ON THE EFFICIENCY OF DISPLACEMENT VENTILATION, with Particular Reference to the Influence of Human Physical Activity

Abstract: The efficiency of room ventilation by the displacement principle was studied with respect to some influential factors, in particular that of physical activity. The study was experimental and performed in two full-scale test rooms, one of office-size and one of classroom-size. Physical (mainly walking) activity was executed in these rooms by person simulators and by humans. The ventilation efficiency was quantified by tracer gas measurements.In the performed tests, the activity of a walking person generally 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 air from the upper part of the room down to lower levels. It appeared, however, that it takes a rather high level of physical activity to completely abolish the displacement effect. Especially the air quality in the breathing zone of non-moving occupants tended to remain significantly better than at perfect-mixing conditions. Completely mixed room air occurred nevertheless when the activity was intense, but the displacement flow pattern was re-established fairly quickly after ceasing of the activity.The contaminant distribution showed substantial horizontal variations in the "classroom". A contaminant released in the occupied zone was effectively extracted from the room when the source was situated on the same side of the room as the extract terminal(s), whereas, when situated on the opposite side, the contaminant accumulated in the upper part of the room. It was further shown that the air supplied from displacement diffusers tends to reach all occupants fairly quickly, also in relatively large and densely populated rooms such as classrooms.The temperature stratification of the room air, and free-convection currents along the walls are crucial for the appearing air flow pattern and contaminant distribution. Transfer and accumulation of heat in materials tend further to make the thermal conditions indoors more or less transient at all times. This entails, it was shown, that also the ventilation efficiency is time dependent. In tests where people suddenly entered the "classroom", the ventilation efficiency improved with time. This generally caused the highest contaminant exposures to occur at the beginning of the stay in the room.Tests with two different ceiling heights showed lower ventilation efficiency with the higher ceiling, involving a higher contaminant exposure of the occupants. The reason for this appears to be enhanced down-flow along the walls, caused by a higher section of relatively low surface temperature, and possibly by the appearance of merging plumes, enhancing the vertical recirculation process.A wall temperature deviating from that of the indoor air impaired the ventilation efficiency, whereas an increase in ventilation rate improved it. In all test cases in this study, however, the air quality in the breathing zone of seated occupants remained significantly better than that at perfect-mixing conditions – a sports-activity test being the only exception.