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This is a jump relation for the entropy in the normal shock wave, being adiabatic the stagnation temperature is constant but not the stagnation pressure. The states are determined indipendently from the path. |
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Yes, in general you start from Gibbs relation to integrate between the state 1 and 2. |
The key assumption is that the the process going from the static to stagnation conditions is via an isentropic process.
The entropy change from 1 to 01 is 0 because the change from static-to-stagnation conditions is (assumed to be) isentropic. The change from 2 to 02 is also 0. Hence the change from 1 to 2 is equal to the change from 10 to 20. Recall also that for a simple system, the thermodynamic state is fully characterized by two intensive properties (static temperature and static pressure). The stagnation conditions at state 1 or 2 are fully specified by knowing their static conditions and vice versa, hence the entropy change between 1 and 2 or 01 and 02 must be equal or you do not have a thermodynamic system. It is common in engineering applications to assume that the change from static to a (hypothetical) stagnation conditions is isentropic. This condition is explicitly noted in your quote of Denton. The usage of stagnation properties for performance calculations in turbomachinery is also consistent with this assumption. |
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