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

7–10 ■ REVERSIBLE STEADY-FLOW WORK

The work done during a process depends on the path followed as well as on

the properties at the end states. Recall that reversible (quasi-equilibrium)

moving boundary work associated with closed systems is expressed in terms

of the fluid properties as

We mentioned that the quasi-equilibrium work interactions lead to the max-

imum work output for work-producing devices and the minimum work

input for work-consuming devices.

It would also be very insightful to express the work associated with

steady-flow devices in terms of fluid properties.

Taking the positive direction of work to be from the system (work out-

put), the energy balance for a steady-flow device undergoing an internally

reversible process can be expressed in differential form as

But

Substituting this into the relation above and canceling dhyield

Integrating, we find

(7–51)

When the changes in kinetic and potential energies are negligible, this equa-

tion reduces to

(7–52)

Equations 7–51 and 7–52 are relations for the reversible work outputassoci-

ated with an internally reversible process in a steady-flow device. They will

wrev

2

1

v¬dP¬¬ 1 kJ>kg 2

wrev

2

1

v¬dP¢ke¢pe¬¬ 1 kJ>kg 2

dwrevv¬dP
dke
dpe

dqrevT¬ds

T¬dsdhv¬dP

1 Eq. 7–16 2

1 Eq. 7–24 2

f¬dqrevdhv¬dP

dqrevdwrevdh
dke
dpe

Wb

2

1

P¬dV

362 | Thermodynamics

T, R

s

1

2

780

510

P 1 = 14 psia

P^2

Isentropic

compression

T 2 = 780 R

P 2 =?

He

COMPRESSOR

P 1 = 14 psia

T 1 = 510 R

FIGURE 7–40

Schematic and T-sdiagram for
Example 7–11.

SEE TUTORIAL CH. 7, SEC. 10 ON THE DVD.

INTERACTIVE

TUTORIAL