Airflow in Rooms Dominated by Supply Jets

In rooms where energy is introduced primarily by supply air jets, air distri­bution methods are referred to as mixing type. With a perfect mixing-type air distribution, airflow pattern and air velocity at any point in the room are governed by supply jet momentum. In this case, if the air supply and air ex­haust openings are located close to each other, a large proportion ot supply — air is extracted from the room without passing the occupied zone. Such a sit­uation, called short-circuiting, results from poor design and leads to undesir­able airflow patterns.

Current mixing-type air distribution methods typically consider ventila­tion of the occupied zone with jets intercepting its upper boundary (e. g., Fig. 7.6a, c). Also, the occupied zone can be ventilated by the reverse flow pro­duced as the supply jet degrades above the occupied zone level (Fig. 7.6b). Mixing-type air distribution methods include air supply with jets projected vertically downward, inclined jets, jets directed vertically upward, and hori­zontal jets along room surfaces.

In the latter case, the jet reaches the opposite wall/ceiling and follows room surfaces until it reaches the occupied zone (Fig. 7.7a). If the combina­tion of room sizes (height, length, and width) allows such an airflow pattern, this room is considered to be “short.”5 The room where an air jet dissolves before it reaches the opposite wall is considered to be “long.” In such rooms, the occupied zone is ventilated by “reverse” flow, and secondary and tertiary vortexes (Figure 7.7b, c).Buoyant forces, e. g., when supply air is heated, can significantly affect the airflow pattern created by supply jets (Fig. 7.6). Applying proper design principles prevents warm air from rising to the upper zone of the room without heating the occupied zone. More de­tailed discussion of airflow created in confined spaces with mixing-type air supply can be found in Section 7.4.5.