The Effect of Aspect Ratios on Critical Velocity in Tunnel Fires

The influence of aspect ratio, A s (height/width for a tunnel of rectangular cross section), on the critical velocity in tunnel fires was studied by computational fluid dynamics (CFD) simulation. According to previous researches, the influence of aspect ratio on critical velocity presents different tendencies. Someone believes the critical velocity increases with A s ; others think that the critical velocity decreases when A s is larger than unity. According to previous experimental results and CFD results, in this paper, it is inappropriate to use aspect ratio as the only factor which affects the critical velocity. Both A s and the tunnel width should be considered. By introducing the characteristic fire diameter, the difference of relationship between critical velocity and aspect ratio is unified, and a new critical velocity model is developed. The critical velocity varies with the one-fifth power of the heat release rate for under-medium fires.