This preview shows page 1. Sign up to view the full content.
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 ––
= – 0.3844 kJ/kg.k
s2 – s1 = Cp, av ln –– – R ln ––
= – 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
Cυ υ1 ln T2
T1 ln T2
T1 which can be rearranged as
υ2 R/Cυ (7–41) or
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.
- Spring '09