Comparison of displacement ventilation and mixing ventilation systems with regard to ventilation effectiveness in offices

Air quality in offices depends on the ventilation system ability to remove contaminants from the occupied zone. In a low polluted building air quality mainly depends on the human presence and carbon dioxide is normally used as indicator of human bioeffluents.
The aim of this paper is to investigate, by using computational fluid dynamics (CFD) methods, the effect of the supply and exhaust positions on the contaminant distributions in an office equipped with a ceiling cooling system. Mixing ventilation was compared with different displacement ventilation solutions, adopting either floor or wall mounted displacement diffusers. Exhaust vents in the mixing ventilation cases were placed directly under the supply or on the opposite wall; in the displacement ventilation cases, exhaust vents were placed on the upper part of a sidewall or on the ceiling in different positions. Besides the percentage of dissatisfied PD [%] and the contaminant removal effectiveness, a discomfort index for the whole office was introduced and calculated. It resulted that, for the considered scenario, displacement ventilation performance was very sensitive to the position of exhaust grilles; moreover, it was found that displacement ventilation does not result always in better air quality than mixing ventilation.