Studies On The Development Of Ceiling Jet Flame In Tunnel Fire Under The Function Of Stack Effect

In case of fire in an urban traffic tunnel, due to its structural features, a lot of heat and smoke generated in fire is difficult to discharge. By providing natural ventilation shaft and mechanical exhaust equipment can effectively solve this problem. Because the location and height of fire happened in tunnel are random and uncertain, fire can reach the tunnel ceiling in a relatively short period of time and formed ceiling jet flame when the vehicle run to the shaft and fire height is higher.With the development of fire, the ceiling jet flame spread to the shaft and formed shaft jet flame. Both the ceiling jet flame and shaft jet flame can cause serious damage to the tunnel structure and bring obstacles to the fire rescue work and evacuation. Therefore, it is of great significance to study the ceiling jet flame and shaft jet flame in tunnel fire.This paper is based on the similarity principle of small size 1:10 experimental platform, studying on the characteristics of the development of ceiling jet flame and shaft jet flame in tunnel fire. In the study of ceiling jet flame in tunnel fire, results show that the pool fire combustion process in tunnel fire under the function of stack effect can divided into ceiling jet flame stage, shaft jet flame development stage and shaft jet flame stable burning stage. The mass burning rate in shaft jet flame development stage continued to increase, in the other two stage can be taken as a constant. The stack effect can make the vertical fire plume and plume impingement point inclined to the shaft side, and make the heat release rate increased 2-3.3 times higher than the control experimental group. Both without shaft and with shaft(no shaft side of the tunnel) the dimensionless total flame extension length are all proportional to 2/5 power of dimensionless heat release rate and the coefficient are1.04 and 0.97 respectively. Affected by the stack effect, the ceiling temperature is lower than the control experimental group and appeared a sudden drop in the downstream direction of shaft entrance. The maximum temperature rise of flame zone beneath the tunnel ceiling is inclined to the shaft side. The maximum temperature rise decreases linearly with decreasing dimensionless fire distance from shaft andincreasing dimensionless shaft height.In allusion to the study on development characteristics of shaft jet flame, results show that the combustion form of shaft jet flame can divided into the free diffusion combustion under thermal buoyancy and the turbulent mixing combustion under forced ventilation. Richardson number Ri=1.18 can determine whether the ceiling jet flame plug-holing occurred. The strength of stack effect can judged by the ceiling jet flame plug-holing, namely, stack effect is strong when the ceiling jet flame plug-holing occurred and the opposite one is weak. Stack effect can promote fire combustion and increase mass burning rate, the shaft jet flame will be significantly elongated. The dimensionless total flame extension length in the side of shaft is still proportional to 2/5 power of dimensionless heat release rate and the coefficient is between1.19and1.41.