Handbook of air conditioning and refrigeration / Shan K

Stratified Mixing Flow
AIR SYSTEMS: SPACE AIR DIFFUSION 18.25
FIGURE 18.18 Mixing flow using sill outlet.(Adapted with permission from
ASHRAE Handbook 1989, Fundamentals.)
with reverse airstreams and cooled. During heating, a stagnant zone may form below the ascending
induced airstream.
Mixing flow with sill or floor outlets has been widely used in buildings with large window areas
or in raised-floor offices. The direction and amount of airflow from floor diffusers usually can be
adjusted according to the requirements of the occupants.
In a building with a high ceiling, it is more economical to stratify the building vertically into two
zones (the stratified upper zone and the cooled lower zone) or three zones (upper, transition, and
lower zones) during cooling. The upper boundary of the lower zone is at the level of the supply outlet
where the air jet projects horizontally.
Gorton and Sassi (1982) and Bagheri and Gorton (1987) performed a series of model studies
and experiments about stratified space air diffusion. Figure 18.19a shows the elevation view of the
nuclear reactor facility in which the experiments were conducted. The ceiling height was 41 ft
(12.5 m). An air-handling unit was located at floor level, and two diffusers were mounted on
the supply riser 16 ft (4.9 m) above the floor level. The supply velocity at the outlet was about
1000 fpm (5 m/s), and the supply temperature was around 60°F (15.6°C).
Figure 18.19b shows the space air temperature profile at 4 p.m. at various height levels during
cooling. This temperature profile could be divided into
● An upper zone, from the ceiling down to about 22 ft (6.7 m) from the floor level, in which air
temperature varied from 82.5 to 79°F (28 to 26.1°C)
● A transition zone, between 16 and 22 ft (4.9 and 6.7 m) from the floor, in which air temperature
varied from 79 to 74.5°F (26.1 to 23.6°C)