lect-7 - nptel · 2017-08-04 · turbine in the afterburner, the temperatures can be taken to much...
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![Page 1: Lect-7 - Nptel · 2017-08-04 · turbine in the afterburner, the temperatures can be taken to much higher values than that at turbine entry. Prof. Bhaskar Roy, Prof. A M Pradeep,](https://reader033.vdocuments.net/reader033/viewer/2022042100/5e7d1116d31f1625e4362015/html5/thumbnails/1.jpg)
1
Lect-7
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Prof. Bhaskar Roy, Prof. A M Pradeep, Department of Aerospace, IIT Bombay2
Lect-7
In this lecture...
• Jet engine cycles for aircraft propulsion– Turbojet engine– Turbojet engine with afterburning– Turbofan and its variants– Turboprop and turboshaft engines– Ramjet engines
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Prof. Bhaskar Roy, Prof. A M Pradeep, Department of Aerospace, IIT Bombay3
Lect-7
Gas Turbine Cycles
• Gas turbine engines operate on Braytoncycles.
• Ideal Brayton cycle is a closed cycle, whereas gas turbines operate in the open cycle mode.
• Ideal cycle assumes that there are no irreversibilities in the processes, air behaves like an ideal gas with constant specific heats, and that there are no frictional losses.
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Prof. Bhaskar Roy, Prof. A M Pradeep, Department of Aerospace, IIT Bombay4
Lect-7
Ideal cycle for jet engines• All air-breathing jet engines operate on the
Brayton cycle (open cycle mode).• The most basic form of a jet engine is a
turbojet engine. • Some of the parameters of a jet engine
cycle are usually design parameters and hence often fixed a priori: eg. compressor pressure ratio, turbine inlet temperature etc.
• Cycle analysis involves determining the performance parameters of the cycle with the known design parameters.
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Prof. Bhaskar Roy, Prof. A M Pradeep, Department of Aerospace, IIT Bombay5
Lect-7
Ideal cycle for jet engines
a 1 2 3 4 5 6 7
CompressorDiffuser
Combustion chamber/burner
Turbine
Afterburner
Nozzle
Schematic of a turbojet engine and station numbering scheme
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Prof. Bhaskar Roy, Prof. A M Pradeep, Department of Aerospace, IIT Bombay6
Lect-7
Ideal cycle for jet engines• The different processes in a turbojet cycle are
the following:• a-1: Air from far upstream is brought to the
air intake (diffuser) with some acceleration/deceleration
• 1-2: Air is decelerated as is passes through the diffuser
• 2-3: Air is compressed in a compressor (axial or centrifugal)
• 3-4 The air is heated using a combustion chamber/burner
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Prof. Bhaskar Roy, Prof. A M Pradeep, Department of Aerospace, IIT Bombay7
Lect-7
Ideal cycle for jet engines
• 4-5: The air is expanded in a turbine to obtain power to drive the compressor
• 5-6: The air may or may not be further heated in an afterburner by adding further fuel
• 6-7: The air is accelerated and exhausted through the nozzle.
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Prof. Bhaskar Roy, Prof. A M Pradeep, Department of Aerospace, IIT Bombay8
Lect-7
Ideal cycle for jet engines
s
T
2
4
5
3
a
7
Ideal turbojet cycle (without afterburning) on a T-s diagram
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Prof. Bhaskar Roy, Prof. A M Pradeep, Department of Aerospace, IIT Bombay9
Lect-7
Ideal cycle for jet engines• Afterburning: used when the aircraft needs a
substantial increment in thrust. For eg. to accelerate to and cruise at supersonic speeds.
• Since the air-fuel ratio in gas turbine engines are much greater than the stoichiometricvalues, there is sufficient amount of air available for combustion at the turbine exit.
• There are no rotating components like a turbine in the afterburner, the temperatures can be taken to much higher values than that at turbine entry.
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Prof. Bhaskar Roy, Prof. A M Pradeep, Department of Aerospace, IIT Bombay10
Lect-7
Ideal cycle for jet engines
s
T
2
4
5, 6
3
a
Ideal turbojet cycle with afterburning on a T-s diagram
6a
7a
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Prof. Bhaskar Roy, Prof. A M Pradeep, Department of Aerospace, IIT Bombay11
Lect-7
Turbofan engine• Propulsion efficiency is a function of the
exhaust velocity to flight speed ratio.• This can be increased by reducing the effective
exhaust velocity.• In a turbofan engine, a fan of a larger
diameter than the compressor is used to generate a mass flow higher than the core mass flow.
• This ratio is called the bypass ratio. • Turbofan engines have a higher propulsion
efficiency as compared with turbojet engines operating in the same speed range.
( )hotcold mm /
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Prof. Bhaskar Roy, Prof. A M Pradeep, Department of Aerospace, IIT Bombay12
Lect-7
Ideal turbofan engine
Schematic of an unmixed turbofan engine and station numbering scheme
a 1 2 3 4 5 6 7
3’ 7’
Compressor
Diffuser
Combustion chamber/burnerTurbine
Primary nozzle
Fan
Secondarynozzle
2’
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Prof. Bhaskar Roy, Prof. A M Pradeep, Department of Aerospace, IIT Bombay13
Lect-7
Ideal turbofan engine
Schematic of a mixed turbofan engine and station numbering scheme
a 1 2 3 4 5 6 7
3’ 7’
Compressor
Diffuser
Combustion chamber/burnerTurbine
Fan
Nozzle2’
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Prof. Bhaskar Roy, Prof. A M Pradeep, Department of Aerospace, IIT Bombay14
Lect-7
Ideal turbofan engine• The different processes in an unmixed
turbofan cycle are the following:• a-1: Air from far upstream is brought to the
air intake (diffuser) with some acceleration/deceleration
• 1-2’: Air is decelerated as is passes through the diffuser
• 2’-3’: Air is compressed in a fan• 2-3: Air is compressed in a compressor (axial
or centrifugal)
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Prof. Bhaskar Roy, Prof. A M Pradeep, Department of Aerospace, IIT Bombay15
Lect-7
Ideal turbofan engine
• 3-4: The air is heated using a combustion chamber/burner
• 4-5: The air is expanded in a turbine to obtain power to drive the compressor
• 5-6: The air may or may not be further heated in an afterburner by adding further fuel
• 6-7: The air is accelerated and exhausted through the primary nozzle.
• 3’-7’: The air in the bypass duct is accelerated and expanded through the secondary nozzle.
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Prof. Bhaskar Roy, Prof. A M Pradeep, Department of Aerospace, IIT Bombay16
Lect-7
Ideal turboprop and turboshaftengines
• Turboprop engines generate a substantial shaft power in addition to nozzle thrust.
• Turboshaft engines, generate only shaft power. These engines are used in helicopters. The shaft power is used to drive the main rotor blade.
• In a turboprop engine, the advantages and limitations are those of the propeller.
• Both turboprops and turboshafts have applications at relatively lower speeds.
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Prof. Bhaskar Roy, Prof. A M Pradeep, Department of Aerospace, IIT Bombay17
Lect-7
Ideal turboprop and turboshaftengines
CompressorPropeller
Combustion chamber/burner
Power turbine
Nozzle
Propellerpitch control
Gear box Compressor-turbine
Schematic of typical turboprop engine
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Prof. Bhaskar Roy, Prof. A M Pradeep, Department of Aerospace, IIT Bombay18
Lect-7
Ideal turboprop and turboshaftengines
• Turboprops and turboshafts usually have a free-turbine or power turbine to drive the propeller or the main rotor blade (turboshafts).
• Stress limitations require that the large diameter propeller rotate at a much lower rate and hence a speed reducer is required.
• Turboprops may also have a thrust component due to the jet exhaust in addition to the propeller thrust.
• In turboshafts, however, there is no thrust component due to the nozzle.
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Prof. Bhaskar Roy, Prof. A M Pradeep, Department of Aerospace, IIT Bombay19
Lect-7
Ideal turboprop and turboshaftengines
• In turboprops, thrust consists of two components, the propeller thrust and the nozzle thrust.
• The total thrust of a propeller is equal to the sum of the nozzle thrust and the propeller thrust.
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Prof. Bhaskar Roy, Prof. A M Pradeep, Department of Aerospace, IIT Bombay20
Lect-7
Ideal turboprop and turboshaftengines
s
h5
6
7
ΔhΔhα
05
06
7
P05
P06
Pa
Enthalpy-entropy diagram for power turbine-exhaust nozzle analysis
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Prof. Bhaskar Roy, Prof. A M Pradeep, Department of Aerospace, IIT Bombay21
Lect-7
• Ramjet is the simplest of all the airbreathingengines.
• It consists of a diffuser, combustion chamber and a nozzle.
• Ramjets are most efficient when operated at supersonic speeds.
• When air is decelerated from a high Mach number to a low subsonic Mach number, it results in substantial increase in pressure and temperature.
• Hence Ramjets do not need compressors and consequently no turbines as well.
Ideal ramjet engines
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Prof. Bhaskar Roy, Prof. A M Pradeep, Department of Aerospace, IIT Bombay22
Lect-7
Ideal ramjet enginesDiffuser Combustion chamber Nozzle
Supersonic compression
Subsonic compression
Flame holders
Schematic of typical ramjet engine
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Prof. Bhaskar Roy, Prof. A M Pradeep, Department of Aerospace, IIT Bombay23
Lect-7
Ideal ramjet engines
s
T4
2
a
7
Ideal ramjet cycle on a T-s diagram
a-2: Isentropic compression in the intake2-4: Combustion at constant pressure4-7: Isentropic expansion through the nozzle
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Prof. Bhaskar Roy, Prof. A M Pradeep, Department of Aerospace, IIT Bombay24
Lect-7
Ideal ramjet engines• In a ramjet, there are no compressors and
turbines and hence the analysis is simpler.• Since ramjets depend upon the ram
compression without the use of compressors, ramjets cannot generate static thrust.
• Therefore ramjets have to be taken to a sufficiently high speed at which ramjets can start generating thrust of its own.
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Prof. Bhaskar Roy, Prof. A M Pradeep, Department of Aerospace, IIT Bombay25
Lect-7
In this lecture...
• Jet engine cycles for aircraft propulsion– Turbojet engine– Turbojet engine with afterburning– Turbofan and its variants– Turboprop and turboshaft engines– Ramjet engines
![Page 26: Lect-7 - Nptel · 2017-08-04 · turbine in the afterburner, the temperatures can be taken to much higher values than that at turbine entry. Prof. Bhaskar Roy, Prof. A M Pradeep,](https://reader033.vdocuments.net/reader033/viewer/2022042100/5e7d1116d31f1625e4362015/html5/thumbnails/26.jpg)
Prof. Bhaskar Roy, Prof. A M Pradeep, Department of Aerospace, IIT Bombay26
Lect-7
In the next lecture...
• Cycle components and component performance– Intake– Compressor/fan– Combustion chamber– Turbine– Nozzle