Characteristic length scale of critical ventilation velocity in tunnel smoke control

This paper investigates the characteristic length scale in an analytical correlation of critical ventilation velocity. The critical ventilation velocity is defined as the minimum airflow velocity to prevent smoke backlayering and is often used for smoke control in tunnels. Using a one-dimensional assumption of uniform mixing, the correlation of critical ventilation velocity was derived from the Froude number, which considered tunnel height as the characteristic length scale. Using numerical modelling, this study examines the effects of enclosure blockage ratio and tunnel width or aspect ratio on critical ventilation velocity. In particular, the results suggest that the correlation with tunnel hydraulic diameter may provide a better characterization of critical ventilation velocity. This is supported from the experimental data reported by others on the effect of tunnel width. The practical implications of using hydraulic diameter in determining critical ventilation velocity are discussed.