On the backlayering length of the buoyant smoke in inclined tunnel fires under natural ventilation

Abstract

In this study, simple analysis, numerical simulations and verified experiments are conducted to investigate the smoke backlayering length in inclined tunnel fires under natural ventilation. The backlayering length is the result of the competition between the inertia force of longitudinal flow induced by the stack effect and the buoyancy force of the backlayering flow. The results indicate that the longitudinal velocity upstream of the fire source is determined by the fire location, the tunnel inclination angle, and the source buoyancy flux. Furthermore, the tunnel inclination angle and the fire source location have a significant impact on the backlayering length. The increase in the tunnel inclination angle or the distance between the fire source and the downstream smoke outlet can enhance the stack effect and therefore give rise to a reduction in the backlayering length. However, the effect of the heat release rate of the fire on the backlayering length is negligible, especially when the stack effect is significant. A model regarding the backlayering length is proposed, which quantitatively demonstrates that the backlayering length can be estimated as long as the tunnel inclination angle and the fire location are determined. The study is useful for emergency rescue in inclined tunnel fires.

Keywords

• Backlayering length
• Inclined tunnel
• Fire location
• Natural ventilation
• Longitudinal ambient velocity