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Therefore, knowing the angle for expansion allows
us to perform simple calculations on the jet such as the distance travelled before the mean velocity drops to an imperceptible level. Similarly, if the supply air is initially at a different temperature to the room air, the entrainment will also change the temperature of the jet along its length due to mixing, and we can estimate the mixed-jet temperature using the same rule of thumb. An isothermal jet in free air will travel in a straight
line. As air is entrained into the jet there must be momentum perpendicular to the jet axis, as indicated by the inward streamlines in Figure 1. As the entrainment occurs equally from all directions, these momentums balance, satisfying conservation of momentum and creating an all-round pressure that constrains the jet to travel in a straight line. A jet at a different temperature to the room air will still expand at the same rate as an isothermal jet, but its axis will follow a trajectory dictated by the buoyancy of the air in the jet relative to the surrounding air. A buoyant plume is very similar to a jet in its
Figure 1: A jet is a stream of air created by a pressure difference across an orifice, as in most supply air situations, even a breeze through an open window. The jet has a turbulent interface with the static air through which it passes, resulting in mixing between the two air masses
characteristics, being a stream of air from an orifice whose propagation into the room is driven by a temperature difference rather than pressure (see Figure 2). A buoyant plume has the same turbulent mixing at the boundary layer, although with a slightly reduced angle of expansion of 10.6C. It is rare to find genuine plumes in ventilation systems as the air is almost always introduced to the room with some pressure difference. Plumes may, however, be encountered in natural ventilation design, for instance where a room has an opening roof-light admitting cool air. Plumes were also sometimes used to introduce mixing ventilation in Victorian theatres, where there was sufficient headroom for cool air to mix with room air before entering the occupied zone. The aim of air conditioning design, particularly
in cooling, is to ensure that the supply air jet has adequately mixed with the room air and its velocity has decayed sufficiently so that it does not give rise to discomfort through cold draughts. One well-established way to increase the distance travelled by a jet or plume is to use the ceiling effect. It has been observed that if a cold supply grill is located close to a ceiling, the resulting jet travels further before succumbing to negative buoyancy (see Figure 3). If a jet is introduced close to a surface, it tends to
Figure 2: A buoyant plume (left) is similar to a jet, but is created when air passes through an orifice driven by a difference in density. A laminar plume (right) occurs when the change in density is caused by a heat source within the room
attach to the surface and expand more slowly, travelling further than a jet in free field. As the jet propagates, it is unable to expand in all directions; it is constrained by the surface on one side, but expands freely in the opposite direction. As room air is only being entrained into the jet from
one direction, this results in unbalanced momentum, creating a pressure that keeps the jet pressed against the surface. This unbalanced momentum probably has as much to do with jet attachment to surfaces as the Coanda Effect, the tendency of a flowing fluid to follow a surface.
64 CIBSE Journal October 2010
www.cibsejournal.com
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