Question

Steam at 8 MPa and 500C flows through a nozzle where it exits at 100kPa. What is the entropy change of this process?

Answer 1

If the process is reversible then the entropy change is zero but in reality the process is irreversible adiabatic

So enthalpy across nozzle is same.

Now from steam table enthalpy at 8 MPa and 500 Degc = 3399.37 KJ/kg

Now corresponding to same enthalpy, temperature for 100 kpa = 458 Degc from steam table

Now entropy change for this process = entropy at state 2- entropy at state 1

From steam table:

Delta entropy = 8715.86-6724.39 =1991.47 J/Kgk or 35.846 kJ/kmolk