2.3.1 gt fundamentals

2.3.1 Fundamentals of Gas Turbine Operation

GTFUN2-0 2.3.1-1

GTFUN2-0

ROTOR

SIMPLIFIED GAS TURBINE ARRANGEMENTS

ROTOR

HOT GASGENERATOR

HOT GASGENERATOR

FREE POWERTURBINE

FUELSUPPLY

FUELSUPPLY

INLET AIR

INLET AIR

ELECTRIC GENERATOR

ELECTRIC GENERATOR

ELECTRIC GENERATOR

OCEAN BREEZE

ROTOR

WINDTURBINE

FIGURE 1

FIGURE 2

FIGURE 2

1st TURBINE2nd TURBINE

COMPRESSOR

2.3.1 Fundamentals of Gas Turbine Operation

Figure 1 above depicts an electric generator driven by a wind turbine. In this application, the wind turbine must be capable of rotating the generator at 3,000 or 3,600 RPM and also apply sufficient torque on the generator shaft to produce electrical power. Production of electrical power using this method is very economical since it does not consume fuel. The practical problem with this system, however, is that winds strong enough and reliable enough to operate a large electric generator are difficult to find ( fortunately ).

Figure 2 is a simplified representation of another method. It involves replacement of the wind source with a gas generator, which draws in ambient air, heats the air to accelerate it and then delivers hot gas to the power turbine wheel, which drives the electric generator. This arrangement is practical for driving large electric generators.

In order for the gas generator to function, it uses an axial flow compressor to draw in the ambient air, a combustion section to heat the air, and an internal turbine wheel to extract some of the hot gas energy to drive its compressor so that it will draw in more air. The remaining hot gas energy exhausting from the gas generator is delivered to the power turbine to drive the electric generator. Figure 2 represents what is referred to as a two-shaft gas turbine engine. The gas generator and power turbine are said to be thermodynamically coupled together. The rotating shaft within the gas generator is not mechanically connected to the power turbine shaft.

Figure 3 represents a single shaft gas turbine engine. In this arrangement the gas generator and power turbine shafts are mechanically coupled and rotate at the same speed. The hot gas extracted by the first turbine stage converts enough hot gas energy into mechanical energy to drive the axial flow compressor and the remaining hot gas energy is converted into mechanical energy by the second stage turbine to drive the electric generator.

GTFUN2-1LM60TA 2.3.1-2

GT2-0BJSIEMENS V94.2 GAS TURBINE ENGINE

COMPRESSOR

COMPRESSORINLET CASE

TURBINEEXHAUST CASE

COMBUSTOR

TURBINE

2.3.1 Fundamentals of Gas Turbine Operation (Cont'd)

The illustration above is a cutaway view of the gas turbine engine. It is a single-shaft machine having a sixteen stage axial flow compressor a four stage turbine to drive the compressor and the electric generator from the air inlet end of the engine.. The engine is designed to operate at a shaft speed of 3,600 RPM ( 60 HZ ) or 3,000 RPM ( 50 HZ ) and produces approximately 155,000 shaft horsepower ( 116 megawatts ). The four stages of turbine blades convert the hot gas energy, developed by the dual silo combustors, into mechanical energy to drive the gas turbine engine rotor and the electric generator. Approximately 40% of the hot gas energy developed by the combustor is consumed by the turbine to drive the axial flow compressor. The remaining 60% is converted by turbine to drive the electric generator. Since the turbine is not 100% efficient some of the 60% of energy is exhausted to atmosphere.

GTFUN2-2 2.3.1-3

GTFUN2-2

LM 6000 GAS TURBINE ENGINEFORCE= MASS X ACCELERATION

GTFUN2-2

LM 6000 GAS TURBINE ENGINEFORCE= MASS X ACCELERATION

NOZZLENOZZLEPRESSUREREACTION

ACTION

REACTION

ACTIONPRESSURE

ACTION

REACTION

DIRECTION OF TRAVEL

F = m x a(Force = mass x acceleration)

FIG. 2FIG. 1


In order to fully understand how a gas turbine engine operates it is necessary to review the basic principles of operation. The illustrations above are of a simple toy balloon. Figure 1 shows the balloon filled with air and the back nozzle tied with a string to prevent the air from escaping. The pressure exerted by the air on the inside of the balloon is equal in all directions. When the string is removed, as shown in Figure 2, the air flows from the nozzle of the balloon to atmospheric pressure, and the balloon travels in the opposite direction of the flow of escaping air.

The travel of the balloon is not caused by the escaping air pushing on atmospheric air, but rather by two physical laws of nature:

r For every action there is an equal and opposite reaction. This is Newton's 3rd Law of Motion.r Force equals mass x acceleration of the mass. This is Newton's 2nd Law of Motion

In this example, as the air escapes from the nozzle of the balloon it accelerates from zero velocity to some higher velocity. Acceleration is defined as the rate of change in velocity. Mass is defined as anything that occupies space and has molecules. Air satisfies this definition. The total force of the air escaping the balloon (the action force) can be calculated using the Force = Mass x Acceleration equation. The reaction force is equal in strength and opposite in direction. The reaction force pushes on the forward inside wall of the balloon and propels it in the opposite direction of the air escaping from the nozzle.

To increase the balloon's force we can increase the mass or increase the acceleration of the mass, or we can increase both. The two basic ways to increase mass are to use colder air (higher density) for a given space (volume) or increase the quantity of air compressed into the space, which is reflected by higher static pressure within the balloon. For a given space, colder air has more mass than warm air. This is because as things become colder they contract. Because of this, colder air contains more molecules than warm air.

GTFUN2-3 2.3.1-4

GTFUN2-3

NOZZLE

F = M X A(FORCE = MASS X ACCELERATION)

NOZZLE

PROPULSION

NOZZLE

FIGURE 3

FIGURE 5

FIGURE 4

PRESSURE


FUEL

FUEL



The balloon's distance of travel in Figure 2 of the previous illustration was limited by the amount of air that it contained and the time required for the all of the air to escape to atmosphere through the nozzle. If the air within the balloon were to be replaced at a faster rate or even at the same rate as the air escaping from the nozzle, then the balloon's distance and duration of travel would be extended indefinitely.

Figure 3 is a simplified possibility for accomplishing this objective. The obvious disadvantage of this machine, however, is that the air supply line to the balloon is too small to replenish the balloon's air supply at a rate equal to the air escaping from the nozzle, unless the dog is very fast on the bicycle pump.

The machine in Figure 4 is an improvement in the design. The air inlet to the balloon has been increased significantly and air is being supplied by an electric motor driven fan. The fan forces more mass into the balloon so the force of the balloon is also increased. The disadvantage here is that the distance of travel is limited by the length of the fan's electrical cord.

In Figure 5 the force of the balloon has been further increased by increasing the acceleration part of the Force Equation. This is accomplished by the addition of heat, expansion and acceleration of the mass by the combustion of fuel.

GTFUN2-4 2.3.1-5

GTFUN2-4

LM 6000 GAS TURBINE ENGINESINGLE SPOOL GAS GENERATOR

DIRECTION OF TRAVEL

FUEL SUPPY

FUEL SUPPY

TURBINE


TO POWER TURBINE

EXHAUST NOZZLE

FAN(COMPRESSOR)


The illustration above represents the basic elements of a single- shaft gas generator that is used in a two shaft gas turbine engine design, previously discussed. The balloon's fan is no longer driven by an electric motor, but rather by a turbine wheel that is located between the combustion zone and the exhaust nozzle. As the hot combustion gas accelerates across the turbine wheel, the turbine wheel converts the hot gas energy into mechanical energy and drives the compressor. This, in turn, draws more air into the gas generator for heating and expansion, completing a cycle that makes it self sufficient without the need for any outside services, except for the fuel supply.

GTFUN2-4A 2.3.1-6

SINGLE SHAFT GAS TURBINE ENGINE

COMPRESSOR


DIRECTION OF THRUST

EXHAUSTNOZZLE

FUELSUPPLY

FUELSUPPLY

TURBINE WHEELS

AIR

ROTOR

ELECTRICAL GENERATOR

SINGLE SHAFT GAS TURBINE ENGINE

GTFUN2-5

COMPRESSOR


DIRECTION OF THRUST

EXHAUSTNOZZLE

FUELSUPPLY

FUELSUPPLY

TURBINE WHEELS

AIR

ROTOR

ELECTRICAL GENERATOR


The illustration above represents a dual spool gas generator. This arrangement has two separate compressors that rotate independently of each other. A dual spool gas generator typically develops more force in the exhaust gas than does a single spool gas generator. This is because the two compressors draw in much more air than a single compressor does. More air means more Mass (Force = Mass X Acceleration).

This configuration of the gas generator and the previously described single spool gas generator are basic aircraft jet engines. These engines are mounted on the aircraft wing and propel the aircraft forward. For electrical power generation applications, the engine is bolted down securely and hot gas accelerating through the exhaust nozzle is used to rotate a power turbine which, is mechanically connected to an electric generator.

GTFUN2-6 2.3.1-7

LM 6000 GAS TURBINE ENGINE

GTFUN2-6CONVERGENT AND DIVERGENT DUCTS


FLOW

DIVERGENT DUCTDIRECT

ION

OF TRA

VEL

CONVERGENT DUCT

FIRE CRACKER

FIRE CRACKER

PIPE CAP

OPEN PIPE

PIPE CLOSED AT ONE END

(VELOCITY INCREASES)(STATIC PRESSURE DECREASES)

(VELOCITY DECREASES)(STATIC PRESSURE INCREASES)

FORCE

RAM AIR FUEL SUPPLY

FUEL SUPPLY

RAM JET

COMBUSTIONZONE

EXHAUST NOZZLE

FORCE FORCE (THRUST)


Figure 1 represents the simplest form of a gas turbine engine, the ram jet. The ram jet does not have any moving parts and is the most efficient turbine engine ever designed. It does not have a compressor or a turbine to drive the compressor, only a combustor. As a result of this, much more hot gas is accelerated through the exhaust nozzle. The only disadvantage of the ram jet is that it will not operate on the ground. It must first be placed into a forward motion at about 500 miles per hour in order for initial combustion to occur. The forward motion causes ram air pressure to increase ( mass ) within the engine. Once sufficient pressure is developed by the forward motion, fuel is then introduced and ignited in the combustion zone. This heats the air, causing it to expand and accelerate through the exhaust nozzle.

In Figure 2, the top pipe is open at both ends. When a fire cracker is dropped in the hole, in the middle of the pipe, combustion occurs within the pipe. The forces created by the combustion are equal in all directions. The equal forces exhaust from both ends of the pipe and the pipe does not travel in any direction. This is what would occur in the ram jet if it were not first placed in forward motion (500 miles per hour) before initiating the combustion process. Flames would exhaust from both ends of the pipe.

The bottom pipe in Figure 2 is capped at one end. When combustion occurs, the forces are equal in all directions, as before. However, the force in Figure 2 exhausts through the open end of the pipe, while the opposite and equal force pushes on the inside wall of the pipe cap, causing the pipe to travel in the opposite direction of the exhaust gases exiting the open end of the pipe.

This example is analogous to what propels the ram jet forward. Figure 2 explains why the ram jet must first be placed in a forward motion so that a wall of high pressure air can be developed just prior to the combustion zone of the engine. It is this wall of high pressure air that functions the same way as the cap on the pipe and prevents the hot combustion gases from exhausting from the front of the engine.

It is critical that the forces created by combustion do not exceed the pressure of the air upstream of the combustion zone. By carefully increasing fuel flow to the engine, the acceleration of the exhaust hot gases is increased, resulting in an increase in the speed of the engine through the air. The faster speed causes higher ram air pressure, and now more fuel can be injected into the engine, causing further acceleration without upsetting the pressure/force relationship between the wall of high pressure air at the inlet of the combustor and the forces created by the combustion process.

GTFUN2-6 2.3.1-8




FLOW


ION

OF TRA

VEL

CONVERGENT DUCT

FIRE CRACKER

FIRE CRACKER

PIPE CAP

OPEN PIPE




FORCE

RAM AIR FUEL SUPPLY

FUEL SUPPLY

RAM JET

COMBUSTIONZONE

EXHAUST NOZZLE



To better understand how air pressure and flow are developed within the ram jet, the convergent and divergent ducts shown in Figure 3 must be explained. As air or hot gas flows through a convergent duct it is accelerated to a higher velocity. As this occurs the static pressure decreases. Static pressure is the pressure exerted on the inside wall of the pipe. The force of the flowing air increases because it has been accelerated ( Force = Mass x Acceleration ). An example of this is the common garden hose with an adjustable nozzle. When the nozzle opening is decreased the force of the water flowing from the nozzle increases. The opposite and equal reaction force on the water hose can be felt in your hand. The exhaust nozzle of the ram jet is a convergent duct. As the hot gases accelerate through the exhaust nozzle, the ram jet is propelled.

When air or hot gas flows through a divergent duct it is decelerated to a slower velocity. As this occurs the static pressure increases. The shape of the ram jet is a divergent duct just upstream of the combustion zone. This area of the engine is referred to as the diffuser section. The diffuser section serves to increase static pressure and develop the wall of high pressure air upstream of the combustor, as previously described.

The forward end of the ram jet is a convergent duct that accelerates the air to fill the combustion zone at the same rate as the hot gases exiting the exhaust nozzle. Otherwise the combustion zone would be depleted of air by the exhaust nozzle and the engine would no longer be propelled forward.

GTFUN2-6 2.3.1-9




FLOW


ION

OF TRA

VEL

CONVERGENT DUCT

FIRE CRACKER

FIRE CRACKER

PIPE CAP

OPEN PIPE




FORCE

RAM AIR FUEL SUPPLY

FUEL SUPPLY

RAM JET

COMBUSTIONZONE

EXHAUST NOZZLE



Several key points are essential for the operator in order to understand the fundamentals of gas turbine engine operation.

r The conventional gas turbine engine functions identical to the ram jet except that an axial flow compressor and a turbine(s) to drive the compressor are installed, as the means to provide air to the combustor for heating and expansion. This allows for the engine to operate self sufficiently, without the need of forward movement to provide the air supply.

r The relationship between pressure created by combustion and the static air pressure created by the diffuser section of the engine must be such that the diffuser pressure is greater than the combustion pressure at all times. Otherwise flames will exhaust from both ends of the engine. This is commonly referred to as an engine stall. Stall is an air flow reversal within the Engine, which can cause catastrophic damage.

r Convergent ducts increase velocity and decrease static pressure. Divergent ducts decrease velocity and increase static pressure.

r Force = Mass x Acceleration. The force may be changed by changing the mass, such as colder air, or by changing the acceleration of the mass, by changing the combustor temperature as a result of changing fuel flow.

r For every action there is an equal and opposite reaction.

GTFUN2-1LM60TA 2.3.1-10

GT2-0BJSIEMENS V94.2 GAS TURBINE ENGINE

COMPRESSOR

COMPRESSORINLET CASE

TURBINEEXHAUST CASE

COMBUSTOR

TURBINE


The illustration above is a cutaway view of the Siemens V94.2 gas turbine engine. The air flow path through the axial flow compressors is a convergent duct. The air flow path changes to a divergent duct just prior to the combustor, which is the diffuser section of the engine. As the air is expanded by the combustion of fuel, the hot gas exits the combustion section through a convergent duct. The air flow path through the turbine sections is a divergent duct.

The gas turbine engine is comprised of three functional sections. The compressor, for the purpose of drawing in the mass and delivering it to the combustor. The combustor, for the purpose of heating, expanding and accelerating the mass, and delivering it to the turbine section.

The purpose of the turbine section is to convert the hot gas energy into mechanical energy to drive the axial flow compressor and the electric generator using the four stage turbine.

The following pages will discuss the fundamentals of operation for these three functional sections.

GTFUN2-7 2.3.1-11

GTFUN2-7TYPICAL AXIAL FLOW COMPRESSOR

SMALL VOLUME

HIGH VOLUME

HIGH PRESSURE

LOW PRESSURE

HIGH VELOCITYLOW VELOCITY ROTOR


Compressor

The above is a cross- sectional view of a typical axial flow compressor. The V94.2 engine has a sixteen stage axial flow compressor. The purpose of the compressor is to draw air into the engine and compress it into a smaller space to increase mass flow to the combustor.

The compressor is comprised of two basic elements, which are the rotor with compressor blades attached ( shown in red ), and the compressor case with stationary vanes attached ( shown in yellow ). The general shape of the compressor is a convergent duct. When the compressor rotor and blades rotate they draw in a large volume of air, compressing it into a smaller space or volume. The stator vanes increase the static pressure of the air as it flows from the inlet end of the compressor to the compressor discharge end. Since the pressure is increased as it flows through the compressor, the temperature also increases due to compression.

GTFUN2-8 2.3.1-12

GTFUN2-8AXIAL FLOW COMPRESSOR

STATOR CASE

INLET GUIDE VANES

AIR INLET

AIR INLET

STATIONARY STATOR VANES

ROTATING BLADES


Compressor (Contd)

To better understand how the compressor works, attention must be given to the shape of the spaces between each of the rotating blades, and also the spaces between each of the stationary ( stator ) vanes. These spaces are actually divergent ducts. The spaces between the stator vanes are more exaggerated divergent ducts than the spaces between the rotating blades.

As the shaft is rotated, the compressor blades draw the air into the engine through the convergent duct flow path. Because of the divergent shape between the blades, static pressure is increased. Pressure created by flow also increases. The static pressure and pressure created by flow, when added together is referred to as total pressure.

When the air enters the row of stator vanes immediately behind the blades, the divergent spaces between the vanes further increase static pressure. The compressor has been designed to produce the required flow rate and compression ratio at an operating speed of 3000 RPM. The problem is, however, that while operating at below design operating speeds, the static pressure developed by the stators can become greater than the upstream total pressure developed by the rotating blades, causing an air flow reversal ( stall ) within the compressor.

To prevent compressor stall while operating below the design operating speed, the compressor is provided with several compressor bleed doors. These valves are open during low speed (low air flow) operation and bleed off low pressure compressor discharge air. This arrangement prevents a stall condition from occurring within the compressor .

ECOMB2-1LM60TA 2.3.1-13

GT2-9BJ

FUEL INJECTIONNOZZLES

DIAGONALSWIRLERS

FLAMEDETECTOR

(TOTAL OF 2)FLAME

DETECTOR(TOTAL OF 2)

LEFTCOMBUSTOR

FIRST STAGETURBINE COMPRESSORDISCHARGE AIR

RIGHTCOMBUSTOR

FLAMECYLINDER

FLAMECYLINDER

AIRAPERTURES

DILUTIONAIR

PRIMARYAIR

COMBUSTOR


Combustor

The purpose of the combustor is to heat and accelerate the air ( mass ), delivered by the compressor. The combustor is of the twin silo design. Air flow combustion enters the combustor from the top and is mixed with the fuel for combustion by the diagonal swirlers. Dilution air enters through the side of the combustor, via the air apertures.

The flame temperature at full load is approximately 3200F. The temperature of the hot gas as it enters the first stage turbine nozzles is approximately 2100F.

The outlet of the combustor is designed as a convergent duct. Here, the hot gases are accelerated before entry into the turbine section of the engine.

GTFUN2-10 2.3.1-14

GTFUN2-10

FIG. 2FIG. 1

FUNDAMENTAL OF HIGH PRESSURE TURBINE( TYPICAL FOR LOW PRESSURE TURBINE )

HOT GAS IN

ROTATION

AIR

ROTATION ROTATION

1st STAGEFIXED NOZZLES

2nd STAGEFIXED NOZZLES

1st STAGEROTATING BLADES

2nd STAGEROTATING BLADES

CONVERGENT

CONVERGENT


Turbine

The V94.2 gas turbine engine is designed with four stage turbine. This is an impulse-reaction type of turbine. Its purpose is to convert the hot gas energy delivered from the combustor into mechanical energy, as the means to drive the engine's axial flow compressor and the electric generator rotor.

The illustration above represents the fundamentals of the turbine section's operation. Only two stages have been shown for the sake of simplicity. Hot gas enters the first stage turbine nozzles, which are stationary vanes, arranged such that the space between each of the vanes forms a convergent duct. Because of this the hot gas from the combustor is further accelerated before striking the first stage rotating blades, which are located immediately downstream of the first stage nozzles.

When the hot gas exits the first stage nozzles it strikes the first stage blades. With the impact ( impulse ) of the hot gas force, the blade is pushed, causing rotation. The second force occurs when the hot gas accelerates away from the first stage blades toward the second stage nozzle assembly. This action produces an equal and opposite reaction that also pushes on the first stage blades, contributing to rotation.The operating fundamentals for the remaining stages within the engine's turbine section are identical to the first stage.

NONE 2.3.1-15

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2.3.1 gt fundamentals

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