Ventilation inlets design based on ventilation performance assessment using a dimensionless time scale

Abstract

Ventilation configurations greatly influence indoor airflow and pollutant dispersion. This work aims to investigate the effect of vent inlet size (indicated as size ratio D/L, inlet vent length to the cubic root of the room volume) on ventilation performance. A dimensionless time scale ratio (proportional to $Ri/\alpha$ , Richardson number Ri, mass flux ratio α) was proposed to represent ventilation performance assessment. Both methods of experiment and simulation were employed. Studies were conducted for five configurations with various inlet sizes (i.e. D/L ratios equal to 0.2, 0.4, 0.6, 0.8 and 1) under three different air change rates per hour (ACHs; 4, 10 and 16). When keeping the same ACH, the smaller the inlet size (0.2 to 0.6), the larger the inlet ventilation rate. The larger the ACH, the smaller the average CO₂ concentration at the outlet, but not within the occupied zone (for D/L ratios of 0.8 and 1, CO₂ concentration firstly decreased with ACH of 10 and then increased with ACH of 16). The results demonstrated that $Ri/\alpha$ is smaller for cases with a small size ratio (with the critical value below 10⁻³) than with larger ones, showing a better pollutant removal performance. These findings can facilitate and improve the initial design for building ventilation.