Week5-EngineCycle2

8/7/2019 Week5-EngineCycle2

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AIRCRAFT

PROPULSIONEME4543

Week 5 Engine Cycle

Semester II 2010/2011

EME4543 AIRCRAFT PROPLSION


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Engine Performance Parameters


y Propulsion efficiency, ratio thrust power to add kineticenergy

y Thermal efficiency, ratio added kinetic energy to totalenergy consumption

y Total efficiencyy Thrust Specific Fuel Consumption

a

p 2 2e a

a f a

Tvv v

m m m

2 2

L ! & & &

2 2

e aa f a

th

f R

v vm m m

2 2m Q

L !

& & &

&

total th propL ! L Lf mTSFC

T! &


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Tu rbine Engine Th ermodynamic Cycle


y AnalysisE nergy control volume per engine component Pressure and temperature changes for ideal engine

With efficiency definitions: pressure and temperature changesfor non-ideal engine

Control Volume over complete engine: Momentum balance=> thrust, propulsion efficiency E nergy balance or thermo analysis:

Brayton cycle: Thermal efficiency


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Th ermodynamic Principles


Figure 5.1 Simple-cycle, single-shaft gas turbine


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y ISO conditions -The standard conditions used by the gasturbine industry are 59 F / 15 C, 14.7 psia / 1.013 bar and60% relative humidity

y P oint 1 ; Air entering the compressor at ambient condition iscompressed to some higher pressure.

y No heat is added ; compression raises the air temperatureso that the air at the discharge of the compressor is athigher temperature and pressure.

y P oint 2; fuel is injected and combustion occurs (constantpressure)

y P oint 3; the combustion mixture leaves the combustionsystem and enters the turbine at a mixed average temp.




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y In the turbine section of the gas turbine, the energy of hotgasses is converted into work.

y Conversion takes place in two steps ;In the nozzle section ; the hot gasses are expanded and a portion of thethermal energy is converted into kinetic energy.In the subsequent bucket section ; a portion of the kinetic energy istransferred to the rotating buckets and converted to work.

y Work developed used to drive the compressor and theremainder is available for useful work at the output flangeof the gas turbine.

y More than 50% of the work developed used to power theaxial flow compressor.




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Figure 5.2 Simple-cycle, two-shaft gas turbine


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y The low-pressure or power turbine rotor is mechanicallyseparate from the high-pressure turbine and compressor rotor.

y The low pressure rotor is said to be aerodynamicallycoupled.

y All of the work developed by the power turbine is availableto drive the load equipment since the work developed bythe high-pressure turbine supplies all the necessary energy

to drive the compressor.y The starting requirements for the gas turbine load train are

reduced because the load equipment is mechanicallyseparate from the high-pressure turbine.




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I dealized air-standard Brayton cycle


1 2 3 4 5 6

diffuser

compressor

combustion chamber turbine nozzle

1

2

3

4

5

6

P=constant

P=constant

qout

qin

T

s

1- 2 Isentropic compression in diffuser 2 -3 Isentropic compression through compressor 3 -4 onstant pressure heat addition

in combustion chamber 4 -5 Isentropic expansion through turbine 5 -6 Isentropic expansion in nozzle 6 -1 onstant pressure heat rejection


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Th e Brayton Cycle


Figure 5.3 Brayton Cycle


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Th e Brayton Cycle

y Path 1 to 2 represents the compression occurring in thecompressor.

y Path 2 to 3 represents the constant-pressure addition of heat in the combustion systems.

y Path 3 to 4 represents the expansion occurring in theturbine.

y Path 4 back to 1 on the Brayton cycle diagrams indicates aconstant-pressure cooling process.

y In the gas turbine, this cooling is done by the atm at point 1on a continuous basis in exchange for the hot gassesexhausted to the atm at point 4.



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Th e Brayton Cycle

y Brayton cycle can be characterized by two significantparameters: pressure ratio and firing temperature.

y The pressure ratio of the cycle is the pressure ratio at point2 (compressor discharge pressure) divided by the pressureat point 1 (compressor inlet pressure).

y In an ideal cycle, this pressure ratio is also equal to thepressure at point 3 divided by the pressure at point 4.

y However, in actual cycle there is some slight pressure lossin the combustion system and hence, the pressure at point3 is slightly less than at point 2.



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G as t u rbine t h ermodynamics


Figure 5.4 Gas turbine thermodynamics


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Tu rbojet cycle


G eneral numberingsystem for theturbojet engine

Thermodynamic cyclefor the turbojet engineoperating with amatched nozzle andconstant pressurecombustor

1 2 3 4 5 6 7

p0

p2p1

p5p4=p 3

1

2

3

4

56

7

p6h

s


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Efficiency of adiabatic compression


p3

p2h t,3

h t,2

s 2 s 3 s

h t

, 3 , 2 , 3 , 2,

, 3 , 2 , 3 , 2

t t t t ad c

t t t t

h h T T

h h T T L

d d ! !

ht,2


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W ork req u ired for compression


2

2

1

, 3

, 2

,

, 3

, 2

1

1

t

t

ad ct

t

p

p

K K

L

!

1

, 3 , 3

, 2 , 2

t t

t t

T p

T p

K K d

!

2

2

1

, 2 , 2 , 3

, , 2

1p t t c

ad c t

c T pW p

K K

L

!

Isentropic relation

Compressor work


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A diabatic efficiency of expansion


p t,4

h t,4h t,5

s 4 s 5 s

adiabatic-frictionprocess

isentropicprocess

4

4

, 5

, 4, 1

, 5

, 4

1

1

t

t ad e

t

t

T

T

p

p

K K

L !

4

4

1

, 5, , 4 , 4

, 4

1 t t ad e p t

t

pW c

p

K K L

! Turbine work output

p t,5

ht,5


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A diabatic efficiency for t u rbine


The ratio of the actual work output of the turbine to the work outputthat would be achieved if the process between the inlet state andthe exit state was isentropic.


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A diabatic efficiency for compressor


The ratio of the work input required to raise the pressure of a gas toa specified value in an isentropic manner to the actual work input.


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A diabatic efficiency for nozzle


The ratio of the actual kinetic energy of the fluid at the nozzle exit tothe kinetic energy value at the exit of an isentropic nozzle for thesame inlet state and exit pressure.

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Week5-EngineCycle2