Local air velocity

Local air velocity The air velocity in the zone in which the design conditions have to be met. Or, the air velocity recorded at a specific location in a space or in a jet stream. 

Mamuro and Hattori (M6) dismissed their annulus data and used only the local air-velocity results obtained in the spout to calculate the air distribution between the spout and the annulus. The calculation, however, requires separate knowledge of spout diameter and of the vertical voidage profile in the spout. While the former could be directly measured, for the latter they had to rely on the values estimated by Mathur and Gishler for a wheat bed 25 in, deep and 6 in. in diameter with the further assumption that the same profile (voidage versus reduced bed level) is valid, not only for wheat beds of different depths, but also for soma sand. This assumption is speculative and weakens the reliability of their calculated air-distribution results as a means for verifying Eq, (37). The earlier experimental results based on pressure-drop measurements (iMlO, Tl), on the other hand, are free from the above objection, and these provide some support for the theoretical derivation of Mamuro and Hattori (Fig. 12), especially for Eq. (36), although less so for its arbitrary extension to Eq. (37). 

It is often possible to reduce air-input requirements- by removing the hazardous material at the point of discharge by loccd ventilation. This lowers the ta value in Eq. (8-5), which assumes possible disposal of hazardous material within the entire enclosed volume of the enclosure being ventilated. Hoods and exhaust ducts are placed over such equipment as open filter presses, pulverizers, open tanks, and over laboratory benches and equipment to catch the maximum amount of vapor or dust without interfering with normal operation and maintenance. Local air velocities in the region of pickup will depend on density of the hazardous material or its particle size if a dust or fume. Air velocities greater than 200 fpm are usually employed for industrial operations, while chemical laboratory fume hoods range from 70 to 125 fpm when fully opened. 

Lagrangian trajectory models can be viewed as foUowing a column of air as it is advected in the air basin at the local wind velocity. Simultaneously, the model describes the vertical diffusion of poUutants, deposition, and emissions into the air parcel as shown in Eigure 4. The underlying equation being solved is a simplification of equation 5 ... 

Aside from the general thermal state of the body, a person may find the thermal environment unacceptable or intolerable if local influences on the body from asymmetric radiation, air velocities, vertical air temperature differences, or contact with hot or cold surfaces (floors, machinery, tools, etc.) are experienced. 

The three categories in Table 6.3 apply to spaces where persons are exposed to the same thermal environment. It is advantageous if some kind of individual control over the thermal environment can be established for each person in a space. Individual control of the local air temperature, mean radiant temperature, or air velocity may contribute to reducing the rather large differences between individual requirements and therefore provide fewer dissatisfied. 

Figure 6.6 and Tables 6.4-6.6 give ranges for local thermal discomfort parameters for the three categories listed in Table 6.3. The acceptable mean air velocity is a function of local air temperature and turbulence intensity. 7 he turbulence intensity may vary between 30% and 60% in spaces with mixed flow air distribution. In spaces with displacement ventilation or without mechanical ventilation, the turbulence intensity may be lower. 

FIGURE 6.6 Acceptable mean air velocity as a function of local air temperature and turbulence intensity for the three categories of thermal environment. 

I he flow quantities may include air velocity, temperature, or contaminant concentrations. The term contaminant is used here as a general expression for other species at low concentrations carried by the air, such as smoke, COi, or toxic gases even the local age of air can be treated in a similar way. 

For the models described, the usual assumption for air nodes in regard to the room air distribution is still valid. This means that each air node represents a volume of perfectly mixed air. Thus, the same limitations as for thermal and airflow models apply Local air temperatures and air velocities as well as local contaminant concentrations can he neither considered nor determined. This also means that thermal comfort evaluations in terms of draft risk cannot be performed. 

In industrial ventilation the majority of air velocity measurements are related to different means of controlling indoor conditions, like prediction of thermal comfort contaminant dispersion analysis adjustment of supply airflow patterns, and testing of local exhausts, air curtains, and other devices. In all these applications the nature of the flow is highly turbulent and the velocity has a wide range, from O.l m in the occupied zone to 5-15 m s" in supply jets and up to 30-40 m s in air curtain devices. Furthermore, the flow velocity and direction as well as air temperature often have significant variations in time, which make measurement difficult. 

In air ducts, the measurement of the local air vekxiity is used to determine the flow rate in the duct. The duct flow is usually more stable and the flow direction under better control than in the room space. Different types of disturbances in the ductwork, such as bends, tees, or dampers, will influence the nature of the flow and cause swirl and other problems in velocity measurement. 

It is also well known that there exist different extinction modes in the presence of radiative heat loss (RHL) from the stretched premixed flame (e.g.. Refs. [8-13]). When RHL is included, the radiative flames can behave differently from the adiabatic ones, both qualitatively and quantitatively. Figure 6.3.1 shows the computed maximum flame temperature as a function of the stretch rate xfor lean counterflow methane/air flames of equivalence ratio (j) = 0.455, with and without RHL. The stretch rate in this case is defined as the negative maximum of the local axial-velocity gradient ahead of the thermal mixing layer. For the lean methane/air flames,... 

The combustion air is introduced at two levels, 60 to 70 percent being introduced through the distributor and 30 to 40 percent above the bed. This staged entry results in the lower reaches operating substoichiomet-rically, which helps to reduce NO emissions but tends to reduce the fluidizing velocity at the base of the combustor. To compensate for this and to increase solids mixing by increasing the local gas velocity, the portion of the combustor below the secondary air entry points is tapered. 

The acceptable air velocity range for local air movement in the tropics. 

The use of a vertical venturi separator was successfully used to clean steel and synthetic fibres from rubber crumb produced from an automotive tyre recycling plant. The major advantage of the venturi separator was an inherent positive feedback loop that exists at the throat. If more material was introduced to the throat then a higher velocity was generated at the throat to help retain particles (dependant upon air mover characteristics). The more conventional cleaning units observed had a negative feedback inherent in their design, with an increased local load of product the air velocity would... 

Air flows over a wide t-m long flat plate which has a uniform surface temperature of 80°C, the temperature of the air ahead of the plate being 20°C. The air velocity is such that the Reynolds number bas J on the length of the plate is 5 x 106. Derive an expression for the local wall heat flux variation along the plate. Use the Reynolds analogy and assume the boundary layer transition occurs at a Reynolds number of 10 ... 

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