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

456 | Thermodynamics

The useful work is the difference between these two:

That is, 1.43 kJ of the work done is available for creating a useful effect

such as rotating a shaft.

Also, the heat transfer from the furnace to the system is determined from

an energy balance on the system to be

Net energy transfer Change in internal, kinetic,

by heat, work, and mass potential, etc., energies

(b) The exergy destroyed during a process can be determined from an exergy

balance, or directly from Xdestroyed
T 0 Sgen. We will use the second approach

since it is usually easier. But first we determine the entropy generation by

applying an entropy balance on an extended system(system + immediate

surroundings), which includes the temperature gradient zone between the

cylinder and the furnace so that the temperature at the boundary where heat

transfer occurs is TR
1200 K. This way, the entropy generation associated

with the heat transfer is included. Also, the entropy change of the argon gas

can be determined from Q/Tsyssince its temperature remains constant.

Net entropy transfer Entropy Change

by heat and mass generation in entropy

Therefore,

and

(c) The reversible work, which represents the maximum useful work that

could be produced Wrev,out, can be determined from the exergy balance by

setting the exergy destruction equal to zero,

Net exergy transfer Exergy Change

by heat, work, and mass destruction in exergy

 0 WsurrT 0

Q

Tsys

 1 U 2 U 12 P 01 V 2 V 12 T 01 S 2 S 12

a 1 

T 0

Tb

bQWrev,out
X 2 X 1

XinXout  Xdestroyed 
¢Xsystem

Xdestroyed
T 0 Sgen
 1 300 K 21 0.00405 kJ>K 2 
1.22 kJ/K

Sgen

Q

Tsys



Q

TR

2.43 kJ

400 K



2.43 kJ

1200 K

0.00405 kJ

Q

TR

Sgen
¢Ssystem

Q

Tsys

SinSout¬ Sgen
¢Ssystem

Qin
Wb,out
2.43 kJ

QinWb,out
¢U
mcV ¢TQ

0


 0

EinEout¬
¬ ¢Esystem

Wu
WWsurr
2.43 1 
1.43 kJ

⎭⎪⎬⎪⎫ ⎭⎪⎬⎪⎫

⎭⎪⎬⎪⎫ ⎭⎬⎫ ⎭⎪⎬⎪⎫

0 (reversible)

⎭⎪⎬⎪⎫ ⎭⎪⎪⎪⎪¡⎬⎪⎪⎪⎪⎫ ⎭⎪⎬⎪⎫