Unformatted text preview: eat values evaluated at the average temperature (Fig. 7–32). The results obtained in this way usually are
sufficiently accurate if the temperature range is not greater than a few hundred
degrees.
The entropychange relations for ideal gases under the constantspecificheat assumption are easily obtained by replacing Cυ(T) and Cp(T) in Eqs. 7–31
and 7–32 by Cυ, av and Cp, av, respectively, and performing the integrations. We
obtain
s2 s1 Cυ, av ln T2
T1 R ln υ2
υ1 (kJ/kg · K) (7–33) and
s2 s1 Cp, av ln T2
T1 R ln P2
P1 (kJ/kg · K) (7–34) Entropy changes can also be expressed on a unitmole basis by multiplying
these relations by molar mass:
–
s2 –
s1 T2
–
C υ, av ln
T1 Ru ln υ2
υ1 (kJ/kmol · K) (7–35) –
s2 –
s1 T2
–
C p, av ln
T1 Ru ln P2
P1 (kJ/kmol · K) (7–36) and Cp Actual Cp
Average Cp Cp, av T1 Tav T2 T FIGURE 7–32
Under the constantspecificheat
assumption, the specific heat is
assumed to be constant at some
average value. cen54261_ch07.qxd 11/18/03 9:57 AM Page 298 298
FUNDAMENTALS OF THERMALFLUID SCIENCES Variable Specific Heats (Exact Analysis)
When the temperature change during a process is large and the specific heats
of the ideal gas vary nonlinearly within the temperature range, the assumption
of constant spe...
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 Spring '09

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