Stability Criteria Based on Internal Energy

Stability criteria based on internal energy The stability of the original single phase in comparison to the two phases demands that the internal energy change be positive (see Fig. 4.15)  

For AU to be positive, since %N N/N is positive, the second-order variations with respect to s and c have to he positive. In case the second-order variations are zero, higher-order even variations should be positive. In other words, the first nonvanishing terms of order two or higher-even order should be positive. Therefore, stability of the single-phase state demands that 

Therefore Cp 0. Note that since cy and Cf are positive for a stable system, the implication is that Cp 0 when a system is stable. 

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