3.1. CFD numerical simulation
The results of the simulations show how the air moves towards the building, generating a small turbulence in the lower part of the enclosure (Figure 3
a) due to the impossibility of going out in the angle created between the façade and the ground. In Figure 3
b, the displacement of some air particles is observed, shown in red.
The simulations make it possible to observe the air particles in movement on the basis of infographics or videos. Figure 4
shows snapshots of this sequence, in which the air is introduced into the chamber through the lower part, increases its temperature, and rises until it leaves the upper part.
shows the temperatures and relative humidity, both with low (×1) and moderate (×3) air velocities, for all the air chamber thicknesses studied: 1, 3, 5, 10 and 25 cm. In particular, the cutting plane is located on the outside of the air chamber, that is, on the inside of the stone cladding.
Determining temperature and humidity makes it possible to locate the areas that may be susceptible to condensation (low temperatures and high humidity). This condensation is liable to deteriorate the stone material. A better knowledge of the areas in which this condensation may be produced will greatly improve the durability of the material and, therefore, the façade.
This figure makes it possible to establish a qualitative analysis of the hygrothermal behaviour of the air chamber in a stone cladding enclosure with different thicknesses of air chamber and different speeds of external air.
When the thickness of the air chamber is 1 cm, the inside temperature of the building is affected a lot, and the air temperature of the chamber is very high. Infiltrations of cold air from outside are clearly visible. When the air velocity is moderate (×3), a clear decrease of the temperature inside the chamber is also shown. The relative humidity is high when the air enters the chamber, but it is drastically reduced inside the chamber. If the thickness of the chamber varies from 3 to 10 cm, the temperature decreases with greater thickness of the chamber, and the relative humidity that penetrates the chamber increases with the thickness of the chamber. For chamber thicknesses of 25 cm, being a large dimension, it can be observed that strong convection movements occur, mixing the air at different temperatures before leaving the chamber.
Temperatures are higher and relative humidity is lower on the ground floor than in the upper two (Figure 5
). This effect can be explained because the air does not access the air chamber with such ease through the lower part of the enclosure. As can be seen in Figure 3
, turbulence occurs in this area.
The following figures make it possible to analyse the data obtained by the simulations.
shows the temperature of a horizontal plane of the air chamber, depending on its thickness and for each of the three floors of the building. The higher temperatures are logically registered together with the thermal insulation (15 °C approx.). Inside the air chamber, the decrease in temperature is more pronounced close to the thermal insulation, and it is less pronounced away from it.
shows the evolution of the temperature inside the air chamber according to its thickness for the ground floor of the building, and considering low air speed (×1) and moderate speed (×3). In this way, it is sought to see what the influence is of air velocity on the interior temperature of the air chamber.
In cases where the air chamber has a thickness of 1 cm, there is no appreciable difference, since the curves on the graph are superimposed. When the camera is 3 or 5 cm, a greater speed of the external air implies a reduction of the interior temperature of the camera when entering a greater volume of cold air. However, for a camera 10 cm thick, this phenomenon does not occur on the ground floor.
The external air velocity (×1) and the thickness of the air chamber (3 cm) were kept constant in Figure 8
. The data was taken in the outermost part of the air chamber (inner face of the stone cladding). This figure compares the relative temperature and humidity values in the three floors of the building. When the temperature is lower and the relative humidity is higher, condensation can occur—liquid water that can affect the material, and which persists over time. In the case studied, the area to be analysed more closely is on the right side of the graph, coinciding with the horizontal joint of 15 mm between the ground floor and the first floor.
shows the variation in temperature and relative humidity for different thicknesses of the air chamber and the different floors of the building, comparing the speed of the soft (×1) and moderate (×3) air. The reduction in temperature is more pronounced on the ground floor, with air chamber thicknesses of 3 and 5 cm, up to 27% and 29%, respectively (Table 4
a). Regarding relative humidity, the greatest increase occurs in floor 1, with a 1 cm air chamber, up to 29%. Also noteworthy is the 17% increase in the ground floor for the 3 cm air chamber (Table 4
shows the variation of temperature and relative humidity by comparing different thicknesses of the air chamber for smooth (×1) and moderate (×3) air velocity. With a soft air velocity (×1), the temperature is drastically reduced if we compare the thickness of the air chamber of 3 cm with that of 1 cm, with up to a 47% reduction in plant 1 (Table 5
a). When the air velocity was moderate (×3) this reduction in temperature increased to 60% with the same air chamber thicknesses (Table 5
Regarding relative humidity, this increases to 96% if we compare the 3 cm chamber with the 1 cm chamber in plant 1 with the soft air velocity (×1) (Table 5
b). For moderate air velocity (×3), the increase is not so sharp, but importantly, there was an up to 74% increase in relative humidity in the ground floor when comparing the 3 cm air chamber and the 1 cm air chamber (Table 5
compares the temperatures of the air chamber as a function of height, in the interior part (next to the thermal insulation), in the central part of the chamber, and in the exterior part (interior area of the stone cladding). The analysis covers the chamber thicknesses analysed: 1, 3, 5 10 and 25 cm. In all cases, the 1-cm-thick air chamber keeps the temperature very high.
In the inner part of the air chamber, the highest temperatures are reached with the 1, 3 and 25 cm chambers, with the lowest temperatures corresponding to the 5 and 10 cm chambers. In the central part of the chamber, the temperature is clearly reduced, as we increase the thickness of the air chamber. On the outside of the chamber, the temperature reached is very similar regardless of the thickness of the chamber, except for 1 cm, where the temperature remains high.
3.2. Application of IRT
This research seeks a better knowledge of thermal and moisture behaviour in ventilated enclosures. In addition, it tries to locate those areas susceptible to condensation. IRT facilitates the knowledge of the exterior temperature of each one of the points of the façade. Thus, this real value is used to validate the CFD models.
The thermographic picture makes it possible to obtain the temperature distribution of the enclosure. Figure 10
shows, as an example, a real building with stone cladding. When the results obtained in the simulation vary considerably in a particular area with respect to the thermography, a possible pathology is identified. The climatic conditions of reality differ slightly from those used in the simulations of the previous sections. When thermograms were registered, the outside temperature was 8 °C and the relative humidity was 65%. For this reason, in order to compare, the boundary conditions of reality have been established in a new simulation (Figure 10
Infrared thermography makes it possible to compare real temperature data with simulations to provide more information to the thermodynamic study of the façades of buildings. In Figure 10
b, the vertical section S1 can be seen on the thermal picture, whose temperatures are represented in the graph of Figure 10
c. This graph shows the variations in temperature, in relative value with respect to the lower temperature of the stone panels. Some peaks in the temperature are observed coinciding with the joints of the stone panels. Figure 10
a corresponds to a simulation performed with the boundary conditions of a real building. When examining Figure 10
a,b, it is observed that the simulation performed coincides with reality.