T --Jp -1 ) Jp
(
()
Tamb Tamb
ηc
Figure 2-2 shows the effect on the overall cycle efficiency of the increasing pressure ratio and the firing temperature. The increase in the pressureratio increases the overall efficiency at a given firing temperature; however， increasing the pressure ratio beyond a certain value at any given firing temperature can actually result in lowering the overall cycle efficiency. lt should also be noted that the very high-pressure ratios tend to reduce the operating range of the turbine compressor. This causes the turbine compres-sor to be much more intolerant to dirt build up in the inlet air filter and on the compressor blades and creates large drops in cycle efficiency and perform-ance. ln somecases， it can lead to compressor surge， which in turn can leadto a flameout， or even serious damage and failure of the compressor blades and the radial and thrust bearings of the gas turbine.
To obtain a more accurate relationship between the overall thermalefficiency and the inlet turbine temperatures， overall pressureratios， andoutputwork， consider the following relationships. For maximum overallthermal cycle efficiency， the following equation gives the optimum pressure ratio for fixed inlet temperatures and efficiencies to the compressor and turbine:
(Jp)eopt二{T1T3ηt -1 T1T3 + T2 [T1T3ηt
1

                                      
古古-1
-(T1T3ηt)2 -(T1T3ηt -T1T3 + T2)(T2ηcηt -T1T3ηcηt + T1T3ηt)]}
13
(2-10)
55
50
45
40
35
30
25
20
15
10
05 10152025303540

Pressure Ratio
the above equation for no losses in the compressor and turbine (ηc二 ηt二 1) reduces to:
/\古
古-1
(Jp)eopt二 TT1T23 (2-11)1
The optimum pressure ratio for maximum output work for a turbine taking into account the efficiencies of the compressor and the turbine expander section can be expressed by the following relationship:
古-1
Jpwopt二 F/T23Tηc1 ηt \ + 21 『古(2-12)
Figure 2-3 shows the optimum pressure ratio for maximum efficiency or work per lb (kg) of air. The optimum pressure ratio based on work occurs at a lower pressure ratio than the point of maximum efficiency at the same firing Temperature.
Thus， a cursory inspection of the efficiency indicate that the overallefficiency of a cycle can be improved by increasing the pressure ratio， orincreasing the turbine inlet temperature， and the work per lb (kg) of air canbe increased by increasing the pressureratio， or increasing the turbine inlettemperature， or by decreasing the inlet temperature.
3000
2500
2000
1500
1000

500 0 0 5 101520253035404550
Based on M
Based o n Max --Work  ax --Efficiency 

Pressure Ratio
Figure 2-.. Pressure ratio based on maximum efficiency or work at various firing temperatures. Based on a compressor efficiency of 87% and a turbine efficiency of 92%.

Regeneration Effect
ln a simple gas turbine cycle the turbine exit temperature is nearly always appreciably higher than the temperature of the air leaving the compressor.Obviously， the fuel requirement can be reduced by the use of a regenerator in which the hot turbine exhaust gas preheats the air between the compressor and the combustion chamber. Figure 2-4 shows a schematic of the regen-erative cycle and its performance in the T-S diagram. ln an ideal case the flow through the regenerator is at constant pressure. The regenerator effect-iveness is given by the following relationship:

ηreg二 T3 -T2 (2-13)
T5 -T2
　
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本文鏈接地址：燃氣渦輪工程手冊 Gas Turbine Engineering Handbook 1(28)