If there is no influence of the walls, ceiling, or obstructions on the air jet, it can be considered a free jet. If the air jet is attached to a surface, it is called an attached air jet.
Characteristics of the air jet in the room might be influenced by reverse flows, created by the jet entraining the ambient air. This air jet is called a confined jet. If the temperature of the supplied air is equal to the temperature of the ambient room air, the jet is an isothermal jet. A jet with an initial temperature different from the temperature of the ambient air is called a nonisothermal jet. The air temperature differential between supplied and ambient room air generates buoyancy forces in the jet, affecting the trajectory of the jet, the location at which the jet attaches and separates from the ceiling/floor, and the throw of the jet. The significance of these effects depends on the relative strength of the thermal buoyancy and inertial forces (characterized by the Archimedes number).
Air jets can be classified according to the diffuser type as follows (Fig. 7.19):’
1. Compact air jets are formed by cylindrical tubes, nozzles, and square or rectangular openings with a small aspect ratio that are unshaded or shaded by perforated plates, grills, etc. Compact air jets are three-dimensional and axisymmetric at least at some distance from the diffuser opening. The maximum velocity in the cross-section of the compact jet is on the axis.
2. Linear air jets are formed by slots or rectangular openings with a large aspect ratio. The jet flows are approximately two-dimensional. Air velocities are symmetric in the plane at which air velocities in the cross-section are maximum. At some distance from the diffuser, linear air jets tend to transform into compact jets.
3. Radial air jets are formed by ceiling cylindrical air diffusers with flat disks or multidiffusers that direct the air horizontally in all directions.
4. Conical air jets are formed by cone-type or regulated multidiffusers in ceiling air distribution devices, and have an axis of symmetry. The vectors of
FIGURE 7.19 Types of diffuser jets: (a) compact; (b) linear; (c) radial; (d) incomplete radial; (e) conical.1
Air velocities are parallel to the conical surface (with an angle at the top of the cone equal to 120°). Maximum velocities in cross-sections perpendicular to the axis occur at the conical surface.
5. Incomplete radial jets are supplied through outlets with grills having diverging vanes, and have a coerced angle of expansion. At some distance this kind of jet tends to transform into a compact one.
6. Swirling jets are supplied to the room through air diffusers with vortex-forming devices creating rotation motion, and have tangential as well as radial velocity vectors. Depending upon the type of air diffuser, swirling jets can be compact, conical, or radial.
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