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Unformatted text preview: nd substituting, we find
s2 s1 s°
1 R ln [(1.79783 P2
P1 1.66802) kJ/kg · K] (0.287 kJ/kg · K) ln 600 kPa
100 kPa 0.3844 kJ/kg · K
(b) The entropy change of air during this process can also be determined approximately from Eq. 7–34 by using a Cp value at the average temperature of
37°C (Table A–2b) and treating it as a constant: s2 s1 T2
(1.006 kJ/kg · K) ln
0.3842 kJ/kg · K
Cp, av ln R ln 600 kPa
(0.287 kJ/kg · K) ln
100 kPa Discussion The two results above are almost identical since the change in temperature during this process is relatively small (Fig. 7–35). When the temperature change is large, however, they may differ significantly. For those cases, Eq.
7–39 should be used instead of Eq. 7–34 since it accounts for the variation of
specific heats with temperature. Isentropic Processes of Ideal Gases
Several relations for the isentropic processes of ideal gases can be obtained by
setting the entropy-change relations developed above equal to zero. Again,
this is done first for the case...
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This document was uploaded on 11/28/2012.
- Spring '09