Study On Downward Evacuation In Road Tunnel Fire With Coupling Action Of Water Mist And Mechanical Smoke Exhaust

Nowadays, road tunnels have become one of the main transport hubs in city. With the rapid progress of urbanization, more and more road tunnel occurs. Currently road tunnel traffic is growing worldwide for the reason that security issues attendant also has been concerned. When the tunnel fire occurs, effective control of fire development, smoke spreads, to get sufficient time for evacuation, and create the most favorable environment for escape has become an important research topic. Water mist system has several advantages, such as environmental protection, high efficiency, water conservation, good insulation. But the function of water mist will depress smoke, thus increasing the smoke layer. The smoke and fire control in road tunnel fire with the coupling effect of water mist and mechanical smoke exhaust is to be studied, at the same time, the optimization analysis on the space of downward evacuation escape stairs is processed.First of all. the 18MW fire model is established in the 1000m road tunnel by using FDS+Evac software, the spacing of escape stairs of road tunnel with downward evacuation is analyzed. Based on the comparison of the development of temperature field, the spread of smoke, visibility changes, as well as the RSET(Required Safe Egress Time) of different stairs spacing and the Available Safe Egress Time(ASET) under the different spacing of escape stairs, i.e. 60m、80m、100m、120m、140m and 160m, people evacuation simulations of different spacing of escape stairs were processed without any fire prevention measures. Based on the aforesaid fire performance analysis, it is found that the maximum of escape stairs spacing should not exceed 100m under the condition of no any fire protections in 1000m unidirectional single lane tunnel.Secondly, the smoke control in tunnel fire under the central mechanical smoke exhaust effect is analyzed. Fire temperature, coverage of smoke, concentration of CO, as well as the visibility changes is compared in different central smoke exhaust modes, it is found that the central smoke exhaust mode has a better effect on smoke control, and in these conditions,flow velocity 8m/s, vent area 8m2,8 vents is the most effective one.Besides, the effect on fire with the affection of water mist is studied.25m as a protection division of water mist is made, the effect of water mist with the different number of protection division of 2,3 and 4 is studied. By the comparison of the development of temperature, spread of smoke, visibility about these 3 conditions, it is decided to choose 3 protection divisions(75m) as the mode of water mist.Finally, the optimal scheme under the coupling action of water mist system and central smoke exhaust system is studied; the spacing of escape stairs under this condition is further optimized. By comparing the effects of different conditions with the action of water mist and central smoke exhaust, it is found that the optimal scheme is 3 water mist protections(75m), flow rate lOL/min,8 vents, vent area 8m2, flow velocity 8m/s. Under this condition, the further optimization on spacing of escape stairs is completed. When the spacing is 120m, the total Required Safe Egress Time(RSET) is 355s, congestion at each stair is not serious, people of number 4 escape stair finished evacuation within ASET; when the spacing is 140m, congestion at stair 3 and stair 4 is serious, congestion time is longer, and the actual required total escape time exceeds 360s, which cannot meet the safety requirement. So I the optimum spacing of escape stair can be extended to 120m with the coupling effect of water mist and mechanical smoke discharge system.