Optimal ventilation rate for effective displacement ventilation

TL;DR

This study uses direct numerical simulations to analyze how varying air change rates affect displacement ventilation efficiency, revealing optimal conditions for contaminant removal and challenging the assumption that stronger flow always improves ventilation.

Contribution

It provides a physics-based understanding of the relationship between ACH and interface height, offering guidelines to optimize displacement ventilation systems.

Findings

Interface height scales as ACH^{3/5} at low ACH

CO2 concentration decreases with ACH in weak ventilation

Beyond a threshold, increased ACH does not improve contaminant removal

Abstract

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.