On the efficiency of displacement ventilation

Abstract: The efficiency of room ventilation by the displacementprinciple was studied with respect to some influential factors,in particular that of physical activity. The study wasexperimental and performed in two full-scale test rooms, one ofoffice-size and one of classroom-size. Physical activity wasexecuted in these rooms by person simulators and by humans. Theventilation efficiency was quantified by tracer gasmeasurements.In the performed tests, the activity of a walking persongenerally proved detrimental to the ventilation efficiency.Particularly the air quality in the occupied zone was impaired,due to down-wash of air in the wake behind the moving person,causing transportation of relatively old and contaminated airfrom the upper part of the room down to lower levels. Itappeared, however, that it takes a rather high level ofphysical activity to completely abolish the displacementeffect. Especially the air quality in the breathing zone ofnon-moving occupants tended to remain significantly better thanat perfect-mixing conditions. Completely mixed room airoccurred nevertheless when the activity was intense, but thedisplacement flow pattern was re-established fairly quicklyafter ceasing of the activity.The contaminant distribution showed substantial horizontalvariations in the "classroom". A contaminant released in theoccupied zone was effectively extracted from the room when thesource was situated on the same side of the room as the extractterminal(s), whereas, when situated on the opposite side, thecontaminant accumulated in the upper part of the room. It wasfurther shown that the air supplied from displacement diffuserstends to reach all occupants fairly quickly, also in relativelylarge and densely populated rooms, likeclassrooms.The temperature stratification of the room air, andfree-convection currents along the walls are crucial for theappearing air flow pattern and contaminant distribution.Transfer and accumulation of heat in materials tend further tomake the thermal conditions indoors more or less transient atall times. This entails, it was shown, that also theventilation efficiency is time dependent. In tests where peoplesuddenly entered the "classroom", the ventilation efficiencyimproved with time. This generally caused the highestcontaminant exposures to occur at the beginning of the stay inthe room.Tests with two different ceiling heights showed lowerventilation efficiency with the higher ceiling, involving ahigher contaminant exposure of the occupants. A walltemperature deviating from that of the indoor air impaired theventilation efficiency, whereas an increase in ventilation rateimproved it. In all test cases in this study, however, the airquality in the breathing zone of seated occupants remainedsignificantly better than that at perfect-mixing conditions?a sports-activity test being the only exception.Key words:Displacement ventilation, Ventilationefficiency, Physical activity, Movements, Transient behaviours,Ceiling height, Wall temperature.