aircraft gas turbine engines_thai technologies

8/7/2019 AIRCRAFT GAS TURBINE ENGINES_Thai Technologies

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AIRCRAFT GAS TURBINE ENGINES

ENGINE TYPES and APPLICATIONS

Introduction

Most of modern passenger and military aircraft are powered by gas turbine engines, which are also called jet

engines. There are several types of jet engines, but all jet engines have some parts in common . Aircraft gas turbine

engines can be classified according to (1) the type of compressor used and (2) power usage produces by the engine.

Compressor types are as follows:

1. Centrifugal flow

2. Axial flow3. Centrifugal-Axial flow.

Power usage produced are as follows:

1. Turbojet engines

2. Turbofan engines.

3. Turboshaft engines.

Centrifugal Compressor Engines

Centrifugal flow engines are compress the air by accelerating air outward perpendicular to the longitudinal axis ofthe machine. Centrifugal compressor engines are divided into Single-Stage and Two-Stage compressor. The amount

of thrust is limited because the maximum compression ratio.

Principal Adventages of Centrifugal Compressor1. Light Weight

2. Simplicity

3. Low cost.

Axial Flow Compressor EnginesAxial flow compressor engines may incorporate one , two , or three spools (Spool is defined as a group ofcompressor stages rotating at the same speed) . Two spool engine , the two rotors operate independently of one

another. The turbine assembly for the low pressure compressor is the rear turbine unit . This set of turbines is

connected to the forward , low pressure compressor by a shaft that passes through the hollow center of the high

pressure compressor and turbine drive shaft.


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Adventages and DisadventagesAdventages: Most of the larger turbine engines use this type of compressor because of its ability to handle large

volumes of airflow and high pressure ratio.

Disadventages: More susceptable to foreign object damage , Expensive to manufacture , and It is very heavy in

comparision to the centrifugal compressor with the same compression ratio.

Axial-Centrifugal Compressor EngineCentrifugal compressor engine were used in many early jet engines , the efficiency level of single stage centrifugal

compressor is relatively low . The multi-stage compressors are some what better , but still do not match with axial

flow compressors. Some small modern turbo-prop and turbo-shaft engines achieve good results by using a

combination axial flow and centrifugal compressor such as PT6 Pratt and Whitney of canada which very popular in

the market today and T53 Lycoming engine.

Characteristics and Applications

The turbojet engine : Turbojet engine derives its thrust by highly accelerating a mass of air , all of which goesthrough the engine. Since a high " jet " velocity is required to obtain an acceptable of thrust, the turbine of turbo jet

is designed to extract only enough power from the hot gas stream to drive the compressor and accessories . All of

the propulsive force (100% of thrust ) produced by a jet engine derived from exhaust gas.


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The turboprop engine : Turboprop engine derives itspropulsion by the conversion of the majority of gas

stream energy into mechanical power to drive thecompressor , accessories , and the propeller load. The

shaft on which the turbine is mounted drives the propeller

through the propeller reduction gear system .

Approximately 90% of thrust comes from propeller and

about only 10% comes from exhaust gas.

The turbofan engine : Turbofan engine has a ductenclosed fan mounted at the front of the engine and driven

either mechanically at the same speed as the compressor ,

or by an independent turbine located to the rear of the

compressor drive turbine . The fan air can exit seperately

from the primary engine air , or it can be ducted back to

mix with the primary's air at the rear . Approximately

morethan 75% of thrust comes from fan and less than25% comes from exhaust gas.

The turboshaft engine : Turboshaft engine derives its propulsion by the conversion of the majority of gas streamenergy into mechanical power to drive the compressor , accessories , just like the turboprop engine but The shaft on

which the turbine is mounted drives something other than an aircraft propeller such as the rotor of a helicopter

through the reduction gearbox . The engine is called turboshaft.

ENGINE THEORY

:

OPERATION

The jet engines are essentially a machine designed for the purpose of producing high velocity gasses at the jet nozzle

. The engine is started by rotating the compressor with the starter , the outside air enter to the engine . The

compressor works on this incoming air and delivery it to the combustion or burner section with as much as 12 times

or more pressure the air had at the front . At the burner or combustion section , the ignition is igniting the mixture of

fuel and air in the combustion chamber with one or more igniters which somewhat likes automobile spark plugs.When the engine has started and its compressor is rotating at sufficient speed , the starter and igniters are turn off.

The engine will then run without further assistance as long as fuel and air in the proper proportions continue to enter

the combustion chamber. Only 25% of the air is taking part in the actual combustion process . The rest of the air ismixed with the products of combustion for cooling before the gases enter the turbine wheel . The turbine extracts a

major portion of energy in the gas stream and uses this energy to turn the compressor and accessories . The engine's

thrust comes from taking a large mass of air in at the front and expelling it at a much higher speed than it had when

it entered the compressor . THRUST , THEN , IS EQUAL TO MASS FLOW RATE TIMES CHANGE INVELOCITY .


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The more air that an engine can compress and use , the greater is the power or thrust that it can produce . Roughly75% of the power generated inside a jet engine is used to drive the compressor . Only what is left over is available to

produce the thrust needed to propel the airplane .

JET ENGINE EQUATION

Since Fuel flow adds some mass to the air flowing through the engine , this must be added to the basic of thrust

equation . Some formular do not consider the fuel flow effect when computing thrust because the weight of air

leakage is approximately equal to the weight of fuel added . The following formular is applied when a nozzle of

engine is " choked " , the pressure is such that the gases are treveling through it at the speed of sound and can not be

further accelerated . Any increase in internal engine pressure will pass out through the nozzle still in the form of

pressure . Even this pressure energy cannot turn into velocity energy but it is not lost .

FACTORS AFFECTING THRUST

The Jet engine is much more sensitive to operating variables . Those are:

1.) Engine rpm.

2.) Size of nozzle area.3.) Weight of fuel flow.

4.) Amount of air bled from the compressor.

5.) Turbine inlet temperature.6.) Speed of aircraft (ram pressure rise).

7.) Temperature of the air.

8.) Pressure of air

9.) Amount of humidity.

Note ; item 8,9 are the density of air .

ENGINE STATION DESIGNATIONS

Station designations are assigned to the varius sections of gas turbine engines to enable specific locations within the

engine to be easily and accurately identified. The station numbers coincide with position from front to rear of the


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engine and are used as subscripts when designating different temperatures and pressures at the front , rear , or inside

of the engine. For engine configurations other than the picture below should be made to manuals published by the

engine manufacturer.

N = Speed ( rpm or percent )

N1 = Low Compressor Speed

N2 = High Compressor Speed

N3 = Free Turbine Speed

P = Pressure

T = Temperature

t = Total

EGT = Exhaust Gas Temperature

EPR = Engine Pressure Ratio ( Engine Thrust in term of EPR ). Pt7 / Pt2

Ex.: Pt 2 = Total Pressure at Station 2 ( low pressure compressor inlet )

Pt 7 = Total Pressure at Station 7 ( turbine discharge total pressure )

ENGINE CONSTRUCTION

AIR INLET DUCT

An engine's air inlet duct is normally considered an airframe part and made by aircraft manufacturer . During flight

operation , it is very important to the engine performance . Engine thrust can be high only if the inlet duct supplies

the engine with the required airflow at the highest posible pressure . The inlet duct has two engine functions and one

aircraft function .First : it must be able recover as much of the total pressure of the free air stream as posible and deliver this

pressure to the front of the engine compressor .Second : the duct must deliver air to the compressor under all flight conditions with a little turbulance .

Third : the aircraft is concerned , the duct must hold to a minimum of the drag.

The duct also usually has a diffusion section just ahead of the compressor to change the ram air velocity into higher

static pressure at the face of the engine . This is called ram recovery . The inlet duct is built generally in thedivergent shape (subsonic diffuser).


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Centrifugal compressorCentrifugal compressors operate by taking in outside air near their hub and rotating it by means of an impeller . The

impeller , which is usually an aluminum alloy , guides the air toward the outer circumference of the compressor ,

building up the velocity of the air by means of high rotational speed of the impeller . The compressor consists ofthree main parts:

1) Impeller

2) A Diffuser3) A Comprssor Manifold

Air leaves the Impeller at high speed , and flows through the diffuser which converts high velocity , kinetic energy

to low velocity , high pressure energy . The diffuser also serves to direct airflow to the compressor manifold which

acts as collector ring. They also delivery air to the manifold at a velocity and pressure which will be satisfactory for

use in the burner section of the engine.

Axial compressorThe air in an axial compressor flows in an axial direction through a series of rotating rotor blades and stationary

stator vanes. The flow path of an axial compressor decreases in cross-section area in the direction of flow , reducing

the volume of the air as compression progresses from stage to stage of compressor blades .

The air being delivered to the face of compressor by the air inlet duct, the incoming air passes through the inlet

guide vanes . Air upon entering the first set of ratating blades and flowing in axial direction, is deflected in thedirection of rotation . The air is arrested and turn as it is passed on to a set of stator vanes , following which it is

again picked up by another set of rotating blades , and so on , through the compressor . The pressure of the air

increases each time that it passes through a set of rotors and stators .

The aerodynamic principles are applied to the compressor blade design in order to increase efficiency . The blades

are treated as lifting surfaces like aircraft wings or propeller blades . The cascade effect is a primary consideration in

determining the airfoil section , angle of attack , and the spacing between blades to be used for compressor bladedisign . The blade must be designed to withstand the high centrifugal forces as well as the aerodynamic loads towhich they are subjected . The clearance between the rotating blades and their outer case is also very important . The

rotor assembly turns at extreamely high speed , and must be rigid , well aligned and well balance .

Compressor Surge and Compressor StallThis characteristic has been called both " Surge " and " Stall " in the past , but is more properly called SURGE when

it is response of the entire engine. The word stall applies to the action occuring at each individual compressor

blade. Compressor surge , also called Compressor stall , is a phenomenon which is difficult to understand because it

is usually caused by complex combination of factors . The basic cause of compressor surge is fairly simple , each


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blade in an axial flow compressor is a miniature airplane wing which , when subjected to a higher angle of attack ,

will stall just as an airplane stalls. Surge may define as results from an unstable air condition within the compressor.

Pilot or engine operator has no instrument to tell him that one or more blades are stalling. He must wait until the

engine surges to know that. The unstable condition of air is often caused from air piling up in the rear stages of thecompressor. Surge may become sufficiently pronounce to cause lound bangs and engine vibration. In most case ,

this condition is of short duration , and will either correct itself or can be corrected by retarding the throttle or powerlever to Idle and advanncing it again , slowly. Among other things , to minimize the tendency of a compressor tosurge , the compressor can be "unload" during certain operating conditions by reducing the pressure ratio across the

compressor for any giving airflow. One method of doing this is bybleeding air from the middle or toward the rear of

the compressor. In dual axial compressor engines , air is often bled from between the low and the high pressure

compressor. Air bleed ports are located in the compressor section. These ports are fitted with automatic , overboard

bleed valves which usually operate in a specified range of engine RPM. Some large engine have been provided with

variable-angle stators ( variable stators) in a few of the forward compressor stages. The angle of these vanes change

automatically to prevent the choking of the downstream compressor stages as engine operating conditions vary.

Turbofan Fan SectionThey are considered as part of the compressor section in dual axial flow compressor engines because the fan is

formed by the outer part of the front stages of the low compressor. The fan also seperate from the forwardcompressor and is driven by a freely rotating turbine of it own. The forward fan design is now used by most of

engine manufacturers. In dual compressor engines , the fan is often integral with the relatively slow turning low

compressor , which allows the fan blades to rotate at low tip speed.

DIFFUSER SECTION

The diffuser has an expanding internal diameter to decrease the velocity and increase thestatic pressure of air . The air leaving compressor , then through a diffuser section . The

diffuser prepares the air for entry the combustion section at low velocity to permit proper

mixing with fuel . Ports are built in the diffuser case through which compressor discharge

air is bled off from the aircraft engine .


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On dual compressor engines , bleed air for service functions is also taken from additional

ports located between the low and high compressors , or at intermediate stages in the high

pressure compressor case . Air is bled from most engine vented over board out of the

primary air flow path during certain engine operating conditions to preventcompressor

surge .This is called over board and must not be confused with the air remove from the

engine to perform service function .

FUEL MANIFOLDS and NOZZLES

Fuel is introduced into the air stream at the front of the burners in spray form , suitable for

rapid mixing with air for combustion. The fuel is carried from outside the engine , by

manifold system , to nozzles mounted in the burner cans .

Primary and secondary fuel manifolds are often used on large engines . The primary

manifold provides sufficient fuel for low thrust operation. At high thrust , the secondary ,

or main manifold cuts in , and fuel commences to flow through both primary and

secondary elements of double-orifice nozzle. Usually , primary fuel is sprayed through a

single orifice at the center of nozzle. Secondary fuel is sprayed through a number of orifices

in a ring around the center orifice.

COMBUSTION CHAMBERS OR BURNER SECTION

There are three basic types of burner systems in use today. They are can type , annular

type and can-annular type. Fuel is introduced at the front end of the burner. Air flows in

around the fuel nozzle and through the first row of combustion air holes in the liner. The

air entering the forward section of the liner tends to recirculate and move up stream

against the fuel spray. During combustion , this action permits rapid mixing and prevents

flame blowout which acts as a continuous pilot for the rest of the burner.


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There are usually has only two igniter plugs in an engine. The igniter plug is usually locate

in the up stream region of the burner. About 25 percent of the air actually takes part in the

combustion process. The gases that result from the combustion have temperatures of 3500

degree F. Before entering the turbine , the gases must be cooled to approximately half this

value , up to the designed of turbine materials involved. Cooling is done by diluting the hot

gases with secondary air that enters through a set of relative large holes located toward therear of the liner.

TURBINE SECTION

The turbine in all modern jet engines , regardless of the type of compressor used , are of

axial flow design.

The turbine extract kinetic energy from the expanding gases as the gases come from the

burner , converting this energy into shaft horsepower to drive the compressor and the

engine accessory. Nearly three fourths of all energy available from the product of


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combustion is needed to drive the compressors.

The turbine wheel is one of the most highly stressed parts in the engine. Not only must it

operateat temperature 1700 degree F, but it must do so under severe centrifugal loads

imposed by high rotational speeds of over 40000 rpm for small engines to 8000 rpm for a

larger engines.The engine speed and turbine inlet temperature must be accurately

controlled to keep the turbine within safe operating limits.

The turbine assembly is made of two main parts , the disk and the blades. The disk orwheel is statically and dynamically balanced and unit specially alloyed steel usually

containing large percentages of chromium , nickle , and cobalt. The blades are attached to

the disk by means of a " fir tree " design to allow for different rates of expansion between

the disk and the blade while still holding the blade firmly against centrifugal loads. The

blade is kept from moving axially either by rivets , special locking tabs or devices , or

another turbine stage.

The blade is shrouded at the tip. The shrouded blades form a band around the perimeter of

the turbine which serves to reduce blade vibrations. The shrouds improve the airflow

characteristics and increase the efficiency of the turbine. The shrouds also serve to cut

down gas leakage around the tips of the turbine blades.

EXHAUST DUCT OR EXHAUST PIPE

A larger total thrust can be obtained from the engine if the gases are discharged from the

aircraft at a higher velocity than is permissible at the turbine outlet. An exhaust duct is

therefore added , both to collect and straighten the gas flow as it comes from the turbine

and to increase the velocity of the gases before they are discharged from the exhaust nozzle


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at the rear of the duct.

Increasing the velocity of the gases increases their momentum and increase the thrust

produced.The duct is essentially a simple , stainless steel , conical or cylinder pipe .

The tail cone helps smooth the flow. A conventional convergent type of exhaust duct is

capable of keeping the flow through the duct constant at velocity not to exceed Mach 1.0 at

the exhaust nozzle.

AFTER BURNING

The afterburner , whose operation is much like a ram-jet , increases thrust by adding fuelto the exhaust gases after they have passed through the turbine section. At this point there

is still much uncombined oxygen in the exhaust. Only approximately 25 percent of the air

passing through the engine is consumed by the combustion. The remainder or 75 percent ,

of the air is capable of supporting additional combustion if more fuel is added. The

resultant increase in the temperature and velocity of gases therefore boosts engine thrust.

Most afterburners will produce an approximately 50 percent more thrust. Afterburning or

" hot " operation or " reheating " is used only for a time limited operation of takeoff ,

climb , and maximum burst speed.

aircraft gas turbine engines_thai technologies

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