Microsoft Word - Cengel and Boles TOC _2-03-05_.doc

place between states 5 and 6. The gas enters the first stage of the turbine at

state 6 and expands isentropically to state 7, where it enters the reheater. It is

reheated at constant pressure to state 8 (T 8 
T 6 ), where it enters the second

stage of the turbine. The gas exits the turbine at state 9 and enters the regener-

ator, where it is cooled to state 10 at constant pressure. The cycle is completed

by cooling the gas to the initial state (or purging the exhaust gases).

It was shown in Chap. 7 that the work input to a two-stage compressor is

minimized when equal pressure ratios are maintained across each stage. It

can be shown that this procedure also maximizes the turbine work output.

Thus, for best performance we have

(9–26)

In the analysis of the actual gas-turbine cycles, the irreversibilities that are

present within the compressor, the turbine, and the regenerator as well as the

pressure drops in the heat exchangers should be taken into consideration.

The back work ratio of a gas-turbine cycle improves as a result of inter-

cooling and reheating. However, this does not mean that the thermal effi-

ciency also improves. The fact is, intercooling and reheating always

decreases the thermal efficiency unless they are accompanied by regenera-

tion. This is because intercooling decreases the average temperature at

which heat is added, and reheating increases the average temperature at which

heat is rejected. This is also apparent from Fig. 9–44. Therefore, in gas-

turbine power plants, intercooling and reheating are always used in conjunc-

tion with regeneration.

P 2

P 1

P 4

P 3

¬and¬

P 6

P 7

P 8

P 9

518 | Thermodynamics

Regenerator

Compressor wnet

II

Compressor

I Turbine I Turbine II

Combustion

chamber Reheater

Intercooler

10

1

2

3

4

5

6 7

8 9

FIGURE 9–43

A gas-turbine engine with two-stage compression with intercooling, two-stage expansion with

reheating, and regeneration.

s

T

3

4

6

qin

1 qout

2

10

8

9

7

5

qregen = qsaved

qregen

FIGURE 9–44

T-sdiagram of an ideal gas-turbine
cycle with intercooling, reheating, and
regeneration.