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

7–4 ■ ISENTROPIC PROCESSES

We mentioned earlier that the entropy of a fixed mass can be changed by
(1) heat transfer and (2) irreversibilities. Then it follows that the entropy of
a fixed mass does not change during a process that is internally reversible
and adiabatic(Fig. 7–14). A process during which the entropy remains
constant is called an isentropic process.It is characterized by

Isentropic process: (7–13)

That is, a substance will have the same entropy value at the end of the
process as it does at the beginning if the process is carried out in an isen-
tropic manner.
Many engineering systems or devices such as pumps, turbines, nozzles,
and diffusers are essentially adiabatic in their operation, and they perform
best when the irreversibilities, such as the friction associated with the
process, are minimized. Therefore, an isentropic process can serve as an
appropriate model for actual processes. Also, isentropic processes enable us
to define efficiencies for processes to compare the actual performance of
these devices to the performance under idealized conditions.
It should be recognized that a reversible adiabaticprocess is necessarily
isentropic (s 2 s 1 ), but an isentropicprocess is not necessarily a reversible
adiabatic process. (The entropy increase of a substance during a process as
a result of irreversibilities may be offset by a decrease in entropy as a result
of heat losses, for example.) However, the term isentropic processis cus-
tomarily used in thermodynamics to imply an internally reversible, adia-
batic process.

¢s 0 ¬or¬s 2 s 1 ¬¬ 1 kJ>kg#K 2

Chapter 7 | 343

Therefore, the entropy change of water during this process is

5.105 Btu/R

¢Sm 1 s 2 s 12  1 3 lbm 21 1.77610.07459 2 Btu>lbm#R

s 2 = s 1

No irreversibilities

(internally reversible)

No heat transfer

(adiabatic)

s

Steam

1

FIGURE 7–14

During an internally reversible,

adiabatic (isentropic) process, the

entropy remains constant.

EXAMPLE 7–5 Isentropic Expansion of Steam in a Turbine

Steam enters an adiabatic turbine at 5 MPa and 450°C and leaves at a pres-

sure of 1.4 MPa. Determine the work output of the turbine per unit mass of

steam if the process is reversible.

Solution Steam is expanded in an adiabatic turbine to a specified pressure

in a reversible manner. The work output of the turbine is to be determined.

Assumptions 1 This is a steady-flow process since there is no change with

time at any point and thus mCV0, ECV0, and SCV0. 2 The

process is reversible. 3 Kinetic and potential energies are negligible. 4 The

turbine is adiabatic and thus there is no heat transfer.

Analysis We take the turbineas the system (Fig. 7–15). This is a control

volumesince mass crosses the system boundary during the process. We note

that there is only one inlet and one exit, and thus m

.

1 m

.

2 m

.

.

STEAM

TURBINE

P 1  5 MPa

T 1  450 C

P 2  1.4 MPa

s 2  s 1

wout ?

T

s

1

2

s 2 = s 1

Isentropic

expansion

1.4 MPa

5 MPa

FIGURE 7–15

Schematic and T-sdiagram for

Example 7–5.

SEE TUTORIAL CH. 7, SEC. 4 ON THE DVD.

INTERACTIVE

TUTORIAL