these little cycles approach the Carnot cycle as their number increases. The
efficiency of such a Carnot cycle is given by the relationship
ηCARNOT二 1 - T內(nèi) Tp (2-17)
Notice that if the specific heats are constant， then
古-1
T3 T4 二 T內(nèi) Tp 二 T2 T1 二 P2 P1/\ 古 (2-18)
All the Carnot cycles making up the simple gas turbine cycle have the sameefficiency. Likewise， all of the Carnot cycles into which the cycle α-b-c-2-α might similarly be divided have a common value of efficiency lowerthan the Carnot cycles which comprise cycle 1-2-3-4-1. Thus， the addition ofan intercooler， which adds α-b-c-2-αto the simplecycle， lowers the efficiency of the cycle.
The addition of an intercooler to a regenerative gas turbine cycle increases the cycle"s thermal efficiency and output work because a larger portion of the heat required for the process c-3 in Figure 2-7 can be obtained from the hot turbine exhaust gas passing through the regenerator instead of from burning additional fuel.
The reheat cycle increases the turbinework， and consequently the network of thecycle， can be increased without changing the compressor work or the turbine inlet temperature by dividing the turbine expansion into two or more parts with constant pressure heating before each expansion. This cycle modification is known as reheating as seen in Figure 2-8. By reasoningsimilar to that used in connection with lntercooling， it can be seen that thethermal efficiency of a simple cycle is lowered by the addition ofreheating，while the work output is increased. However， a combination of regenerator and reheater can increase the thermal efficiency.
Actual Cycle Analysis
The previous section dealt with the concepts of the various cycles. Work output and efficiency of all actual cycles are considerably less than those ofthe corresponding ideal cycles because of the effect of compressor， combus-tor， and turbine efficiencies and pressure losses in the system.
The Simple Cycle
The simple cycle is the most common type of cycle being used in gas turbines in the field today. The actual open simple cycle as shown in Figure 2-9 indicates the inefficiency of the compressor and turbine and the loss in pressure through the burner. Assuming the compressor efficiency is ηc and the turbine efficiency is η1， then the actual compressor work and the actual turbine work is given by:
Wca二內(nèi)的α(h2 -h1)/ηc (2-19)
Wta二(內(nèi)的α +內(nèi)的)(h3α -h4)ηt (2-20)
T
Thus， the actual total output work is
Wact二 Wta -Wca (2-21)
The actual fuel required to raise the temperature from 2α to 3α is
內(nèi)的二 h3α -h2α (2-22)
(LH )ηb
Thus， the overall adiabatic thermal cycle efficiency can be calculated from the following equation:
ηc二 Wact(2-23)
內(nèi)的(LH. )
Analysis of this cycle indicates that an increase in inlet temperature to the turbine causes an increase in the cycle efficiency. The optimum pressure ratio for maximum efficiency varies with the turbine inlet temperature from an optimum of about 15.5:1 at a temperature of 1500 oF (816 oC) to about 43:1 at a temperature of about 2400 oF (1316 oC). The pressure ratio for max-imumwork， however， varies from about 11.5:1 to about 35:1 for the same respective temperatures.
S
50
45
40
35
30
25
20
15
10
5
0
Thus， from Figure2-10， it is obvious that for maximum performance， a pressure ratio of 30:1 at a temperature of 2800 oF (1537 oC) is optimal. .se of an axial-flow compressor requires 16-24 stages with a pressure ratio of 1.15-1.25:1 per stage. A 22-stage compressor producing a 30:1 pressure ratio is a relatively conservative design. lf the pressure ratio were increased to
1.252:1 per stage， the number of stages would be about 16. The latter pressure ratio has been achieved with high efficiencies. This reduction in number of stages means a great reduction in the overall cost. Turbinetemperatures increases give a great rise in efficiency andpower， so tempera-tures in the 2400 oF (1316 oC) range at the turbine inlet are becoming the state-of-art.
中國航空網(wǎng) m.k6050.com
航空翻譯 www.aviation.cn
本文鏈接地址：燃氣渦輪工程手冊 Gas Turbine Engineering Handbook 1(30)
