As for Open Systems

Dividing Equation (3.12) by Af and taking the limit as the time step becomes zero, the second law becomes  

At steady-state, the entropy change of the system is zero  

Applying Equations (3.16) and (3.17), at steady-state Equation (3.15) becomes  

For unsteady-state problems, the entropy change of the system, (dS/d )sys, must be included. Adding this term to Equation (3.18) gives  

Using Entropy to Help Calculate the Exit Velocity from a Nozzle 

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