Laboratory Experiments with General and Local Ventilation
This section shows some examples of the use of model experiments. The examples cover the range from natural ventilation of buildings, comfort ventilation of an exhibition hall, to local ventilation in industrial areas.
12.4.6.1 The Danish Pavilion in Seville
The Danish Pavilion at the EXPO ’92 World Exhibition is shown in Fig. 12.32. It has two main elements: a steel-framed structure, facing west, with a floor area of 45.0 m x 2.5 m and a height of 24 m, and a fiberglass construction, facing east, which leans against the steel structure. The large room formed between the fiberglass surface and the steel building is enclosed by glass walls to the north and to the south. This room is the exhibition hall and it was visited by more than 800000 people during EXPO ’92.
The occupied zone design load of the exhibition hall is 48 kW, corresponding to 300 persons in the pavilion. The equipment for slides and video will generate another 130 kW, which is expected to move upward in convective flows, causing a high temperature in the upper part of the pavilion.
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Elements in ihe south gabie. |
The ventilation system is based on an extract fan in the north top of the exhibition hall (smoke ventilation) and on exposed cooling elements in the south gable; see Fig. 12.32. Air is drawn through the cooling elements, where it cools and falls to the floor. It is difficult to give an estimate of the downdraft from the 12 m high diffuser in the gable, and therefore it was decided to carry out scale-model experiments and CFD predictions of the flow.
The model experiments were carried out in a model on the scale of 1 to 10. Experience with measurements on flow from wall-mounted diffusers for displacement ventilation indicates that it is possible to ignore the level of the Reynolds number at the given dimensions, which will enable reasonable temperature differences in the model experiments.
Figure 12.33 shows the velocity distribution down through the full length of the exhibition hall. It is quite obvious that the flow is a stratified flow with the highest velocity in the occupied zone. Smoke measurements show that the cold air from the cooling device accelerates down into the occupied zone, due to gravity, and moves horizontally as a stratified flow along the floor in the restaurant and exhibition section.
Measurements show that the velocity has a fairly constant level in the occupied zone even far downstream from the wall with the cooling device. The flow is plane, and general experience indicates that the velocity in a plane stratified flow is constant and independent of the distance from the inlet device. Prediction of the flow by computational fluid dynamics shows a similar velocity level in the hall.13 The full-scale measurements shown in Fig. 12.33 indicate a very low velocity in most of the hall due to the practical difficulties in obtaining a correct load during the full-scale experiment.
Experiments with natural ventilation of large constructions such as shopping arcades and atria necessitate a set of boundary conditions outside the
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FIGURE 12.33 Velocity distribution in the restaurant and exhibition hall of the Danish Pavilion.
Openings in the building because the pressure distribution (and flow) around a building are an important part of the problem.
These experiments have to be carried out in an environmental wind tunnel.3’14 A roughness on the surface in such a wind tunnel generates a typically vertical profile, and the whole building and the neighboring buildings are exposed to the flow. Figure 12.34 illustrates an experiment. The flow around a model of a school building in Tanzania is shown by the streamlines of lightweight particles in the air. It is possible to study the flow in the double ceiling, the flow through the window openings, and the recirculating flow in the classroom.
The contaminant source considered in this section is a tilling machine for paint. Figure 12.35a shows the machine with a filling tube (to the left in the figure) and equipment for closing of the cans (to the right in the figure), l’he machine was originally delivered without exhaust equipment, but an exhaust opening has been mounted behind the filling tube. Measurements at the operator’s workplace show an exhaust flow rate of 180 m3 h‘1 and an acceptable air quality in the operator’s breathing zone.
Figure 12.35b shows a model of the machine on the scale of 1 to 1. Many details and surfaces are made in the correct size and location to achieve a good reproduction of the actual capturing zone. The capture efficiency A is used for the evaluation of the system and it is defined as the ratio between the flow rate of the contaminant SE directly captured from the process and the total flow rate of the contaminant S released from the process:
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FIGURE 12.34 Scale-model experiment with natural ventilation Of a school building. |
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Figure 12.36 shows the capture efficiency as a function of the exhaust flow rate <7C both with the emission source at the filling position and with the emission source at the closing position. The last position shows the lowest values in capture efficiency due to the distance to the exhaust opening. The shaded area in the figure corresponds to the position between the filling and the closing of the cans. The exhaust air represents a high energy consumption and therefore it is important to have a design with a high capture efficiency at low flow rates. Figure 12.37 shows a new design of the exhaust opening where parts of the machine are integrated into the opening and in this way act as flanges for the |
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FIGURE 12.35 (o) Filling machine from the paint industry and (b) full-scale model of the machine.
Section in the upper part. The newly developed films leave the air-drying section through the opening shown in the upper part of the figure. Air is supplied to the drying section from ventilators and is blown out into the surroundings through the opening for the films and, consequently, causes low air quality in the room. In addition, the operator of the developing machine will be highly exposed when standing in front of the machine and changing the films.
The initial solution to this problem was to install an exhaust channel on the opening for the films, as in Fig. 12.38. Laboratory experiments show that the principle is inexpedient and it is possible only to obtain a capture efficiency of 70% at a flow rate of 100 m3 h_t.
The experiments show that it is impossible to make a simple design of the opening to improve the air quality in the room. A proper solution should have been chosen in the design phase of the machine. It would, for example, be obvious to reverse the direction of the airflow and extract the air from the room through the machine.
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