Dispersion pattern of contaminants in a displacement ventilated room - implications for demand control.

A passive tracer gas technique has been used in an experimental study of the distribution of contaminants in a room with displacement ventilation. Humans are simulated by heated metallic bodies and the tracer concentration in the breathing zone (exposure) is shown to be greatly influenced by both the position of the tracer source and the air convection current around the bodies. It is shown that pollutants emitted close to a body are completely and directly transported to the upper mixed zone and not mixed into the lower zone. Pollutants emitted at a small heat source or close to a wall in the lower zone are transported to, but do not directly penetrate the boundary between the two zones, thus accumulating below the interface. By natural convection currents, occupants will draw uncontaminated air from the lower zone, and experience a better air quality at the breathing level than that of the surrounding air - even if the interface is below the head. It is concluded that air quality demand control of the supply air flow rate is a suitable means of securing the excellent air quality possible in a displacement ventilated room. A carbon dioxide sensor should preferably be positioned, so that the interface height can be maintained at a level slightly above the head of the occupants.