Critical velocity and back-layering length of arched tunnels with large sectional aspect ratios under different fire-source–side-wall distances

Abstract

With the continuous advancement of shield tunneling technology, the size of shield machines has also increased significantly. It is necessary to study the fire behavior in arched highway tunnels with large sectional aspect ratios under longitudinal ventilation. In a vehicle fire, the vehicle typically stops near the tunnel sidewall. This work used experiments and numerical simulations to investigate the influence of different fire source offset positions on the critical velocity in arched tunnels with large sectional aspect ratios. The results showed that: (1) In an arched tunnel, the critical velocity first decreases and then increases with increasing distance between the fire source center and the sidewall. (2) For car fires in arched tunnels, if the fire source is located in the center of the tunnel, the critical velocity is similar to that of the near-wall fire, and it does not always increase as the fire source approaches the sidewall. The predictive model for the critical velocity of arched tunnel offset fires with different sectional aspect ratios has been established. (3) In large section aspect ratio arched tunnels, the influence of different fire source distances from the sidewall on the back-layering length is more pronounced than in tunnels with small aspect ratios. In near-wall fires, the dimensionless control velocity still has a linear relationship with the dimensionless back-layering length in arched tunnels.