Information
This is a medium model that is similar to
Modelica.Media.Air.MoistAir but
it has a constant specific heat capacity.
In particular, the medium is thermally perfect, i.e.,
-
it is in thermodynamic equilibrium,
-
it is chemically not reacting, and
-
internal energy and enthalpy are functions of the temperature only.
In addition, the gas is calorically perfect, i.e., the
specific heat capacities at constant pressure
and constant volume are both constant (Bower 1998).
References
Bower, William B. A primer in fluid mechanics: Dynamics of flows in one
space dimension. CRC Press. 1998.
Package Content
Types and constants
constant Integer Water=1
"Index of water (in substanceNames, massFractions X, etc.)";
constant Integer Air=2
"Index of air (in substanceNames, massFractions X, etc.)";
constant Real k_mair = steam.MM/dryair.MM "ratio of molar weights";
constant Buildings.Media.PerfectGases.Common.DataRecord dryair=
Buildings.Media.PerfectGases.Common.SingleGasData.Air;
constant Buildings.Media.PerfectGases.Common.DataRecord steam=
Buildings.Media.PerfectGases.Common.SingleGasData.H2O;
Parameters
| Type | Name | Default | Description |
|---|
| Advanced |
| Boolean | preferredMediumStates | false | = true if StateSelect.prefer shall be used for the independent property variables of the medium |
Modelica definition
redeclare replaceable model extends BaseProperties(
T(stateSelect=if preferredMediumStates then StateSelect.prefer else StateSelect.default),
p(stateSelect=if preferredMediumStates then StateSelect.prefer else StateSelect.default),
Xi(stateSelect=if preferredMediumStates then StateSelect.prefer else StateSelect.default))
/* p, T, X = X[Water] are used as preferred states, since only then all
other quantities can be computed in a recursive sequence.
If other variables are selected as states, static state selection
is no longer possible and non-linear algebraic equations occur.
*/
MassFraction x_water "mass of total water/mass of dry air";
Real phi "relative humidity";
protected
constant SI.MolarMass[2] MMX = {steam.MM,dryair.MM}
"Molar masses of components";
MassFraction X_liquid "Mass fraction of liquid water";
MassFraction X_steam "Mass fraction of steam water";
MassFraction X_air "Mass fraction of air";
MassFraction X_sat
"Steam water mass fraction of saturation boundary in kg_water/kg_moistair";
MassFraction x_sat
"Steam water mass content of saturation boundary in kg_water/kg_dryair";
AbsolutePressure p_steam_sat "Partial saturation pressure of steam";
equation
assert(T >= 200.0 and T <= 423.15, "
Temperature T is not in the allowed range
200.0 K <= (T ="
+ String(T) + " K) <= 423.15 K
required from medium model \"" + mediumName + "\".");
MM = 1/(Xi[Water]/MMX[Water]+(1.0-Xi[Water])/MMX[Air]);
p_steam_sat = min(saturationPressure(T),0.999*p);
X_sat = min(p_steam_sat * k_mair/max(100*Modelica.Constants.eps, p - p_steam_sat)*(1 - Xi[Water]), 1.0)
"Water content at saturation with respect to actual water content";
X_liquid = max(Xi[Water] - X_sat, 0.0);
X_steam = Xi[Water]-X_liquid;
X_air = 1-Xi[Water];
h = specificEnthalpy_pTX(p,T,Xi);
R = dryair.R*(1 - X_steam/(1 - X_liquid)) + steam.R*X_steam/(1 - X_liquid);
//
u = h - R*T;
d = p/(R*T);
/* Note, u and d are computed under the assumption that the volume of the liquid
water is neglible with respect to the volume of air and of steam
*/
state.p = p;
state.T = T;
state.X = X;
// this x_steam is water load / dry air!!!!!!!!!!!
x_sat = k_mair*p_steam_sat/max(100*Modelica.Constants.eps,p - p_steam_sat);
x_water = Xi[Water]/max(X_air,100*Modelica.Constants.eps);
phi = p/p_steam_sat*Xi[Water]/(Xi[Water] + k_mair*X_air);
end BaseProperties;
Steam water mass fraction of saturation boundary in kg_water/kg_moistair
Inputs
| Type | Name | Default | Description |
|---|
| ThermodynamicState | state | | shermodynamic state |
Outputs
| Type | Name | Description |
|---|
| MassFraction | X_sat | steam mass fraction of sat. boundary [kg/kg] |
Modelica definition
function Xsaturation = Modelica.Media.Air.MoistAir.Xsaturation
"Steam water mass fraction of saturation boundary in kg_water/kg_moistair";
Thermodynamic state as function of p, T and composition X
Inputs
Outputs
| Type | Name | Description |
|---|
| ThermodynamicState | state | |
Modelica definition
redeclare function setState_pTX
"Thermodynamic state as function of p, T and composition X"
extends Modelica.Media.Air.MoistAir.setState_pTX;
end setState_pTX;
Thermodynamic state as function of p, h and composition X
Information
Function to set the state for given pressure, enthalpy and species concentration.
This function needed to be reimplemented in order for the medium model to use
the implementation of T_phX provided by this package as opposed to the
implementation provided by its parent package.
Inputs
Outputs
| Type | Name | Description |
|---|
| ThermodynamicState | state | |
Modelica definition
redeclare function setState_phX
"Thermodynamic state as function of p, h and composition X"
extends Modelica.Icons.Function;
input AbsolutePressure p "Pressure";
input SpecificEnthalpy h "Specific enthalpy";
input MassFraction X[:] "Mass fractions";
output ThermodynamicState state;
algorithm
state := if size(X,1) == nX then ThermodynamicState(p=p,T=T_phX(p,h,X),X=X) else
ThermodynamicState(p=p,T=T_phX(p,h,X), X=cat(1,X,{1-sum(X)}));
end setState_phX;
Thermodynamic state as function of d, T and composition X
Inputs
Outputs
| Type | Name | Description |
|---|
| ThermodynamicState | state | |
Modelica definition
redeclare function setState_dTX
"Thermodynamic state as function of d, T and composition X"
extends Modelica.Media.Air.MoistAir.setState_dTX;
end setState_dTX;
Gas constant (computation neglects liquid fraction)
Inputs
| Type | Name | Default | Description |
|---|
| ThermodynamicState | state | | thermodynamic state |
Outputs
Modelica definition
redeclare function gasConstant
"Gas constant (computation neglects liquid fraction)"
extends Modelica.Media.Air.MoistAir.gasConstant;
end gasConstant;
Saturation curve valid for 273.16 <= T <= 373.16. Outside of these limits a (less accurate) result is returned
Inputs
| Type | Name | Default | Description |
|---|
| Temperature | Tsat | | saturation temperature [K] |
Outputs
Modelica definition
function saturationPressureLiquid =
Modelica.Media.Air.MoistAir.saturationPressureLiquid
"Saturation curve valid for 273.16 <= T <= 373.16. Outside of these limits a (less accurate) result is returned";
Saturation curve valid for 223.16 <= T <= 273.16. Outside of these limits a (less accurate) result is returned
Inputs
| Type | Name | Default | Description |
|---|
| Temperature | Tsat | | sublimation temperature [K] |
Outputs
Modelica definition
function sublimationPressureIce =
Modelica.Media.Air.MoistAir.sublimationPressureIce
"Saturation curve valid for 223.16 <= T <= 273.16. Outside of these limits a (less accurate) result is returned";
Saturation curve valid for 223.16 <= T <= 373.16 (and slightly outside with less accuracy)
Inputs
| Type | Name | Default | Description |
|---|
| Temperature | Tsat | | saturation temperature [K] |
Outputs
Modelica definition
redeclare function extends saturationPressure
"Saturation curve valid for 223.16 <= T <= 373.16 (and slightly outside with less accuracy)"
algorithm
psat := Buildings.Utilities.Math.spliceFunction(saturationPressureLiquid(Tsat),sublimationPressureIce(Tsat),Tsat-273.16,1.0);
end saturationPressure;
Gas pressure
Inputs
Outputs
Modelica definition
redeclare function pressure "Gas pressure"
extends Modelica.Media.Air.MoistAir.pressure;
end pressure;
Gas temperature
Inputs
Outputs
Modelica definition
redeclare function temperature "Gas temperature"
extends Modelica.Media.Air.MoistAir.temperature;
end temperature;
Gas density
Inputs
Outputs
| Type | Name | Description |
|---|
| Density | d | Density [kg/m3] |
Modelica definition
redeclare function density "Gas density"
extends Modelica.Media.Air.MoistAir.density;
end density;
Specific entropy (liquid part neglected, mixing entropy included)
Inputs
Outputs
Modelica definition
redeclare function specificEntropy
"Specific entropy (liquid part neglected, mixing entropy included)"
extends Modelica.Media.Air.MoistAir.specificEntropy;
end specificEntropy;
Enthalpy of vaporization of water
Inputs
| Type | Name | Default | Description |
|---|
| Temperature | T | | temperature [K] |
Outputs
Modelica definition
redeclare function extends enthalpyOfVaporization
"Enthalpy of vaporization of water"
algorithm
r0 := 2501014.5;
end enthalpyOfVaporization;
Specific heat capacity of water (liquid only) which is constant
Inputs
Outputs
Modelica definition
function HeatCapacityOfWater
"Specific heat capacity of water (liquid only) which is constant"
extends Modelica.Icons.Function;
input Temperature T;
output SpecificHeatCapacity cp_fl;
algorithm
cp_fl := 4186;
end HeatCapacityOfWater;
Enthalpy of liquid (per unit mass of liquid) which is linear in the temperature
Inputs
| Type | Name | Default | Description |
|---|
| Temperature | T | | temperature [K] |
Outputs
Modelica definition
redeclare replaceable function extends enthalpyOfLiquid
"Enthalpy of liquid (per unit mass of liquid) which is linear in the temperature"
annotation(derivative=der_enthalpyOfLiquid);
algorithm
h := (T - 273.15)*4186;
end enthalpyOfLiquid;
Temperature derivative of enthalpy of liquid per unit mass of liquid
Inputs
Outputs
Modelica definition
replaceable function der_enthalpyOfLiquid
"Temperature derivative of enthalpy of liquid per unit mass of liquid"
extends Modelica.Icons.Function;
input Temperature T "temperature";
input Temperature der_T "temperature derivative";
output SpecificHeatCapacity der_h "derivative of liquid enthalpy";
algorithm
der_h := 4186;
end der_enthalpyOfLiquid;
Enthalpy of steam per unit mass of steam
Inputs
| Type | Name | Default | Description |
|---|
| Temperature | T | | temperature [K] |
Outputs
Modelica definition
redeclare function enthalpyOfCondensingGas
"Enthalpy of steam per unit mass of steam"
annotation(derivative=der_enthalpyOfCondensingGas);
extends Modelica.Icons.Function;
input Temperature T "temperature";
output SpecificEnthalpy h "steam enthalpy";
algorithm
h := (T-273.15) * steam.cp + enthalpyOfVaporization(T);
end enthalpyOfCondensingGas;
Derivative of enthalpy of steam per unit mass of steam
Inputs
Outputs
Modelica definition
replaceable function der_enthalpyOfCondensingGas
"Derivative of enthalpy of steam per unit mass of steam"
extends Modelica.Icons.Function;
input Temperature T "temperature";
input Temperature der_T "temperature derivative";
output SpecificHeatCapacity der_h "derivative of steam enthalpy";
algorithm
der_h := steam.cp;
end der_enthalpyOfCondensingGas;
Enthalpy of gas mixture per unit mass of gas mixture
Inputs
Outputs
Modelica definition
redeclare replaceable function extends enthalpyOfGas
"Enthalpy of gas mixture per unit mass of gas mixture"
algorithm
h := enthalpyOfCondensingGas(T)*X[Water]
+ enthalpyOfDryAir(T)*(1.0-X[Water]);
end enthalpyOfGas;
Enthalpy of dry air per unit mass of dry air
Inputs
| Type | Name | Default | Description |
|---|
| Temperature | T | | temperature [K] |
Outputs
Modelica definition
replaceable function enthalpyOfDryAir
"Enthalpy of dry air per unit mass of dry air"
annotation(derivative=der_enthalpyOfDryAir);
extends Modelica.Icons.Function;
input Temperature T "temperature";
output SpecificEnthalpy h "dry air enthalpy";
algorithm
h := (T - 273.15)*dryair.cp;
end enthalpyOfDryAir;
Derivative of enthalpy of dry air per unit mass of dry air
Inputs
Outputs
Modelica definition
replaceable function der_enthalpyOfDryAir
"Derivative of enthalpy of dry air per unit mass of dry air"
extends Modelica.Icons.Function;
input Temperature T "temperature";
input Temperature der_T "temperature derivative";
output SpecificHeatCapacity der_h "derivative of dry air enthalpy";
algorithm
der_h := dryair.cp;
end der_enthalpyOfDryAir;
Specific heat capacity of gas mixture at constant pressure
Inputs
Outputs
Modelica definition
redeclare replaceable function extends specificHeatCapacityCp
"Specific heat capacity of gas mixture at constant pressure"
algorithm
cp := dryair.cp*(1-state.X[Water]) +steam.cp*state.X[Water];
end specificHeatCapacityCp;
Specific heat capacity of gas mixture at constant volume
Inputs
Outputs
Modelica definition
redeclare replaceable function extends specificHeatCapacityCv
"Specific heat capacity of gas mixture at constant volume"
algorithm
cv:= dryair.cv*(1-state.X[Water]) +steam.cv*state.X[Water];
end specificHeatCapacityCv;
dynamic viscosity of dry air
Inputs
Outputs
Modelica definition
redeclare function extends dynamicViscosity
"dynamic viscosity of dry air"
algorithm
eta := 1.85E-5;
end dynamicViscosity;
Thermal conductivity of dry air as a polynomial in the temperature
Inputs
Outputs
Modelica definition
redeclare function extends thermalConductivity
"Thermal conductivity of dry air as a polynomial in the temperature"
algorithm
lambda := Polynomials_Temp.evaluate({(-4.8737307422969E-008), 7.67803133753502E-005, 0.0241814385504202},
Cv.to_degC(state.T));
end thermalConductivity;
Compute specific enthalpy from pressure, temperature and mass fraction
Inputs
Outputs
Modelica definition
function h_pTX
"Compute specific enthalpy from pressure, temperature and mass fraction"
extends Modelica.Icons.Function;
input SI.Pressure p "Pressure";
input SI.Temperature T "Temperature";
input SI.MassFraction X[nX] "Mass fractions of moist air";
output SI.SpecificEnthalpy h "Specific enthalpy at p, T, X";
protected
SI.AbsolutePressure p_steam_sat "Partial saturation pressure of steam";
SI.MassFraction x_sat "steam water mass fraction of saturation boundary";
SI.MassFraction X_liquid "mass fraction of liquid water";
SI.MassFraction X_steam "mass fraction of steam water";
SI.MassFraction X_air "mass fraction of air";
SI.SpecificEnthalpy hDryAir "Enthalpy of dry air";
algorithm
p_steam_sat :=saturationPressure(T);
x_sat :=k_mair*p_steam_sat/(p - p_steam_sat);
X_liquid :=max(X[Water] - x_sat/(1 + x_sat), 0.0);
X_steam :=X[Water] - X_liquid;
X_air :=1 - X[Water];
/* THIS DOES NOT WORK --------------------------
h := enthalpyOfDryAir(T) * X_air +
Modelica.Media.Air.MoistAir.enthalpyOfCondensingGas(T) * X_steam + enthalpyOfLiquid(T)*X_liquid;
--------------------------------- */
/* THIS WORKS!!!! +++++++++++++++++++++
h := (T - 273.15)*dryair.cp * X_air +
Modelica.Media.Air.MoistAir.enthalpyOfCondensingGas(T) * X_steam + enthalpyOfLiquid(T)*X_liquid;
+++++++++++++++++++++*/
hDryAir := (T - 273.15)*dryair.cp;
h := hDryAir * X_air +
((T-273.15) * steam.cp + 2501014.5) * X_steam +
(T - 273.15)*4186*X_liquid;
end h_pTX;
Specific enthalpy
Inputs
Outputs
Modelica definition
redeclare function extends specificEnthalpy "Specific enthalpy"
algorithm
h := h_pTX(state.p, state.T, state.X);
end specificEnthalpy;
Specific internal energy
Inputs
Outputs
Modelica definition
redeclare function extends specificInternalEnergy
"Specific internal energy"
extends Modelica.Icons.Function;
algorithm
u := h_pTX(state.p,state.T,state.X) - gasConstant(state)*state.T;
end specificInternalEnergy;
Specific Gibbs energy
Inputs
Outputs
Modelica definition
redeclare function extends specificGibbsEnergy
"Specific Gibbs energy"
extends Modelica.Icons.Function;
algorithm
g := h_pTX(state.p,state.T,state.X) - state.T*specificEntropy(state);
end specificGibbsEnergy;
Specific Helmholtz energy
Inputs
Outputs
Modelica definition
redeclare function extends specificHelmholtzEnergy
"Specific Helmholtz energy"
extends Modelica.Icons.Function;
algorithm
f := h_pTX(state.p,state.T,state.X) - gasConstant(state)*state.T - state.T*specificEntropy(state);
end specificHelmholtzEnergy;
Compute temperature from specific enthalpy and mass fraction
Inputs
Outputs
Modelica definition
function T_phX
"Compute temperature from specific enthalpy and mass fraction"
input AbsolutePressure p "Pressure";
input SpecificEnthalpy h "specific enthalpy";
input MassFraction[:] X "mass fractions of composition";
output Temperature T "temperature";
protected
package Internal
"Solve h(data,T) for T with given h (use only indirectly via temperature_phX)"
extends Modelica.Media.Common.OneNonLinearEquation;
redeclare record extends f_nonlinear_Data
"Data to be passed to non-linear function"
extends Buildings.Media.PerfectGases.Common.DataRecord;
end f_nonlinear_Data;
redeclare function extends f_nonlinear
algorithm
y := h_pTX(p,x,X);
end f_nonlinear;
// Dummy definition has to be added for current Dymola
redeclare function extends solve
end solve;
end Internal;
algorithm
/* The function call below has been changed from
Internal.solve(h, 200, 6000, p, X[1:nXi], steam);
to
Internal.solve(h, 200, 6000, p, X, steam);
The reason is that when running the problem
Buildings.Media.PerfectGases.Examples.MoistAirComparison
then an assertion is triggered because the vector X had the wrong
dimension. The above example verifies that T(h(T)) = T.
*/
T := Internal.solve(h, 200, 6000, p, X, steam);
end T_phX;
HTML-documentation generated by Dymola Tue Sep 30 14:24:47 2008.
BaseProperties
Xsaturation
setState_pTX
T_phX
Buildings.Media.PerfectGases.MoistAir.BaseProperties
Buildings.Media.PerfectGases.MoistAir.setState_pTX