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 C-2 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 Q from 0.01 to 0.1 m(3) s(-1) person(-1). For this ventilation scheme, a cool lower zone is established beneath a warm upper zone with interface height h, which depends on Q. For weak ventilation, we find the scaling relation h similar to Q(3/5), as suggested by Hunt & Linden (Build. Environ., vol. 34,1999, pp. 707-720). Also, the CO2 concentration decreases with Q within this regime. However, for too strong ventilation, the interface height h becomes insensitive to Q, and the ambient averaged CO2 concentration decreases towards the ambient value. At these values of Q, 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.