A common principle for the productiun of smoke for this purpose is to evaporate a mineral oil by electrically heating it and to mix the vapor into air. The oil will then condense and form a mist. Different such apparatus can be found on the market. Some of them are aimed for the visualization of airflow but others are intended for special effects in theaters, discotheques, etc. Figure 12.3 shows one such apparatus commonly used for this purpose.
The smoke generator shown in Fig. 12.3 allows the user to adjust the flow rate of the smoke and also to connect different types of spreaders through a several-meter-long tube. This makes it possible to simulate different types of sources, such as a point source with low or high momentum, a line source, a surface source, or any other source with any geometry. Some examples are IlLustrated in Fig. 12.4.
An application of the smoke generation principle described above is the “smoke wire.” This simulates a line source and makes it possible to effectively study the airflow patterns in a layer, which is often desirable. A thin steel wire is
Wetted with the same type of oil used in the smoke generator described above. The wire is stretched and heated by passing an electric current through it, controlled by an adjustable transformer.4 This technique has proved to be especially useful for studies of airflow patterns near obstacles in wind tunnel experiments.
Two other principles that have been made commercially available are (1) the reaction between pyrosulfuric acid (H2S2O7) and water (water vapor in air) to form sulfuric acid aerosol and (2) the reaction between titanium tetrachloride (TiCl4) and water to form titanium dioxide (TiOz) and hydrogen chloride (HC1). Figure 12.5 shows two different hand-held, disposable smoke emitters.
These methods are the simplest, cheapest, and practically most accessible for airflow visualization. They are sold in the form of small glass tubes or plastic bottles through which air is pumped manually. Time for use of one unit is typically one hour or up to one day. One drawback of these two principles is the fact that the emitted smoke is strongly irritating if inhaled and also corrosive. Therefore, they must be used with some care, but this will normally not lead to any major restrictions. Another limitation is the low amount of smoke that is emitted.
Smoke emitters developed for fire exercises, pressure-testing of chimneys, etc. are sometime used for airflow studies. They normally take the form of big tablets that are put in fire and emit large amounts of smoke (Fig. 12.6;. Also, smoke bombs giving different colors of smoke are available.
Another method that has been used is heating/melting of tablets of metacetalde — hyde, which then form a large number of particles that spread out from the source. By combining this with a photographic technique it is possi ble to determine both,;ir velocity7 and airflow direction at single points.-1 The use of that principle is, how ever, limited since the generated smoke is harmful to persons. Magnesium carbonate (MgCO;) powder has also been used combined with a laser light sheet.0
Dry ice (frozen carbon dioxide) can also be used to generate ‘smoke’ tor the same purpose.7 By just letting air pass over a piece of dry ice, a mi t is formed that is neither corrosive nor especially toxic if used in moderate amounts.
As said earlier in this chapter, the use of properly arranged i lumination will improve the visibility of the smoke markedly. Extra light should be arranged so that the light beams are directed almost directly into the eves of the observer or into the lens of a camera. Direct dazzling must be avoided with the help ot some shield. The use of a laser beam expanded to a sheet makes it possible to visualize the airflow in a special layer in the room. This technique makes it possible to study the airflow in more detail, e. g., near an enclosure or around a machine.8
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