An experimental study of the effect of the aspect ratio on the critical velocity in longitudinal ventilation tunnel fires

In this study, the critical velocity was investigated with various aspect ratios (0.5, 0.667, 1.0, 1.5, 2.0) of the tunnel cross-section in tunnel fires. Reduced-scale experiments were carried out under the Froude scaling using ethanol pool fire. Temperatures were measured under the ceiling and vertical direction along the center of the tunnel. The effects of aspect ratio on critical velocity were analyzed from smoke movement. It has been found that the critical velocity increases with aspect ratio, where the aspect ratio is height/width for a tunnel of rectangular cross-section. This study used new dimensionless velocity and dimensionless heat release rate parameters considering aspect ratio of the tunnel. The critical velocity varied with the one-third power of the heat release rate for under-medium fires. The results of this study are compared with other small-scale and full-scale experiments. A good agreement has been obtained.