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

388 | Thermodynamics

Substituting, the rate of entropy generation is determined to be

Discussion Note that entropy is generated during this process at a rate of

8.65 Btu/min · R. This entropy generation is caused by the mixing of two

fluid streams (an irreversible process) and the heat transfer between the

mixing chamber and the surroundings through a finite temperature difference

(another irreversible process).

8.65 Btu/min#R

¬

180 Btu>min

530 R

 1 322.7 0.18174 300 0.0360922.7 1.7406 2 Btu>min#R

S

#

genm

#

3 s 3 m

#

1 s 1 m

#

2 s 2 

Q

#

out

Tb

EXAMPLE 7–21 Entropy Generation Associated with Heat

Transfer

A frictionless piston–cylinder device contains a saturated liquid–vapor

mixture of water at 100C. During a constant-pressure process, 600 kJ of

heat is transferred to the surrounding air at 25C. As a result, part of the

water vapor contained in the cylinder condenses. Determine (a) the entropy

change of the water and (b) the total entropy generation during this heat

transfer process.

Solution Saturated liquid–vapor mixture of water loses heat to its surround-

ings, and some of the vapor condenses. The entropy change of the water and

the total entropy generation are to be determined.

Assumptions 1 There are no irreversibilities involved within the system

boundaries, and thus the process is internally reversible. 2 The water

temperature remains constant at 100C everywhere, including the

boundaries.

Analysis We first take the water in the cylinderas the system (Fig. 7–69).

This is a closed systemsince no mass crosses the system boundary during

the process. We note that the pressure and thus the temperature of water in

the cylinder remain constant during this process. Also, the entropy of the

system decreases during the process because of heat loss.

(a) Noting that water undergoes an internally reversible isothermal process,

its entropy change can be determined from

(b) To determine the total entropy generation during this process, we

consider the extended system, which includes the water, the piston–cylinder

device, and the region immediately outside the system that experiences a

¢Ssystem

Q

Tsystem



600 kJ

1100 
273 K 2


1.61 kJ/K

T = 100°C

H 2 O

600 kJ

Tsurr = 25°C

FIGURE 7–69

Schematic for Example 7–21.