Do increased flow rates in displacement ventilation always lead to better results?

Indoor ventilation is essential for a healthy and comfortable living environment. A key issue is to discharge anthropogenic air contamination such as CO22 gas or, of potentially more direct consequence, airborne respiratory droplets. Here, by employing direct numerical simulations, we study mechanical displacement ventilation with a wide range of ventilation rates QQ from 0.01 to 0.1 m33 s−1−1 person−1−1. For this ventilation scheme, a cool lower zone is established beneath a warm upper zone with interface height hh, which depends on QQ. For weak ventilation, we find the scaling relation h∼Q3/5h∼Q3/5, as suggested by Hunt & Linden (Build. Environ., vol. 34, 1999, pp. 707–720). Also, the CO22 concentration decreases with QQ within this regime. However, for too strong ventilation, the interface height hh becomes insensitive to QQ, and the ambient averaged CO22 concentration decreases towards the ambient value. At these values of QQ, the concentrations of pollutants are very low and so further dilution has little effect. We suggest that such scenarios arise when the vertical kinetic energy associated with the ventilation flow is significant compared with the potential energy of the thermal stratification.