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Previous lectures

• Chapter 1 Theoretical basis–Thermodynamics

–Aerodynamics

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Chapter 2Principle of Gas Turbine Engine

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Chapter 2Principle of Gas Turbine Engine

• §2.1 Thermodynamic cycles of gas turbine engines– 1. Ideal cycle – 2. Real cycle

• §2.2 Thrust– 1. Propulsion power and propulsion efficiency– 2. Total efficiency– 3. Parameter evolution along flow passage– 4. Thrust distribution and delivery in components

• §2.3 Gas engine performance and specifications– 1. Performance characteristics – 2. Specifications – 3. Future development

• §2.4 Variations of Aircraft engines

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Turbofan engine with afterburner

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5 components of turbo engines

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Compressor

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Combustion chamber

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Turbine

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§2.1 Thermodynamic cycles of gas turbine engines

• 1. Ideal cycle (Brayton cycle)

– 0-2 Isentropic compression– 2-3 Isobar heating– 3-9 Isentropic expansion– 9-0 Isobar cooling

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1. Ideal cycle (Cont’d)

• 0-2 Isentropic compression

– Diffuser and compressor– 0-1 speed pressure rise. 0 atmosphere

condition. Add dynamic energy to substance and to increase pressure to 1. Area 011'0'0 presents dynamic energy difference.

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1. Ideal cycle (Cont’d)

• 0-2 Isentropic compression

– 1-2: compressor. Pressure from 1 to 2, work added is the area 122'1'1.

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Bernoulli function

02

1 21

22

2

1 fWWvv

dp

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1. Ideal cycle (Cont'd)

• 2-3 Isobar heating

– Combustion chamber– Burn ideally kerosene at constant pressure in

combustion chamber and substance properties unchanged.

– Total temperature T2* T3

* 。

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1. Ideal cycle (Cont'd)

• 3-9 Isentropic expansion

– Turbine and nozzle– 3-4 presents expansion in turbine, heatme

chanical energy giving to compressor. Area 344'2'3=Area 122'1'1, total pressure p3

*p4*.

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1. Ideal cycle (Cont'd)

• 3-9 Isentropic expansion

– 4-9 complete expansion in exhaust system. Heat changes kinetic energy in substance, exits from the nozzle.

– As diffuser, kinetic energy change can be seen as output work.

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1. Ideal cycle (Cont'd)

• 9-0 Isobar heat release

– Dash line, accomplished in atmosphere. This process is unavoidable, The cycle is closed.

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1. Ideal cycle (Cont'd)

• Specific heat added in the cycle

• Specific heat release

• Specific work in the cycle

)( *2

*31 TTcq p

)( 092 TTcq p

21 qqW

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1. Ideal cycle (Cont'd)

• Thermal efficiency in the cycle

Where pressure ratio

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21

1

11

q

qq

q

Wt

0

*2p

p

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1. Ideal cycle (Cont'd)

• Cycle work as mechanical energy

if WT=WC

2 29 0

2 2T C

v vW W W

2

20

29 vv

W

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§2.1 Thermodynamic cycles of gas turbine engines

• 2. Real cycle

– 0-2 Compression (non isentropic)– 2-3 Heating (non isobar)– 3-9 Expansion (non isentropic)– 9-0 Isobar heat release

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2. Real cycle (Cont'd)

• 0-2 Compression– Stagnation in diffuser and compression in

compressor have many types of losses. – Non isentropic and n >

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2. Real cycle (Cont'd)

• 2-3 Non isobar heating– Existing flow losses and thermal

resistance losses lows the pressure in combustion chamber.

– Composition of substance changes.

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2. Real cycle (Cont'd)

• 3-9 Expansion– There are always losses in turbine and

nozzle.

– Non isentropic and n <

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2. Real cycle (Cont'd)

• Heat added, area 2’233’2’

• Heat released, area 0’099’0’

cp’ gas specific heat

)( *2

*31 TTcq p

)( 09'

2 TTcq p

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2. Real cycle (Cont'd)

• Efficiency

• Work

21 qqW

1

21

q

qqt

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2. Real cycle (Cont'd)

• If T3* lower, q1=q2, then t=0, no output work.

• Work can be presented by mechanical energy, same as ideal cycle:

2

20

29 vv

W

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2. Real cycle (Cont'd)

• Under the same pressure ratio and the same T3*, work is smaller in real cycle than ideal cycle.

• Note that area in diagram T-s is heat, not work.

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2. Real cycle (Cont'd)

• If not take account of composition change and mass flow increase, differences between real cycle and ideal cycle are:– Friction and flow losses

– Total pressure loss

– Heating resistance

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2. Real cycle (Cont'd)

• Finally, in nozzle gas kinetic energy is smaller, velocity of air jet is smaller.

• To improve engine’s efficiency, use the components of high efficiency and high performance.

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§2.2 Thrust generation

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§2.2 Thrust generation

• Turbo-engine thrust overcomes airplane drag or accelerates airplane.

• Usually called effective thrust.

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§2.2 Thrust generation (Cont’d)

• Thrust = momentum of air + static pressure differences

• Usually, p9≈p0 and neglecting fuel flow, then

)( 09 vvqF m

)(

)()(

09909

00009909

ppAvqvq

ppAppAvqvqF

mmg

mmg

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§2.2 Thrust generation (Cont'd)

• 1. Propulsion power and efficiency– Propulsion power Fv0 , v0 flying speed

or air velocity in engine inlet.

– Thermal cycle of engine produces power

2

20

29 vv

qP m

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§2.2 Thrust generation (Cont'd)

– Since thrust , propulsion efficiency (percentage of propulsion power in thermal cycle power)

（ 2-16 ）

– Work provided by engine are divided into 2 parts: one pushes airplane forward; another jets gas backward. The second part is (/kg air)

)( 09 vvqF m

0

920

29

0

1

2

2 vvvv

q

Fv

m

P

2

)()(

2

209

009

20

29 vv

vvvvv

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§2.2 Thrust generation (Cont'd)

• 2. Total efficiency– Propulsion power over burning fuel heat

t Thermal eff (0.25~0.40), P propulsion eff (0.50~0.75), total eff (0.20~0.30)

– How to improve total efficiency• T3*

• Bypass ratio

Ptmqq

Fv

1

00

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§2.2 Thrust generation (Cont'd)

• 3. Parameter evolution along air passage

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§2.2 Thrust generation (Cont'd)

• 4. Thrust distribution in the components

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4. Thrust distribution (Cont'd)

WP6 bearing configuration （ 1-2-0 ）

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Summary

• §2.1 Thermodynamic cycles of gas turbine engines1. Ideal cycle

2. Real cycle

• §2.2 Thrust generation1. Propulsion power and efficiency

2. Total efficiency

3. Parameter evolution along air passage

4. Thrust distribution in the components