On The Characteristics Of Smoke Stratified Transportation With Smoke Control In Tunnel

Urban transportation facilities have improved significantly with urbanization of China. A large number of tunnels have been built in recent years. The usage of urban tunnel have reduced the cost of transportation and boosted development of the economy. However, usages of tunnels have also brought some new problems of fire prevention and passenger protection. Statistics [1] have shown that smoke and toxic gases are the most fatal factors in fires; about 85% of people killed in building fires are killed by toxic smoke. In a tunnel fire, more toxic gases are produced because combustion is incomplete due to lack of oxygen supply. Taking appropriate measures to control dispersion of smoke in case of a fire is a serious concern for smoke management in tunnels.With the increasing demand for higher safety in tunnels, more reliable measures need to be taken for smoke control in events of tunnel fires. Protective effects of smoke control measures have drawn much academic attention. In this context, understanding of characteristics of smoke flow and its interaction with controls such as ventilation and mitigation is necessary. The aim of this study is to examine variations of smoke flow characteristics with smoke control in tunnel on the basis of a two layer model.Firstly, both experimental and theoretical studies were conducted to investigate characteristics of buoyancy driven smoke flow in tunnel. Vertical temperature distributions in tunnel cross-sections and longitudinal temperature distributions were presented. Temperature at each point of the tunnel can be deduced from empirical correlations reported in this work. Theoretical studies of heat flux and radiant flux were performed, based on results of smoke layer characteristics.Secondly, effects of mechanical extraction and ventilation on smoke flow were studied experimentally. A linear correlation is proposed for representing the relationship between defined interception velocity V * and interception distance L. Effects of heat release rate on interception velocity were also deduced as a power law correlation. Moreover, actuation of smoke extraction system can decrease smoke temperature and raise smoke layer height, which can mitigate the radiation of smoke layers to ground level targets.Thirdly, interactions of water mist spray and smoke layer were investigated. Cooling effects of water mist were studied theoretically and experimentally, and empirical models were proposed for cooling of smoke with water mist under three different situations: thin smoke layer, thick smoke layer and heavy smoke layer. The theoretical analysis has revealed that cooling efficiencies increase with increase of tilt angles. Heat absorbed by water mist increases with increase of water flow rates. Heat absorbed by water mist also increases linearly with increase of the convective heat release rate.Finally, efficiency of water mist in diluting smoke density was investigated parametrically. Removal efficiency was found to increase with increase of water spray pressure. The mechanisms of interaction between water mist droplets and smoke particles were presented. Particles clinging to water droplets are dependent on surface tensions between particle/air, particle/water and air/water.This work presents correlations of distributions of smoke temperature in tunnel on the basis of a two layer model. Results indicate that distributions are yield to exponential functions. Correlations between interception distance and interception velocity were derived for different heat release rates. A numerical model was proposed to analyze the heat absorbed by the water mist system. A simpler model was introduced to analyze smoke particles clinging to water droplets. It was found that the maximum value of removal efficiency could reach 99.7% when water spray pressure was relatively high. However, smoke cluster sizes with water mist were 10 times larger than without water spray.Other issues discussed in this work include deflection of flames in the tunnel due to the high rate of heat release by fire, mass loss rate due to smoke extraction and radiant heat transfer from smoke to the exposed objects, the plug-holing effects on smoke extraction and smoke density variations on actuation of water mist.