# With this definition the last relation becomes p2 p1

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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 ideal-gas 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.

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