The fluid mechanics of DV

Displacement ventilation (DV) is characterized by the supply of cool air at low levels and the extraction of air at high levels. The ventilation can either be:

• Natural displacement ventilation where the flow is driven through openings by natural pressure forces (stack and wind).
• Mechanical displacement ventilation where the flow is driven mechanically.

The principle behind DV is that by introducing supply air at low levels higher cooling loads can be met and improved comfort and air quality are achieved when compared with traditional overhead air-conditioning system. In a traditional system the cool air introduced at high levels in a space falls under buoyancy to the floor and the resulting motion mixes the space. As a result temperatures within the space are fairly uniform - this is known as mixing ventilation (MV). In contrast, the cool air introduced at low levels in a DV system spreads out across the floor, with little mixing. This layer moves upwards as new air is introduced and 'displaces' the air above it.

These two systems are illustrated in the figure on the right. Internal heat loads from occupants and equipment produce thermal plumes that rise above the person or piece of equipment until they reach the ceiling where they spread out forming a warm layer. In DV this motion is unimpeded, and so a strong thermal stratification develops. In MV this stratification is largely destroyed by the descending cold air and the heat form the internal sources is distributed through out the space. Consequently, the return temperature TR is significantly greater in DV than in MV.