Methodology for the Evaluation of Natural Ventilation in ... - Cham

For airflow visualization, the particles must be highly reflective so that they are able to becaptured on film for analysis. In general, for smoke and fog generating machines, the particlesize is approximately 0.5μm (Smits and Lim 2000). The working fluid used to introduce thematerial for flow visualization should also be neutrally buoyant. For air, this requires that thematerial introduced be at the same temperature as the inlet air for the model. First orderapproximation of buoyancy effects due to particle size becomes negligible when the diameter isless than 1μm, and visibility of particles is possible when the particles are larger than 0.15μm(Merzkirch 1987). A system comprised of several outlets is often used in wind tunnels,providing a series of equally spaced lines of smoke, parallel to the main air stream.In water bath models, flow visualization and imaging is more easily carried out than with airmodels. The use of clear Plexiglas for constructing the model allows light to penetrate throughthe model, easily projecting the flow visualization onto a screen behind the model. Oftenprojectors are used as the light source, illuminating the entire water model setup and using adigital video camera to record flow patterns. Using water as the modeling fluid also enables amuch wider range of tracer materials to be used for visualization of flow patterns. Typicallycolored dye, mixed with water at the same temperature as the ambient fluid is used. Multiplecolors of dye can be used to help visualize more complex fluid flow patterns and the interactionbetween flows. However, with water models there are potentially problems with matching keydimensionless parameters, which will be discussed in Chapter 5.Capturing the flow patterns created with the air modeling methods requires appropriate lightingand imaging equipment. Proper lighting can affect the visibility of the flow patterns and theability to record them on film. Two main light sources are used for flow visualization: a)conventional light sources, such as spotlights, halogen lamps, mercury lamps, and b) lasers.Mirrors and lenses can be used with either type of light source in order to brighten, expand, andlocate the desired shape of light. Using a combination of a mirror and cylindrical lens, a circularbeam of light from a laser can be transformed into a light sheet to visualize the structure offlows. This is one of the more common techniques used in flow visualization when air is used asthe fluid. Light sheets are created by aiming a beam from the light source at a cylindrical lens,which then, due to its geometry, creates a linear beam of light or a light sheet. The smaller thediameter of the cylinder, the larger the beam spread.However, capturing these airflow patterns on film can prove to be a difficult task. Camera andlight source position can affect which cross-section is observed and the image quality. Theavailability of higher speed 35mm and digital cameras has improved the ability to capture flowvisualization images. Often the exposure or shutter time, resolution, aperture size can affect thequality of the resulting image. High-speed cameras are of some concern with reduced-scalemodeling using air as the fluid, as they require increased illumination, which often leads toadditional heat dissipation.4.2.2 Application to BuildingsEffective ventilation is important to thermal and occupant comfort when evaluating anddesigning spaces for use. Indoor air quality is also influenced by airflow. It is difficult to predictthese flow patterns without the use of visualization techniques. A variety of methods have beendeveloped for a wide range of application types (Merzkirch 1987), but here the focus is on78