Adiabatic Turbine PV & HS Diagrams

Ch 8, Lesson C, Page 6 - Adiabatic Turbine PV & HS Diagrams

• What do you notice right away in the PV Diagram?
• Well, the isentropic path is steeper than the isothermal path and as a result the specific volume at state 2S is smaller than the specific volume at state 2T.
• That makes sense since the temperature at 2T is greater than the temperature at 2S.
• Ok. Does anything seem odd about the HS Diagram ?
• Well, it seems strange that the specific enthalpy at 2T is greater than the specific enthalpy at 1, doesn’t it ?
• This occurs because for real fluids, the specific enthalpy IS a function of pressure. Do you remember that from back in Lesson 3A ?
• If our working fluid were an ideal-gas or a perfectly incompressible liquid, the specific enthalpy at state 2T would EQUAL the specific enthalpy at state 1, because specific enthalpy for ideal-gases and incompressible liquids does not depend on pressure.
• For real fluids, the specific enthalpy can increase or decrease slightly when the pressure decreases.
• This seems odd, but Hhat  = Uhat +P Vhat and as the pressure drops the specific volume increases and the specific internal energy goes either up or down depending on the chemistry of the working fluid and temperature and pressure of the system.
• But what you need to know is that the specific enthalpy can go up or down a little bit in an irreversible turbine that is both adiabatic and isothermal.
• We haven’t used the HS diagram since it was introduced back in Lesson 7B, but it is the most useful phase diagram for the analysis of SS, SISO, adiabatic processes.
• So, which processes fall into this category ? The big 4 are: pumps, compressors, turbines and nozzles.
• We’ve covered turbines, so now we will consider the isentropic efficiency of the other 3 processes.
• Let’s begin with…. Nozzles.