PIA Module 15 (Gas Turbine Engine) Sub Module 15.1 (Fundamentals)

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Module 15 – GAS TURBINE ENGINE

Category – Aerospace – Aerospace

Sub Module 15.1 – 15.1 – Fundamentals

MODULE 15 Sub Module 15.1 15.1

FUNDAMENTALS

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ISO 9001:2008 Certified

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CAA Approval No: HQCAA/2231/44/AW Dated: 11 th Sept, 09

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Sub Module 15.1 – 15.1 – Fundamentals OBLIQUE SHOCK WAVE CHARACTERISTICS --------------------------------- 18

Contents

FUNDAMENTALS -------------------------------------------------------------------- 1 NEWTON’S LAWS OF MOTION -------------------------------------------------- 3 APPLICATION OF NEWTON'S LAWS -------------------------------------------- 7 WORK---------------------------------------------------------------------------------- 8 POWER -------------------------------------------------------------------------------- 8 ENERGY ------------------------------------------------------------------------------- 8 PRESSURE----------------------------------------------------------------------------- 8 HEAT ---------------------------------------------------------------------------------- 10 TEMPERATURE ---------------------------------------------------------------------10 ---------------------------------------------------------------------10 INTRODUCTION TO GAS PHYSICS ---------------------------------------------- 11

SUPERSONIC AIRFLOW THROUGH A CONVERGENT-DIVERGENT DUCT ----------------------------------------------------------------------------------------- 19 THERMODYNAMICS -------------------------------------------------------------- 20 EFFICIENCY ------------------------------------------------------------------------- 20 HEATING PROCESS IN THERMODYNAMICS --------------------------------- 21 CYCLE -------------------------------------------------------------------------------- 24 THE OTTO CYCLE ------------------------------------------------------------------ 24 THE BRAYTON CYCLE ------------------------------------------------------------- 25 THRUST ------------------------------------------------------------------------------ 27 BASIC CONSTRUCTION AND OPERATION OF THE TURBOPROP ENGINE ----------------------------------------------------------------------------------------- 31

PERFECT GAS EQUATION -------------------------------------------------------- 11

BASIC CONSTRUCTION AND OPERATION OF THE TURBO-SHAFT ENGINE ------------------------------------------------------------------------------ 33

GAS D YNAMICS -------------------------------------------------------------- ------13

PROPULSIVE EFFICIENCY OF THE TURBO-PROP AND THE TURBO-JET 34

STATIC AND DYNAMIC PRESSURE --------------------------------------------- 13 SUBSONIC AIRFLOW --------------------------------------------------------------15 --------------------------------------------------------------15 CONTINUITY THEOREM ---------------------------------------------------------- 15 CONVERGENT DUCT --------------------------------------------------------------15 --------------------------------------------------------------15 DIVERGENT DUCT ----------------------------------------------------------------- 15 BERNOULLI’S THEOREM --------------------------------------------------------- 16 SUPERSONIC AIRFLOW ----------------------------------------------------------- 17 NORMAL SHOCK WAVE CHARACTERISTICS --------------------------------- 18 For Training Purpose Only


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Sub Module 15.1 – 15.1 – Fundamentals



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Category – Aerospace

Sub Module 15.1 – Fundamentals

FUNDAMENTALS

ACCELERATION

DYNAMICS In physics, dynamics is the study of forces (why objects move). When studying dynamics, vectors are used to describe forces .

Acceleration is the change in velocity over time. In common speech, the term acceleration commonly is used for an increase in speed (the magnitude of velocity); a decrease in speed is called deceleration.

MOTION

DISPLACEMENT

Motion is change in the location of a body. Change in motion is the result of applied f orce. Motion is typically described in te rms of velocity, acceleration, displacement, and time. An object’s velocity cannot change unless it is acted upon by a force, as described by Newton's first law also known as Inertia.

Displacement is the shortest directed distance between any two points. TIME Time is a component of the measuring system used to sequence events, to compare the durations of events and the intervals between them, and to quantify the motions of objects.

VELOCITY Velocity is the rate of change of position. It is measured in meters per second. Absolute Velocity Absolute velocity refers to the velocity of an object as compared to a fixed ob ject or surface.

SPEED Speed is a measure of how fast an object is moving. Speed involves only the length of the path traveled by an object and the time taken to travel the path. Units: Miles per Hour (MPH), Kilometers per Hour (KPH), Meters per Second (mps).

Relative Velocity MASS Relative velocity refers to the velocity of an object as compared to another moving object.

Mass is a measure of the amount of fundamental matter of which an object is composed. Units: Kilogram (Kg), Pound (lb)



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WEIGHT The weight of an object is the effect of gravity on it. Units: Kgms2 ', Ibfts2 MOMENTUM Momentum is defined as the product of the mass of a body and its velocity. It is the property of a moving body, which determines the length of time required to rest under the a ction of a constant force. Units: Kilogram Meters per Second



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NEWTON’S LAWS OF MOTION

NEWTON’S SECOND LAW OF MOTION

NEWTON’S FIRST LAW OF MOTION

The rate of change of momentum of a body is proportional to the total force acting upon it and occurs in the direction of the force. i.e. the effect of a force depends on its mass, speed and direction, As shown in the following figure.

Every object continues in its state of rest or uniform motion in a straight line unless it is compelled to change that state by an external force acting upon it. i.e. You cannot move, stop or steer anything unless you apply a force to it, As shown in the following figure.

This law defines force FORCE That which attempts to changes the state or position of a body. Types of forces are gravity, friction, electrical, etc. Units: Newton, Pound Force (Ibf) F=ma Where, F: force m: mass a: acceleration

This law gives rise to concept of inertia. INERTIA

Centrifugal force

Inertia is resistance of a body to change of state of momentum. Example :( A ball kept on a level table will remain motionless until it is made to move by some external action such as a gust of wind or a push by a person's hand.

Centrifugal force is the force, which tends to causes an object in circular motion to move further away from the centre of the circle, as shown in the following figure. For Training Purpose Only


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Centripetal force

NEWTON’S THIRD LAW OF MOTION

Centripetal force is the force a restraining agent apply on a object In rotary motion, As shown in the following figure.

To every action there is an equal and opposite reaction . This law defines reaction. The term action means the force exerted by one body on another, while the reaction means the force the second body exerts on the first. These forces always occur in pairs but never cancel each other because, although equal in magnitude, they always act on different objects. As an example, when a person jumps from a boat, it is pushed backwards with the same force that pushes the person forward. The person gains the same amount of momentum as the boat receives, but in opposite directions. Some examples of Newton's Third Law are shown below. -

When a balloon is blown up and released, it will travel at a high speed because of the air expelled in the opposite direction.

-

When a gun is fired, as the bullet moves at high speed in one direction, the gun is forced in the other direction, which is called recoil, As the figure shows.



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Reaction engines A family of engines uses the principle of the Newton's third law to create a force called thrust to move vehicles. These engines are called reaction engines. Because, the force these engines generate is a reaction to the expelling of hot gasses in the opposite directi on. This family of engines includes engines such as Ramjet, Pulsejet, Gas Turbine Jet engine and Rocket engine, As shown in the figure. PULSE JET

RAM JET ROCKET ENGINE

GAS TURBINE JET For Training Purpose Only


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APPLICATION OF NEWTON'S LAWS When a balloon is inflated, the inside air pressure, which is stretching the skin, is greater than the outside pressure. If the stem is tied closed, the inside air pushes equally in all directions and the balloon will not move. Releasing the stem removes a section of the skin on that side of the balloon against which the air has been pushing. On the side directly opposite the stem, however, the air continues to push on an equal area of the skin. It is this unbalanced push on this area that causes the balloon to move in the direction away f rom the stem. The balloon's flight is short because the pressure within the skin is lost quickly. This handicap can be overcome by pumping air into the balloon with a bicycle pump so the pressure and airflow are maintained.

of the balloon, which is in effect a jet nozzle. The transformation is now complete and the ba lloon 'jet engine' can operate as long as there is fuel to burn. The action that Newton's third law refers to is the acceleration of the escaping air from the rear of the balloon. The reaction to this acceleration is a force in the opposite direction acting on the balloon. Since the forces always occur in pairs, it can be said that if it takes a certain force to accelerate a mass rearwards, the reaction to this force is thrust in the opposite direction , As shown in the following figure.

To transform this apparatus into a self contained jet engine, the hand pump is replaced with a compressor. And if the compressor is turned at high speed, a large quantity of air is passed through the balloon while holding a high pressure inside. For energy to turn the compressor, a burner is placed in the stream. Burning fuel raises the air temperature rapidly, and the air volume is greatly increased. Since the compressor pressure blocks the forward flow, the air can only move rearwards on the less restricted path leading to the exit. By placing a turbine in the path of the heated air, some of this energy is used to spin the turbine, which in turn, spins the compressor by means of a connecting shaft. The remaining energy is expended in expelling the hot gases through the stem For Training Purpose Only


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WORK

PRESSURE

Work is done when force acting on a body causes it to move through any distance.

Pressure is defined as force per unit area.

Work = Force x Distance Units: Newton Meters (Nm), Pound force feet (Ib-fft)

Units: Pounds per square Inch (psi), Bar (B), Inches of Mercury (InHg), Pascal (Pa) Absolute pressure

POWER

A pressure reading measured with reference to a complete vacuum.

Power is the rate of doing work. Power = Work Power = Force x Distance / Time Units: Newton Meters per Second (Nm/S), Pound force feet per second (Ibf ft/s)

Units: psia Gauge pressure Gauge pressure is the pressure reading that is obtained from a pressure gauge that is calibrated to indicate zero at atmospheric pressure or ambient pressure.

ENERGY Energy is the capacity to do work. Units: Joule, Foot Pounds

Therefore Gauge pressure is pressure above ambient. Potential Energy Gauge pressure = Absolute pressure - Ambient pressure Potential Energy is the energy a body possesses due to its condition, position, or its chemical state. Potential energy = Weight x Height

Units: psig Differential Pressure

Kinetic Energy Kinetic energy is the energy possessed by a body because of its motion. Kinetic Energy = 1/2 x Mass x Square of Velocity , as shown in the following figure.

Differential pressure is the difference between two pressures. The term Delta P refers to this pressure. Units: psid For Training Purpose Only


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HEAT

Where:

Heat is a form of energy the addition of which results in an increase of internal energy (kinetic energy of molecules) of a body.

°C = Centigrade

TEMPERATURE

°K = Kelvin

Temperature is a measure of the kinetic energy of molecules of a body.

°R = Rankine

°F = Fahrenheit

Static Temperature Units: Celsius, Kelvin, and Fahrenheit Static temperature is a measure of the heat in a gas or liquid. Temperature Scales Conversion Total Temperature The temperature scale normally used in thermodynamics is the Kelvin scale. American engine manufacturers commonly use degrees Fahrenheit when describing their engines. It will be useful, therefore, to remember the conversions:

Total temperature is a measure of the energy in the gas or liquid.

For interpolation 1°C = 1·8°F °C = 5/9 (°F – 3 °K = °C + 273 °F = (9/5°C) + 3 °R = °F + 460



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INTRODUCTION TO GAS PHYSICS

COMBINED GAS LAW

In order to attain a fuller understanding of the functioning of g as turbines, it is essential to have an appreciation of the basic gas laws.

Derived by combining Charles's and Boyle's laws for a unit mass of gas. Pressure (P) x Volume (V) = Constant (C) Temperature (T)

BOYLE’S LAW 'The volume of a given mass of gas, whose temperature is maintained constant, is inversely proportional to the gas pressure’.

PV/T=C

What this means is that if the pressure of a given mass of gas i s doubled, its volume is halved, or if the pressure is halved, the volume will be doubled, provided that the temperature of the gas remains constant, as shown in the following figure.

PERFECT GAS EQUATION

If the mass is kept constant.

For a unit mass of air (11b.), the constant (C) of above equation becomes a specific constant known as the Universal Gas Constant (R).

Pressure (P) x Volume (V) = Constant(C) If Mass and Temperature kept constant. PV = C

Pressure (P) x Volume M = UGC (R) Temperature (T) PV = MRT

CHARLES’ LAW If the pressure of a given mass of a gas is maintained constant, the volume of gas increases a s its temperature is increased’. Volume (V) = Constant (C) Temperature (T) If Mass and Pressure is kept constant. For Training Purpose Only


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CHARLES'S LAW



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GAS DYNAMICS

STATIC AND DYNAMIC PRESSURE

Up to this Point, we have been primarily concerned with the internal energy of the gasses or air that is due to molecular activity within the body. But a body of gas can possess energy that is not due to individual molecular activity. When any body is moving, its movement represents a considerable amount of energy. The energy of movement is separate from the energy inside the body. This concept is one of the most important in understanding the performance of a turbo-jet engine.

Static Pressure

There is one very significant difference between the internal molecular energy and the external energy of flow. The molecules are charging around in random directions so the energy they possess is exerted in all directions. The whole body of moving air is moving in only one direction so the energy of flow can be exerted in only one direction. In other words the static Pressure inside the body pushes outward from the center of the body in all directions whereas the pressure of flow is only in the downstream direction, As shown in the following figure.

Static Pressure refers to the pressure, exerted by a mass of stationary air, equally on all of the walls of a container. Dynamic Pressure Dynamic Pressure refers to the pressure that would be exerted if a flow of air were brought to a stop. When the static pressure and the flow pressure are added together we get what is commonly known as Total Pressure. This is important because the total pressure represents a combination of internal and external pressure energy. The total temperature is likewise the sum of the static and flow temperatures. If we neither add nor subtract energy from a body of flowing gas, we still can change internal energy into flow energy and vice versa. Thus the total temperature and pressure will stay the same even though the static temperature and pressure do change into flow energy in the form of velocity, and this leads to: Static Pressure + Dynamic Pressure = Total Pressure Static Temp. + Dynamic Temperature = Total Temperature



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SUBSONIC AIRFLOW At low airspeeds, the study of aerodynamics is greatly simplified by the fact that air may experience relatively small changes in pressure with only negligible changes in density. The airflow is termed incompressible since the air may undergo changes in pressure without apparent changes in density. This is similar to flow of liquid such as water. CONTINUITY THEOREM For fluids in steady motion an identical mass of fluid passes each cross-section of a duct per second. Density x Velocity x Area = Constant

DIVERGENT DUCT A divergent du ct is one which ha s an inlet area which is less than the outlet area. This gives a decrease in velocity with an increase in pressure and temperature, As shown in the figure.

pVA= C Where p is density, V is velocity, A is area and C is a constant CONVERGENT DUCT A convergent duct is one that ha s an area at the inlet greater than the area at the outlet. When air flows through such a duct it incurs a velocity increase at the expense of the static pressure and temperature, As shown in the following figure.



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BERNOULLI’S THEOREM

Then,

At any point in a tube (or a gas passage) through which liquid (or gas) is f lowing, the sum of the pressure energy, the potential energy and the kinetic energy is constant. Thus, if one of the energy factors in a gas flow changes, one or both of the other variables also changes so that the total energy remains constant, As shown in the following figure.

P + pV 2 = C 2

This theorem gives us the relationship between velocity and pressure of a stream of air f lowing through a tube, or duct, such as a gas turbine engine. This theorem is applicable for incompressible fluids only. Air can consider to be abiding by this theorem when velocities are subsonic. From the foregoing theory: Pressure Dynamic Energy Energy

Potential Energy

=

Total Energy

Where: Pressure Energy is energy due to static pressure. Dynamic energy is kinetic energy. P + -pV 2 +pgH = C Where P is Static Pressure, V is velocity, p is Density g is gravitational acceleration H is height and C is a constant. Changes in Potential Energy may be neglected, as it is negligible for a gas and if there will be no appreciable changes in height for a fluid. For Training Purpose Only


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SUPERSONIC AIRFLOW

Speed of sound

So far we have dealt with flow velocities of low speed (or subsonic) nature. The behavior of air under subsonic conditions is easily predictable. However, when at high speeds, airflow behavior is altogether different from that of subsonic airflow. At high airflow speeds, pressure changes that take place are quite large, and significant changes in density occur; this type of airflow is called a compressible flow.

Speed of sound is the rate at which small pressure differences will spread through the air. Speed of sound is not affected by changes in atmospheric pressure but is affected by changes in temperature. Therefore the speed of sound can be calculated as,

If the velocity of the gas flow through a venturi duct is increased beyond a certain point, Bernoulli's equation can no longer be applied. This point occurs at approximately the local speed of sound, beyond which, the effects of convergent and divergent ducts are reversed. This is due to the gas flow reaching a velocity where the flow becomes compressible and no longer follows the theory of incompressible flow (Bernoulli's equation).

Speed of Sound a = (kRT) 1/2 . Study of compressibility effects due to high-speed airflow are of great importance in the design of both aircraft and engines. The compressibility phenomenon and the speed of sound are closely related to each other. As an object moves through the air, velocity and pressure changes occur which create pressure disturbances in the airflow surrounding the object. These pressure disturbances propagate through the air at the local speed of sound. If the object is traveling below the speed of sound, these pressure disturbances propagate ahead of the object and influence the air immediately in f ront of the object. If the object is traveling at the speed of sound, these pressure disturbances will accumulate and form a compression wave or shock wave immediately in front of the object. This type of shock wave is called a normal shock wave, as the shock wave will form perpendicular to the line of travel Shock waves are very narrow areas of discontinuity where changes in velocity, pressure and temperature will take place.



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k = ratio of specific heats) R = gas constant T = absolute temperature ( oK, oR)

NORMAL SHOCK WAVE CHARACTERISTICS Air passing through a Normal shock wave will undergo following changes.

For an ideal gas the speed of sound is proportional to the square root of the absolute temperature.

 

Example - Speed of Sound in Air The speed of sound in air at 0 oC and absolute pressure 1 bar can be calculated as



If the object is traveling at some speed above the speed of sound, the airflow ahead of the object will not be influenced as the pressure disturbances cannot propagate ahead of the object. In this condition the object is out-speeding its own disturbances. In this condition the airflow ahead of the object is not influenced until forced out of the way by a concentrated pressure wave set up by the object. This pressure wave is formed at an angle to the airflow, therefore is called an oblique shock wave.

c = (1.4 (287 J/K kg) (273 K)) 1/2 = 331.2 (m/s) Where k = 1.4 And R = 287 (J/K kg)

OBLIQUE SHOCK WAVE CHARACTERISTICS

The speed of sound in air at 20 oC and absolute pressure 1 bar can be calculated as c = (1.4 (287 J/K kg) (293 K)) 1/2 = 343.1 (m/s)



Air velocity decreased from supersonic to subsonic. Air static pressure increases. Magnitude of change depends on the strength of the shock wave. Air temperature increases. Direction of airflow will not change.

Air passing through an oblique shock wave will undergo following changes.    

Air velocity will decrease but will remain supersonic. Static pressure of air will increase. Temperature of air will increase. Direction of airflow may change.



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SUPERSONIC AIRFLOW DIVERGENT DUCT


THROUGH

A

CONVERGENT-

The characteristics of a supersonic airflow through a convergent-divergent duct are the exact opposite to that of a subsonic airflow. Convergent duct Velocity Decreases Pressure Increases

Divergent duct Velocity Increases Pressure Decreases

Mach Number The Mach number refers to the speed at which a body is traveling in relation to the local speed of sound. The speed of sound varies according to the temperature of the air and therefore, we must add to the definition the fact that the speed of sound must be that corresponding to the temperature of the air in which the body is actually traveling.

oblique shock wave

MACH NO = V/a Where, V=actual speed A=speed of sound



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THERMODYNAMICS

Thermal Efficiency

Thermodynamics is the study of heat flow and heat exchange. For example, the heat flow from one level to another and the exchange of heat energy for mechanical energy and mechanical work for heat energy.

The overall thermal efficiency of a gas turbine engine is equal to the useful work output divided by the heat of combustion of the fuel used, when both quantities are measured in the same units, such as BTU's or horsepower.

1. Laws of thermodynamics I (Conservation of energy) Energy cannot be created nor destroyed, but can on ly be transformed from one form to another.

The thermal efficiency of an engine may be expressed as the ratio of heat energy supplied from fuel combustion to the kinetic energy as represented by the jet nozzle velocity of engine.

2. Laws of thermodynamics II (Transformations of energy) Heat can only transfer from a warmer body to a cooler body.

Propulsive Efficiency

EFFICIENCY A number of Efficiencies are present in gas turbine engine operation. However, all of them express the ratio of the work obtained from the machine (or process), to the work or energy put into each, in producing the desired result. Briefly stated, efficiency is the output divided by the input, and each component of the gas turbine such as the compressor, the burners, the turbines and the exhaust nozzle, has its own efficiency.

The efficiency of conversion of kinetic energy to propulsive work is termed the propulsive or external efficiency and this is affected by the amount of kinetic energy wasted, by the propelling mechanism. Wasted energy dissipated in the jet wake, which represents a loss, can be exp ressed as, Energy wasted; Ew = —MaV Where V is wasted velocity and V = Vj - Va Vj is Jet velocity Va is aircraft velocity Ma is mass airflow

Mechanical Efficiency Mechanical efficiency is the ratio of the useful work output of a machine to the work or energy input. This difference is due chiefly to mechanical friction losses, but also includes gas and other losses.

It is therefore apparent that at the aircraft lower speed range the pure jet stream wasted considerably more energy than a propeller stream and consequently is less efficient over this range. However, this factor changes as aircraft speed increases, because although the jet stream continues to i ssue at a high For Training Purpose Only


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velocity from the engine its velocity relative to the surrounding atmosphere is reduced and, in consequence, the waste energy loss is reduced. Briefly, propulsive efficiency (Pe) may be expressed as: Pe =

Work done on the aircraft Energy imparted to engine airflow

Pe=

______ Work done __________ Work done + work wasted in exhaust

CYCLE EFFICIENCY Briefly, cycle efficiency can be defined as the ratio of the amount of useful work obtained from the gas turbine engines actual cycle to the amount of work that would be obtained from the same ideal cycle. The ideal work of a cycle would be obtained if all component efficiencies - compressor efficiency, combustion efficiency, mechanical efficiency, thermal efficiency, jet nozzle efficiency, etc. were 100 per cent, but since none of the components are perfect, the actual cycle efficiency is always less than the ideal.

Work done is the net thrust multiplied by the aircraft speed.

Specific Fuel Consumption

Therefore, progressing from the net thrust equation given above, the following equation is arrived at:

Specific Fuel Consumption (SFC) or thrust specific fuel consumption (TSFC) which is the measure of amount of fuel used for generation of a unit of thrust. This in turn can be used as a measure of how efficient an engine is at developing thrust. TSFC will enable a person to judge how efficient an engine is relative to another simply by comparing TSFC; even between engines classed in two different thrust levels.

Pe =

2Va Va + Vj

Where Va is aircraft velocity Vj is jet velocity

HEATING PROCESS IN T HERMODYNAMICS

From the above equation we see that if, aircraft speed {Va) is '0', the efficiency will be zero. If Va equals Vj efficiency will be 100%. But this will never happen as we notice that If Va equals Vj there will be no acceleration of air through the engine which means no momentum change i.e. no reaction force, and no thrust produced. Therefore, the propulsive efficiency must always be less than 100%.

Here we will discuss briefly, different ways of heating and expanding a gas. They are referred to as thermodynamic processes. Constant Volume Heating (Isochoric) A gas, heated in a fixed enclosed space will remain at constant volume. The heating will be accompanied by a rise in pressure and temperature. For Training Purpose Only


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As there is no change in volume there will be no external work done by the gas; the whole of the heat supplied will be stored in the gas in the form of internal energy. Constant Pressure Heating (Isobaric) A gas may be heated whilst enclosed in a cylinder containing a sliding piston, on which a constant force is acting; this will maintain a constant pressure in the cylinder, the heating of a gas under constant pressure causes an increase in volume and temperature. External work is done owing to the increase in volume.

Adiabatic Expansion When a gas expands, doing external work, in such a manner that no heat is supplied or rejected during the expansion, the expansion is called adiabatic. The adiabatic expansion may take place in an engine cylinder, in which case no heat passes through the cylinder walls and work is done on the piston as the gas expands. The following three conditions must be satisfied in this type of expansion: (a) No heat is supplied or rejected during the expansion. (b) Work of some nature, must be done by the gas in expanding. (c) The expansion is assumed to be frictionless.

Constant Temperature Heating (Isothermal Expansion) If heat is supplied to a gas whilst maintaining temperature constant the gas will expand, doing external work equal to the amount of heat supplied. An expansion at constant temperature is known as an isothermal expansion. When a gas is compressed or expanded in accordance with the Boyle's Law, the change is said to take place isothermally.

As the gas is not receiving heat, the work done must be performed at the expense of its own supply of internal energy. Hence, it follows, that the temperature will fall during an adiabatic expansion, and will rise during an adiabatic compression

Such a process can only be approached in practice if the change takes place slowly so that all the heat of compression or expansion was absorbed or supplied by an external source. In practice changes takes place quickly and there is always a change in temperature so that the isothermal change has no practical significance in connection with our consideration of the Gas Turbine Engine.



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CYCLE

Due to the heating stages of each cycle take place in different conditions, so different types of cycle are resulted.

A cycle is a process that begins with some known conditions and ends with those same conditions. There are mainly two forms of cycle; they are the open cycle and the close cycle. The typical example for a close cycle is the operation of a piston engine, and the example for an open cycle is the operation of a jet engine. The same series of events, i.e. intake, compression, power, and exhaust take place in both, the main difference being that in the jet engine, all these events are happening simultaneously but each event take place in separate component design for its particular function, and the cycle is continuous as long as the engine is running. This is why the cycle for a jet engine is called a open cycle or a continuous flow cycle. For the case of a piston engine, all the events take place one after or each event must follow the preceding one, and they all occurs in the same chamber i.e. engine cylinder, and the cycle is constantly repeating i tself. We come to look at two different types of cycle which we usually come across, they are the: 1. Constant volume cycle or the Otto cycle. 2. Constant pressure cycle or the Brayton cycle.

THE OTTO CYCLE (THE CONSTANT VOLUME CYCLE) The reciprocating engine operates on what is commonly termed a closed cycle. 1. In the series of events shown in Fig. 1-39, air is drawn in on the intake stroke at point 1, where compression by this piston raises the temperature and decreases the volume of the gases. 2. Near the end of the compression stroke, point 2, ignition occurs, which greatly increases the temperature of the mixture. The term "constant volume" is derived from the fact that from point 2 to 3 there is no appreciable change of volume while the mixture is burning. 3. Point 3 to 4 represents the expansion stroke with a loss of temperature and pressure, and a corresponding increase in volume. It might be noted that this is the only stroke of the four from which power may be extracted. 4. When the exhaust valve opens near the end of the power stroke (point 4 to 1), the gases lose their remaining pressure and temperature and the closed cycle starts all over again.

These cycles will commence with a certain known conditions and goes through the usual events of intake compression, combustion, expansion and exhaust, then end up with more or less the same conditions as it began. For Training Purpose Only


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THE BRAYTON CYCLE (THE CONSTANT PRESSURE CYCLE) Since all of the events are going on continuously, it can be said that the gas turbine engine works on what is commonly called an open cycle. Refer Fig. 1. As in the reciprocating engine, air is drawn in and compressed (point 1 to 2) with a corresponding rise in pressure and temperature, and a decrease in volume. 2. Point 2 to 3 represents the change caused by the burning of the fuel air mixture in the combustion chamber at an essentially constant pressure, but with a very large increase in volume. This increase in volume shows up as an increase in velocity because the engine area does not change much in this section. 3. From the burner, the gases expand through the turbine wheel causing an increase in volume and a decrease in temperature and pressure (Point 3 to 4). 4. This process continues from point 4 to 5 through the exhaust nozzle.



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THRUST Distribution of Thrust The thrust developed within a gas turbine engine is, as previously stated, the acceleration of a mass airflow within the engine. The acceleration can be rearward as well as forward. The total forward thrust is the gross thrust developed by the engine but the forward thrust less the rearward thrust is the net thrust developed. Since the rearward thrust can be over 60% of the total this means that, with other losses, there is less than 40% left over to drive the aeroplane forwards. The force of the thrust is felt against all the static and rotating parts of the engine. The thrust felt against the rotating parts has to be transferred to the static part and then passed to the airframe to drive the aircraft forwards. Not all of the thrust is felt in the same place; it is spread variously along the engine so that some is developed at the compressor and some at the combustion section, and so on.



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Basic Construction and Operation of the Pure Jet The modern jet engine is basically cylindrical in shape as it is essentially a duct into which the necessary parts are fitted. The parts from front to rear are the: Compressor Combustion system Turbine assembly Exhaust system A shaft connects the turbine to the compressor and fuel burners are positioned in the combustion system. Initial ignition is provided once the air flow is produced by rotation of the compressor; the pressure of the mass ensures the expanding gas travels in a rearward direction. Once ignition is achieved, the flame will be continuous, providing fuel is supplied and the ignition device can be switched off. The hot gases crossing the turbine produce torque to drive the compressor; therefore the starter can also be switched off.

Stage Centrifugal Flow Turbojet For Training Purpose Only


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Add a higher by-pass ratio, the engine shown above is a ‘low by-pass’ of around 1:1, to make the engine more efficient at lower altitudes and speeds and the turbofan comes into existence. These have ratios of over 1:1 to more than 6 5 and are said to be ‘high by-pass’ engines. Note that the British term is a ‘twin spool’ engine but the Americans use the phrase ‘dual axial’ . On some high by-pass engines the ‘cold’ air and the ‘hot’ air emerge as separate streams; on others, the streams are combined in what is called a ‘common nozzle’ . The high bypass turbofan engine is essentially a fixed pitch, multi-bladed, ducted propeller. It shifts a very large mass of air faster than a conventional propeller but much more slowly than a pure jet. It is able to move this large mass of air quite quickly because, unlike a propeller blade, the fan blade can cut through the air at supersonic velocities – the tip is usually quoted as moving at around Mach 1.3.

Triple Spool Front Fan turbojet (High By-Pass Ration) It is for this reason that some turbofans have mid-span shrouds (snubbers or clappers) at some stage along their length to support them and stop them ‘whipping’ or ‘ flapping’ in the airflow. Wide chord fan blades do not require these supports. As stated above, the air mass flow is cooler leaving a by-pass engine so that the thrust is achieved by moving the air through a larger area of nozzle. By making a comparison between pure jet engines and by-pass engines more differences can be found.



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The turbines of pure jet engines are heavy because they are

smaller overall spool sizes and permits a closer matching between the components, this, in turn, leads to less stages in both the compressor and the turbine to perform the same tasks. For a given mass flow of air through the engine a by-pass engine produces less thrust due to the lower exit velocity. To obtain the same thrust, a by-pass engine must be scaled to move a greater mass flow of air than a pure jet engine. The weight of the engine is still less than the equivalent pure jet engine because of the reduced size of the HP section which still gives an improvement in the power-weight ratio as well as a lower specific fuel consumption (this will be dealt with later). BASIC CONSTRUCTION TURBOPROP ENGINE

dealing with the whole mass flow of air through the engine. Bypass engines are only using a proportion of the air through the turbines which mean, in turn, that the HP compressor and the combustor can now be made smaller and, therefore, lighter. In order to obtain the same power at the turbine to drive the compressors and the accessories, the turbine inlet temperature is elevated and the pressure ratio is also increased. The core engine is not only narrower but shorter and the use of modern materials and improved gas flow characteristics makes for a considerably lighter engine. The weight reduction on a typical low by-pass engine over a pure jet of similar mass flow is around 20%. Curiously, the number of parts in a triple spool engine is less than those in a twin spool. This is brought about by having

OPERATION

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The turbo-prop (turbo-propeller engine) is a combination of a gas turbine and a propeller. Turbo-props are basically similar to turbo-jet engines in that both have a compressor, combustion chamber(s), turbine and jet nozzle, all of which operate in the same manner on both engines. However, the difference is that the turbine in the turbo-prop engine usually has more stages than that in the turbo-jet engine. In addition to operating the compressor and accessories the turbo-prop turbine transmits increased power forward, through a shaft and a gear train, to drive the propeller. The increased power is generated by the exhaust gases passing additional stages of the turbine. The exhaust gases and reaction within the engine also contribute to engine power output through jet reaction; the amount of energy available for jet thrust is roughly 10% on most modern engines at ISA (SL). For Training Purpose Only


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Single Entry Two Stage Centrifugal Flow Turbo propeller The turbo propeller pictured above is known as a ‘fixed turbine’ unit. This is because the turbine drives the compressor, accessory gearbox and reduction gearbox (propeller) as one mechanically coupled unit. This is a very simple system. It is light for the power output obtained and relatively simple to maintain. Most turbo propeller engines are now ‘free turbine’ units. This is a design where there is one turbine to drive the compressor and the accessory gearbox and another turbine to drive the reduction gear and propeller. The only link between the ‘core’ engine (the turbine, compressor and accessory gearbox) and the propeller drive is energy rich gas. Free power turbines are connected to the gas generator solely by a stream of energy enriched gas, there is no mechanical coupling.

The turbo propeller engine shown above is a combination of the two because it has a free turbine that also drives a LP compressor and a HP compressor driven by its own turbine. The accessories are driven from the gas generator ( ‘core’ ) – the HP section. The typical turbo-prop engine can be broken down into assemblies as follows: The power section assembly which contains the usual major components of gas turbine engines (compressor, combustion chamber, turbine and exhaust system). For Training Purpose Only


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The reduction gear or gear-box assembly which contains those sections peculiar to turbo-prop configurations. The torque meter assembly which transmits the torque from the engine to the gear box to the reduction section. The accessory drives housing assembly. The turbo-prop engine can be used in many different configurations. It is often used in transport aircraft, but can be adapted for use in single-engine aircraft.

Free Turbine Turbopropeller

BASIC CONSTRUCTION TURBO-SHAFT ENGINE

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OPERATION

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A gas turbine engine that delivers power through a shaft to operate something other than a propeller is referred to as a turbo shaft engine. Turbo-shaft engines are similar to turbo-prop engines. The power take-off may be coupled directly to the engine turbine or the shaft may be driven by a turbine. The free turbine is located downstream of the engine turbine. The free turbine independently, being connected to the main engine only by the hot stream of gases. This principle is used in the majority of turbo-shaft engines currently produced and is being used extensively in helicopters and hovercraft.

Free Turbine Turbo shaft

Twin spool Turbo shaft with Free Turbine For Training Purpose Only


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PIA Module 15 (Gas Turbine Engine) Sub Module 15.1 (Fundamentals)

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