Issue with the derivative annotation in the NF produces grossly incorrect results

[Francesco Casella]

Please check ​Modelica.Fluid.Examples.TraceSubstances.RoomCO2. The simulation fails after about 20 seconds. Analysis of the transient reveals that the temperature in the volume unexpectedly increases above 300 degC, while it is supposed to remain roughly constant trhough the transient, which mainly involves the concentration of CO2.

Further analysis of the -d=optdaedump output reveals that in the process of changing the state variables from volume.m, volume.U to volume.medium.p, volume.medium.T, carried out via the Pantelides algorithm, when the flattening is carried out by NF, the backend computes

boundary1.ports[2].h_outflow = 
  Modelica.Fluid.Examples.TraceSubstances.RoomCO2.Medium.specificEnthalpy_pTX(
  volume.medium.p, 293.15, {0.01})
der(volume.ports[2].h_outflow) = 0.0

This is plain wrong and causes the derivatives of pressure and temperature to be computed in a grossly incorrect way, leading to a totally unphysical transient.

When the OF is used, the Pantelides algorithm instead correctly computes

boundary1.ports[2].h_outflow = 
  Modelica.Fluid.Sources.MassFlowSource_T$boundary1.Medium.specificEnthalpy_pTX(
    volume.medium.p, 293.15, {0.01})
der(volume.ports[2].h_outflow) = 
  Modelica.Fluid.Vessels.ClosedVolume$volume.Medium.h_pTX_der(
  volume.medium.p, volume.medium.T, {volume.medium.Xi[1], 
  1.0 - volume.medium.Xi[1]}, der(volume.medium.p), 
  der(volume.medium.T), {der(volume.medium.Xi[1]), 
  -der(volume.medium.Xi[1])})

and eventually produces the correct simulation results.

It is apparent that

• the enthalpy function is flattened differently in the two cases
• its derivative is correctly computed when starting from the OF, while it is computed to zero when starting from the NF.

Unfortunately the flat output is stripped of annotations, so I can't see if the Derivative annotation was flattened correctly, but I guess there's some problem with it.

@perost, can you please investigate and report?