Test sample
In the test wall a calcium silicate sample is positioned (20cm x 20cm, thickness 10cm). The test sample is placed directly opposite to the air inlet of the test room. Figure 4.6(a) represents a section of the test sample, Figure 4.6(b) shows a front
(a) 0 10 20 30 40 50 60 70 80 90 100 1,2 1,7 2,2 2,7 3,2 3,7 Re lativ e Humid ity [% ] Voltage [V] (b)
Figure 4.5: (a) Calibration setup. (b) Comparison between chilled mirror calibration (△)
view of the calcium silicate sample in the test wall. The sample is sliced into four layers of 10mm, 15mm, 25mm and 50mm thickness, respectively. Between each two material layers, in the middle, a thermocouple and a small capacitance relative humidity sensor (Honeywell HIH-4000) are placed. The positions of the thermocouples and relative humidity sensors are indicated in Figure 4.6(a). The layers are then pressed back together to ensure good contact. To examine the impact of the slicing of the sample and the insertion of the sensors, a preliminary test was performed that will be discussed in the next section. The test sample is placed in a Plexiglas box. The four sides and the back side are sealed with paraffin to avoid moisture exchange. At the sides and the back the sample is insulated with 4cm mineral wool to avoid heat exchange with the surroundings. These measures should ensure 1D moisture transport in the material sample.
The calcium silicate used in the tests is a highly hygroscopic material which makes it suitable for use in the validation experiments of a coupled CFD-HAM model where vapour transport is studied. The material properties of the calcium silicate were extensively measured by different laboratories during the HAMSTAD-project (Heat, Air and Moisture Standards Development) [107, 108]. Some of these properties are listed in Appendix A.
In the calcium silicate material a fine and a coarse pore system were distinguished. The measurements showed an open porosity ψ0ranging from 84% to 90%. The open porosity ψ0is defined as the ratio between the pore volume open for moisture transport and the total volume of the material sample. The material properties measured by the KU Leuven laboratory were used in the validation study. A dry vapour resistance factor of 5.42 and a dry thermal conductivity of 0.06W /mK were registered. The measured saturation moisture content wsatwas 894kg/m3. Note that the water vapour resistance factor is quasi independent of the RH. The minimum and maximum values of the material properties measured by the other laboratories are not presented in Appendix A but were used in a sensitivity study performed by Steeman et al. [109]. From this sensitivity analysis it was found that the measured value of 5.42 for the dry vapour resistance factor was too high and not in correspondence with measurements by other laboratories. Therefore Steeman et al. proposed a value of 3. This value was also adopted in this work.
Preliminary experiment: effect of sample cutting
Preliminary experiments were carried out to check whether sample cutting may have an effect on the temperature and relative humidity profiles which are measured on different depths in the sample. Due to cutting of the calcium silicate sample at the different positions (at 10mm, 25mm and 50mm) for the installation of the thermocouples and a relative humidity sensors, it is possible that small air layers arise between the different material parts when the sample is assembled again. Because of this, the material properties may no longer be homogeneous along the depth of the material, which can affect the overall permeability and sorption of the sample. In turn this may influence the temperature and relative humidity profiles measured inside the sample. To check the possible effect of
(a) (b)
Figure 4.6: Schematic representation of the calcium silicate test sample (a) and view on the test sample (b)
the cutting edges in the sample, a new sample is prepared which is only sliced at 25mm depth. In this material sample the influence of a possible air layer at 10mm depth is hence excluded. Both samples are successively installed in the test wall and with each sample an identical experiment is performed. Before each experiment the calcium silicate sample is preconditioned for four days by supplying air (10ACH) at 25°C and 50% relative humidity until the temperature and relative humidity differences inside the sample are smaller than the uncertainty interval of the sensors (i.e. ±0.1°C and ±1.4%RH). During the experiment the supply air temperature is kept at 25°C, while a relative humidity step is imposed from 50% to 70%: 8 hours of high relative humidity (70%) are followed by 16 hours of low relative humidity (50%). This cycle is repeated five times. Apart from the calcium silicate sample in the test wall no other porous material is present in the room.
Figure 4.7 compares the measured temperature and relative humidity profile at 25mm for the last three cycles. The first two cycles are not depicted here. During the first two cycles some transition and startup phenomena occur that are not relevant for the current comparison.
Note that the latent heat of evaporation has a strong effect on the temperatures inside the sample: the temperature in the sample rises if water vapour is absorbed by the sample while a temperature decrease is noted when water vapour is released from the sample. The relative humidity measured at 25mm in both samples is quasi identical. Also the associated temperature measured in the sample is comparable, in the third cycle a difference of about 0.2°C is noted.
The temperature and relative humidity curves at the front and the back side of the sample show that in both experiments the boundary conditions are similar (Figure 4.7d and Figure 4.7f). At the front side of the sample, the relative humidity between the two tests differs from 0.5%RH up to 1.4%RH when the relative humidity of the supply air is 70%. The maximum difference between the temperatures measured at the front side is 0.1°C. At the back side, the average difference in relative humidity is smaller than 0.5%RH while the maximum temperature difference is about 0.2°C. Note that the thermocouple and relative humidity sensor at the front and back side of the sample do not exactly register the conditions at the sample surface but rather measure the temperature in a small air layer near the surface.
Consequently, the experiments demonstrate that the cutting edges do not have a considerable effect on the temperature and relative humidity profile measured in the sample. Furthermore the experiments have shown to be reproducible.