8-13
8-39 An insulated rigid tank contains a saturated liquid-vapor mixture of water at a specified pressure. An
electric heater inside is turned on and kept on until all the liquid vaporized. The entropy change of the
water during this process is to be determined.
Analysis From the steam tables (Tables A-4 through A-6)
x1
100 kPa ½ v1 v f x1v fg 0.001 0.251.6941 0.001 0.4243 m3/kg
¾
0.25
¿ s1 s f x1s fg 1.3028 0.256.0562 2.8168 kJ/kg ˜ K
v2
v1
P1
½
¾ s2
sat. vapor ¿
H2O
2 kg
100 kPa
We
6.8649 kJ/kg ˜ K
Then the entropy change of the steam becomes
'S
ms 2 s1 (2 kg )(6.8649 2.8168) kJ/kg ˜ K
8.10 kJ/K
8-40 CD EES A rigid tank is divided into two equal parts by a partition. One part is filled with compressed
liquid water while the other side is evacuated. The partition is removed and water expands into the entire
tank. The entropy change of the water during this process is to be determined.
Analysis The properties of the water are (Table A-4)
P1
T1
300 kPa ½ v 1 # v f @60qC
¾
60qC
¿ s1 s f @60qC
0.001017m 3 /kg
0.8313 kJ/kg ˜ K
1.5 kg
compressed
liquid
Noting that
20.001017 v2
2v 1
P2
15 kPa
v2
½° x 2
¾
3
0.002034 m /kg °¿
s2
300 kPa
60qC
3
0.002034 m /kg
v 2 v f
v fg
s f x 2 s fg
Vacuum
0.002034 0.001014
0.0001018
10.02 0.001014
0.7549 0.00010187.2522 0.7556 kJ/kg ˜ K
Then the entropy change of the water becomes
'S
ms2 s1 1.5 kg 0.7556 0.8313 kJ/kg ˜ K
0.114 kJ/K
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