Adding fluids ideal gas reference

[Justin Begay]

Hello all, 

I am attempting to get familiarized with the CoolProp function that allows you to add your own fluids. Simply put, I am looking for a simple way of defining a fluid’s idea gas reference. After reviewing the ideal gas reference documentation, I am curious about 3 terms within the alpha0 function.  For the type, “IdealGasHelmholtzLogTau”, I am wondering how the “a” term is defined and the physical and/or mathematical meaning behind it. Does this “a” term have any relation to the heat capacity? For the type, “IdealGasHelmholtzLead”, I am puzzled about the “a1” and “a2” terms. Is there a physical or mathematical meaning for how these terms are defined for a given fluid? Any information on these inquiries will be greatly appreciated!

Best regards,

Justin

[Ian Bell]

There's some info here:  http://www.coolprop.org/fluid_properties/more_reading.html#conversion-from-ideal-gas-term-to-helmholtz-energy-term .  a1 and a2 are related to the reference state.

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[Justin Begay]

Hello Ian, 

Let’s say we have a simple example: I want to analyze a Rankine cycle that isentropically expands through a turbine. To determine the thermal efficiency of the cycle, the enthalpy difference between the state of the fluid before and after expansion is desired. My question is: can we arbitrarily define these alpha0 terms (a1 and a2) and still generate accurate enthalpy differences and thus accurate thermal efficiencies? Or is there a thermodynamic principle that dictates the values of a1 and a2?

-Justin

[Ian Bell]

a1 and a2 have no impact on enthalpy differences at given pairs of temperature and pressure.  Try it yourself to confirm!

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