An Introduction To Gas Turbine Types , Classification ,Function, And Applications
Introduction
The gas turbine is an internal combustion engine that uses air as the working fluid. The engine extracts chemical energy from fuel and converts it to mechanical energy using the gaseous energy of the working fluid (air) to drive the engine and propeller, which, in turn, propel the airplane.
Worldwide trends in power generation and electricity conversion processes and the role of gas turbines to minimise CO2 emissions are addressed. Gas turbines are essential and crucial to reduce emissions both in aviation and in power production. Technologies for improving gas turbine and system efficiency
A cycle describes what happens to air as it passes into, through, and out of the gas turbine.
The cycle usually describes the relationship between the space occupied by the air in the
system (called volume, V) and the pressure (P) it is under. The Brayton cycle (1876), shown
in graphic form in Fig. 2 as a pressure-volume diagram, is a representation of the properties
of a fixed amount of air as it passes through a gas turbine in operation. These same points
are also shown in the engine schematic in Fig. 3.
Air is compressed from point 1 to point 2. This increases the pressure as the volume of space
occupied by the air is reduced.
The air is then heated at constant pressure from 2 to 3 . This heat is added by injecting fuel into the combustor and igniting it on a continuous basis
The hot compressed air at point 3 is then allowed to expand (from point 3 to 4) reducing the
pressure and temperature and increasing its volume. this represents
flow through the turbine to point 3’ and then flow through the power turbine to point 4 to turn a
shaft or a ship’s propeller. the flow from point 3’ to 4 is through the exit nozzle to produce thrust. The "useful work" in Fig. 2 is indicated by the curve 3’- 4. This is the energy available to cause output shaft power for a land-based gas turbine , or thrust for a jet aircraft.
GAS TURBINE SECTIONS
Inlet
The air inlet duct must provide clean and unrestricted airflow to the engine. Clean and undisturbed inlet airflow extends engine life by preventing erosion, corrosion, and foreign object damage (FOD).
Consideration of atmospheric conditions such as dust, salt, industrial pollution, foreign
objects (birds, nuts and bolts), and temperature (icing conditions) must be made when designing the inlet system. Fairings should be installed between the engine air inlet
housing and the inlet duct to ensure minimum airflow losses to the engine at all airflow conditions.
The inlet duct assembly is usually designed and produced as a separate system rather than as part of the design and production of the engine.
Compressor
The compressor is responsible for providing the turbine with all the air it needs in an efficient manner. In addition, it must supply this air at high static pressures. The example of a large turboprop axial flow compressor will be used. The compressor is assumed to contain fourteen stages of rotor blades and stator vanes.
In general terms, the compressor rotor blades convert mechanical energy into gaseous energy. This energy conversion greatly increases total pressure (PI) Figure 5. Most of the increase is in the form of velocity (Pi), with a small increase in static pressure (Ps) due to the divergence of the blade flow paths.
Diffuser
Air leaves the compressor through exit guide vanes, which convert the radial component of the air flow out of the compressor to straight-line flow. The air then enters the diffuser section of the engine, which is a very divergent duct. The primary function of the diffuser structure is aerodynamic. The divergent duct shape converts most of the air’s velocity (Pi) into static pressure (PS). As a result, the highest static pressure and lowest velocity in the entire engine is at the point of diffuser discharge and combustor inlet. Other aerodynamic design considerations that are important in the diffuser section arise from the need for a short flow path, uniform flow distribution, and low drag loss.
In addition to critical aerodynamic functions, the diffuser also provides:
∎ Engine structural support, including engine mounting to the nacelle
∎ Support for the rear compressor bearings and seals
∎ Bleed air ports, which provide pressurized air for:
∗ air frame "customer" requirements (air conditioning, etc.)
∗ engine inlet anti-icing
∗ control of acceleration bleed air valves
∎ Pressure and scavenge oil passages for the rear compressor and front turbine bearings.
∎ Mounting for the fuel nozzles.
Combustors
A successful combustor design must satisfy many requirements and has been
a challenge from the earliest gas turbines of Whittle and von Ohain. The relative importance
of each requirement varies with the application of the gas turbine, and of course, some
requirements are conflicting, requiring design compromises to be made. Most design
requirements reflect concerns over engine costs, efficiency, and the environment. The basic
design requirements can be classified as follows:
1. High combustion efficiency at all operating conditions.
2. Low levels of unburned hydrocarbons and carbon monoxide, low oxides of nitrogen at
high power and no visible smoke. (Minimized pollutants and emissions.)
3. Low pressure drop. Three to four percent is common.
4. Combustion must be stable under all operating conditions.
5. Consistently reliable ignition must be attained at very low temperatures, and at high
altitudes (for aircraft).
6. Smooth combustion, with no pulsations or rough burning.
7. A low temperature variation for good turbine life requirements.
8. Useful life (thousands of hours), particularly for industrial use.
9. Multi-fuel use. Characteristically natural gas and diesel fuel are used for industrial
applications and kerosene for aircraft.
10.Length and diameter compatible with engine envelope (outside dimensions).
11.Designed for minimum cost, repair and maintenance.
12.Minimum weight (for aircraft applications).
A combustor consists of at least three basic parts: a casing, a flame tube and a fuel injection
system. The casing must withstand the cycle pressures and may be a part of the structure of
the gas turbine. It encloses a relatively thin-walled flame tube within which combustion takes
place, and a fuel injection system.
Turbine
The turbine converts the gaseous energy of the air/burned fuel mixture out of the combustor into mechanical energy to drive the compressor, driven accessories, and, through a reduction gear, the propeller.
The turbine converts gaseous energy into mechanical energy by expanding the hot, high-pressure gases to a lower temperature and pressure.
Each stage of the turbine consists of a row of stationary vanes followed by a row of rotating blades.
This is the reverse of the order in the compressor. In the compressor, energy is added to the gas by the rotor blades, then converted to static pressure by the stator vanes. In the turbine, the stator vanes increase gas velocity, and then the rotor blades extract energy.
Exhaust
After the gas has passed through the turbine, it is discharged through the exhaust. Though most of the gaseous energy is converted to mechanical energy by the turbine, a significant amount of power remains in the exhaust gas. This gas energy is accelerated through the convergent duct shape of the exhaust to make it more useful as jet thrust - the principle of equal and opposite reaction means that the force of the exhausted air drives the airplane forward.
Types of gas turbines
1-Jet engines
This done in order to produce the thrust needed to overcome the aerodynamic drag of an aircraft. A jet engines are gas turbines optimized to produce thrust from the exhaust gases, or from ducted fans connected to the gas turbines. Jet engines that produce thrust primarily from the direct impulse of exhaust gases are often called turbojets.
The original design of the jet engine is known as a “Whittle” (Fig. 8) developed by Sir Frank Whittle in 1930’s. The first flight of a jet engine of his design was in 1941. The main modification in the gas turbine jet engine is the addition of the jet pipe and propelling nozzle. The earliest commercial jet aircrafts used a single-spool turbojet engine
2-Auxiliary power units
APUs are small gas turbines designed for auxiliary power of larger machines, such as those inside an aircraft. They supply compressed air for aircraft ventilation (with an appropriate compressor design), start-up power for larger jet engines, and electrical and hydraulic power.
3-Industrial gas turbines for power generation
Industrial gas turbines differ from aeronautical designs in that the frames, bearings, and blading are of heavier construction. They are also much more closely integrated with the devices they power—electric generator—and the secondary-energy equipment that is used to recover residual energy (largely heat). Its thermal efficiency is around the 30%.
4-industrial gas turbines for mechanical drive
Industrial gas turbines that are used solely for mechanical drive or used in collaboration with a recovery steam generator differ from power generating sets in that they are often smaller and feature a "twin" shaft design as opposed to a single shaft. The power range varies from 1 megawatt up to 50 megawatts. These engines are connected via a gearbox to either a pump or compressor assembly, the majority of installations are used within the oil and gas industries. Mechanical drive applications provide a more efficient combustion raising around 2%. Oil and Gas platforms require these engines to drive compressors to inject gas into the wells to force oil up via another bore, they're also often used to provide power for the platform.
5-Turbo shaft engines
Turbo shaft engines are often used to drive compression trains (for example in gas pumping stations or natural gas liquefaction plants) and are used to power almost all modern helicopters Fig.11. The primary shaft bears the compressor and the high speed turbine (often referred to as the Gas Generator), while a second shaft bears the low-speed turbine (a power turbine or freewheeling turbine on helicopters, especially, because the gas generator turbine spins separately from the power turbine). In effect the separation of the gas generator, by a fluid coupling (the hot energy-rich combustion gases), from the power turbine is analogous to an automotive transmission's fluid coupling. This arrangement is used to increase power-output flexibility with associated highly-reliable control mechanisms.
6-Microturbines
Also known as:
⋆ Turbo alternators
⋆ Turbo generator
Microturbines are small electricity generators that burn gaseous and liquid fuels to create highspeed rotation that turns an electrical generator. Today’s microturbine technology is the result of development work in small stationary and automotive gas turbines, auxiliary power equipment, and turbochargers,
They are touted to become widespread in distributed power and combined heat and power applications. They are one of the most promising technologies for powering hybrid electric vehicles. They range from hand held units producing less than a kilowatt, to commercial sized systems that produce tens or hundreds of kilowatts. Basic principles of microturbine are based on micro combustion. Microturbines are ideally suited for distributed generation applications due to their flexibility in connection methods, ability to be stacked in parallel to serve larger loads, ability to provide stable and reliable power, and low emissions. Types of applications include:
⋆ Peak shaving and base load power (grid parallel)
⋆ Combined heat and power
⋆ Stand-alone power
⋆ Backup/standby power
⋆ Ride-through connection
⋆ Primary power with grid as backup o Microgrid
⋆ Resource recovery
gas turbines classification
A- Open cycle gas turbines:
1-Single shaft gas turbine
It's the simplest form of land base gas turbines where compressor & turbine are connected via the same shaft yet they have the same speed of rotation.
2-Twin spool gas turbine
More complex configuration of gas turbine, in this engine there are two concentric shafts the first shaft is low pressure shaft & the other is high pressure shaft & both shafts are rotating with different speeds, the main advantages of this configuration is that the star up torque required to turn the machine is minimized compare to single shaft with the same load since only high pressure shaft needed to be turned, also compressor surge is minimized in this configuration, also its is shorter smaller & lighter than single shaft engine & has less number of blow off lines ,the main disadvantage of this configuration is additional complexity to the design & added cost.
B-Gas generator & power turbine:
In this configuration, the gas turbine is used as GAS GENERATOR, the gas turbine provide stream of
hot gases which turns the power turbine on the left & the power turbine turns the load.
C-Combined cycle gas turbines:
1- Single shaft combined cycle (Steam & gas turbines are on the same shaft via synchro self shifting clutch
2-Two shaft combined cycle
D-Closed cycle gas turbines:
Closed cycle gas turbines are not so common like open cycles, on these engines the working fluid that is exit from turbine is goes through heat rejection process & recycled a gain as input for compressor, examples of working fluid used in this cycles are hydrogen, helium.
Advantages and Disadvantages of Gas Turbine:
Advantages:
⋇ Very high power-to-weight ratio, compared to reciprocating engines.
⋇ Smaller than most reciprocating engines of the same power rating.
⋇ Moves in one direction only, with far less vibration than a reciprocating engine.
⋇ Fewer moving parts than reciprocating engines.
⋇ Greater reliability, particularly in applications where sustained high power output is required.
⋇ Waste heat is dissipated almost entirely in the exhaust. This results in a high temperature exhaust stream that is very usable for boiling water in a combined cycle, or for cogeneration.
⋇ Low operating pressures.
⋇ High operation speeds.
⋇ Low lubricating oil cost and consumption.
⋇ Can run on a wide variety of fuels.
⋇ Very low toxic emissions of CO and HC due to excess air, complete combustion and no "quench" of the flame on cold surfaces.
Disadvantages:
⋇ Cost is very high
⋇ Less efficient than reciprocating engines at idle speed
⋇ Longer startup than reciprocating engines
⋇ Less responsive to changes in power demand compared with reciprocating engines
⋇ Characteristic whine can be hard to suppress.
What is the main deference between land base gas turbines & gas turbine used in aircraft's?
Methods of turbine blades cooling
Turbine blades must be cooled to protect them from melting due to high turbine inlet temperature, highestturbine inlet temperature raised to 1370 ºC in new designs. There are two main techniques used to cool down turbine blades which are air cooling & water cooling (steam).
The range for air cooling technique is up to turbine inlet temperature of 1150 ºC & for water cooling technique is up to turbine inlet temperature of 1315 ºC . In some hybrid designs both technique are used where water cooling is used for turbine first fixed stage & air cooling is used for the other blades.
m = the mass flow rate
Introduction
The gas turbine is an internal combustion engine that uses air as the working fluid. The engine extracts chemical energy from fuel and converts it to mechanical energy using the gaseous energy of the working fluid (air) to drive the engine and propeller, which, in turn, propel the airplane.
Worldwide trends in power generation and electricity conversion processes and the role of gas turbines to minimise CO2 emissions are addressed. Gas turbines are essential and crucial to reduce emissions both in aviation and in power production. Technologies for improving gas turbine and system efficiency
THE GAS TURBINE CYCLE
The basic principle of the airplane turbine engine is identical to any and all engines that extract energy from chemical fuel. The basic 4 steps for any internal combustion engine are (Figure 1):
1. Intake of air (and possibly fuel).
2. Compression of the air (and possibly fuel).
3. Combustion, where fuel is injected (if it was not drawn in with the intake air) and burned to convert the stored energy.
4. Expansion and exhaust, where the converted energy is put to use.
 |
| FIG 1 |
The cycle usually describes the relationship between the space occupied by the air in the
system (called volume, V) and the pressure (P) it is under. The Brayton cycle (1876), shown
in graphic form in Fig. 2 as a pressure-volume diagram, is a representation of the properties
of a fixed amount of air as it passes through a gas turbine in operation. These same points
are also shown in the engine schematic in Fig. 3.
 |
| FIG 2 |
 |
| FIG 3 |
occupied by the air is reduced.
The air is then heated at constant pressure from 2 to 3 . This heat is added by injecting fuel into the combustor and igniting it on a continuous basis
The hot compressed air at point 3 is then allowed to expand (from point 3 to 4) reducing the
pressure and temperature and increasing its volume. this represents
flow through the turbine to point 3’ and then flow through the power turbine to point 4 to turn a
shaft or a ship’s propeller. the flow from point 3’ to 4 is through the exit nozzle to produce thrust. The "useful work" in Fig. 2 is indicated by the curve 3’- 4. This is the energy available to cause output shaft power for a land-based gas turbine , or thrust for a jet aircraft.
GAS TURBINE SECTIONS
 | |
Inlet
The air inlet duct must provide clean and unrestricted airflow to the engine. Clean and undisturbed inlet airflow extends engine life by preventing erosion, corrosion, and foreign object damage (FOD).
Consideration of atmospheric conditions such as dust, salt, industrial pollution, foreign
objects (birds, nuts and bolts), and temperature (icing conditions) must be made when designing the inlet system. Fairings should be installed between the engine air inlet
housing and the inlet duct to ensure minimum airflow losses to the engine at all airflow conditions.
The inlet duct assembly is usually designed and produced as a separate system rather than as part of the design and production of the engine.
Compressor
The compressor is responsible for providing the turbine with all the air it needs in an efficient manner. In addition, it must supply this air at high static pressures. The example of a large turboprop axial flow compressor will be used. The compressor is assumed to contain fourteen stages of rotor blades and stator vanes.
In general terms, the compressor rotor blades convert mechanical energy into gaseous energy. This energy conversion greatly increases total pressure (PI) Figure 5. Most of the increase is in the form of velocity (Pi), with a small increase in static pressure (Ps) due to the divergence of the blade flow paths.
 |
| FIG 5 |
Diffuser
Air leaves the compressor through exit guide vanes, which convert the radial component of the air flow out of the compressor to straight-line flow. The air then enters the diffuser section of the engine, which is a very divergent duct. The primary function of the diffuser structure is aerodynamic. The divergent duct shape converts most of the air’s velocity (Pi) into static pressure (PS). As a result, the highest static pressure and lowest velocity in the entire engine is at the point of diffuser discharge and combustor inlet. Other aerodynamic design considerations that are important in the diffuser section arise from the need for a short flow path, uniform flow distribution, and low drag loss.
In addition to critical aerodynamic functions, the diffuser also provides:
∎ Engine structural support, including engine mounting to the nacelle
∎ Support for the rear compressor bearings and seals
∎ Bleed air ports, which provide pressurized air for:
∗ air frame "customer" requirements (air conditioning, etc.)
∗ engine inlet anti-icing
∗ control of acceleration bleed air valves
∎ Pressure and scavenge oil passages for the rear compressor and front turbine bearings.
∎ Mounting for the fuel nozzles.
Combustors
A successful combustor design must satisfy many requirements and has been
a challenge from the earliest gas turbines of Whittle and von Ohain. The relative importance
of each requirement varies with the application of the gas turbine, and of course, some
requirements are conflicting, requiring design compromises to be made. Most design
requirements reflect concerns over engine costs, efficiency, and the environment. The basic
design requirements can be classified as follows:
1. High combustion efficiency at all operating conditions.
2. Low levels of unburned hydrocarbons and carbon monoxide, low oxides of nitrogen at
high power and no visible smoke. (Minimized pollutants and emissions.)
3. Low pressure drop. Three to four percent is common.
4. Combustion must be stable under all operating conditions.
5. Consistently reliable ignition must be attained at very low temperatures, and at high
altitudes (for aircraft).
6. Smooth combustion, with no pulsations or rough burning.
7. A low temperature variation for good turbine life requirements.
8. Useful life (thousands of hours), particularly for industrial use.
9. Multi-fuel use. Characteristically natural gas and diesel fuel are used for industrial
applications and kerosene for aircraft.
10.Length and diameter compatible with engine envelope (outside dimensions).
11.Designed for minimum cost, repair and maintenance.
12.Minimum weight (for aircraft applications).
A combustor consists of at least three basic parts: a casing, a flame tube and a fuel injection
system. The casing must withstand the cycle pressures and may be a part of the structure of
the gas turbine. It encloses a relatively thin-walled flame tube within which combustion takes
place, and a fuel injection system.
Turbine
The turbine converts the gaseous energy of the air/burned fuel mixture out of the combustor into mechanical energy to drive the compressor, driven accessories, and, through a reduction gear, the propeller.
The turbine converts gaseous energy into mechanical energy by expanding the hot, high-pressure gases to a lower temperature and pressure.
Each stage of the turbine consists of a row of stationary vanes followed by a row of rotating blades.
This is the reverse of the order in the compressor. In the compressor, energy is added to the gas by the rotor blades, then converted to static pressure by the stator vanes. In the turbine, the stator vanes increase gas velocity, and then the rotor blades extract energy.
 |
| FIG 6 |
After the gas has passed through the turbine, it is discharged through the exhaust. Though most of the gaseous energy is converted to mechanical energy by the turbine, a significant amount of power remains in the exhaust gas. This gas energy is accelerated through the convergent duct shape of the exhaust to make it more useful as jet thrust - the principle of equal and opposite reaction means that the force of the exhausted air drives the airplane forward.
Types of gas turbines
1-Jet engines
 |
| FIG 7 |
This done in order to produce the thrust needed to overcome the aerodynamic drag of an aircraft. A jet engines are gas turbines optimized to produce thrust from the exhaust gases, or from ducted fans connected to the gas turbines. Jet engines that produce thrust primarily from the direct impulse of exhaust gases are often called turbojets.
 |
| FIG 8 |
The original design of the jet engine is known as a “Whittle” (Fig. 8) developed by Sir Frank Whittle in 1930’s. The first flight of a jet engine of his design was in 1941. The main modification in the gas turbine jet engine is the addition of the jet pipe and propelling nozzle. The earliest commercial jet aircrafts used a single-spool turbojet engine
2-Auxiliary power units
APUs are small gas turbines designed for auxiliary power of larger machines, such as those inside an aircraft. They supply compressed air for aircraft ventilation (with an appropriate compressor design), start-up power for larger jet engines, and electrical and hydraulic power.
 |
| FIG 9 |
3-Industrial gas turbines for power generation
Industrial gas turbines differ from aeronautical designs in that the frames, bearings, and blading are of heavier construction. They are also much more closely integrated with the devices they power—electric generator—and the secondary-energy equipment that is used to recover residual energy (largely heat). Its thermal efficiency is around the 30%.
 |
| FIG 10 |
Industrial gas turbines that are used solely for mechanical drive or used in collaboration with a recovery steam generator differ from power generating sets in that they are often smaller and feature a "twin" shaft design as opposed to a single shaft. The power range varies from 1 megawatt up to 50 megawatts. These engines are connected via a gearbox to either a pump or compressor assembly, the majority of installations are used within the oil and gas industries. Mechanical drive applications provide a more efficient combustion raising around 2%. Oil and Gas platforms require these engines to drive compressors to inject gas into the wells to force oil up via another bore, they're also often used to provide power for the platform.
5-Turbo shaft engines
Turbo shaft engines are often used to drive compression trains (for example in gas pumping stations or natural gas liquefaction plants) and are used to power almost all modern helicopters Fig.11. The primary shaft bears the compressor and the high speed turbine (often referred to as the Gas Generator), while a second shaft bears the low-speed turbine (a power turbine or freewheeling turbine on helicopters, especially, because the gas generator turbine spins separately from the power turbine). In effect the separation of the gas generator, by a fluid coupling (the hot energy-rich combustion gases), from the power turbine is analogous to an automotive transmission's fluid coupling. This arrangement is used to increase power-output flexibility with associated highly-reliable control mechanisms.
 |
| FIG 11 |
Also known as:
⋆ Turbo alternators
⋆ Turbo generator
Microturbines are small electricity generators that burn gaseous and liquid fuels to create highspeed rotation that turns an electrical generator. Today’s microturbine technology is the result of development work in small stationary and automotive gas turbines, auxiliary power equipment, and turbochargers,
 |
| FIG 12 |
They are touted to become widespread in distributed power and combined heat and power applications. They are one of the most promising technologies for powering hybrid electric vehicles. They range from hand held units producing less than a kilowatt, to commercial sized systems that produce tens or hundreds of kilowatts. Basic principles of microturbine are based on micro combustion. Microturbines are ideally suited for distributed generation applications due to their flexibility in connection methods, ability to be stacked in parallel to serve larger loads, ability to provide stable and reliable power, and low emissions. Types of applications include:
⋆ Peak shaving and base load power (grid parallel)
⋆ Combined heat and power
⋆ Stand-alone power
⋆ Backup/standby power
⋆ Ride-through connection
⋆ Primary power with grid as backup o Microgrid
⋆ Resource recovery
gas turbines classification
A- Open cycle gas turbines:
1-Single shaft gas turbine
It's the simplest form of land base gas turbines where compressor & turbine are connected via the same shaft yet they have the same speed of rotation.
 |
| FIG 13 |
2-Twin spool gas turbine
More complex configuration of gas turbine, in this engine there are two concentric shafts the first shaft is low pressure shaft & the other is high pressure shaft & both shafts are rotating with different speeds, the main advantages of this configuration is that the star up torque required to turn the machine is minimized compare to single shaft with the same load since only high pressure shaft needed to be turned, also compressor surge is minimized in this configuration, also its is shorter smaller & lighter than single shaft engine & has less number of blow off lines ,the main disadvantage of this configuration is additional complexity to the design & added cost.
 |
| FIG 14 |
B-Gas generator & power turbine:
In this configuration, the gas turbine is used as GAS GENERATOR, the gas turbine provide stream of
hot gases which turns the power turbine on the left & the power turbine turns the load.
 |
| FIG 15 |
C-Combined cycle gas turbines:
1- Single shaft combined cycle (Steam & gas turbines are on the same shaft via synchro self shifting clutch
 |
| FIG 16 |
2-Two shaft combined cycle
 |
| FIG 17 |
D-Closed cycle gas turbines:
Closed cycle gas turbines are not so common like open cycles, on these engines the working fluid that is exit from turbine is goes through heat rejection process & recycled a gain as input for compressor, examples of working fluid used in this cycles are hydrogen, helium.
 |
| FIG 18 |
Advantages and Disadvantages of Gas Turbine:
Advantages:
⋇ Very high power-to-weight ratio, compared to reciprocating engines.
⋇ Smaller than most reciprocating engines of the same power rating.
⋇ Moves in one direction only, with far less vibration than a reciprocating engine.
⋇ Fewer moving parts than reciprocating engines.
⋇ Greater reliability, particularly in applications where sustained high power output is required.
⋇ Waste heat is dissipated almost entirely in the exhaust. This results in a high temperature exhaust stream that is very usable for boiling water in a combined cycle, or for cogeneration.
⋇ Low operating pressures.
⋇ High operation speeds.
⋇ Low lubricating oil cost and consumption.
⋇ Can run on a wide variety of fuels.
⋇ Very low toxic emissions of CO and HC due to excess air, complete combustion and no "quench" of the flame on cold surfaces.
Disadvantages:
⋇ Cost is very high
⋇ Less efficient than reciprocating engines at idle speed
⋇ Longer startup than reciprocating engines
⋇ Less responsive to changes in power demand compared with reciprocating engines
⋇ Characteristic whine can be hard to suppress.
What is the main deference between land base gas turbines & gas turbine used in aircraft's?
 |
| FIG 19 |
Turbine blades must be cooled to protect them from melting due to high turbine inlet temperature, highestturbine inlet temperature raised to 1370 ºC in new designs. There are two main techniques used to cool down turbine blades which are air cooling & water cooling (steam).
The range for air cooling technique is up to turbine inlet temperature of 1150 ºC & for water cooling technique is up to turbine inlet temperature of 1315 ºC . In some hybrid designs both technique are used where water cooling is used for turbine first fixed stage & air cooling is used for the other blades.
 |
| FIG 20 |
Gas Turbine Design Considerations
Mechanical Drive Power
Power at the gas turbine shaft is the product of the torque and speed, with appropriate unit conversion
constants. (U.S. Customary Units)
where
SHP = shaft horsepower, with Torque in lbf-ft and
Speed in rpm
(SI Units)
Power =Torque ×Speed ×2𝛑/60
where Power is in watts, with Torque in N-m, and Speed in rpm
Efficiency
Heat rate, conventionally used for generator drives, may not be the preferred parameter for mechanical drives. If thermal efficiency is required, it is
where heat input (Btu/hr in this case) . Heat rate, in Btu/hp-hr, can be calculated from
Heat Input/SHP. In SI Units, thermal efficiency can be calculated by
where power is in watts, and heat input (kJ/hr in this case) . Heat rate, in kJ/W-hr, can
be calculated from Heat Input/Power.
Gas Turbine Power Plant
A simple gas turbine plant was depicted schematically in Fig. 21. This operates on the Joule/Brayton cycle and represented on the temperature entropy diagrarn as shown in Fig. 22. The gas turbine cycle thermal efficiency𝛈gt can be expressed as
 |
| FIG 21 |
 |
| FIG 22 |
Cpg = the specific heat at constant pressure
T0 = the stagnation temperature.
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