Gas Power Cycle - Jet Propulsion Technology, A Case Study

Ideal Brayton Cycle

Air

Fuel

Products

Compressor

Turbine

Combustor

1

2 3

4

P

v

T

s

1

23

4 1

2

3

4

• 1-2 Isentropic compression• 2-3 Constant pressure heat addition• 3-4 Isentropic expansion• 4-1 Constant pressure heat rejection

Ideal Brayton Cycle - 2

4 1th

3 2

4 1 1 4 1

3 2 2 3 2

The thermal efficiency of the ideal Brayton cycle is

1 1

( ) ( / 1)1 1 equation (1)

( ) ( / 1)

Processes 1-2 and 3-4 are isen

net out in in out out

in in in in

p

p

h hw w w q q q

q q q q h h

c T T T T T

c T T T T T

( 1) / ( 1) /

3 32 2

1 1 4 4

32 2 12 3 4 1

1 4 3 4

1 4th ( 1) / (

2 3 2

1

tropic (adiabatic), therefore

,

Also, P and P , therefore and

1 1Equation (1) becomes 1 1 1 1

k k k k

k k kp

T PT Pand

T P T P

TT T TP P

T T T T

T T

T T rPP

1) /

2p

1

where r is the pressure ratio of the compressor and the turbine

k

P

P

Jet Propulsion Cycle

1

2

3

4

5

6

P=constant

P=constant

qout

qin • 1-2, inlet flow decelerates in the diffuser; pressure and temperature increase

• 5-6, outlet flow accelerates in the nozzle section, pressure and temperature decrease

T

s

Propulsive PowerJet Engine

Mass flow in

Inlet velocity, Vin

Mass flow out

Exit velocity, Vexit

Action force, FA

Reaction force, FR

Due to the action force F the momentum of the air flowing

through the engine increases:

F linear momentum change) = [d

dtFrom Newton' s third law: F Propulsive force

F

Propulsive Power

W = F V

A

A

A

R

P R aircraft

,

( ( )] [ ( )]

( )

( )

mVd

dtmV mV mV

F

m V V

m V V V

exit in exit in

R

exit in

exit in aircraft

Gas Turbine Improvements

• Increase the gas combustion temperature (T3) before it enters the turbine since th = 1 - (T4/T3)

Limited by metallurgical restriction: ceramic coating over the turbine blades Improved intercooling technology: blow cool air over the surface of the blades (film cooling), steam cooling inside the blades.

• Modifications to the basic thermodynamic cycle: intercooling, reheating, regeneration • Improve design of turbomachinery components: multi-stage compressor and turbine configuration. Better aerodynamic design on blades (reduce stall).

PW8000 Geared Turbofan Engine

• Twin-spool configuration: H-P turbine drives H-P compressorL-P turbine drives L-P compressor, on separated shafts• Gearbox to further decrease the RPM of the fan• More fuel efficiency• Less noise• Fewer engine parts From Machine Design Magazine

Nov. 5, 1998