3844 kjkgk t2 p2 s2 s1 cp av ln r ln t1 p1 03842

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Unformatted text preview: of constant specific heats and then for the case of variable specific heats. AIR T1 = 290 K T2 = 330 K P2 s2 – s1 = s° – s1 – R ln –– ° 2 P1 = – 0.3844 kJ/kg.k T2 P2 s2 – s1 = Cp, av ln –– – R ln –– T1 P1 = – 0.3842 kJ/kg.k FIGURE 7–35 For small temperature differences, the exact and approximate relations for entropy changes of ideal gases give almost identical results. cen54261_ch07.qxd 11/18/03 9:57 AM Page 300 300 FUNDAMENTALS OF THERMAL-FLUID SCIENCES Constant Specific Heats (Approximate Analysis) When the constant-specific-heat assumption is valid, the isentropic relations for ideal gases are obtained by setting Eqs. 7–33 and 7–34 equal to zero. From Eq. 7–33, R υ2 ln Cυ υ1 ln T2 T1 ln T2 T1 which can be rearranged as ln υ1 υ2 R/Cυ (7–41) or T2 T1 s const. υ1 υ2 k1 (ideal gas) (7–42) since R Cp Cυ, k Cp/Cυ, and thus R/Cυ k 1. Equation 7–42 is the first isentropic relation for ideal gases under the constant-specific-heat assumption. The second is...
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This document was uploaded on 11/28/2012.

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