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

The T-sdiagram is particularly useful as a visual aid in the analysis of ideal

power cycles. An ideal power cycle does not involve any internal irre-

versibilities, and so the only effect that can change the entropy of the work-

ing fluid during a process is heat transfer.

On a T-sdiagram, a heat-additionprocess proceeds in the direction of

increasing entropy, a heat-rejectionprocess proceeds in the direction of

decreasing entropy, and an isentropic (internally reversible, adiabatic)

process proceeds at constant entropy. The area under the process curve on a

T-sdiagram represents the heat transfer for that process. The area under the

heat addition process on a T-sdiagram is a geometric measure of the total

heat supplied during the cycle qin, and the area under the heat rejection

process is a measure of the total heat rejected qout. The difference between

these two (the area enclosed by the cyclic curve) is the net heat transfer,

which is also the net work produced during the cycle. Therefore, on a T-s

diagram, the ratio of the area enclosed by the cyclic curve to the area under

the heat-addition process curve represents the thermal efficiency of the

cycle. Any modification that increases the ratio of these two areas will also

increase the thermal efficiency of the cycle.

Although the working fluid in an ideal power cycle operates on a closed

loop, the type of individual processes that comprises the cycle depends on

the individual devices used to execute the cycle. In the Rankine cycle, which

is the ideal cycle for steam power plants, the working fluid flows through a

series of steady-flow devices such as the turbine and condenser, whereas in

the Otto cycle, which is the ideal cycle for the spark-ignition automobile

engine, the working fluid is alternately expanded and compressed in a piston–

cylinder device. Therefore, equations pertaining to steady-flow systems

should be used in the analysis of the Rankine cycle, and equations pertaining

to closed systems should be used in the analysis of the Otto cycle.

9–2 ■ THE CARNOT CYCLE AND ITS VALUE

IN ENGINEERING

The Carnot cycle is composed of four totally reversible processes: isother-

mal heat addition, isentropic expansion, isothermal heat rejection, and isen-

tropic compression. The P-v and T-s diagrams of a Carnot cycle are

replotted in Fig. 9–6. The Carnot cycle can be executed in a closed system

(a piston–cylinder device) or a steady-flow system (utilizing two turbines

and two compressors, as shown in Fig. 9–7), and either a gas or a vapor can

490 | Thermodynamics

PT

v s

1

2 3

4

1

2

3

4

wnet wnet

FIGURE 9–5

On both P-vand T-sdiagrams, the
area enclosed by the process curve
represents the net work of the cycle.

P

T

s

v

1

2

3

4

1 2

4 3

q (^) out
q (^) in
Isentropic Isentropic
TH
TL
q (^) in
Isentropic
q (^) out
TH = const.
TL = const.
Isentropic
FIGURE 9–6
P-vand T-sdiagrams of a Carnot
cycle.