Airflow visualization

The initial performance test for all local ventilation systems is a smoke test, which provides easy airflow visualization between the source and the hood, it helps to identify, with little effort, the main features of airflow patterns. Such a test, recorded by a video camera, allows performance comparisons to be made before and after improvements. Real contaminant or tracer gas measurements are necessary in the case of more detailed testing. 

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 re strictions. Another limitation is the low amount of smoke that is emitted. 

Airflow visualization (To verify required airflow patterns) All classes 24 months Tests to demonstrate airflows from clean to dirty areas do not cause cross-contamination uniformly from laminar flow units. Demonstrated by actual or videotaped smoke tests. In accordance with ISO 14644-3 Annex B7 ... 

The airflow pattern in rooms ventilated by linear attached jets with L/H ratio greater than that for effectively ventilated rooms was studied by Schwenke and Muller. The results of their air velocity measurements ami visualization studies indicate that there are secondary vortexes formed downstream in the room and in the room corners. The number of downstream vortexes and their size depend upon the room length (Fig. 736b). Mas,s transfer between the primary vortex and the secondary vortex depends upon the difference in characteristic air velocities in the corresponding flows (/, and Ui and can be described using the Stanton number, St . ... 

Visualization of airflow in the room with smoke and silk threads was used to describe airflow patterns in rooms with inclined jet supply. Airflow created... 

Evaluation can be performed by measuring capture efficiency using real contaminants and applying the real process or by substituting with tracer materials. A simpler, but qualitative, method of evaluation is the visualization of the airflow. If the relationship between capture efficiency and airflow rate is known, a measurement of the airflow rate can be used for frequent evaluation. See Section 10.5. 

It has also been shown, using visual illustrative methods, that accumulation can occur in the wake of people or objects, provided that the contaminants are emitted in the vortex region. Special consideration must be taken with instabilities and vortices generated by the working person. Vortices can also occur in empty open unidirectional airflow benches. 

When open unidirectional airflow benches are being used in production sensitive to contamination, a thorough function check should always he carried out before the start of production. Studies of air movements with visual illustrative methods give both quick and valuable information. 

An established design method for this type of system is not available. The practical design of the low-momentum supply with exterior hood system described in the previous part of this section used the flow ratio method. How-evec, the actual exhaust flow rate was adjusted visually to the appropriate value in order to exhaust only the contaminants transported by the supply airflow. 

TTie ability of the ventilation system to protect the worker efficiently can readily be determined by personal samples. The PIMEX method (see Chapter 12) can be used to determine the worker s exposure during various work phases. The capture efficiency as well as the supply air fraction can be measured using tracer gas techniques. Simple evaluation is carried out visually with smoke tube or pellet tests. Daily system evaluation is recommended using airflow or static pressure measurements at appropriate parts of the system. The air velocities, turbulence intensities, air temperature, mean radiant temperature, and air humidity should also be measured to provide an assessment ol thermal comfort. 

Different methods to measure efficiency are described m Section 10.5.2. Alethods to visualize contaminants and airflow are described in Section 10.5.3. 

In order to control the movements of contaminants it is useful to be able to see how both the contaminant and the induced airflows move. A number of flow visualization methods have been developed some are more suitable for laboratory research applications whereas others are quite widely used in industrial situations. We are primarily interested in this latter category. The methods involve releasing a tracer (for example gas, aerosol, or heat) and making visible its path. While in most cases the methods are subjective, their use is invaluable. Ideally the tracer should be nontoxic, nonirritating, inexpensive, and highly visible at low concentrations. The system should be easily portable, self-contained, easy to use, and be controllable. 

This is a clear liquid that vaporizes and, on contact with damp air, combines with w ater to produce a dense acid mist. Titanium tetrachloride can be painted on to surfaces, such as fume cupboard sills, from which it will evaporate over a period of several seconds showing the airflow patterns close to the surface. (Airflow patterns close to a surface could also be visualized by fastening short filaments of wool or cotton to the surface). Titanium tetrachloride can also be used, when soaked onto a cotton swab, in a similar way to a smoke tube. It is a simple and inexpensive method but the production of smoke, which is toxic and corrosive, is uncontrollable. 

Kennedy describes a method using an ultrasonic nebulizer to generate a fog of water droplets w hich is used in the same way as smoke to visualize airflows. Several types of nebulizers are available but they require an electrical connection and are not hand-held. Food dye can be added to the water to produce colored fog. The nebulizers are expensive (about 1500 ECU) but have negligible operating costs. Although the amount of smoke produced is small, it is nontoxic and nonirritating. 

For integral building design and performance assessments (thermal comfort, indoor air quality, visual comfort), either integrated building simulation tools must be used or the thermal building simulation must be complemented by airflow and daylighting simulations. 

All three experimental approaches are presented in this chapter visualization of airflow and contaminant dispersion (Section 12.2), measurement techniques including laser-based-techniques (Section 12.3), and scale model experiments (Section 12.4). 

Clean air and many air contaminants are under normal conditions invisible. It is, however, often desired to actually see the movement of air or the emission and transport of contaminants in order to ensure good air quality.. Methods aimed at the visualization of airflow, contaminants emission to the air, and their transportation out in the workplace and to the breathing zone of the worker are therefore important tools for designers of industrial ventilation systems. 

The main purpose with flow visualization is to make the airflow field or the emission and transport of air contaminants visible and thereby possible to study. In technical terms, flow visualization gives possibilities to study airflow field and contaminant dispersion and changes in it depending on general changes in geometry, boundary conditions, inlet and exhaust airflow, etc. It is... 

The reasons for using airflow or contaminant dispersion visualization techniques may be divided into purely technical and pedagogical, both with the purpose to give a base for the design and proper use of a contaminant control technology for workplaces with a good air quality. 

In this chapter the most common and/or well-evaluated techniques for the visualization of airflow and contaminant dispersion are described. The strategy for its use as a pedagogic tool is not included this can be found elsewhere. To separate the different methods according to either airflow or contaminant dispersion studies is meaningless and not always possible. For example, smoke may many times be used for both purposes. The emitted smoke acts like a substitute for the contaminant in question and visualizes its dispersion to, e.g., the... 

There are of course a number of methods that can be classific d a- methods for the visualization of airflow and contaminant dispersion. This i.hapter describes some of these that are useful for designers of industrial vcntilatiou. Methods that not are presented in more detail here are, for example, to fill small soap bubbles or ordinary balloons with helium in order to stuiiy the airflow field in large rooms. A large number of textbooks focus on flow- visualization. The research in this area can also be followed in The lournal of Floif Visualization and Image Processing. -... 

A common principle for the prt)duction of smoke for this purpose is to cvat>-orate a mineral oil by electrically hearing 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 t f airflow but others are intended lor special effects in theaters, discotheques, etc. Figure 12.3 shows one such apparatus commonly used for this purpose. 

There was more. A strain gauge on each mask would indicate the airflow, and a small tube would divert a sample of the aerosol to the spectrophotometers, designed to calculate cumulative dosage in real time. Airtight Plexiglas windows would provide visual access to the volunteers. Auditory input would feed into each soldier s earpiece from a microphone in the booth. 

Webster and Weissburg (2001) visualized instantaneous versus time-averaged odor plumes by laser-induced fluorescence. The spatially varying plume at any particular point in time matters more to an animal than an average plume shape. The mean direction and speed of airflow may be relatively constant, but the animal may extract information from concentration differences on very small temporal and spatial scales. 

Airflow/helium bubble analog flow visualization. 

Af John Zink Company s COOLflozv Physical Modeling Facilify, a differenf flow visualization fech-nique has been successfully applied to scale model sfud-ies. In fhis technique, small quanfifies of minufe helium bubbles are suspended in fhe airflow and used as fracer spheres. The helium bubbles are neufrally buoyanf wifh a nominal densify close to fhaf of fhe air af ambienf femperafures. Figure 10.4 shows fhe helium bubble flow visualization technique applied to a scale model. 

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