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

Entropy Generation Associated

with a Heat Transfer Process

In Example 7–21 it is determined that 0.4 kJ/K of entropy is generated dur-
ing the heat transfer process, but it is not clear where exactly the entropy
generation takes place, and how. To pinpoint the location of entropy genera-
tion, we need to be more precise about the description of the system, its sur-
roundings, and the system boundary.
In that example, we assumed both the system and the surrounding air to
be isothermal at 100°C and 25°C, respectively. This assumption is reason-
able if both fluids are well mixed. The inner surface of the wall must also be

Chapter 7 | 389

temperature change so that the entire boundary of the extended system is at

the surrounding temperature of 25C. The entropy balance for this extended

system(system 
immediate surroundings) yields

or

The entropy generation in this case is entirely due to irreversible heat transfer

through a finite temperature difference.

Note that the entropy change of this extended system is equivalent to the

entropy change of water since the piston–cylinder device and the immediate

surroundings do not experience any change of state at any point, and thus

any change in any property, including entropy.

Discussion For the sake of argument, consider the reverse process (i.e., the

transfer of 600 kJ of heat from the surrounding air at 25C to saturated water

at 100C) and see if the increase of entropy principle can detect the

impossibility of this process. This time, heat transfer will be to the water

(heat gain instead of heat loss), and thus the entropy change of water will be


1.61 kJ/K. Also, the entropy transfer at the boundary of the extended

system will have the same magnitude but opposite direction. This will result

in an entropy generation of 0.4 kJ/K. The negative sign for the entropy

generation indicates that the reverse process is impossible.

To complete the discussion, let us consider the case where the surrounding

air temperature is a differential amount below 100C (say 99.999... 9C)

instead of being 25C. This time, heat transfer from the saturated water to

the surrounding air will take place through a differential temperature

difference rendering this process reversible. It can be shown that Sgen0 for

this process.

Remember that reversible processes are idealized processes, and they can

be approached but never reached in reality.

Sgen

Qout

Tb

¢Ssystem

600 kJ

125 
 2732 K

 1 1.61 kJ>K 2 0.40 kJ/K



Qout

Tb

Sgen¢Ssysem

SinSout¬ 
¬ Sgen¬¬¢Ssystem

123

Net entropy transfer

by heat and mass

123

Entropy

generation

123

Change

in entropy