Moist Air Variables
The temperature variable solved by the Heat Transfer interfaces corresponds to the dry bulb temperature. This is the temperature measured by a thermometer with a dry sensor and screening to prevent from deviation due to external radiation like solar radiation.
When the presence of water vapor is accounted for in the model, other temperatures may be considered, depending on vapor pressure.
Dew Point Temperature
The dew point temperature of a sample of air with water vapor pressure pv is the temperature to which it must be cooled to become fully saturated.
The variable T_dp is defined in Ref. 1 by:
where phi is the Relative Humidity variable. See Saturation State for the definition of saturation pressure psat as a function of temperature. See also Saturation Pressure for the definition of the variable psat.
Equivalent Temperature
The equivalent temperature is obtained by adiabatically condensing all the water vapor of a sample of air with initial vapor pressure pv. In this process, the latent heat decrease due to total removal of the vapor is balanced by a increase of the sensible heat and temperature.
The variable T_eq is approximated in Ref. 1 by:
where phi is the Relative Humidity, and γTeq (SI unit: Pa/K) is the psychrometer constant, defined in Ref. 1 by:
where p is the total pressure, Cp, a is the heat capacity at constant pressure of dry air at temperature T, Lv is the latent heat of evaporation at temperature T (see Latent Heat of Evaporation), and Ma and Mv are the molar mass of dry air and water vapor, respectively.
See also Saturation Pressure for the definition of the variable psat.
These definitions are illustrated on Figure 3-1.
Figure 3-1: Relation between dry bulb, dew point, equivalent, and wet bulb temperatures.
The wet bulb temperature is also represented on Figure 3-1. It is obtained by adiabatically reaching saturation state for a sample of air with initial water vapor pressure pv. In this process, the latent heat increase due to evaporation is balanced by a decrease of the sensible heat and temperature. It is not available as a predefined variable, but it can be approximated by solving the following equation:
The psychrometer constant is again evaluated at temperature T.
Saturation Pressure
The variable psat is defined by:
where T is the temperature. See Functions for the definition of the function fpsat.
Relative Humidity
The variable phi is defined by:
Latent Heat of Evaporation
The variable Lv is defined by:
where T is the temperature. See Functions for the definition of the function Lv.