Fix Redlich-Kwong partial molar properties

[speth]

Changes proposed in this pull request

• Remove incorrect implementation of RedlichKwongMFTP::getPartialMolarCp
• Correct implementation of RedlichKwongMFTP::getPartialMolarIntEnergies

If applicable, fill in the issue number this pull request is fixing

Closes #998

Checklist

• The pull request includes a clear description of this code change
• Commit messages have short titles and reference relevant issues
• Build passes (scons build & scons test) and unit tests address code coverage
• Style & formatting of contributed code follows contributing guidelines
• The pull request is ready for review

[codecov]

Codecov Report

Merging #1218 (c72f095) into main (719275e) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main    #1218   +/-   ##
=======================================
  Coverage   65.43%   65.44%           
=======================================
  Files         320      320           
  Lines       46249    46250    +1     
  Branches    19657    19659    +2     
=======================================
+ Hits        30265    30267    +2     
+ Misses      13454    13453    -1     
  Partials     2530     2530           

Impacted Files	Coverage Δ
include/cantera/thermo/RedlichKwongMFTP.h	100.00% <100.00%> (ø)
src/thermo/RedlichKwongMFTP.cpp	71.65% <100.00%> (+0.46%)	⬆️
src/thermo/MixtureFugacityTP.cpp	37.03% <0.00%> (-0.47%)	⬇️

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[decaluwe]

Thanks for tackling this one, @speth. It looks good to me.

Is the idea that hopefully some day we will remove the check_cp flag from the checkThermo definition? I think it's the right call, right now, but does seem odd that just that specific property is excepted (and I certainly wouldn't want it viewed as a precedent for anyone who just doesn't feel like implementing a full thermo feature set, but I think that's easy enough for us to head off).

[decaluwe]

Nice! I was fearing some long derivation on which I'd have to go and run due diligence to check 😂

[speth]

Ah, that's an interesting point -- as I had it, it would have probably gone unnoticed to remove the check_cp=False settings from those tests if getPartialMolarCp was later implemented, so I changed it to test that an exception is raised when check_cp=False.

I'm not necessarily worried about thermo models not providing a "complete" interface. I'd be more interested in defining what the minimal interface that a model needs to provide is, say to be compatible with the reactor network or 1D flame models. As it is, I don't know when you actually need to know the partial molar specific heats -- they appear in the 1D flame for the ideal gas case, but end up being replaced by the partial molar enthalpies in the non-ideal formulation (according to #1079).

[decaluwe]

Ah, good point - the partial molar enthalpies being the generally correct approach, I would be slightly curious to see if there is a speed difference, or if we could just replace the partial molar specific heats in the ideal gas reactor with partial molar enthalpies, and be no worse for it.

IIRC, one of J. Shepherd's students originally wrote asking for access to these partial molar entities, for use with non-ideal phases in SD-ToolBox.

[ischoegl]

@decaluwe ... is this ready to be merged? If not, just dismiss my approval.