1D:Dialog EditHygrothermalSources

Dialog: Edit Hygrothermal Sources

This dialog serves to specify the location and the properties of the sources you have added to the current layer. A sink is simply a source with negative source strength.

First, you have to specify the location and the extent of the source. A source may be point-like or extended. Accordingly, select whether the source shall be spread over one grid element [##HREF?##], several grid elements or the whole layer.
In the first case, specify the depth of the source in the layer; in the second case, specify the start depth and the end depth of its extension in the layer (in meters, counted from the left surface of the layer). WUFI will assign the source to the closest grid element(s). If the source is to extend over the whole layer, it will be distributed evenly over all grid elements of the layer.

Several heat or moisture sources may reside in one layer. However, heat sources may not overlap each other, and moisture sources may not overlap each other either (heat sources may overlap with moisture sources, however). Two neighboring sources for which you have specified non-overlapping extents may still happen to overlap if WUFI assigns their respective ends to the same grid element (error message: "overlapping sources"). In this case you must adjust the positions of the sources slightly.
An air change source contains both a heat and a moisture source, and it always extends over the whole layer. Thus, in such a layer no more heat, moisture or air change sources are allowed.

Furthermore, you have to specify how the source strength behaves in time:

• For heat sources the source strength may be read from a file, or it may be derived from the weather conditions as a specified fraction of the incident solar radiation.
  

 

• For moisture sources the source strength may be read from a file, or it may be derived from the weather conditions as a specified fraction of the incident driving rain.
  

 

• For air change sources the source strength is read from a file. According to the specified air changes, an air change source replaces part of the air at its location by outdoor or indoor air. Depending on the temperature and humidity conditions in the component and in the outdoor or indoor air, heat and moisture may be transported into or out of the component. For details on how WUFI computes the source strength from the air changes see the appendix below.
  

 

Files controlling the source strength are text files with one column for the time axis (in h; values must be in ascending order) and one column for the source strength (in W/m² for heat sources, kg/s m² for moisture sources and 1/h for air change sources; the decimal separator is always the decimal point, independent of your system settings).

The time column indicates the point in time until which the corresponding source strength prevails. For example, a file for a heat source may have the content

1		1
14		0.2
15		20
24		0.2

This files specifies that for one hour (i.e. until the end of the first hour) the source strength is 1 W/m². Then for 13 hours (until the end of hour 14) it is 0.2 W/m², for another hour (until the end of hour 15) it is 20 W/m², and finally for 9 hours (until the end of hour 24) it is 0.2 W/m² again. This curve is shown in the above screenshot.

The file is read as often as necessary, starting over when its end is reached. The example file thus describes a behavior which repeats daily (every 24 hours). Of course, a file may also contain a time period of, say, two hours, or a whole year.
 

Note: if a moisture source/sink is being used, it may happen that it tries to put water into a layer which is already completely filled, or it may try to remove water from a layer which has already dried up. Both attempts must fail as this is physically not possible and there is no mathematical solution for the transport equations in this situation. In such a case, WUFI stops the calculation and indicates "diverging results". You may be able to avoid the problem by spreading the source/sink over a wider area, otherwise you must decrease the source/sink strength.

Version notice: in WUFI light no sources can be specified. WUFI ORNL/IBP allows no sources except one moisture source.
 

Appendix: Determination of the source strengths of an air change source

The source strength of a heat or moisture source is directly determined by the user, or it is directly derived from the weather conditions as a user-defined fraction of solar radiation or driving rain. In the case of an air change source, however, the user only specifies the air changes per hour and WUFI determines from this and some boundary conditions the heat and moisture source strengths of the air change source. This is done as follows:

Determination of the air change heat source:

	Qt	[W/m²]	:	heat source strength
	rout	[kg/m³]	:	density of the outdoor air
	ACH	[1/h]	:	air change rate of the ventilated cavity
	dcavity	[m]	:	thickness of the ventilated cavity
	Cp,Air	[J/kg K]	:	specific heat capacity of dry air at constant pressure
				(effect of moisture content neglected)
	Tout	[K]	:	outdoor temperature
	Tcavity	[K]	:	temperature of the ventilated cavity
				(mean value of all involved grid elements)

Determination of the air change moisture source:

	Qm	[kg/m²s]	:	moisture source strength
	cout	[kg/m³]	:	water vapor concentration of the outdoor air
	ccavity	[kg/m³]	:	water vapor concentration of the cavity air
				(mean value of all involved grid elements)

The water vapor concentrations are determined from the respective air temperatures and relative humidities.

If the cavity is connected with the indoor air rather than the outdoor air, the relevant variables have to be replaced accordingly.