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Description
INTRODUCTION
Evaluation of exposures to contaminants and assessment of the risk of draft require that a number of physical quantities, such as concentration of gases and particles, temperature, and air velocity, be monitored. Traditional measuring methods utilize point-measuring techniques, e.g., thermocouples for recording the temperatures, thermistors or hot-wire anemometers for recording velocities, and sampling bags for recording air quality. A general overview of traditional methods with a focus on fluid mechanics is presented in Goldstein (1996); a recent review is Tavoularis (2005), which also deals with whole-field methods. With point-measuring techniques, only one sensor measurement is done at a time at a certain position. Access to several sensors makes it possible to simultaneously measure at several positions. However, the measurements can only be carried out in discrete positions.
Whole-field measuring techniques, where measurements are undertaken simultaneously at different locations, are new techniques developed during the last two decades. Particle streak velocimetry (PSV) is an old visualization method that now has become a quantitative method. The individual flow markers give rise to streaks on the image. The light is modulated to give a sequence of streaks of different lengths to enable identification of the flow direction. One of the first examples of recording the three-dimensional velocity field using stereophotogrammetry in room airflow was reported by Kaga et al. (1990).
Particle image velocimetry (PIV) relies on a light source of short pulse duration and high repetition frequency, resulting in a series of dots of an individual flow marker on the image. By statistical techniques, the displacement of groups of dots are analyzed. Suitable laser light sources became available in the 1990s, which made it possible to use PIV outside research laboratories.
Infrared cameras have been used for recording the temperature of a screen surrounded by air (Sundberg 1993; Stetz 1993). The use of infrared cameras has become easier since there is no longer a need for cooling by liquid nitrogen.
SOME PROPERTIES OF AIR MOTIONS IN ROOMS
The different sources of air movements are described in detail in Etheridge and Sandberg (1996). The ventilation system itself is a source of air motions. Velocities supplied through the supply devices range from about 0.5 m/s (low-velocity diffusers for displacement ventilation) to 10 m/s (diffusers for mixing ventilation). In the occupied zone the velocities are low, typically in the range of 0.05-0.5 m/s. Velocities as low as 0.15 m/s may be experienced as a draft. The relative turbulence intensity (standard deviation/mean velocity) in the occupied space is around 25%, and the majority of the velocity fluctuations have a frequency of less than 2 Hz. The spatial resolution of the flow is given by the Kolmogorov microscale, which in the occupied zone has been estimated to be around 1 mm (Etheridge and Sandberg 1996, pp. 223). The flow pattern is in many cases unstable. A plume is very vulnerable to disturbances and is always meandering around its own axis. The trajectory of a jet may change due to a change in, for example, the temperature of a wall. Another feature of room flow is that usually there is a strong recirculation and, therefore, the direction of the velocity is not known beforehand.
EXAMPLES OF MEASUREMENTS WITH WHOLE-FIELD METHODS
The first example of a measurement with whole-field methods is the monitoring of the temperature and velocity field generated by a supply of cold air directly into the occupied zone with the supply device shown in Figure 1. The supply device was located above floor level, and the device consisted of nozzles whose direction could be changed individually (Elvsen and Sandberg 2007). Because the air is discharged directly into the occupied zone, the temperature and velocity must be quantified in the vicinity of the supply device. The combination of a complex supply diffuser and a buoyant flow is an example of a situation where the use of computational fluid dynamics (CFD) will lead to difficulties.
[FIGURE 1 OMITTED]
The left-hand side of Figure 2a shows the temperature distribution with an infrared camera and a measuring screen. The screen can be discerned in the upper part of the picture. The right-hand side of Figure 2a shows the streaks of neutrally buoyant soap bubbles.
[FIGURE 2 OMITTED]
Figure 2b shows an enlargement of PIV measurements indicated by insets in the right-hand side of Figure 2a. The area covered by the PIV is in this case only 10 x 13 cm, which is a very small area in the context of airflow pattern in rooms. Therefore, PIV is only suitable for documenting details in room flow.
Due to its ability to provide detailed information of small regions, PIV has been used to document details of the flow around the human body. Figure 3 (Murakami 2002) shows a measurement of the velocity field generated by the human respiration sequence.
[FIGURE 3 OMITTED]
A severe malfunction of ventilation is the short-circuiting of air, which is always a risk when heated air is supplied at ceiling level. Figure 4 displays an image of a supply of warm air.
[FIGURE 4 OMITTED]
Currently there is an interest in studies of air quality and the spread of contaminants in commercial aircraft cabins. Interesting examples of the application of PSV for characterization of airflow patterns in aircraft cabins can be found in Sun et al. (2005) and Zhang et al. (2005).
PARTICLE DISPLACEMENT
In both PIV and PSV, the velocity is determined by measuring the displacement [DELTA]X in room coordinates [DELTA]X = X(t + [DELTA]t) - X(t) = [X(t + [DELTA]t) - X(t), Y(t + [DELTA]t) - Y(t), Z(t + [DELTA]t) - Z(t)] of seeded particles in a time step [DELTA]t.
For a known time step [DELTA]t, the components of the velocity vector U(X, t) are derived from the fundamental definition of velocity:
u(X,t) = [[[DELTA]X]/[[DELTA]t]] = [[X(t + [DELTA]t) - X(t)]/[[DELTA]t],[[Y(t + [DELTA]t) - Y(t)]/[[DELTA]t]],[Z(t + [DELTA]t) - Z(t)]/[[DELTA]t]] (1)
Equation 1 is the definition of velocity; both PSV and PIV are therefore direct methods. For these methods, a planar light sheet is used and the velocity components lying in the light sheet are the in-plane components. Traditionally the notation assumes that the X and Y components are the in-plane components, while the Z component is the out-of-plane component.
STEREOPHOTOGRAMMETRY FOR DETERMINING THE OUT-OF-PLANE VELOCITY COMPONENT
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