Jet Propulsion

Lecture-4

Ujjwal K Saha, Ph. D.Department of Mechanical Engineering

Indian Institute of Technology Guwahati

Prepared underQIP-CD Cell Project

Turbojet :

( ) ( )1aj CCmF −=

( )22

aj

ap CC

C+

=η

j pC η⇒High Lower

pη of Turbojet is quite low except at high flight speed.

decreased. be must high For p jC⇒η

Extract more power in the turbine

jLowC

without increasing the power to drive the compressor.

Extra power to drive a FAN for additional compression of air.

TURBOFAN

Increase of without fuel supply.

am

TSFC becomes lower.

More economic and efficient.

Accelerates more in to a lower velocity.

m

mac

cm jcc

cm jcc

hmjhc

High frontal area (due to fan)Gives more drag at high speed.

More weight

High specific weight.

Front FanTwin spool configuration.

( )

( )( )

3

1

41

c h

c

h

h h h

h

m m mmBm

m Bm m m Bmm

B

= +

=

= + = +

∴ =+

B y P ass R a tio ,

;1c

mm mB

= ++

c

c

mB

Bm

mB

m

=+

=⎟⎠⎞

⎜⎝⎛

+−

1

111Again,

( )51

mB

Bmc +=∴

Total Thrust = Thrust (Cold stream + Hot stream)

( ) ( )ajhhajcc CCmCCmF −+−=( ) ( )hcajhhjcc mmCCmCm +−+=

( )6mCCmCmF ajhhjcc −+=

Thrust = Cold Nozzle F + Hot Nozzle F= Fan F + Core Engine F= 75% + 25%

mac

cm jcc

cm jcc

hmjhc

B = 4 to 6 For transport aircraft

Applications : DC-10AirbusBoeing 737, 747

B = 8-9 GE 90 entered in service 1995

FPR (Fan Pressure ratio) : 1 to 3

Higher FPR Low By Pass Ratio EnginesLower FPR High By Pass Ratio Engines

Turbofan: Large increase in F is obtained by taking a turbojet engine and adding a Fan to it.

This lowers the exit velocityless noise.

Increases

No fuel flow for this added thrustand therefore Lower TFSC.

pη

Types :Non-mixed type(separate streams)

Mixed type(Pressure Balanced)

Static pressure must be balanced at the point of mixing for both the streams

Engines may incorporate one, two, or three spools (Spool is defined as a group of compressor stages rotating at the same speed). Two spool engine , the two rotors operate independently of one another. The turbine assembly for the low pressure compressor is the rear turbine unit . This set of turbines is connected to the forward, low pressure compressor by a shaft that passes through the hollow center of the high pressure compressor and turbine drive shaft.

Civil Engines: Single Stage FanPressure Ratio : 1.5- 1.8

Flight conditions, Cycle Parameters

Military Engines: Two/Three Stage FanPressure Ratio : 4

Original Concept: AFT-FAN Configuration

Problems:Blading of high costSealing between the two streams

Turbine fan was mounted downstream of GG turbine

Turboprop :

Combines the advantages of a turbojet and propulsion efficiency of a Propeller.

Turbojet:

o F from a large momentum change of small

o Turbine extracts power to drive compressor + accessories.

o F is from purejet (100%).

am

Turboprop:

o F is from a small momentum change of large .

o Turbine extractspower to drive compressor + accessories + propeller shaft.

o F is from jet (20%)+ propeller (80%).

am

airspeed% may vary altitude

other engineparameters

Turboprop

Additional turbineFree-turbine/ Power turbine

configuration

Flexibility to meet a range of demand.

Reduction gear : 9:115:1

To rotate the large diameter Propeller atLow RPM

Additional weight: PropellerPitch-change mechanismPower turbineReduction gear

1.5 times heavier than a turbojet ofsimilar GG size

Stresslimitations

Allison EngineAE2100

General Electric CT7

Allison Engine, T56

In production for 44 years, the T56 is most often seen on large military turboprop aircraft

Allied Signal, TPE331

The 331 has been in service for nearly 30 years, on a large variety of aircraft, from agricultural planes to executive planes to puddle jumping commuters

Benefits : Short take offLow speed operationLow TSFCLow noise level

Turbopropand

Turbofan

Similar in principleGeared propeller is replacedby a ducted fan driven atengine speed.

Turboprop Applications:

Small aircraftCapacity: 10-60 seatsSpeed: 400-600 km/hrDuration: 60-90 minutesRange: 400-500 km

Till1990s

Combination Of Shaft Power and Jet thrust

jet thrustThrust

propeller thrust

Propeller efficiency:

Good upto M = 0.5

At high speeds, operation becomes noisy, and propeller efficiency falls because of formation of shockwave and flow separation on the outer part.

Turbojet Rated onTurbofan the basis

of Thrust

Turboprop: SP+Thrust

No directcomparison

Possibilities :SP to thrust for TP

ConvertThrust to SP for TJ/TF

HOW ??

Under static conditions1kW of power input produces a thrustof 8.5 N (Experimental evidence)

TP = Thrust Power = ( ) apropeller FCSP +η

Equivalent PowerEffective Power propeller

a

propeller

FCSPTPEP

ηη+==

When (Static condition)

EP = SP

0=aC

propellerη falls beyond M = 0.5

0 200 400 600 800 1000

20

40

60

80

100

Airspeed mph

pη

TP TFTJ

Turboprop

Unsuitable for longerhaul aircraft.

Early 80sSerious drive towards a new design

M = 0.8

Competitor to turbofan enginesin terms of saving fuels

8.0=propellerη

New Design:Used highly swept supersonicbladed propeller to reduce the danger of shockwave and flow induced separation.Propfans (8-12 blades)

Objectionable??because of transmission of propeller noise to the passenger cabin.

Advanced Version : FanjetsUse of PUSHER configurationi.e., propellers were mountedbehind the passenger cabin.

Alternative configuration:General ElectricTwo Counter-rotating Fans coupled to Two Counter-rotating Turbines

UnductedFan Engines

Long range Cruise missiles

No statorsNo gearbox

Noise & cabinvibrations

Refused by airlines

Fanjets are similar in concept to a turboprop. Instead of powering a conventional propeller, they have a high-chord, low-span fan that is designed to operate at jet velocities. They promise much higher fuel efficiency than a turbofan, but high development cost, unit cost, noise problems, and relatively inexpensive fuel have kept the fanjet from entering production. If it did, it would fill a middle range in performance between low-speed, high-efficiency turboprops and high-speed, lower-efficiency turbofans.

Characteristics of Turbojet

Thrust is purely from the reaction of the jet

For the same thrust, engine is smaller in size and weight i.e., low specific weight

TSFC is high

Comparatively simple in design - Low drag

Smaller engine size permits better ground clearance

Produces excessive noise

Characteristics of TurbopropThrust is from two sources, viz. a) Propeller Thrust (80%), b) Jet Thrust (20%)

For the same thrust, engine is larger in size and weight i.e., more Specific weight.

TSFC is low

More complicated and heavier TJ. Additional Turbine Stages, Presence of Gearbox

Less ground clearance due to larger diameter of propeller

Low noise level. Propeller efficiency is goodupto M = 0.5

Characteristics of Turbofan

• Thrust is from two sources viz., a) Fan Thrust (75%), b) Core engine Thrust (25%)

• Specific weight falls between TJ and TP

• TSFC falls between TJ and TP

• High drag at high speeds (upto M = 1)

• Ground clearance is less than TP, but not as good as TJ

• Noise level reduction by 10-20% than TJ

35

References1. Hill, P.G., and Peterson, C.R., (1992), Mechanics and

Thermodynamics of Propulsion, Addison Wesley.2. Saravanamuttoo, H.I.H, Rogers, G.F.C, and. Cohen, H,

(2001), Gas Turbine Theory, Pearson Education.3. Oates, G.C., (1988), Aerothermodynamics of Gas Turbine and

Rocket Propulsion, AIAA, New York.4. Mattingly, J.D., (1996), Elements of Gas Turbine Propulsion,

McGraw Hill.5. Cumpsty, N.A., (2000), Jet Propulsion, Cambridge University

Press.6. Bathie, W.W., (1996), Fundamentals of Gas Turbines, John

Wiley.7. Treager, I.E., (1997), Aircraft Gas Turbine Engine Technology,

Tata McGraw Hill. 8. Anderson, J. D. Jr., (2000), Introduction to Flight, 4th Edition,

McGraw Hill. 9. Sutton, G.P. and Biblarz, O., (2001), Rocket Propulsion

Elements, John Wiley & Sons.10. Zucrow, M.J., (1958), Aircraft and Missile Propulsion, Vol. II,

John Wiley.

36

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