Experimental Study On Flame Ejecting Behavior Of Carriage Fires And Evolution Characteristics Of Thermal Smoke Layer Thickness Under Different Blockage–fire Distances In A Longitudinal Ventilation Tunnel

Since the new century,the transportation industry has developed rapidly with the growth of China's economy.Tunnels play an increasingly important role in mountainous areas,river crossings,sea crossings and railway transportation.The tunnel safety operation problem cannot be ignored.Because the tunnel is a long and relatively closed space structure,once the tunnel fire accident occurs,it is easy to cause a large number of casualties and serious property damage.The longitudinal ventilation system of tunnel plays an important role in controlling the spread of smoke plume and maintaining the environment for emergency ventilation in the tunnel.This paper focused on the ejected fire plume behavior from carriage in the longitudinal ventilation tunnel and the evolution mechanism of the smoke layer under the blocking condition.Firstly,the fire occurred inside carriage in the tunnel,it is taken as the starting point of the research.By theoretical analysis and reduced scaled tunnel experiment,the critical state characteristics of ejected carriage fire plume,ejected flame shape and the effect of blockage–fire distances on the evolution characteristics of the smoke layer thickness in the ventilation tunnel were studied.The initial fire combustion and smoke movement characteristics of the carriage in the longitudinal ventilation tunnel were clarified and revealed,which can provide a theoretical reference for the smoke control design in the tunnel.The main research contents of this thesis were as follows:In this paper,the intermittent flame ejecting behavior of carriage fires was an experimentally investigated and statistically characterized.It was found that the temperature inside the carriage increased firstly and then decreased with the increasing fire heat release rates(HRRs),and finally remains unchanged.The fire heat release rates of critical ejecting state increased with carriage ventilation factor in tunnel and decreased with the increasing longitudinal wind speeds.The mathematical relationship model between the fire heat release rates of critical ejecting state and carriage ventilation factor in tunnel was built,and it has good fitting correlation.Secondly,the evolution characteristics of the ejected flame height and the horizontal length from carriage under different longitudinal wind speeds were revealed by experimental and theoretical analysis.It was found that,for the given same ventilation factor,the ejected flame height from carriage increased firstly with the increasing longitudinal wind speeds,and the ejected horizontal flame length from carriage increased firstly and then decreased with the increasing longitudinal wind speeds until the blowout occurs.At this point,the ejected flame height and flame horizontal length from carriage in the tunnel are reduced to zero.Based on the physical model,the new Froude number was defined and used to correlate the ejected fire plume from carriage in the tunnel.According to the coupling competition between the inertia force induced by the tunnel longitudinal ventilation and the combustion thermal buoyancy of the ejected fire plume from carriage,the ejected flame height from carriage in the longitudinal ventilation tunnel was quantified.The evolution of ejected horizontal flame length from carriage presented a piecewise function relationship.In the end,a dimensionless representation model between ejected flame height,horizontal flame length from carriage and the new Froude number was established.Finally,the evolution characteristics of the smoke layer thickness in the one-dimensional plume region under different vehicle-fire blocking distances in longitudinal ventilation tunnels were studied.It was found that the smoke layer thickness in the tunnel decreased with the increase of the longitudinal wind speeds when the vehicle-fire blocking distances was constant.When the longitudinal wind speed was constant,the smoke layer thicknesses decreased with the blocking distance increased.