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

454 | Thermodynamics

(a) Noting that no energy enters or leaves the system during the process, the

application of the energy balance gives

Net energy transfer Change in internal, kinetic,

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

By using the specific-heat values for water and iron at room temperature

(from Table A–3), the final equilibrium temperature Tfbecomes

which yields

(b) Exergy Xis an extensive property, and the exergy of a composite system

at a specified state is the sum of the exergies of the components of that sys-

tem at that state. It is determined from Eq. 8–15, which for an incompress-

ible substance reduces to

where Tis the temperature at the specified state and T 0 is the temperature

of the surroundings. At the initial state,

Similarly, the total exergy at the final state is

That is, the exergy of the combined system (water iron) decreased from

315 to 95.6 kJ as a result of this irreversible heat transfer process.

X2,total
X2,ironX2,water 
 0.595.1
95.6 kJ

X2,water
95.1 kJ

X2,iron
0.5 kJ

X1,total
X1,ironX1,water
 1 245.269.8 2 kJ
315 kJ

69.8 kJ

X1,water
1 100 kg 21 4.18 kJ>kg#K2c1 303  2932 K 1 293 K 2 ln

303 K

293 K

d

245.2 kJ

X1,iron
1 5 kg 21 0.45 kJ>kg#K2c1 623  2932 K 1 293 K 2 ln

623 K

293 K

d

mcaTT 0 T 0 ln

T

T 0

b

mc 1 TT 02 T 0 mc ln

T

T 0

 0

X
 1 UU 02 T 01 SS 02 P 01 VV 0 2

Tf
31.7°C

 1 100 kg 21 4.18 kJ>kg#°C 21 Tf30°C 2

0
1 5 kg 21 0.45 kJ>kg#°C 21 Tf350°C 2

0
3 mc 1 TfTi (^24) iron 3 mc 1 TfTi (^24) water
0
1 ¢U (^2) iron 1 ¢U (^2) water
0
¢U
EinEout¬
¬ ¢Esystem
0
¡
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