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 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 - 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.


Name Description
ThermodynamicState ThermodynamicState record for moist air
BaseProperties Base properties
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


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