.Modelica.Media.Air.MoistAir

Information

Thermodynamic Model

This package provides a full thermodynamic model of moist air including the fog region and temperatures below zero degC. The governing assumptions in this model are:

All extensive properties are expressed in terms of the total mass in order to comply with other media in this library. However, for moist air it is rather common to express the absolute humidity in terms of mass of dry air only, which has advantages when working with charts. In addition, care must be taken, when working with mass fractions with respect to total mass, that all properties refer to the same water content when being used in mathematical operations (which is always the case if based on dry air only). Therefore two absolute humidities are computed in the BaseProperties model: X denotes the absolute humidity in terms of the total mass while x denotes the absolute humidity per unit mass of dry air. In addition, the relative humidity phi is also computed.

At the triple point temperature of water of 0.01 °C or 273.16 K and a relative humidity greater than 1 fog may be present as liquid and as ice resulting in a specific enthalpy somewhere between those of the two isotherms for solid and liquid fog, respectively. For numerical reasons a coexisting mixture of 50% solid and 50% liquid fog is assumed in the fog region at the triple point in this model.

Range of validity

From the assumptions mentioned above it follows that the pressure should be in the region around atmospheric conditions or below (a few bars may still be fine though). Additionally a very high water content at low temperatures would yield incorrect densities, because the volume of the liquid or solid phase would not be negligible anymore. The model does not provide information on limits for water drop size in the fog region or transport information for the actual condensation or evaporation process in combination with surfaces. All excess water which is not in its vapour state is assumed to be still present in the air regarding its energy but not in terms of its spatial extent.

The thermodynamic model may be used for temperatures ranging from 190 ... 647 K. This holds for all functions unless otherwise stated in their description. However, although the model works at temperatures above the saturation temperature it is questionable to use the term "relative humidity" in this region. Please note, that although several functions compute pure water properties, they are designed to be used within the moist air medium model where properties are dominated by air and steam in their vapor states, and not for pure liquid water applications.

Transport Properties

Several additional functions that are not needed to describe the thermodynamic system, but are required to model transport processes, like heat and mass transfer, may be called. They usually neglect the moisture influence unless otherwise stated.

Application

The model's main area of application is all processes that involve moist air cooling under near atmospheric pressure with possible moisture condensation. This is the case in all domestic and industrial air conditioning applications. Another large domain of moist air applications covers all processes that deal with dehydration of bulk material using air as a transport medium. Engineering tasks involving moist air are often performed (or at least visualized) by using charts that contain all relevant thermodynamic data for a moist air system. These so called psychrometric charts can be generated from the medium properties in this package. The model PsychrometricData may be used for this purpose in order to obtain data for figures like those below (the plotting itself is not part of the model though).


Legend: blue - constant specific enthalpy, red - constant temperature, black - constant relative humidity

Contents

Name Description
 ThermodynamicState ThermodynamicState record for moist air
 BaseProperties Moist air base properties record
 setState_pTX Return thermodynamic state as function of pressure p, temperature T and composition X
 setState_phX Return thermodynamic state as function of pressure p, specific enthalpy h and composition X
 setState_dTX Return thermodynamic state as function of density d, temperature T and composition X
 setSmoothState Return thermodynamic state so that it smoothly approximates: if x > 0 then state_a else state_b
 Xsaturation Return absolute humidity per unit mass of moist air at saturation as a function of the thermodynamic state record
 xsaturation Return absolute humidity per unit mass of dry air at saturation as a function of the thermodynamic state record
 xsaturation_pT Return absolute humidity per unit mass of dry air at saturation as a function of pressure p and temperature T
 massFraction_pTphi Return steam mass fraction as a function of relative humidity phi and temperature T
 relativeHumidity_pTX Return relative humidity as a function of pressure p, temperature T and composition X
 relativeHumidity Return relative humidity as a function of the thermodynamic state record
 gasConstant Return ideal gas constant as a function from thermodynamic state, only valid for phi<1
 gasConstant_X Return ideal gas constant as a function from composition X
 saturationPressureLiquid Return saturation pressure of water as a function of temperature T in the range of 273.16 to 647.096 K
 saturationPressureLiquid_der Derivative function for 'saturationPressureLiquid'
 sublimationPressureIce Return sublimation pressure of water as a function of temperature T between 190 and 273.16 K
 sublimationPressureIce_der Derivative function for 'sublimationPressureIce'
 saturationPressure Return saturation pressure of water as a function of temperature T between 190 and 647.096 K
 saturationPressure_der Derivative function for 'saturationPressure'
 saturationTemperature Return saturation temperature of water as a function of (partial) pressure p
 enthalpyOfVaporization Return enthalpy of vaporization of water as a function of temperature T, 273.16 to 647.096 K
 HeatCapacityOfWater Return specific heat capacity of water (liquid only) as a function of temperature T
 enthalpyOfLiquid Return enthalpy of liquid water as a function of temperature T(use enthalpyOfWater instead)
 enthalpyOfGas Return specific enthalpy of gas (air and steam) as a function of temperature T and composition X
 enthalpyOfCondensingGas Return specific enthalpy of steam as a function of temperature T
 enthalpyOfNonCondensingGas Return specific enthalpy of dry air as a function of temperature T
 enthalpyOfWater Computes specific enthalpy of water (solid/liquid) near atmospheric pressure from temperature T
 enthalpyOfWater_der Derivative function of enthalpyOfWater
 pressure Returns pressure of ideal gas as a function of the thermodynamic state record
 temperature Return temperature of ideal gas as a function of the thermodynamic state record
 T_phX Return temperature as a function of pressure p, specific enthalpy h and composition X
 density Returns density of ideal gas as a function of the thermodynamic state record
 specificEnthalpy Return specific enthalpy of moist air as a function of the thermodynamic state record
 h_pTX Return specific enthalpy of moist air as a function of pressure p, temperature T and composition X
 h_pTX_der Derivative function of h_pTX
 isentropicExponent Return isentropic exponent (only for gas fraction!)
 isentropicEnthalpyApproximation Approximate calculation of h_is from upstream properties, downstream pressure, gas part only
 specificInternalEnergy Return specific internal energy of moist air as a function of the thermodynamic state record
 specificInternalEnergy_pTX Return specific internal energy of moist air as a function of pressure p, temperature T and composition X
 specificInternalEnergy_pTX_der Derivative function for specificInternalEnergy_pTX
 specificEntropy Return specific entropy from thermodynamic state record, only valid for phi<1
 specificGibbsEnergy Return specific Gibbs energy as a function of the thermodynamic state record, only valid for phi<1
 specificHelmholtzEnergy Return specific Helmholtz energy as a function of the thermodynamic state record, only valid for phi<1
 specificHeatCapacityCp Return specific heat capacity at constant pressure as a function of the thermodynamic state record
 specificHeatCapacityCv Return specific heat capacity at constant volume as a function of the thermodynamic state record
 dynamicViscosity Return dynamic viscosity as a function of the thermodynamic state record, valid from 123.15 K to 1273.15 K
 thermalConductivity Return thermal conductivity as a function of the thermodynamic state record, valid from 123.15 K to 1273.15 K
 velocityOfSound
 isobaricExpansionCoefficient
 isothermalCompressibility
 density_derp_h
 density_derh_p
 density_derp_T
 density_derT_p
 density_derX
 molarMass
 T_psX Return temperature as a function of pressure p, specific entropy s and composition X
 setState_psX
 s_pTX Return specific entropy of moist air as a function of pressure p, temperature T and composition X (only valid for phi<1)
 s_pTX_der Return specific entropy of moist air as a function of pressure p, temperature T and composition X (only valid for phi<1)
 isentropicEnthalpy Isentropic enthalpy (only valid for phi<1)
 Utilities Utility functions

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