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Modelica.Media.Air.MoistAir.density_derX is not right #2868

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finback-at opened this issue Mar 29, 2019 · 2 comments · Fixed by #2932
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Modelica.Media.Air.MoistAir.density_derX is not right #2868

finback-at opened this issue Mar 29, 2019 · 2 comments · Fixed by #2932
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@wischhusen
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bug Critical/severe issue L: Media Issue addresses Modelica.Media
Milestone
MSL4.0.0

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@finback-at
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I think that the current algorithms of Modelica.Media.Air.MoistAir.density_derX,

dddX[Water] := pressure(state)*(steam.R - dryair.R)/((steam.R - dryair.R)
  *state.X[Water]*temperature(state) + dryair.R*temperature(state))^2;
dddX[Air] := pressure(state)*(dryair.R - steam.R)/((dryair.R - steam.R)*
  state.X[Air]*temperature(state) + steam.R*temperature(state))^2;

are not right.

Unit check

Mass fraction X [kg/kg] is unitless.
The unit of density_derX should be same as density.
But the unit of abouve equations are,
pressure*R/(R*temperature)^2 = pressure/(R*temperature^2) = density/temperature.
where, density = pressure/(R*temperature) is equation of state for ideal gases.

my derivation

density = p/(R*T)
=>
d(density)/dX = - p/(R^2*T) * dR/dX = {-p*(dR/dX)} / (R^2*T)  --- (a)

(1) dddX[Water]

R = steam.R*X[Water] + dryair.R*(1-X[Water]) 
   = (steam.R - dryair.R)*X[Water] + dryair.R 

dR/dX[Water] = steam.R - dryair.R

numerator of (a) = -p*(dR/dX) = - pressure(state)*(steam.R - dryair.R)
denominator of (a) = R^2*T 
   = ((steam.R - dryair.R)*X[Water] + dryair.R)^2*temperature(state)

dddX[Water] = - pressure(state)*(steam.R - dryair.R)/
    ( ((steam.R - dryair.R)*X[Water] + dryair.R)^2 * temperature(state))

(2) dddX[Air]

R = steam.R*(1-X[Air]) + dryair.R*X[Air]
  = steam.R + (dryair.R - steam.R)*X[Air]

dR/dX[Air] = dryair.R - steam.R

numerator of (*) = -p*(dR/dX) = - pressure(state)*(dryair.R - steam.R)
denominator of (*) = R^2*T
   = (steam.R + (dryair.R - steam.R)*X[Air])^2 * temperature(state)

dddX[Air] = - pressure(state)*(dryair.R - steam.R)/
              ( (steam.R + (dryair.R - steam.R)*X[Air])^2 * temperature(state) )
@tbeu tbeu added the L: Media Issue addresses Modelica.Media label Mar 29, 2019
@beutlich
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That code is in there since MoMoLib was integrated more than 7 years ago:

ModelicaStandardLibrary/Modelica/Media/Air.mo

Lines 3734 to 3745 in ba2f041

redeclare function extends density_derX
algorithm
dddX[Water] := pressure(state)*(steam.R - dryair.R)/((steam.R - dryair.R)
*state.X[Water]*temperature(state) + dryair.R*temperature(state))^2;
dddX[Air] := pressure(state)*(dryair.R - steam.R)/((dryair.R - steam.R)*
state.X[Air]*temperature(state) + steam.R*temperature(state))^2;
annotation (Documentation(revisions="<html>
<p>2012-01-12 Stefan Wischhusen: Initial Release.</p>
</html>"));
end density_derX;

@wischhusen Can you please have a look at this.

@HansOlsson You once told (maybe for a similar fix of molarMass), that unit checks in function are tricky. Can you please cross-check.

@beutlich beutlich changed the title Modelica.Media.Air.MoistAir.density_derX is not right. Modelica.Media.Air.MoistAir.density_derX is not right Mar 29, 2019
@beutlich beutlich added this to the MSL4.0.0 milestone Mar 29, 2019
@wischhusen
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Looks reasonable. Have to revisit the code later.

wischhusen added a commit to wischhusen/Modelica-1 that referenced this issue May 14, 2019
@wischhusen
Corrected issues modelica#2874, modelica#2872, modelica#2868
a96a5a8
@wischhusen wischhusen mentioned this issue May 14, 2019
Merged
beutlich pushed a commit to wischhusen/Modelica-1 that referenced this issue May 14, 2019
@wischhusen @beutlich
Corrected issues modelica#2874, modelica#2872, modelica#2868
13867f9
@beutlich beutlich added the bug Critical/severe issue label May 14, 2019
beutlich pushed a commit that referenced this issue Jul 24, 2019
@wischhusen @beutlich
Corrected issues #2874, #2872, #2868 (#2932)
86516d4
beutlich pushed a commit to beutlich/ModelicaStandardLibrary that referenced this issue Jul 24, 2019
@wischhusen @beutlich
Corrected issues modelica#2874, modelica#2872, modelica#2868
a2507c7
beutlich pushed a commit to beutlich/ModelicaStandardLibrary that referenced this issue Jul 24, 2019
@wischhusen @beutlich
Corrected issues modelica#2874, modelica#2872, modelica#2868 (modelic…
63394b5 
…a#2932)
@beutlich beutlich mentioned this issue Jul 24, 2019
Merged
beutlich pushed a commit to beutlich/ModelicaStandardLibrary that referenced this issue Jul 24, 2019
@wischhusen @beutlich
Corrected issues modelica#2874, modelica#2872, modelica#2868
f85bea9
beutlich pushed a commit to beutlich/ModelicaStandardLibrary that referenced this issue Sep 26, 2019
@wischhusen @beutlich
Corrected issues modelica#2874, modelica#2872, modelica#2868
43f4e9f
beutlich added a commit that referenced this issue Sep 26, 2019
@beutlich
Corrected issues #2874, #2872, #2868 (#3069)
b2c5346
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Labels
bug Critical/severe issue L: Media Issue addresses Modelica.Media
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Fix Media.Air.MoistAir.{density_derX, s_pTX, s_pTX_der} wischhusen/Modelica-1
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@wischhusen @tbeu @beutlich @finback-at