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

3-4 Isentropic expansion (in a turbine)

4-1 Constant-pressure heat rejection

The T-sand P-vdiagrams of an ideal Brayton cycle are shown in Fig. 9–31.

Notice that all four processes of the Brayton cycle are executed in steady-

flow devices; thus, they should be analyzed as steady-flow processes. When

the changes in kinetic and potential energies are neglected, the energy bal-

ance for a steady-flow process can be expressed, on a unit–mass basis, as

(9–15)

Therefore, heat transfers to and from the working fluid are

(9–16a)

and

(9–16b)

Then the thermal efficiency of the ideal Brayton cycle under the cold-air-

standard assumptions becomes

Processes 1-2 and 3-4 are isentropic, and P 2 
P 3 and P 4 
P 1. Thus,

Substituting these equations into the thermal efficiency relation and simpli-

fying give

hth,Brayton
 1  (9–17)

1

r^1 pk^1 2>k

T 2

T 1

a

P 2

P 1

b

1 k 1 2>k


a

P 3

P 4

b

1 k 1 2>k

T 3

T 4

hth,Brayton

wnet

qin
^1 

qout

qin
^1 

cp 1 T 4 T 12

cp 1 T 3 T 22

 1 

T 11 T 4 >T 1  12

T 21 T 3 >T 2  12

qout
h 4 h 1 
cp 1 T 4 T 12

qin
h 3 h 2 
cp 1 T 3 T 22

1 qinqout 2  1 winwout 2 
hexithinlet

508 | Thermodynamics

Compressor

wnet

Turbine

Combustion

chamber

Fresh

air

Exhaust

(^1) gases
2
3
4
Fuel
FIGURE 9–29
An open-cycle gas-turbine engine.
Compressor Turbine
1
2
3
Heat^4
exchanger
Heat
exchanger
wnet
qin
qout
FIGURE 9–30
A closed-cycle gas-turbine engine.
P
s s = const.
= const.
2
1 4
3
s
T
2
3
4
1
P = const.
P = const.
(a) T-s diagram
(b) P-v diagram
qout
qin
qout
qin
v
FIGURE 9–31
T-sand P-vdiagrams for the ideal
Brayton cycle.