Example 11
A simple power cycle is executed in a pistoncylinder
assembly with air as the working fluid. The cycle consists
of three steps:
12
→
constant volume heat addition
23
→
adiabatic, quasiequilibrium, expansion
31
→
constant pressure cooling
At state 1, the temperature and pressure are 70°F and
14.7 psia. At state 2, the temperature is 410°F. The mass
of air in the cylinder is 0.0426 lbm. In all your calculations,
assume constant specific heats evaluated at the average
of
T
1
and
T
2
. Find:
a) pressure and volume at state 2
b) heat and work done in
→
c) volume and temperature at state 3
d) heat and work done in
→
e) heat and work done in
→
f) show that
net
net
QW
=
for this cycle
g) find cycle efficiency
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View Full DocumentSolution:
a) From the ideal gas law
1
11
mRT
PV
M
=
2
22
mRT
M
=
Dividing one equation by the other and noting that
12
VV
=
P
T
P
T
=
2
14.7 psia
(70
460)R
(410
460)R
P
+
=
+
2
24.1psia
P
=
At state 1,
1
3
lbm
density(air,
70,
14.7)
0.0749
ft
TP
ρ
==
=
=
3
1
1
3
0.0426lbm
0.569ft
lbm
0.0749
ft
m
V
=
3
21
b) From the first law for a closed system
UQ
W
∆=−
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 Spring '08
 Kaminski
 Thermodynamics, Qnet, Btu lbm

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