Program and model documentation
The following presentations provide a quick overview of the COND2 model documentation:
Physical principles
Starting from a vapor-permeable construction, which can consist of several layers, the one-dimensional, steady state heat and moisture transport is considered. A constant climate should be maintained on both sides of the construction. This climate is described in simplified terms by temperature and humidity for the respective side of the construction (in the following, the index i denotes the inside and the index e the outside of the construction).
As a result of the temperature gradient, heat flows through the construction. After a certain time, which can sometimes be very long, a stationary heat flow and a temperature profile are established (Fig. 1).
The vapor pressure gradient causes a vapor flow through the construction. Here, too, a stationary state develops after a certain time. COND calculates the steady state vapor and heat flow and the resulting steam pressure and temperature profiles.
If the calculated vapor pressure pv exceeds the saturation vapor pressure psat (which is directly dependent on the temperature) at any point within the structure, condensation occurs (see Figure 2, condensate is formed in the area outlined in red).
The traditional Glaser method is now used to estimate the amount of condensate by calculating the steam flows from the inside to the condensate level and from the condensation area to the outside. The difference between the fluxes is then multiplied by the duration of the condensation period to estimate the expected condensate (see Figure 3).
The Glaser process assumes that the condensate forms between the layers and is also stored there. Regardless of the amount of condensate actually produced, the vapor fluxes and the condensation area remain constant, as Glaser does not take the distribution of condensate into account.
In reality, however, as soon as condensate forms in the construction, liquid water flows away from the condensation area (capillary flux) occur in addition to the vapor fluxes. This process is taken into account in the COND method and allows a more realistic description of the moisture transport processes. Figure 4 shows the overlapping vaporand liquid water fluxes in the vicinity of the condensation area.
Two specifications are now made for calculating the moisture distribution in the steady state:
- A state of global equilibrium prevails, i.e. the vapor flux diffusing into the structure is just as great as the vapor flux diffusing out on the other side.
- In the steady state, a local equilibrium applies, i.e. the superimposed liquid water and vapor flows together are always just as large as the steady (global) vapor flux.
These equilibrium states can be described using equations and the resulting system of equations can be solved analytically. The moisture distribution expected after a certain time is then calculated by taking into account the time-dependent adjustment process.
The capillary distribution of the condensate leads to a hygric reduction in pressure in the construction. By taking this process into account, calculations using the COND method usually result in lower condensate quantities compared to the Glaser-scheme. For internal insulation systems in particular, the Glaser method greatly overestimates the calculated amount of condensate, whereas the COND algorithm tends to describe the actual amounts of condensate that occur and the distribution of the condensate. This is illustrated in Figure 5 using the example of an internally insulated construction with a condensate layer.
Although COND, like the Glaser method, only provides expected values for the condensate that occurs, the calculation is closer to reality, especially for internally insulated constructions, and therefore also allows verification to be carried out for constructions that are classified as critical using the traditional Glaser method.
The diploma thesis, which describes the theoretical principles and the calculation method in detail, is also installed together with the program. However, it can also be downloaded directly from this website (see cond_fundamentals_en.pdf).
The principles of the procedure are briefly described below.
Steady-state temperature profile
Vapor fluxes (light blue) and liquid water fluxes leading away from the condensate level (dark blue)
Validation
Validation refers to the process of model testing, in which the correct implementation of the formulated physics in the numerical model and solution method is checked. Validation also refers to the basic suitability of a software or the model contained therein to reproduce the data measured in the laboratory or field test. Among other things, it is checked whether the model contains the required complexity and extensive physics and whether it can be parameterized sufficiently.