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Unformatted text preview: between the initial and the final states. To perform these
integrations, however, we must know the relationship between du or dh and
the temperature (such as du Cυ dT and dh Cp dT for ideal gases) as well
as the equation of state for the substance (such as the ideal-gas equation of
state Pυ RT). For substances for which such relations exist, the integration
of Eq. 7–25 or 7–26 is straightforward. For other substances, we have to rely
on tabulated data.
The T ds relations for nonsimple systems, that is, systems that involve more
than one mode of quasi-equilibrium work, can be obtained in a similar manner by including all the relevant quasi-equilibrium work modes. 7–8 I ENTROPY CHANGE OF LIQUIDS AND SOLIDS Recall that liquids and solids can be approximated as incompressible substances since their specific volumes remain nearly constant during a process.
Thus, dυ 0 for liquids and solids, and Eq. 7–25 for this case reduces to
T C dT
T (7–27) since Cp Cυ C and du C dT for incompressible substances. Then the
entropy change during a process is determined by integration to be
2 Liquids, solids: s2 s1 C(T)
T Cav ln T2
T1 (kJ/kg · K) (7–28) where Cav is the average specific heat of...
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This document was uploaded on 11/28/2012.
- Spring '09