratio of airflow through the fan

duct to the airflow through the engine

core

For example, if a turbofan has a bypass

ratio of 6 to 1, 7 units of air are entering

the intake duct with 1 unit entering the

engine core and 6 units going through the

fan section only

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Thrust versus A/C speed & drag

Propulsive Efficiency

Compares the work done by the engine on the

air mass with the work done by the engine on

the aircraft.

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Thrust (force) = mass x acceleration

A turbojet gives a large acceleration to a

small mass of air

A turboprop gives a small acceleration to a

large mass of air

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Ratio of exhaust gas velocity to aircraft speed 64

• The turbofan has replaced the turbojet for

commercially operated aircraft

• For a turbojet and turbofan of the same

rated thrust the turbofan will burn less fuel

• The turbofan has less wasted kinetic

energy after exiting the exhaust (exhaust

velocity is closer to aircraft speed)


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Effect of aircraft speed on jet thrust

Thrust = M(V2 – V1)

Airspeed

Ram effect

Resultant thrust

250 kts

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Effect of engine RPM on thrust

% Engine RPM

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Effect of air temperature on thrust

Air Temperature

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Effect of air pressure on thrust

Air Pressure

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Effect of altitude on thrust

Altitude

Stratosphere

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Turbine Engine

© Devinder K Yadav


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Turbine Engine

Design and Construction

Entrance Ducts (Intake)

Compressor Section

Compressor-Diffuser Section

Combustion Section

Turbine Section

Exhaust Section 3

Turbine Engine Entrance Ducts

Properties

Must furnish a uniform supply of air to the

compressor in all conditions

Contributes to stall-free compressor

performance

Must create minimal drag

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Turbine Engine Entrance Ducts

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Gas Turbine Entrance Ducts

A divergent duct from front to back

Increased static pressure

to the compressor

Designed to be efficient at the cruise but

must still operate effectively when the

aircraft is stationary and before RAM

pressure recovery occurs

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Turbojet inlet duct

Single entrance duct

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Turbojet inlet duct

Divided entrance duct

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Turbojet inlet duct Variable geometry ducts

Divergent subsonic inlet duct

Supersonic inlet duct

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Turboprop inlets

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Turbofan engine inlets

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Inlet Guide Vanes

Direct intake duct air onto the first

compressor stage rotor at the correct

angle of attack

Both stationary and variable angle inlet

guide vanes may be used

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Inlet Guide Vanes

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Compressor Section

It’s function is to supply air in sufficient

quantity to satisfy the needs of the

combustor

Compressors operate on the principle of

acceleration of air followed by diffusion to

convert the acquired kinetic energy into a

pressure rise

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Compressor Section

A secondary purpose of the compressor

section is to supply bleed air for use by

the engine and aircraft systems

Common bleed air uses are

Cabin pressurisation

Air Conditioning

Aircraft pneumatic systems

Anti icing, inflating door seals, suction 15

Compressor Section

There are two types of compressors

Centrifugal flow

Axial flow

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Centrifugal Compressor

It consists of an impeller (rotor), a diffuser (stator) and a

manifold.

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The principal differences between the two types of impellers are

size and ducting arrangement.

The double-entry type has a smaller diameter but is usually

operated at a higher rotational speed to ensure enough airflow.

The single-entry impeller permits convenient ducting directly to

the impeller eye (inducer vanes) as opposed to the more

complicated ducting necessary to reach the rear side of the

double-entry type.

Plenum Chamber

This chamber is necessary for a double-entry compressor because

air must enter the engine at almost right angles to the engine axis.

To give a positive flow, air must surround the engine compressor at

a positive pressure before entering the compressor.

Single & double entry impellers

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Impellers

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Air enters the impeller at the hub and then

flows outward through impeller blades

The impeller imparts rotational and

outward velocity to the air which then

flows into the diffuser where divergent

ducts convert velocity into pressure

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Advantages of Centrifugal Compressor

High pressure rise (10:1)

Good efficiency over a wide

rotational speed range

Robust

Low cost

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Disadvantages of Centrifugal Compressor

Large frontal area

More than two stages is not practical

because of the energy losses between

stages

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Two stage centrifugal compressors

Single stage

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Two stage centrifugal compressor

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Most common in rotorcraft and

turboprop aircraft because of their

robustness – more reliable on gravel

runways

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Axial Compressor

The airflow and compression occur parallel

to the rotational axis of the compressor

Air Flow

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Axial Compressor

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Axial Compressor

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Axial Compressor

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Axial Compressor

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Axial Compressor

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Axial Compressor

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Axial Compressor

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Variable stator

vanes operation:

They are

operated by fuel

pressure and

scheduling is

done by main

engine control

(fuel control

unit).

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Axial Compressor

The air flows axially through a number of

rotating rotor blades and fixed

intervening stator vanes

Each set of rotating blades and stator vanes

is known as a compressor stage

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Vector Diagram – complete engine

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Axial compressor roots and tips

Vibration is a problem with any rotational

machinery

The root of the compressor disk is often

only loosely fitted to the hub

As the compressor rotates centrifugal

loading locks the blade in its correct position

and the air stream over the airfoil provides a

shock mounting or cushioning effect 45

To avoid energy losses (including shock

waves) over the tips of the rotor blades,

the clearance between the rotor and the

surrounding shroud must be kept to a

minimum

Newer engines are designed to rotate within

a shroud strip of abradable material

Sometimes during coastdown a high

pitched noise can be heard if the blade

tip and shroud strip are touching 46

Advantages of Axial Flow Compressors

Higher compression available by

addition of compression stages

Small frontal area and lower drag

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Disadvantages of Axial Compressors

High cost of manufacture

Relatively high weight

Higher starting power requirements

Lower pressure rise per stage

Good compression in the cruise and

take off power settings only 48

Combination Compressors

Popular in many small turbine engines

(Pratt and Whitney PT 6) 49

Axial Compressor

There are three designs of axial flow

compressors

Single spool

Double spool

Triple spool

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Axial Compressor

(N1)

(N2)

Spools are not mechanically linked together 51

Multi Spool Compressors

For any given power setting the high

pressure compressor speed is held

constant by the fuel control unit

The low pressure compressor(s) will speed

up and slow down with changes in engine

inlet conditions resulting from atmospheric

changes

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Trent 900 triple spool compressor

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Advantages of multi-spool axial

compressors

Less power required for starting

Less prone to compressor stalling

Quicker acceleration

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Compressor Stall & Surge

Compressor blades, being aerofoils, can

stall at too high an angle of attack

the close proximity of blades in different

stages means that if one stage stalls, so

may the next

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Angle of Attack and compressor stall

Compressor stalls cause air flowing

through the compressor to slow down,

stagnate or reverse direction

this is then know as an engine surge

Any change to the design airflow will have

an effect to all other sections of the gas

turbine engine

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Causes

Excessive fuel flow changes

Turbulent air

Contaminated or damaged compressors

Contaminated or damaged turbine blades

Engine operation outside design RPM

Too rapid movement of throttles 60


Can occur during a cross wind take-off

Can occur during a steep turn

Detected by

Audible noise and/or vibration

Fluctuating RPM

Increased EGT 61


Reverse air flow may result in the compressor

blades bending and contacting the stator

vanes

Sophisticated engines use:

bleed air to reduce the possibility of

compressor stall, or

variable incidence guide vanes 62

Turbine Engine

© Devinder K Yadav

1


2

The Combustion Section

The combustion process must ideally be

able to efficiently convert chemical energy

to heat energy under all operating

situations from engine start to engine shut

down

A chemically correct (stoichiometric)

mixture is approximately 15:1 air/fuel

3

The Combustion Process

The temperature of the gases released by

combustion can be well in excess of 15000C

which will destroy the combustion chamber

and turbine section

About 60% of the air entering the

combustion chamber is used for cooling only

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5


To function properly the combustion

chamber must

1. Provide a proper environment for the

mix of air and fuel

2. Cool the hot gases to a temperature

the turbine section components can

withstand

To accomplish this the airflow through

the combustor is divided into primary

and secondary paths 7


Air from the compressor may enter

the combustion chamber in excess of

500 feet per second (300 knots)

The axial flow of the primary airflow

must be reduced to about 5 feet per

second (3 knots)

Because of the slow flame propagation

rate of jet fuels if the primary velocity

were too high it would blow the flame

out (flame out) 8


The reduction in axial velocity is achieved

by swirl vanes which create radial motion

and retard axial motion

The air from the swirl vanes and secondary

air holes interact and create a region of low

velocity circulation

This forms a toroidal vortex similar to a

smoke ring stabilising and anchoring the

flame 9


10


11


The combustion process is complete in

the first one third of the combustion

liner

In the remaining two thirds of the

combustor length the combusted and

uncombusted gas is mixed to provide an

even heat distribution at the turbine

nozzle 12

Flame Out

Although uncommon in modern engines

they still occur

Some common causes are

Turbulent weather

High altitude

Violent flight maneuvers

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** Be careful when quoting air/fuel ratios**

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Flame Out

Flame out (lean) Usually occurs at low

fuel pressures at low engine speeds in

high altitude flight

Flame out (rich) Usually occurs during

fast engine acceleration in which an over

rich mixture causes combustion pressure

to increase until compressor flow stagnates

Turbulent inlet conditions can also cause stalls 15

Flame Out

To minimise the possibility of flame out it

is essential to have a correct matching of

compression ratio, mass airflow and engine

speed

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Combustion Chamber Types

The various combustion chambers in

use include

Multiple can

Can Annular

Annular reverse flow

Annular

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Multiple Can

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Multiple Can

This type of combustion chamber is more

common with centrifugal flow

compressors and earlier types of axial

flow compressors

The separate flame tubes are

interconnected to allow a constant

pressure and also propagate

combustion around the flame tubes

during starting 19

Can Annular

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Annular

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Annular Reverse Flow

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Annular Reverse Flow

Common in turboprop engines as this

arrangement provides shorter engine

length and also a weight reduction

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Garrett TPE 331 Reverse Flow

Combustion Chamber

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Fuel Supply

Fuel is supplied to the combustion chamber

by one of two methods

The most common is the injection of a

fine atomised spray into the re-circulating

airstream through spray nozzles

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Fuel Supply

The second fuel supply method is based on

the pre-vaporisation of the fuel before it

enters the combustion zone

The fuel/air mix is carried in a vaporising

tube which passes through the primary

flame area of the combustion chamber

More common in low power engines

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The Turbine Section

The turbine section is bolted onto the

combuster and contains nozzle guide vanes,

turbine rotors and turbine stators

The turbine functions to transform a

portion of the kinetic and heat energy in

the exhaust gases into mechanical work to

drive the compressor, propeller, fan and

accessories 28

The turbine section

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The turbine section

Turbine Stator Turbine Rotor 30

The turbine section

• Since energy is extracted from the airflow through a turbine section, pressure decreases across the turbine section

• Hence the boundary layer is much more likely to remain attached than in the compressor section

• Each stage of the turbine section can support several stages of compressor

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The turbine section

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The turbine

section

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The turbine

section

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The turbine

section

35

The turbine

section

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The turbine

section

37

The turbine

section

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The turbine

section

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The turbine

section

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Turbine Blades

Turbine blades extract energy from the

gas stream in two ways

Reaction

Impulse

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Reaction turbine blades

Reaction drives the blades via an

aerodynamic reaction

The gas stream is accelerated by

convergent nozzle guide vanes and directed

to flow over the turbine blades producing

an aerodynamic reaction

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Impulse turbine blades

Impulse turbine blades rotate via impact

of high velocity gas on the blades

The blades of a pure impulse turbine are

bucket shaped to maximise the conversion

of kinetic energy to mechanical energy

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Turbine Blades

Most turbine blades combine both

impulse and reaction principles

The degree of reaction depends on the

type of engine

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Turbine Blades

Turbojets require high exhaust velocities

to produce thrust so they use high reaction

turbine blades to produce maximum

acceleration

Turboprops and APUs use impulse turbine

blades because they are concerned with

power extraction and not thrust

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Turbine Blades

Turbofans use reaction/impulse blades to

extract energy to drive the fan while

maintaining reasonably high exhaust

velocity for core engine thrust

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Turbine Blades

• Higher entry

pressure at the blade

tips means that, to

create a uniform

exit flow, blade

profiles are adjusted

to a reaction profile

at the tip

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Turbine Blade Creep

Turbine blades are subject to enormous

stress loads

A blade weighing only 8 grams may

have to resist a centrifugal force of over

2000 kg

This causes turbine blades to lengthen with

continued use – known as creep 52

Turbine Blade Creep

If manufacturer’s temperature or rpm

limits are exceeded the creep rate increases

and blade life is drastically reduced

Overhauls are timed to ensure that blades

are replaced before tertiary creep begins

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Turbine Temperature Measurement

Ideally temperature probes should be placed

in the turbine inlet to measure turbine inlet

temperature (TIT)

The temperature at the turbine inlet is

usually too hot to place temperature probes

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Turbine Temperature Measurement

Temperature probes are usually placed in an

intermediate stage (ITT) or at the turbine

outlet stage (TOT)

ITT and TOT readings are often

compensated to give an indication of the

temperature at the most critical point – the

turbine inlet

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Turbine blade cooling

• Cooled by internal air cooling system

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Exhaust Section

The exhaust section is located behind the

turbine section and usually consists of a

convergent cone to convert pressure

energy to kinetic energy

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Exhausts

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Exhausts

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Exhausts

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Engine Exhausts

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Exhausts

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Engine exhausts

• Convergent exhaust duct

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Exhausts

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Exhausts

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Exhausts

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Exhausts

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Exhausts

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Exhausts

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Accessory Section

The primary function is to provide space for the

mounting of accessories necessary for operation

and control of the engine. It also includes

accessories concerned with the aircraft, such as

electric generators and fluid power pumps.

Secondary functions include acting as an oil

reservoir and/or oil sump, and housing the

accessory drive gears and reduction gears.

Accessories are usually mounted on common

pads either ahead of or adjacent to the

compressor section. 72

Accessory

Section

73

Accessory

Section

Accessory

Section

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Accessory

Section

75

Accessory

Section

76

Accessory

Section

77

Accessory

Section

78

Accessory Section

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Auxiliary Power Units

80


A gas turbine powerplant

Supplies the aircraft with

Bleed air

Electrical power

Hydraulic power

81


Used mainly during ground operations,

take-off and landing

Most can be used in flight as a back up

supply source but usually have an

operating altitude limit

82


APUs have the following features:

Operate at a constant RPM

Start sequence is fully automatic

Vital parameters are automatically

monitored

Automatic shutdown with any faults 83


A typical cockpit panel consists of:

Start and stop button

Turbine temperature indicator (EGT)

RPM indicator

Control switches for bleed air,

hydraulic and electrical generation 84

Many turbofan engines have two or more spools to

A. improve the cooling of the combustion chamber

walls resulting in a lower turbine temperature

B. assist the compressor sections to rotate closer to

their ideal RPM

C. reduce vibration within the engine core

D. increase spool up time required when compared

to a single spool

And….. the answer is……….. 86

An ideal jet intake delivers air to the compressor

in which state?

A. No turbulence and pressure lower than ambient

B. Increased radial velocity and temperature higher

than ambient

C. Increased temperature and velocity compared

ambient conditions

D. No turbulence and pressure higher than ambient


The row of stator blades after each row of

compressor blades in n axial flow compressor is

designed to

A. longitudinally balance the engine

B. convert axial flow to radial flow before the next

rotating compressor section

C. convert kinetic energy to pressure energy

D. Convert pressure energy to pressure energy


Of a turbofan’s total air passing through the

intake 21% goes through the engine core. The

bypass ratio is closest to

A. 4:1

B. 5:1

C. 1:5

D. 1:4


Which of the following would increase the

maximum possible performance of a jet engine?

A. introduce the air into the engine at a lower speed

B. introduce the air into the engine at a lower

temperature

C. introduce the air into the engine at a higher

temperature

D. introduce the air into the engine at a lower

pressure


Gas decreases in velocity and increases in pressure

when

A. flowing through a convergent duct

B. it is within the last two thirds of the combustion

chamber

C. it is within the nozzle guide vanes prior to the first

turbine rotor section

D. flowing through a divergent duct


What is meant by tertiary creep in a turbine blade

of a gas turbine engine?

A. blade creep experienced on the test bench by the

manufacturer

B. normal blade creep during the acceptable working

life of the turbine blade section

C. blade creep that could be detrimental to

continued use of the turbine section

D. an unruly university aviation student


For a given engine RPM, thrust output from a gas

turbine engine will be greatest

A. At MSL in ISA conditions

B. At high altitude in ISA conditions

C. At high altitude in ISA + conditions

D. At MSL in ISA + conditions


aerospace propulsion study for shenyang aerospace university by lale420 (1)

Engineering