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Unformatted text preview: associated with isentropic processes.
The definition of the first is based on Eq. 7–48, which can be rearranged as
P2
P1 exp s°
1 s2
°
R or
P2
P1 exp(s2/R)
°
exp(s°/R)
1 The quantity exp(s°/R) is defined as the relative pressure Pr. With this definition, the last relation becomes
P2
P1 s const. Pr 2
Pr1 (7–49) Note that the relative pressure Pr is a dimensionless quantity that is a function
of temperature only since s° depends on temperature alone. Therefore, values
of Pr can be tabulated against temperature. This is done for air in Table A–21.
The use of Pr data is illustrated in Fig. 7–37.
Sometimes specific volume ratios are given instead of pressure ratios. This
is particularly the case when automotive engines are analyzed. In such cases,
one needs to work with volume ratios. Therefore, we define another quantity
related to specific volume ratios for isentropic processes. This is done by utilizing the idealgas relation and Eq. 7–49:
P1υ1
T1 P2υ 2
T2 → υ2
υ1 T2...
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This document was uploaded on 11/28/2012.
 Spring '09

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