Enhancing understanding of diffusion fire flame geometry in longitudinally ventilated tunnels: Considering the confinement effect of tunnel space

Spill fires during fuel transportation can be extremely catastrophic when they occur in tunnels due to the confined space. Understanding flame behavior is essential for managing the associated fuel management and thermal processes. This article investigates the flame base drag length and flame tilt angle of diffusion flames resulting from spill fires in tunnels, with an aim to shed light on how the confinement effect in the tunnel space affects the diffusion fire flame morphology. The results show that tunnel walls and ceiling influence flame geometry by increasing the flame tilt angle and extending the flame base along the tunnel floor. In addition, the tunnel cross-sectional aspect ratio, i.e., the height to width ratio, has been determined as an additional influencing factor, particularly affecting the flame tilt angle. Notably, the flame tilt angle tends to increase as the tunnel height increases, while maintaining a constant cross-sectional area. The proposed correlations also suggest that the equivalent ventilation rate, used to correlate flame geometry in open spaces, is approximately 0.91 times the reference wind speed measured at a height of 2 m above the ground in open fields. The findings provide an accurate method for depicting flame morphology in tunnels and for harmonizing flame geometry correlations across diverse indoor and outdoor environments.