Page 414 - Advanced thermodynamics for engineers

404 CHAPTER 17 GAS TURBINES

Temperature, T p p 01
0a

p
1 a
1
T0a
Va
T01 2 V1
V1/2cp
2
Va/2cp Schematic
p a of intake

Ta

Entropy, S
FIGURE 17.18
‘Intake’ loss.
pressure in the nozzle system. Reheat gives a substantial increase in thrust, but a very low efficiency: it
is used in combat aircraft for takeoff (to increase the payload that can be carried) and also for higher
sprint performance. However, a 44% increase in thrust can require a 164% increase in fuel flow rate, so
reheat has to be used sparingly otherwise range is reduced to unacceptable levels. Finally, the nozzle
converts the slow moving gas into a jet which will propel the aircraft forward.
The propelling nozzle is usually a simple convergent one, even though the pressure ratio will often
exceed the critical pressure ratio. Convergent–divergent nozzles are used on the high-pressure ratio

Temperature, T p 05
Reheat fuel p 04
4 5 04 05

V 5
V 2
5
2cp

5
p p a
5 =
Diffuser Jet pipe Nozzle 5’

Entropy, S
FIGURE 17.19
‘Propelling nozzle’ system.