  Indoor ventilation is essential for a healthy and comfortable living
environment. A key issue is to discharge anthropogenic air contamination such
as CO2 gas or, more seriously, airborne respiratory droplets. Here, by
employing direct numerical simulations, we study the mechanical displacement
ventilation with the realistic range of air changes per hour (ACH) from 1 to
10. For this ventilation scheme, a cool lower zone is established beneath the
warm upper zone with the interface height h depending on ACH. For weak
ventilation, we find the scalings relation of the interface height h ~
ACH^{3/5}, as suggested by Hunt & Linden (Build. Environ., vol. 34, 1999, pp.
707-720). Also, the CO2 concentration decreases with ACH within this regime.
However, for too strong ventilation, the interface height h becomes insensitive
to ACH, and the CO2 concentration remains unchanged. Our results are in
contrast to the general belief that stronger flow is more helpful to remove
contaminants. We work out the physical mechanism governing the transition
between the low ACH and the high ACH regimes. It is determined by the relative
strength of the kinetic energy from the inflow, potential energy from the
stably-stratified layers, and energy loss due to drag. Our findings provide a
physics-based guideline to optimize displacement ventilation.
