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

This preview shows page 1. Sign up to view the full content.

This is the end of the preview. Sign up to access the rest of the document.

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...
View Full Document

This document was uploaded on 11/28/2012.

Ask a homework question - tutors are online