Investigation on the critical shaft height of plug-holing in the natural ventilated tunnel fire

In order to study the plug-holing of vertical shaft with different fire-shaft distance in road tunnel, a series of numerical simulations about tunnel fire were carried out. The results have shown that the smoke layer thickness under the ceiling increases with the increase of smoke spreading distance and it is mainly affected by tunnel width when the fire heat release rate is in the range of 3-15 MW. In the wider tunnel, the dimensionless smoke temperature rise decays faster in exponential. While the smoke velocity shows a well correlation with the smoke layer thickness and dimensionless temperature rise beneath the ceiling. Both the wider tunnel and larger area of shaft vent contribute to the occurrence of plug-holing. The fire heat release rates have little effect on the critical shaft height of plug-holing, which is mainly controlled by smoke layer thickness, area of shaft vent, tunnel width as well as the side length of shaft vent. On the whole, the influence of aspect ratio of shaft vent on the critical shaft height of plug-holing is complex. Based on the theoretical analysis and simulation results, a new prediction model considering the factor of fire-shaft distance is deduced to predict the critical shaft height in the tunnel with natural exhaustion through vertical shaft.