Gas Power Cycle - Jet Propulsion Technology, A Case Study
Ideal Brayton Cycle (YAC 7-7)
An closed ideal Brayton Cycle is used to model an open Gas Turbine cycle
using the air statndard assumptions
Combustor
Fuel
3
2
Compressor
Air
• 1-2 Isentropic compression
• 2-3 Constant pressure heat addition
• 3-4 Isentropic expansion
• 4-1 Constant pressure heat rejection
4
1
Turbine
Products
P
3
2
3
T
2
4
1
1
4
s
v
Air Standard Assumptions
• Another common assumption used is that the air has constant specific heats
equal to specific heats at room temperatures.
When this is also used they are collectively referred to as the:
cold-air-standard assumptions
Mod: 11/15/01
Ideal Brayton Cycle - 2
The thermal efficiency of the ideal Brayton cycle is
th 
h  h 
wnet wout  win qin  qout
q


 1  out  1  4 1
qin
qin
qin
qin
 h3  h2 
 1
c p (T4  T1 )
c p (T3  T2 )
 1
T1 (T4 / T1  1)
equation (1)
T2 (T3 / T2  1)
Processes 1-2 and 3-4 are isentropic (adiabatic), therefore
T2  P2 
 
T1  P1 
( k 1) / k
T3  P3 
, and
 
T4  P4 
( k 1) / k
T2 T3
T2 T1
Also, P2  P3 and P4  P1 , therefore

and

T1 T4
T3 T4
Equation (1) becomes th  1 
where rp 
T1
T
1
1
 1 4  1

1

( k 1) / k
T2
T3
rp( k 1) / k
 P2 
 
 P1 
P2
is the pressure ratio of the compressor and the turbine
P1
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).
Jet Propulsion Cycle
T
4
P=constant
qin
3
5
6
2
1
qout
• 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
P=constant
s
Propulsive Power
Jet Engine
Mass flow out
Action force, FA
Mass flow in
Inlet velocity, Vin
Exit velocity, Vexit
Reaction force, FR
Due to the action force F , the momentum of the air flowing
A
through the engine increases:
d
d


F  ( linear momentum change) = [ ( mV )]  [ ( mV )]  mV
 mV
dt
dt
From Newton' s third law: F  F  Propulsive force
A
exit
A
F  m (V  V )
R
exit
in
Propulsive Power
 = F V  m (V  V )V
W
P
R
aircraft
exit
in
aircraft
R
in
exit
in
PW8000 Geared Turbofan Engine
• Twin-spool configuration: H-P turbine drives H-P compressor
L-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