Hi Bryan
Thanks for your question; it's about time someone asked me that, because
you uncovered an dirty little undocumented hack.
Basically, using the Cp0/R expression allows you to integrate to get h
and s, if you assume otherwise ideal gas behaviour pv=nRT.
The calculation of the ideal component of Helmholtz energy, as
extrapolated from the zero-pressure limit for c_p, is fairly well
explained in Ch 11 of Moran & Shapiro, as well as in the Span & Lemmon
paper about nitrogen (cited on the FPROPS page).
As the ideal components of 's' and 'h' are derived from this
zero-pressure specific heat, there are two constants of integration you
can effectively choose freely. I believe that 'correct' values of these
follow from considerations of fluid mixtures and the 3rd law of
thermodynamics, but at present, I have simply chosen values that allow
me to correctly reproduce the output from other software.
Hope that helps.
Cheers
JP
Bryan Karpowicz wrote:
> Hi John,
> I came across FPROPS,and I'm interested in putting in ortho/para
> hydrogen in place of normal hydrogen. I'm having trouble understanding
> where exactly you come up with the c (constant), and m (linear)
> correction factors specifically in this piece of hydrogen.c:
>
> const IdealData ideal_data_hydrogen = {
> -6.0132647014e+03 / HYDROGEN_R /* const */
> , 2.5810400764e+05/HYDROGEN_R/HYDROGEN_TSTAR /* linear */
>
> I've been trying to come up with my own routines, but I'm a little
> confused as to how one gets the ideal part of the helmholtz energy
> from Cpo/R (which Leachman does "somehow", but in reading his thesis
> its not at all clear to me how he does it), so understanding where
> those factors come from (-6.0132647014e+03,2.5810400764e+05) could
> really help me out.
>
> Thanks,
> Bryan
>
--
Dr John Pye
Dept of Engineering
Australian National University
http://pye.dyndns.org/
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