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420 CHAPTER 17 GAS TURBINES

that the stability limits are very narrow. This means that when the engine control system attempts to
compensatefor combustioninefficiencybysupplyingmorefueltothe combustor,thisextrafuelisliable to
cause a ‘rich extinction’ of the flame. Thus, an important design requirement for an aircraft combustor is
thatitissizedlargeenoughtoensureanadequatelevelofcombustionefficiencyduringenginerestartatthe
highest altitude at which relight capability is guaranteed.

17.5 CONCLUDING REMARKS
This chapter has developed the concepts introduced in Chapter 3, when the Joule cycle was introduced.
It has shown that only the closed cycle gas turbines are heat engines, and most of the gas turbines
encountered do not fall into that strictly defined category. However, it is possible to analyse the full
range of gas turbine arrangements, and the basic parameters such as work (or power) output and
efficiency can be evaluated for them. Again, the importance of the temperature range of operation has
been emphasized, and the overriding importance of the mean temperatures of heat (energy) reception
and rejection has been demonstrated. The fact that the working fluid in a gas turbine is a single phase
gas is the biggest downfall of the device, because it results in a low work ratio: the situation is
exacerbated at low loads when the difference between the turbine and compressor work reduces. The
effects of reheating and intercooling were investigated, and it was found that in isolation these did not
improve the efficiency of the device: regeneration (or heat exchange) was the way to improve
efficiency in land-based engine.
Aircraft gas turbines were considered, and the differences between pure jet engines, turboprop and
bypass engines were discussed. While the total spectrum of engines considered was limited to com-
mercial applications, it was apparent that the use of bypass improved the sfc and propulsive efficiency
of aircraft engines.
A brief description of combustion chambers showed that the basic parameters and principles used
in their design relied on the underlying physics introduced in Chapter 15.

17.6 PROBLEMS

P17.1 A gas turbine engine operates between minimum temperature T 1 and maximum temperature
T 3 . Show that the optimum pressure ratio for maximum work output is
k

T 3 2ðk 1Þ
r p ¼
T 1

P17.2 A gas turbine engine operates at temperature between 300 and 1200 K. The pressure ratio is
12 and the working fluid is CO 2 . Assume an isentropic process,
(1) Determine the efficiency and work ratio of the cycle. Assume heat capacity ratio of
¼ 1:3.
CO 2 , k CO 2
(2) Calculate the maximum net work. Assume c ¼ 0:9kJ=kgK
pðCO 2 Þ
[0.436, 0.556, 270 kJ/kg]
P17.3 Assume the maximum pressure ratio. Determine the efficiency and work ratio of the cycle in
P17.1.
[0.75, 0.]