AERO ENGINES NOTES

7/30/2019 AERO ENGINES NOTES

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3.2 Compressor


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3.2 Compressor

Role of the compressorAccording to thermal

cycle analysis,

compressor is aimportant componentwhich increases gaspressure and so that thecycle can outputmechanical work.


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1. Compressor structures and types

1.1 Often seen types

Centrifugal

Axial


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3.2 Compressor (Contd)

Centrifugal Shaft is coupled

directly to

turbine.

Impeller can be

single or double

sided.


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Centrifugal compressor

Rotating guidevane:Air goes

in axially. It can

be made in one

piece withimpeller.

Impeller: Radial

blades increase

air speed and

pressure.


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Centrifugal compressor

Diffuser:Reduces speedand getspressure rise.

Air outletcasing: Turnsair to adaptcombustion

chamber.


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Characteristic

Structure simple and reliable Single stage pressure ratio highmay>12

Performance stable

Efficiency lower Frontal area bigger


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Centrifugal compressor (Cont'd)

Uses Small power

Cruise missiles

UAVs or small airplanes

Helicopters

Often compressor is combined with

axial and centrifugal (last stage)


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Axial compressors


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Axial compressor (Contd)

Single spool Consists in one rotor and one stator.

The rotor may includes blades, disks drum

and shaft. They are assembled togetherand sit on 2 bearings.

Stator has guide vanes and casing.

Axial compressor has pressure ratio lower

than centrifugal, normally 1.15~1.35. Thatwhy multiple stage axial compressor.


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Two spool compressors


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Twin spool compressors The same axe, but different shafts.

Front one is low pressure (LP)

compressor, it rotates with LP Turbine;Rear one is high pressure (HP)

compressor coupled with HP turbine.

Two rotors have no mechanicalconnection, and they have their own

rotational speed.


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1.2 Compressor rotorsResistances

High speed rotation (thousands

~ hundreds k rpm)

Bending moment, torque,

centrifugal forces, vibrations. Require light and enough

strength and stiffness


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1.2 Compressor rotors (Cont'd)

Structure Disks+shaft. One shaft and many disks

where blades are installed. Centrifugal

forces of blades and disks are borne by

the disks and bending stiffness dependson the shaft.

No more used because of

bending stiffness too weak.


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1.2 Compressor rotors (Cont'd)

Strurcture Drum and drum+disks.

Blades are attached

circumferentially or axiallyin drum or disk. Forces

are transferred through

the drum and disks. The

drum insures the bending

stiffness.


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Rolls Royce structure

Meridian plane


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Drum


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Drum+discks


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1.3 Blades Important parts

in axial

compressor,composed by

airfoil and

attachment.


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1.3 Blades

Blades suffer centrifugal,aerodynamic and vibrating forces.

Attachment is also important.

Swallow tail (easy fabrication)

Pivot (No bending stress)


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1.3 Blades (Cont'd)

Blades may be broken due to fatigue,especially vibrating fatigue.

To reduce vibration amplitude, long

blades are often made with a mid-spansupport called snubber or clapper.

centrifugal force

efficiency


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1.3 Blades (Cont'd)

At the end of compressor, temperaturecan reach 500 ~600C, even higher.

Materials used are normally titanium,

aluminum alloys, steels and composites.


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1.4 Compressor stator is the part which does not rotate

and consists of vanes and casings.

bears axial forces, torques,vibration and rotors forcestransferred by bearings.

is part of air passage, bears

pressure and thermal stresscaused by temperature.


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1.4 Compressor stators (Cont'd)

Types of casing Half-half

Good stiffness

Assembly no need disassemble therotor

Heavier

Entire

Must disassemble rotor (blades),normally used in few stagescompressor.

Lighter


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1.5 Anti-icing and axial force redistribution Water droplets may become ice. They may

reduce air passage and break blades when

detached.

Anti-icing methods

Heating (electricity,

hot air)

Hydrophobiccoating


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1.5 Anti-icing and axial force redistribution

Bearing Ball (Axial and radial forces)

Roller (Journal bearing, Radial force only)

Compressor axial force (>total thrust) istoo big for balls.

Coupling with turbine

Creating rooms (Not in air passage)


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1.5 Anti-icing and axial force redistribution

High pressure (push)

Low pressure (pull)

Rooms have one side: rotor; another side: stator.

They are sealed.


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3.2 Compressor

2. Basic equations 2.1 Energy equation

Fixed system

(Ignoring heat)

*

1

*

2

2

1

2

2

12 2 hh

vv

hhWu


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1.1 Energy equation (Cont'd)

Mobile system(Ignoring heat exchange)

Compressor

ublade circumferential velocity

wRelative velocity

22

2

1

2

212

2

1

2

2 wwhhuu


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1.1 Energy equation (Cont'd)

In case of axial compressor (u notsignificant change), so

i.e.

Gas relative total enthalpy unchanged

form inlet to outlet.

*

2

*

1 ww hh

22

2

22

2

11 whwh


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2. Basic equations (Cont'd)

2.2 Bernoulli equation Fixed system

WfLosses

Polytropic work:

fu

Wvvdp

W

2

2

1

2

22

1

)(1

12

2

1TTR

n

ndp


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2.2 Bernoulli equation (Cont'd)

Polytropic work (Compressor)

Isentropic work

11

1

1

21

n

n

nCp

pRT

n

nW

11

1

1

21

ppRTWiC

2 2 B lli i (C 'd)


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Work added by compressor blades usedto accomplish generic compression,

increase air kinetic energy and

overcome flow losses.

In case of isentropic

fnCC Wvv

WW

2

2

1

2

2

2

2

1

2

2 vvWW iCC

2 2 B lli ti (C t'd)


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Due to losses, more work needed

)( iCnCf WWW



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Polytropic work (Turbine)

Isentropic work

n

nnT

p

p

TRn

ndpW

1

2

1

1

'2

1

11

1

'

' 1

2

1

1'

'

'

111

p

p

TRWiT



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Gas expansion work and kineticenergy change generate shaft

work and overcome losses.

Isentropic expansion

fTnT WWvv

W 2

22

21

TiT Wvv

W

2

2

2

2

1



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Work lost in turbine due to losses

More work needed in compressor

)( nTiTf WWW

)( iCnCf WWW



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)( nTiTf WWW )( iCnCf WWW



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Rotational coordinate system

For axial compressor

Relation of velocities and pressures in

inlet and outlet.

fWwwdpuu

222

1

2

22

1

2

1

2

2

02

2

1

2

22

1

fW

wwdp



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In case of isentropicFor compressor,p, w.

For turbine,p, w.

02

2

1

2

22

1

wwdp

3 2 C (C t'd)


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3.2 Compressor (Cont'd)

2. Basic equations 2.3 Efficiency and losses

Due to viscosity, gas flowing in

turbomachines will produce manykinds of losses and they can classed

into 2 :

(1) Airfoil losses

(1) Airfoil losses (Cont'd)


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Boundary layer lossFriction (a) Separation loss (b)



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Tail trace vortexes (c) Shockwave loss (d)

2 3 Efficiency and losses (Cont'd)


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2.3 Efficiency and losses (Cont'd)

(2) Circumferential losses (secondary flow)

Tip and hub circumferential boundary layers

Tip clearance leaking and passage

vortexes


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(2) Circumferential losses (Contd)



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2.3 Efficiency and losses (Cont d)

These losses are presentedW

f inBernoulli equation. But, usually,

efficiency is used to evaluate

performance.

Bernoulli equation can be used for

whole compressor:

)(2

*1*2

2

1

2

2 TTcWvvWW pfnCC



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Isentropic

11

)(2

1

*

1

*

2*

1

*

1

*

2

2

1

2

2

p

pRT

TTcvv

WW ipiCCi



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Compressor efficiency (definition for certain

pressure ratio)

Real work

In general, 0.9 for a single stage, 0.83 forwhole compressor.

*

1

*

2

*

1

*

2*

)Real( TT

TT

W

W i

C

CiC

*1

**

1 /1

1CCC RTW



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The same, for turbine

3-25

)(

2

*

4

*

3

2

4

2

3 TTcWvv

WW pfnTT



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Isentropic

(3-26)

'

' 1

*

4

*3

*

3

'

'

'

*

4

*

3

2

4

2

3

11

1

)(2

p

p

TR

TTcvv

WW ipiTTi



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Turbine efficiency

Real work

0.88 for a stage, 0.92 for whole turbine.

*

4

*

3

*

4

*

3* )(

iTi

TT

TT

TT

W

GenericW

*

1*

*

3'

'

'

'

1

11 T

T

T RTW

2 Basic equations (Cont'd)


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2. Basic equations (Cont d)

2.4. Equation of moment of

momentum

Take an isolated element

from 1-1 to 2-2. After dt, it

moves to 1'-1' to 2'-2'.Between 2-2 and 2'-2',

moment of momentum:

dm2v2ur2 and between 1-1and1'-1', dm1v1ur1 .

2 4 Equation of moment of momentum (Cont'd)


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2.4. Equation of moment of momentum (Cont d)

Because of continuity, dm1=dm2=qmdtIn period dt, Momentum change:

dm2v2ur2- dm1v1ur1 = qmdt(v2ur2-v1ur1)

Based on law of moment ofmomentum: Change is equal to sum of

moments, so the Moment:

)()(

11221122 rvrvq

dt

rvrvdtqM uum

uum

2 4 Equation of moment of momentum (Cont'd)


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2.4. Equation of moment of momentum (Cont d)

Specific work (Euler equation)

If equal radius in inlet and outlet

(3-31)

11221122

1122

)(

)(

uvuvrvrv

dtq

dtrvrvq

dtq

MdW

uuuu

m

uum

m

u

uuuu vuvvuW )( 12

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AERO ENGINES NOTES