GAS TURBINE ENGINE-2

Assignment No:2Assignment Title:Function and operation of gas turbine engines Unit No:83 Unit Title: Aircraft Gas Turbine Engines

TASK: 01 (P4)

1.1

TURBOPROP ENGINE: A turboprop engine is a type of aircraft power

plant which uses a gas turbine to drive a propeller. Turboprop engines

also belong to the group torque turbine engine. Speed of the shaft of

this engine is very high. In this engine a reduction gearbox is

required between the engine shaft and propeller for reducing

invariable requires speed.

The engine's exhaust gases contain little energy and play a minor role

in the propulsion of the aircraft. A turboprop engine can be two types

of engine. One is single spool engine and other is twin spool engine.

Advantage of turboprop engine:

At lower altitude turboprop engine is more efficient than

turbojet engine.

Turboprop engine can give high thrust power with respect of fuel

consumption.

It creates lees noise than turbo jet engine.

Propeller of this engine can rotate in different angles.

It can create more thrust.

When aircraft is on take of position in maximum angle then it can

give maximum thrust.

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This type of engine aircraft takes short field for takeoff and

landing.

Disadvantage of turboprop engine:

Less thrust then turbojet engines.

Providing low speed

Turboprop engines aircraft are flying at lower altitude

efficiently

Used in short distance aircraft

Turbojet engines: As a pure jet called turbojet engine. Turbojets

consist of an air inlet, an air compressor, a combustion chamber,

turbine, exhaust nozzles.

Advantage of turbojet engine:

Turbojets are more efficient then turboprop at higher altitude

It is situated for subsonic and supersonic.

Less time consuming top fly any where

Disadvantage of turbojet engine:

Fuel consumption is very high

Creating noise pollution



Inefficient at the lower altitude

Maintenance cost at higher rate

1.2

APU: It stands for Auxiliary power unit. When an aircraft is on

ground then it is called ground power unit. And in flying condition it

is called airborne Auxiliary power unit.

Construction:

This consists of compressor single entry centrifugal compressor or

axial flow compressor followed by centrifugal compressor, reverse flow

combustion chamber, turbine and exhaust.

Operation:

Air goes into the compressor through intake. The impeller is rotated

at higher speed by the turbine and air flows into the combustion

chamber. Therefore combustion chamber is reversed type. In the

combustion chamber fuel is mixing and combustion takes place. After

combustion the gas flows at higher speed that rotate the turbine. Of

course APU is not provided thrust but it drives some different

accessories.

Installation of APU:

APU is located at the tail section by means of nuts and bolt. There

are some struits to attach with APU.



TASK: 02 (P5)

Various type of air intake used in gas turbine engines now a day. Some

of them are:

Pitot type intake

Side scoop intake

Wing root inlet

Air flow through a Pitot type intake is described below:

This type of intake has divergent duct shaped interior to receive

ram air at the entry and convert to ram pressure as maximum as

possible before delivery to the compressor , which means the intake

increase the pressure and describe the velocity of incoming air.

This type of intake is most suitable to pod installed engine under

the wing and also fuselages side mounted engines.


http://www.cs.mcgill.ca/~rwest/wikispeedia/wpcd/images/74/7434.gif.htm


TASK: 03 (P6)

Centrifugal flow compressor: The centrifugal flow compressor is a

single or two stage units employing an impeller to accelerate the air

and a diffuser to produce the required pressure rise.

It is consist of three main components:

(a) Impeller and the casing

(b) Diffuser

(c) Manifold



FIG: Centrifugal compressor

Construction: The construction of the compressor centers on the

impeller, diffuser and air intake system. The impeller shaft rotates

in ball and roller bearings and is either common to the

turbine shaft or split in the centre or connected by a coupling, which

is usually designed for ease of detachment.

Impellers: The impeller consists of a .forged, disc with integral,

radically disposed vanes on one or both sides forming convergentPage | 6 ZIAD IBNE ANISH ID#ACB-09-04-20

Assignment No:2Assignment Title:Function and operation of gas turbine engines Unit No:83 Unit Title: Aircraft Gas Turbine Engines passages in conjunction with the compressor casing. The vanes may be

swept back, but for ease of manufacture straight radial vanes are

usually employed.

Diffuser: The diffuser assembly may be an integral part of the

compressor casing or a separately attached assembly. In each instance

it consists of a number of vanes formed tangential to the impeller.

The vane passages are divergent to convert the kinetic energy into

pressure energy and the inner edges of the vanes are in line with the

direction of the resultant airflow from the impeller. The clearance

between the impeller and the diffuser is an important factor, as too

small a clearance will set up aerodynamic buffeting impulses that

could be transferred to the impeller and create an unsteady airflow

and vibration.

Operation: The impeller is rotated at high speed by the turbine and

air is continuously induced into the centre of the impeller.

Centrifugal action causes it to flow radically outwards along the

vanes to the impeller tip, thus accelerating the air and also causing

a rise in pressure to occur. The air, on leaving the impeller, passes

into the diffuser section where the passages form divergent nozzles

that convert most of the kinetic energy into pressure energy. To

maintain the efficiency of the compressor, it is necessary to prevent

excessive air leakage between the impeller and the casing; this is

achieved by keeping their clearances as small as possible.


Assignment No:2Assignment Title:Function and operation of gas turbine engines Unit No:83 Unit Title: Aircraft Gas Turbine Engines Axial flow compressor: This type of compressor has one or more rotor

assemblies, which carry blades of airfoil section. The compressor is a

multi-stage unit as the amount of pressure increase by each stage is

small; a stage consists of a row of rotating blades followed by a row

of stator vanes.

Construction: The construction of the compressor centers on the rotor

assembly and casings. The rotor shaft is supported in ball and roller

bearings and coupled to the turbine shaft in a manner that allows for

any slight variation of alignment.

FIG: Axial flow compressor

The cylindrical casing assembly may consist of a number of cylindrical

casings with a bolted axial joint between each stage or the casing may

be in two halves with a bolted centre line joint.



Fig. Rotors of drum and disc construction

Rotors: In compressor designs (fig. 3-10) the rotational speed is such

that a disc is required to support the centrifugal blade load. The

rotor assembly may be of a drum or disc-type construction, or a

combination of both types.

The drum-type rotor consists of a one or two-piece forging on to which

are secured the rotor blades. The disc type rotor has the rotor blades

attached to separate discs, which are then splinted to the rotor shaft

and separated by integral or-individual spacer rings.

Rotor blades: The rotor blades are of airfoil section and usually

designed to give a pressure gradient along their length to ensure that


Assignment No:2Assignment Title:Function and operation of gas turbine engines Unit No:83 Unit Title: Aircraft Gas Turbine Engines the air maintains a reasonably uniform axial velocity. 'Twist' the

blade from root to tip to give the correct angle of attack at each

point. The length of the blades varies from front to rear; the frontal

& low pressure blades are longer then the rare.

Stator blades: The stator vanes are again of airfoil section and are

secured into the compressor casing or into stator vane retaining

rings, which are themselves secured to the casing. The blades are

often mounted in packs in the front stages and may be shrouded at

their tips to minimize the vibration effect of flow variations on the

longer blades. It is also necessary to lock the stator blades in such

a manner that they will not rotate around the casing.

Operation: When the compressor starts rolling at high speed by the

turbine, air is continuously induced into the compressor, which is

then accelerated by the rotating blades and swept rearwards onto the

adjacent row of stator vanes. So, the pressure is increased in every

stage. The pressure ratio of outgoing air & inlet air is between 1:1

and 1:2. Though the pressure ratio of each stage is small but it

increases too much at the end of last stage because it has multistage

compressor. Where high pressure ratios are required from a single

compressor this problem can be overcome by introducing variable stator

vanes in the front stages of the system.

TASK: 04(P7)


Assignment No:2Assignment Title:Function and operation of gas turbine engines Unit No:83 Unit Title: Aircraft Gas Turbine Engines Multiple can: This type of combustion chamber is used on centrifugal

compressor engines and the earlier types of axial flow compressor

engines. The chambers are disposed around the engine. And compressor

delivery air is directed by ducts to pass into the individual

chambers. Each chamber has an inner flame tube around which there is

an air casing. The separate flame tubes are all interconnected.

FIG: Multi can type combustion chamber

Can annular: The can-annular combustion chamber bridges the

evolutionary gap between the multiple and annular types. A number of

flame tubes are fitted inside a common air casing. This arrangement

combines the ease of overhaul and testing of the multiple systems with

the compactness of the annular system.



Annular: This type of combustion chamber consists of a single flame

tube, completely annular in form, which is contained in an inner and

outer casing. The main advantage of the annular chamber is that, for

the same power output, the length of the chamber is only 75 per cent

of that

of a tubo-annular system of the same diameter, resulting in

considerable saving of weight and production cost.



FIG: Annular type combustion chamber

4.1

use of individual components of the combustion chamber:

Case:

Diffuser

Liner

Dome and Swirler

Fuel injector:

4.2

Advantages Page | 13 ZIAD IBNE ANISH ID#ACB-09-04-20

Assignment No:2Assignment Title:Function and operation of gas turbine engines Unit No:83 Unit Title: Aircraft Gas Turbine Engines Multiple can type:

In multiple can combustion chambers, chambers are disposed around

the engine and compressor delivery air is directed by ducts to

pass into the individual chambers.

The separate flame tubes are all interconnected. This allows each

tube to operate at the same pressure and also allows combustion

to propagate around the flame tubes during engine starting.

Can annular type:

The turbo-annular combustion chamber bridges the evolutionary gap

between the multiple and annular types.

A number of flame tubes are fitted inside a common air casing.

This arrangement combines the ease of overhaul and testing of the

multiple systems with the compactness of the annular system.

Annular type:

This type of combustion chamber consists of a single flame tube,

completely annular in form, which is contained in an inner and

outer casing.

For the same power output, the length of the chamber is only 75

per cent of that of a turbo-annular system of the same diameter,

resulting in considerable saving of weight and production cost.

The elimination of combustion propagation problems from chamber

to chamber.



TASK: 05 (P8)

Impulse turbine:

In the stator –nozzles; the entire pressure drops. Pressure in the

rotor is same thought the fluid density is remaining constants

approximately.



Operation:

From figure we can see that The position of the nozzle moving blades,

stationary blades and the second row of moving blades in a Curtis

stage of an impulse turbine. As the impulse turbine is built in single

stages so it has two or more simple turbines on the same shaft. The

blades exert a corresponding force on the fluid to change its momentum

and the fluid exerts a corresponding reactive force on the blades.

This reactive force acts about the axis of rotation to form a torque

and rotors turns continuously.



So the steam is expanded in the nozzle which gives up part of its

energy in pushing the row of blades (A), is redirected in the second

row of moving blades by the stationary row of blades (B). The steam

gives up more of its velocity in the row of moving blades (C). The

stationary row of blades is used to reverse the direction of the steam

flow.

Reaction turbine:

Here the nozzle guide vanes have parallel air passes. This air is

served directly to the rotor blades. And the desired angle of the air

passage of the nozzle of a reaction turbine is slightly convergent,

so it increases in its exit velocity.



Operation:

In reaction turbine from the convergent duct the gas is passage and

accelerates for driving turbine. The area of the rotor passages

decreases from entrance to exit. So the pressure drops and the

velocity increase in the blades. In the other ways the area of stator

passages is approximately constant. As a result force is generated.

This force is trans- mitted to the blades. So the momentum of the

fluid is changed by a force exerted on the blades.

Impulse/ reaction turbine:


Assignment No:2Assignment Title:Function and operation of gas turbine engines Unit No:83 Unit Title: Aircraft Gas Turbine Engines As certain amount of force is always present here so it creates torque

by reacting impulse of gas. After having reaction force on the blades,

then a resultant force is formed. This acts in the plane of rotating

to drive the drive the turbine.

Operation: In the reaction turbine as the gases are passages through

the exhaust nozzle with high acceleration so it discharges the

working. The acceleration of the hot gas is leaving the exhaust

nozzle. So it produces a reaction force. Then the rotor moves the

opposite direction to that gas.

5.1 Turbine creeps and creep failure:

Turbine creeps:Page | 19 ZIAD IBNE ANISH ID#ACB-09-04-20

Assignment No:2Assignment Title:Function and operation of gas turbine engines Unit No:83 Unit Title: Aircraft Gas Turbine Engines Turbine blades increase in length and also in disk diameter as they

are under continues stress in yield point.

Turbine creep depends on three factors RPM, mass of blade and gas

temperature. As mass of blade is constant any change in RPM and/or gas

temperature will change the characteristics of the blade.

Creep failure:

If plastic deformation results about 0.2% then it is said to have been

failed in creep. A curve on creep strain vs. time shows that as more

the turbine blade operates for longer period of time the closer the

time for fracture comes nearer. To make it short the creep failure

will occur after extensive hours of use.

5.2 Film cooling of turbine blades


Assignment No:2Assignment Title:Function and operation of gas turbine engines Unit No:83 Unit Title: Aircraft Gas Turbine Engines After long experiment the designer found that the combustor exit

temperature more will be the thrust. But this exceeding temperature

damages high pressure turbine components specially the turbine blades.

FIG: Film cooling of turbine blades

Turbine blades melt in this exceeding temperature. To prevent this

from happening film cooling was incorporated into the blades. There

are some small holes in the turbine blades through which cool air

comes from the compressor stage. The cool air makes a thin, cool,

insulating blanket of air on the surface of blades which prevent

blades to melt in the excess operating heat.

TASK: 06 (P9)

Exhaust system of turbojet engine:

The exhaust system of gas turbine engines passes the turbine

discharge gases to the atmosphere. The turbojet engine produces

thrust mainly from exhaust jet. Exhaust gases passes to


Assignment No:2Assignment Title:Function and operation of gas turbine engines Unit No:83 Unit Title: Aircraft Gas Turbine Engines atmosphere through propelling nozzle which is a convergent

passage thus increase the jet velocity. At low thrust

conditions the exits velocity is subsonic only but during most

operating conditions the exits velocity reaches speed of sound

in relation to the exhaust gas temperature. At this condition

propelling nozzle is said to be chocked i.e. velocity cannot be

increase unless temperature get increase. Pressure difference

across the exhaust nozzle area gives pressure thrust. This is

additional thrust which is the result of momentum changed of

the gas stream. The velocity and pressure of the exhaust gases

create the thrust in the turbo-jet engine. The design of the

exhaust system therefore, exerts a considerable influence on

the performance of the engine. To reduce these losses, the

turbine rear struts in the exhaust unit are designed to

straighten out the flow before the gases pass into the jet

pipe.

Gas flow through mixed by pass engine:

The by-pass engine mixer unit mixes the hot and cold gas

streams. The operation of mixed by pass engine is to mixing the

hot and cold air stream before they are discharged into the

atmosphere and reduce the noice.In low by-pass ratio engine,

mixer unit thoroughly mixes the two gas flows into the turbine

exhaust gas flow However, an improvement can be made If two gas

flow flows within a common or integrated nozzle assembly. This

partially mixes the gas flows prior to ejection to atmosphere.


Assignment No:2Assignment Title:Function and operation of gas turbine engines Unit No:83 Unit Title: Aircraft Gas Turbine Engines TASK: 07 (M2)

After Burner:

The system used to inject the fuel in the thrust jet is called

afterburner. Afterburners are generally used to provide additional

power to reaction engines, commonly known as jet engines, whenever it

is necessary. Reaction engines, which are some sort of turbine

engines, work by compressing atmospheric air gathered through the air

intake of the engine and using it to create a combustible mix along

with the fuel, which is then lit. The rapid expansion of the

combustible mix generates thrust and power for the turbine at the same

time, which in turn drives the compressor.

Figure: - Afterburner System

Operation of after burner:Page | 23 ZIAD IBNE ANISH ID#ACB-09-04-20

Assignment No:2Assignment Title:Function and operation of gas turbine engines Unit No:83 Unit Title: Aircraft Gas Turbine Engines Afterburning (or reheat) is a method of augmenting the basic thrust of

an engine to improve the aircraft take-off, climb and (for military

aircraft) combat performance. The increased power could be obtained by

the use of a larger engine, but as this would increase the weight,

frontal area and overall fuel consumption, afterburning provides the

best method of thrust augmentation for short periods.

Figure: - Principle of Afterburning

Afterburning consists of the introduction and burning of fuel between

the engine turbine and the jet pipe propelling nozzle, utilizing the

unburned oxygen in the exhaust gas to support combustion. The

resultant increase in the temperature of the exhaust gas gives an

increased velocity of the jet leaving the propelling nozzle and

therefore increases the engine thrust.

Water-Methanol systems:

The maximum power output of a gas turbine engine depends to a large

extent upon the density or weight of the airflow passing through the

engine. There is, therefore, a reduction in thrust or shaft horsepowerPage | 24 ZIAD IBNE ANISH ID#ACB-09-04-20

Assignment No:2Assignment Title:Function and operation of gas turbine engines Unit No:83 Unit Title: Aircraft Gas Turbine Engines as the atmospheric pressure decreases with altitude, and/or the

ambient air temperature increases.

Under these conditions, the power output can be restored or, in some

instances, boosted for take-off by cooling the airflow with water or

water/methanol mixture (coolant). When methanol is added to the water

it gives anti-freezing properties and also provides an additional

source of fuel. There are two basic methods of injecting the coolant

into the airflow. Some engines have the coolant sprayed directly into

the compressor inlet, but the injection of coolant into the

combustion chamber inlet is usually more suitable for axial flow

compressor engines. This is because a more even distribution can be

obtained and a greater quantity of coolant can be satisfactorily

injected.



Figure: - Turbo jet thrust restoration

How thrust augmentation affects engine performance:

The sole purpose of thrust augmentation methods is to increase engine

performance. Two methods of thrust augmentation are mentioned above.

Afterburner and Water injection method. Afterburner burns unborn

oxygen after they have left the turbine, this increases engine

performance because even though it uses more fuel it, greatly increase

the thrust produced by the engine. Water injection also utilizes a

similar method where water methanol mixture is sprayed into the

compressor. The purpose of this too is to increase thrust thus

increasing engine performance.


GAS TURBINE ENGINE-2

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