# .IBPSA.Media.Air

## 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/ρstp = p/pstp,

where pstd and ρstp 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 IBPSA.Airflow.Multizone compute the mass density using the function IBPSA.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 - pstp ⁄ ρstp,

where u is the internal energy per unit mass, h is the enthalpy per unit mass, pstp is the static pressure and ρstp 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 - pstp ⁄ ρstd,

because p ⁄ ρ = pstp ⁄ ρstp in this medium model.

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

## Contents

NameDescription
ThermodynamicStateThermodynamicState record for moist air
BasePropertiesBase properties
densityGas density
dynamicViscosityReturn the dynamic viscosity of dry air
enthalpyOfCondensingGasEnthalpy of steam per unit mass of steam
enthalpyOfGasEnthalpy of gas mixture per unit mass of gas mixture
enthalpyOfLiquidEnthalpy of liquid (per unit mass of liquid) which is linear in the temperature
enthalpyOfNonCondensingGasEnthalpy of non-condensing gas per unit mass of steam
enthalpyOfVaporizationEnthalpy of vaporization of water
gasConstantReturn ideal gas constant as a function from thermodynamic state, only valid for phi<1
pressureReturns pressure of ideal gas as a function of the thermodynamic state record
isobaricExpansionCoefficientIsobaric expansion coefficient beta
isothermalCompressibilityIsothermal compressibility factor
saturationPressureSaturation curve valid for 223.16 <= T <= 373.16 (and slightly outside with less accuracy)
specificEntropyReturn the specific entropy, only valid for phi<1
density_derp_TReturn the partial derivative of density with respect to pressure at constant temperature
density_derT_pReturn the partial derivative of density with respect to temperature at constant pressure
density_derXReturn the partial derivative of density with respect to mass fractions at constant pressure and temperature
specificHeatCapacityCpSpecific heat capacity of gas mixture at constant pressure
specificHeatCapacityCvSpecific heat capacity of gas mixture at constant volume
setState_dTXReturn thermodynamic state as function of density d, temperature T and composition X
setState_phXReturn thermodynamic state as function of pressure p, specific enthalpy h and composition X
setState_pTXReturn thermodynamic state as function of p, T and composition X or Xi
setState_psXReturn the thermodynamic state as function of p, s and composition X or Xi
specificEnthalpyCompute specific enthalpy from pressure, temperature and mass fraction
specificEnthalpy_pTXSpecific enthalpy
specificGibbsEnergySpecific Gibbs energy
specificHelmholtzEnergySpecific Helmholtz energy
isentropicEnthalpyReturn the isentropic enthalpy
specificInternalEnergySpecific internal energy
temperatureReturn temperature of ideal gas as a function of the thermodynamic state record
molarMassReturn the molar mass
temperature_phXCompute temperature from specific enthalpy and mass fraction
thermalConductivityThermal conductivity of dry air as a polynomial in the temperature

## Revisions

• 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 IBPSA.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 IBPSA.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.

Generated at 2024-05-29T18:16:16Z by OpenModelicaOpenModelica 1.22.4 using GenerateDoc.mos