Reformulated to use density difference and temperature difference as …

…a state

For ibpsa/modelica-ibpsa#1412

Buildings/Fluid/Interfaces/ConservationEquation.mo

@@ -98,7 +98,8 @@ model ConservationEquation "Lumped volume with mass and energy balance"
 
   Modelica.SIunits.Mass m(
     min=Modelica.Constants.eps,
-    start=fluidVolume*rho_start)
+    start=fluidVolume*rho_start,
+    stateSelect=StateSelect.never)
     "Mass of fluid";
 
   Modelica.SIunits.Mass[Medium.nXi] mXi(

Buildings/Media/Air.mo

@@ -53,16 +53,16 @@ package Air
   final parameter Boolean standardOrderComponents=true
     "If true, and reducedX = true, the last element of X will be computed from the other ones";
 
-  InputAbsolutePressure p "Absolute pressure of medium";
+  InputAbsolutePressure p(stateSelect=StateSelect.never) "Absolute pressure of medium";
   InputMassFraction[1] Xi(
     start=X_default[1:1],
     nominal={0.01},
     each stateSelect=if preferredMediumStates then StateSelect.prefer else StateSelect.default)
     "Structurally independent mass fractions";
   InputSpecificEnthalpy h "Specific enthalpy of medium";
-  Modelica.Media.Interfaces.Types.Density d "Density of medium";
+  Modelica.Media.Interfaces.Types.Density d
+    "Density of medium";
   Modelica.Media.Interfaces.Types.Temperature T(
-   stateSelect=if preferredMediumStates then StateSelect.prefer else StateSelect.default,
    nominal=100)
    "Temperature of medium";
   Modelica.Media.Interfaces.Types.MassFraction[2] X(start=reference_X)
@@ -92,13 +92,18 @@ package Air
 
     // Declarations for Air only
   protected
-  Modelica.SIunits.TemperatureDifference dT(start=T_default-reference_T)
-    "Temperature difference used to compute enthalpy";
-
+    Modelica.SIunits.TemperatureDifference dT(
+      nominal=10,
+      stateSelect=if preferredMediumStates then StateSelect.prefer else StateSelect.default) = T - reference_T
+      "Temperature difference used to compute enthalpy";
+    // Nominal value is 100/1E5=1E-3
+    Modelica.Media.Interfaces.Types.Density dd(
+      nominal=1E-3,
+      stateSelect=if preferredMediumStates then StateSelect.prefer else StateSelect.default) = d - dStp
+      "Density of medium";
   equation
     MM = 1/(X[1]/steam.MM+(X[2])/dryair.MM);
 
-    T = dT + reference_T;
     h = dT*dryair.cp * X[2] +
        (dT * steam.cp + h_fg) * X[1];
     R = dryair.R*X[2] + steam.R*X[1];

Buildings/Media/Specialized/Air/PerfectGas.mo

@@ -29,6 +29,7 @@ package PerfectGas "Model for air as a perfect gas"
 
   redeclare replaceable model extends BaseProperties(
     u(nominal=1E4),
+    p(stateSelect=StateSelect.never),
     T(start=T_default,
       stateSelect=if preferredMediumStates then StateSelect.prefer else StateSelect.default,
       nominal=100),
@@ -47,6 +48,17 @@ package PerfectGas "Model for air as a perfect gas"
     constant Modelica.SIunits.MolarMass[2] MMX = {steam.MM,dryair.MM}
       "Molar masses of components";
 
+    Modelica.SIunits.TemperatureDifference dT(
+      nominal=10,
+      stateSelect=if preferredMediumStates then StateSelect.prefer else StateSelect.default) = T - reference_T
+      "Temperature difference used to compute enthalpy";
+    // Nominal value is 100/1E5=1E-3
+    Modelica.Media.Interfaces.Types.Density dd(
+      nominal=1E-3,
+      stateSelect=if preferredMediumStates then StateSelect.prefer else StateSelect.default) = d - dStp
+      "Density of medium";
+
+
     MassFraction X_steam "Mass fraction of steam water";
     MassFraction X_air "Mass fraction of air";
   equation
@@ -62,8 +74,8 @@ as required from medium model \"" + mediumName + "\".");
     X_steam  = Xi[Water];
     X_air    = 1-Xi[Water];
 
-    h = (T - reference_T)*dryair.cp * (1 - Xi[Water]) +
-        ((T-reference_T) * steam.cp + h_fg) * Xi[Water];
+    h = dT*dryair.cp * (1 - Xi[Water]) +
+        (dT * steam.cp + h_fg) * Xi[Water];
 
     R = dryair.R*(1 - X_steam) + steam.R*X_steam;
     //