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Humid air is usually modelled with the classical stream type air (Lib-FDBR). The humidity thus can be specified in component 1 (or 33) or with a component 46.
Alternatively, the stream type "Humid air" can be used. When using this stream type, the LibHuAirXiw is used. The composition of the air does not need to be specified. Only the water fraction XI must be specified.
LibHuAirXiw provides the following substance value functions:
These functions from the LibHuAirXiw are also used with standard air streams.
Since LibHuAirXiw also includes the sublimation curve (direct ice/vapour transition), it is also possible to specify a relative humidity at negative temperatures.
It should be noted that this library is based on a different definition of the enthalpy zero point than the classical Lib-FDBR air. For LibHuAirXiw, the enthalpy zero point for H2O is 0°C for liquid water. For Lib-FDBR, the enthalpy zero point for H2O is at 0°C for gaseous water. That is why results on classical (FDBR-) stream types show negative enthalpy values for liquid water.
Therefore, this fluid type was initially only rudimentarily applicable, since no interactions (mixing, combustion) with other fluids and components were implemented yet.
Since both the composition and the zero points for enthalpy and entropy in this library differ from the "air" stream type, no mixing between the two stream types was possible. Component 60 (General mixer) was upgraded to perform the corresponding conversions. The stream type "humid air" must be connected to the mixing input (connection 3).
As main input (connection 1) and output (connection 2) the classical stream types
are possible.
Also for component 21 (combustion with heat release) the stream type "humid air" can be used instead of "air". For this, however, the component must be changed to the new calculation mode FCALC=3 (see chapter Combustion chamber with heat output - component 21).
In streams with material composition the water can be in the liquid or gaseous phase. This distribution is normally calculated based on the partial pressures in the ideal gas approximation. On the other hand, for the calculation of quantities such as relative humidity and wet bulb temperature, the functions from LibHuAirXiw are used, which have a higher accuracy. This could result in minor inconsistencies.
For this reason, LibHuAirXiw is also used for the calculation of the phase equilibrium, if the (dry) composition corresponds to that of the air. Since the phase equilibrium also affects the calculation of enthalpies, slight differences may occur on streams containing liquid and gaseous water, not only in the phase proportions but also in the results.
On stream types whose composition does not correspond to air, but only contains the substances contained in LibHuGas, LibHuGas is used to calculate the phase fractions.
If other gases are present, the partial pressures are calculated according to the ideal gas approximation.