packageAir

Information

This medium package models moist air using a gas law in which pressure and temperature are independent, which often leads to significantly faster and more robust computations. The specific heat capacities at constant pressure and at constant volume are constant. The air is assumed to be not saturated.

This medium uses the gas law

ρ/ρ_stp = p/p_stp,

where p_std and ρ_stp are constant reference temperature and density, rathern than the ideal gas law

ρ = p ⁄(R T),

where R is the gas constant and T is the temperature.

This formulation often leads to smaller systems of nonlinear equations because equations for pressure and temperature are decoupled. Therefore, if air inside a control volume such as room air is heated, it does not increase its specific volume. Consequently, merely heating or cooling a control volume does not affect the air flow calculations in a duct network that may be connected to that volume. Note that multizone air exchange simulation in which buoyancy drives the air flow is still possible as the models in Buildings.Airflow.Multizone compute the mass density using the function Buildings.Utilities.Psychrometrics.Functions.density_pTX in which density is a function of temperature.

Note that models in this package implement the equation for the internal energy as

u = h - p_stp ⁄ ρ_stp,

where u is the internal energy per unit mass, h is the enthalpy per unit mass, p_stp is the static pressure and ρ_stp is the mass density at standard pressure and temperature. The reason for this implementation is that in general,

h = u + p v,

from which follows that

u = h - p v = h - p ⁄ ρ = h - p_stp ⁄ ρ_std,

because p ⁄ ρ = p_stp ⁄ ρ_stp in this medium model.

The enthalpy is computed using the convention that h=0 if T=0 °C and no water vapor is present.

Parameters

Contents

Name	Description
ThermodynamicState	ThermodynamicState record for moist air
BaseProperties	Base properties (p, d, T, h, u, R, MM and X and Xi) of a medium
density	Gas density
dynamicViscosity	Return the dynamic viscosity of dry air
enthalpyOfCondensingGas	Enthalpy of steam per unit mass of steam
enthalpyOfGas	Enthalpy of gas mixture per unit mass of gas mixture
enthalpyOfLiquid	Enthalpy of liquid (per unit mass of liquid) which is linear in the temperature
enthalpyOfNonCondensingGas	Enthalpy of non-condensing gas per unit mass of steam
enthalpyOfVaporization	Enthalpy of vaporization of water
gasConstant	Return ideal gas constant as a function from thermodynamic state, only valid for phi<1
pressure	Returns pressure of ideal gas as a function of the thermodynamic state record
isobaricExpansionCoefficient	Isobaric expansion coefficient beta
isothermalCompressibility	Isothermal compressibility factor
saturationPressure	Saturation curve valid for 223.16 <= T <= 373.16 (and slightly outside with less accuracy)
specificEntropy	Return the specific entropy, only valid for phi<1
density_derp_T	Return the partial derivative of density with respect to pressure at constant temperature
density_derT_p	Return the partial derivative of density with respect to temperature at constant pressure
density_derX	Return the partial derivative of density with respect to mass fractions at constant pressure and temperature
specificHeatCapacityCp	Specific heat capacity of gas mixture at constant pressure
specificHeatCapacityCv	Specific heat capacity of gas mixture at constant volume
setState_dTX	Return thermodynamic state as function of density d, temperature T and composition X
setState_phX	Return thermodynamic state as function of pressure p, specific enthalpy h and composition X
setState_pTX	Return thermodynamic state as function of p, T and composition X or Xi
setState_psX	Return the thermodynamic state as function of p, s and composition X or Xi
specificEnthalpy	Compute specific enthalpy from pressure, temperature and mass fraction
specificEnthalpy_pTX	Specific enthalpy
specificGibbsEnergy	Specific Gibbs energy
specificHelmholtzEnergy	Specific Helmholtz energy
isentropicEnthalpy	Return the isentropic enthalpy
specificInternalEnergy	Specific internal energy
temperature	Return temperature of ideal gas as a function of the thermodynamic state record
molarMass	Return the molar mass
temperature_phX	Compute temperature from specific enthalpy and mass fraction
thermalConductivity	Thermal conductivity of dry air as a polynomial in the temperature
GasPropertiesprotected	Coefficient data record for properties of perfect gases
der_enthalpyOfLiquidprotected	Temperature derivative of enthalpy of liquid per unit mass of liquid
der_enthalpyOfCondensingGasprotected	Derivative of enthalpy of steam per unit mass of steam
enthalpyOfDryAirprotected	Enthalpy of dry air per unit mass of dry air
der_enthalpyOfDryAirprotected	Derivative of enthalpy of dry air per unit mass of dry air
der_enthalpyOfNonCondensingGasprotected	Derivative of enthalpy of non-condensing gas per unit mass of steam
der_specificHeatCapacityCpprotected	Derivative of specific heat capacity of gas mixture at constant pressure
der_specificHeatCapacityCvprotected	Derivative of specific heat capacity of gas mixture at constant volume

Revisions

• June 18, 2024, by Michael Wetter:
  Added start and nominal attributes to avoid warnings in OpenModelica due to conflicting values.
  This is for IBPSA, #1890.
• September 9, 2022, by Michael Wetter:
  Set nominal attribute for BaseProperties.Xi.
  This is for #1634.
• September 28, 2020, by Michael Wetter:
  Reformulated BaseProperties to avoid event-triggering assertions.
  This is for #1401.
• January 11, 2019 by Michael Wetter:
  Reforulated assignment of X_int in setState_psX.
  This is for #1079.
• October 26, 2018, by Filip Jorissen and Michael Wetter:
  Now printing different messages if temperature is above or below its limit, and adding instance name as JModelica does not print the full instance name in the assertion. This is for #1045.
• November 4, 2016, by Michael Wetter:
  Set default value for dT.start in base properties.
  This is for #575.
• June 6, 2015, by Michael Wetter:
  Set AbsolutePressure(start=p_default) to avoid a translation error if Buildings.Fluid.Sources.Examples.TraceSubstancesFlowSource is translated in pedantic mode in Dymola 2016. The reason is that pressures use Medium.p_default as start values, but Modelica.Media.Interfaces.Types sets a default value of 1E-5. A similar change has been done for pressure. This fixes #266.
• June 5, 2015, by Michael Wetter:
  Added stateSelect attribute in BaseProperties.T to allow correct use of preferredMediumState as described in Modelica.Media.Interfaces.PartialMedium. Note that the default is preferredMediumState=false and hence the same states are used as were used before. This is for #260.
• May 11, 2015, by Michael Wetter:
  Removed p(stateSelect=if preferredMediumStates then StateSelect.prefer else StateSelect.default) in declaration of BaseProperties. Otherwise, when models that contain a fluid volume are exported as an FMU, their pressure would be differentiated with respect to time. This would require the time derivative of the inlet pressure, which is not available, causing the translation to stop with an error.
• May 1, 2015, by Michael Wetter:
  Added Inline=true for issue 227.
• March 20, 2015, by Michael Wetter:
  Added missing term state.p/reference_p in function specificEntropy. #193.
• February 3, 2015, by Michael Wetter:
  Removed stateSelect.prefer for temperature. This is for #160.
• July 24, 2014, by Michael Wetter:
  Changed implementation to use Buildings.Utilities.Psychrometrics.Constants. This was done to use consistent values throughout the library.
• November 16, 2013, by Michael Wetter:
  Revised and simplified the implementation.
• November 14, 2013, by Michael Wetter:
  Removed function HeatCapacityOfWater which is neither needed nor implemented in the Modelica Standard Library.
• November 13, 2013, by Michael Wetter:
  Removed non-used computations in specificEnthalpy_pTX and in temperature_phX.
• March 29, 2013, by Michael Wetter:
  Added final standardOrderComponents=true in the BaseProperties declaration. This avoids an error when models are checked in Dymola 2014 in the pedenatic mode.
• April 12, 2012, by Michael Wetter:
  Added keyword each to Xi(stateSelect=...).
• April 4, 2012, by Michael Wetter:
  Added redeclaration of ThermodynamicState to avoid a warning during model check and translation.
• August 3, 2011, by Michael Wetter:
  Fixed bug in u=h-R*T, which is only valid for ideal gases. For this medium, the function is u=h-pStd/dStp.
• January 27, 2010, by Michael Wetter:
  Fixed bug in else branch of function setState_phX that lead to a run-time error when the constructor of this function was called.
• January 22, 2010, by Michael Wetter:
  Added implementation of function enthalpyOfNonCondensingGas and its derivative.
• January 13, 2010, by Michael Wetter:
  Fixed implementation of derivative functions.
• August 28, 2008, by Michael Wetter:
  First implementation.