Handbook of air conditioning and refrigeration / Shan K

duced ambient air at a comparatively higher pressure presses the air jet against the surface, even
when it is a curved surface. Such a phenomenon is called the surface effect or the Conda effect.
Friction between the air jet and the boundary decreases the centerline velocity of confined air
jets, as shown in Fig. 18.4. However, because of the surface effect, the throw of a confined air jet is
longer, and the drop from the horizontal axis smaller, than that of a free air jet.
Airflow Pattern. The flow pattern and characteristics of a confined air jet using a sidewall outlet
were introduced by Russian scientists during the 1950s. In an air conditioned room whose supply
outlet is located above the occupied zone and whose exhaust opening is on the same side of the supply
outlet, the supply air jet clings to the surface of the ceiling and mixes with the room air. An induced
reverse airstream, with more even velocity and temperature distribution than that of the air
jet, covers the occupied zone.
Figure. 18.5 shows the airflow pattern of a typical confined isothermal jet. When supply air is discharged
from the circular outlet and moves along the surface of the ceiling, the fluctuating velocity
components continue to transport momentum across the boundary of the air jet. Therefore, ambient air
is induced into the air jet, and the induced circulating airflow occupies most of the enclosed space.
As the air jet moves forward, its mass flow rate increases and mean air velocity decreases until it
arrives at a cross-sectional plane where dimensionless distance s � 0.22. Term s is defined as
s � (18.12)
ax
√0.5A r
AIR SYSTEMS: SPACE AIR DIFFUSION 18.9
where a � turbulence factor; for a circular nozzle a � 0.076, and for a rectangular outlet without
guide vanes a � 0.15.
FIGURE 18.5 Airflow pattern of a typical confined air jet.