Study On The Development Characteristics Of Tunnel Fire Ceiling Jet Flame Under Blocking Effect

With the development of economy,more and more tunnels were built in recent years.The tunnels brings us not only the solution of the traffic demands,but also severe challenges to fire protection.Tunnel is a kind of long and narrow space,if a large fire occures in tunnel,the fire plume will impinge the ceiling easily,and forming a ceiling jet flame.The ceiling jet flame will release a large amount of heat which would result in tunnel structural damage and mass casualties.A great number of researches on tunnel fires without blockage have been carried out in previous literature,and they obtained a lot of empirical conclusions about the influence of tunnel shape,tunnel inclination,HRR,longitudinal ventilation velocity on the inclination angle and the height of flame,the longitudinal temperature distribution and maximum temperature under the ceiling.While in the case of tunnel fire with blockage,previous literature only investigated the effect of blockage cross section,the distance between the fire source and blockage,and the vehicle arrays on the smoke and critical velocity.However,in actual tunnel fire,tunnel ceiling jet flame is equally important to those,therefore,this paper studies the effect of longitudinal ventilation velocity,the relative position and blocking ratio on the flame characteristics of the ceiling jet flame.In this paper,a 1:10 small-scale experimental setup which is based on the similarity criterion was constructed to study the fire behavior and mass loss rate of the tunnel fire with blockage.The results showed that there is a critical distance between the blockage and fire source,and the critical distance is proportional to the longitudinal ventilation velocity.Within critical distance,the mass loss rate decreases with the distance between the fire source and blockage,furthermore,the mass loss rate reaches the maximum when the fire source attached to the blockage,and the mass loss rate is twice that of the non-blocking condition,which greatly increases fire hazard;the flame is stratified and spread along the sidewall,the analysis of the flow field in these conditions shows that the stratification is a special phenomenon of closing-to-wall burning,the former could be only generated by fire plume entrainment which was induced by the vortex and the side wall,while the latter could only be generated by the single impact of the vortex.When the fire source located outside of the critical distance,the flame shape and mass loss rate is similar to that with no blockage.The research on the development characteristics of the ceiling jet flame based on the blocking ratio and the relative position of blockage shows that within the critical distance,the cos? for flame inclination angle and the flame length linearly decrease,HRR ratio and the maximum temperature ratio of cases with blockage to that with no blockage exponentially decrease with increasing the longitudinal fire-blockage distance,respectively.Moreover,with the increase in horizontal distance,the cos? for flame inclination angle increases linearly,HRR ratio and the maximum temperature ratio of cases with blockage to that with no blockage increase exponentially,the flame length first increases and then approaches a constant value,respectively.In addition,the increase of the blocking ratio will also expand the critical distance and enhance longitudinal temperature attenuation.When the fire source located outside of the critical distance,the cos? for flame inclination angle,the ceiling jet flame length,HRR and the maximum temperature are similar to that with no blockage.