Airflow patterns

Figure 10-4. Flame stabilization created by impinging jets and general airflow pattern. (C Rolls-Royce Limited.)...

Detailed measurements of temperature, humidity, airflow, or other parameters are more appropriate to a later stage of profile development. However, chemical smoke can be used to observe airflow patterns and pressure relationships between special use areas or other identified pollutant sources and surrounding rooms. Odors in inappropriate locations may indicate that ventilation system components require adjustment or repair. 

Pollutant pathway information helps the investigator to understand airflow patterns in and around the complaint area. The pollutant pathway data may indicate a need to enlarge the complaint area, or may direct attention toward contaminant sources that deserve close study. 

Evaluate airflow patterns into and within the complaint area. Because of the complexity and variability of airflow patterns, investigators cannot be expected to understand how air moves within the building under all potential operating conditions. However, data on pathways and driving forces can help to locate potential pollutant sources and to understand how contaminants are transported to building occupants. The discovery of unexpected pollutant pathways can show a need to study areas of the building that may be distant from the original complaint area. 

FIGURE 5.22 Schematic depiction of airflow pattern through the larynx. Note how eddies form downstream as air passes through the tracheal jet created by the vocal cords. This effect varies according to vocal chord position. 

In some cases, the ventilation process in the room can be simplified and mechanisms of air and contaminant movement under the influence of each of the above factors can be described using simplified theoretical principles of fluid mechanics, empirical data, and observations from numerous research studies. In general, the ventilation process in a room is complex and different factors have a joint effect on airflow patterns and characteristics, in continued spaces and in industrial buildings particularly. 

In rooms where energy is introduced primarily by supply air jets, air distribution methods are referred to as mixing type. With a perfect mixing-type air distribution, airflow pattern and air velocity at any point in the room are... 

FIGURE 7.7 Schematics of air supply (o) with inclined jets toward the occupied zone (b) with horizontal jets and occupied zone ventilated by reverse flow (c) with vertical jets. Shaded areas show the effect of buoyant forces on airflow pattern when supply air is excessively heated over the room air" ... 

FIGURE 7.9 Influence of exhausced airflow on airflow pattern in the naturally ventilated room fo> airflow in the convective plume smaller than exhausted airflow (b) airflow in the convective pluime equal to the exhausted airflow (c) airflow In the thermal plume at the stratification level equal to the exhausted airflow (t, air temperature along the room height, t, - average room temperaoire)... 

Experimental Studies of Isothermal Horizontal Jets in Confined Spaces Airflow Pattern, Throw, Velocities... 

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 . ... 

The influence of room transverse cross-section configuration on airflow patterns created by air jets supplied through round nozzles in proximity to the ceiling was studied by Baharev and Troyanovsky and Nielsen (see Fig. 7.37). Based on experimental data, they concluded that when the room width B is less than 3.5H, the jet attaches to the ceiling and spreads, filling the whole width of the room in the manner of a linear jet. The reverse flow develops under the jet. When B > 4H, the reverse flow also develops along the jet sides. Baharev and Troyanovsky indicated that air temperature and velocity distribution in the occupied zone is more uniform when the jet develops in the upper zone and the occupied zone is ventilated by the reverse flow. Thus, they proposed limiting room width to 3-3.5H,. 

FIGURE 7.40 Beam influence on the airflow pattern along the ceiling. Reproduced from Holmes and Sachariewitz. I... 

With a common value of boh /Dj = 1/250, the airflow pattern will be similar to isothermal with a modified Ar number limited to 40. 

Experimental studies conducted by Grimitlyn on heated and chilled confined jets showed that the airflow pattern remains the same as for isothermal... 

Similar studies were conducted by Troyanovsky, who concluded that to maintain the airflow pattern in rooms with heated or cooled air supply as in isothermal conditions, it is necessary that the rise of horizontally supplied jet does not exceed Ay = 0.1 BH at the distance from the outlet X = 0.15K BH) -. From this assumption the following equation for the maximum air temperature difference was derived ... 

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... 

Nielsen, P. V. 1980. The influence of ceiling-mounted obstacles on the airflow pattern in air-conditioned rooms at different heat loads. Building Service Engineering Research and Jeebnoi-ogy, vol. 1, no. 4. 

The size of these wakes and the velocity uniformity level depend on the hood design and the airflow pattern in close proximity to the hood face. 

This section will describe general features of airflow patterns and then present information on the dimensions and locations of recirculating (stagnant) zones around the building envelope, which determine wind pressures and contaminant dilution. This knowledge allows one to select the locations of stacks and air intakes and to calculate infiltration and natural ventilation rates. 

Industrial halls are typically large enclosures—indoor spaces that typically comprise one or more zones of occupancy. A large height combined with heat sources often results in room airflow patterns controlled by buoyancy flows,... 

German guidelines base the division on the resulting airflow pattern within the room rather than distribution methods. They suggest that airflow patterns be divided into four categories hall-filling mixed flow zonewise mixed flow low-momentum, low-turbulence flow for the air supply in the work region and zonewise displacement ventilation. 

Room airflow patterns controlled partly by supply and partly by buoyancy... 

The choice of room airflow pattern and air supply method is subject to study in each separate case. Table 8.9 presents, however, some guidelines for air distribution methods most commonly applied for various cases. 

The momentum flux of the plumes is high enough to penetrate the supply airflow patterns. The penetration depends on the location of the plumes in relation to the supply airflow patterns. 

The plume airflows (q ) are determined as described in Section 7.5. The turbulent mixing iqi,) between zones and the penetration of the plume airflows iq, h,n) through the supply airflow patterns must be determined specially for the air distribution method and devices used as well as the locations of plumes and supply air devices. 

The aim of this section is to provide a basic introduction to the methods by which air may enter a space and be distributed and to consider tlie governing equation for the determination of the air quantity and temperature. The governing equation relating to airflow patterns in a space is not covered in this section, as it is discussed elsewhere in the guides. 

The greater the distance between the low-level inlets and the high-level outlets, the greater the resulting air change rate will be. The resulting airflow patterns in this arrangement will not ensure satisfactory air distribution in many industrial environments. 

Based on the manufacturing process, the existing emission processes, and on workers demands (e.g., ergonomical aspects), different types of booths are possible. Many of these are commonly used. There can be a division according to the booth type (the shape of the booth), the position of the worker, the type of emission process, and the applied types of capture devices with the corresponding airflow pattern inside the booth. 

If acceptable working conditions must be maintained in the enclosure during furnace operation, attention must be given to internal airflow patterns, i.e., minimization of fume recirculation in the enclosure. 

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