Fluid boundaries not working with ideal gas mixtures

[mwetter]

This is to correct the issue reported by @justnielsen through lbl-srg/modelica-buildings#1543

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I am trying to use Modelica Buildings with MSL mixture gasses.

In the simples possible case I connect a mass flow source to a pressure sink and use Modelica.Media.IdealGases.MixtureGases.FlueGasSixComponents.

model SourceToSink
  package Medium = Modelica.Media.IdealGases.MixtureGases.FlueGasSixComponents;
  Buildings.Fluid.Sources.MassFlowSource_T boundary(redeclare package Medium = Medium, nPorts=1);
  Buildings.Fluid.Sources.Boundary_pT bou(redeclare package Medium = Medium, nPorts=1);
equation 
  connect(boundary.ports[1], bou.ports[1]);
end SourceToSink;

This gives a model with 26 equations and 25 unknowns.

Looking into Buildings.Fluid.Sources.BaseClasses.PartialSource_Xi_C.mo, I see some problems around the lines

if not use_X_in then
  X_in_internal[1:Medium.nXi] = Xi_in_internal[1:Medium.nXi];
  X_in_internal[Medium.nX] = 1-sum(X_in_internal[1:Medium.nXi]);
end if;

Since I have chosen not to use an external mass fraction input, I would expect a connection between X_in_internal and X. If I replace the two lines in the if statement with
X_in_internal = X (like in the Modelica.Fluid.Sources) the model works.

Also, if I specify the X as an internal input vector in the simple model above, it fails to work.

[mwetter]

The problem seems to be caused because FlueGasSixComponents has reducedX=false. If it is instantiated with
Modelica.Media.IdealGases.MixtureGases.FlueGasSixComponents(reducedX=true) the model works. While this is not a work-around, it points to the problem.

Note that

  constant Boolean reducedX=true 
    "= true if medium contains the equation sum(X) = 1.0; set reducedX=true if only one substance (see docu for details)";

  constant Boolean fixedX=false 
    "= true if medium contains the equation X = reference_X";

  final constant Integer nS=size(substanceNames, 1) "Number of substances";
  constant Integer nX=nS "Number of mass fractions";
  constant Integer nXi=if fixedX then 0 else if reducedX then nS - 1 else nS 
    "Number of structurally independent mass fractions (see docu for details)";

and in BaseProperties,

      if reducedX and not fixedX then
        X[nX] = 1 - sum(Xi);
      end if;