Experimental Studies On Influences Of Tunnel Sidewall On Characteristics Of Combustion And Ceiling Jet Of Oil Pool Fires

With the speeding up of urbanization,various kinds of tunnels are springing up and playing an increasingly prominent role in the transportation sector in response to the rapid economic development and the shortage of land resources.Compared with other forms of construction,tunnel interior space is relatively narrow and dark.With large traffic volume and fast car speed,it is easy to happen traffic accidents and cause a fire.Because it is difficult to discharge the smoke in a long and narrow space,the toxic and harmful substances accumulate quickly,and the strong radiation from the high temperature gas and flame makes fighting and evacuation hard,which usually result in heavy casualties.Previous researches on tunnel fires usually assume that fire is located in the center line of the tunnel.In fact,fire accidents caused by tunnel vehicles and fuel oil leakage are random and could occur at any position.Currently,the study on pool fires against the wall and forming ceiling jet flame is less,and the effect from fire source shapes is insufficient.Experimental studies were carried out for the square,rectangular and linear pool fires flush with the sidewall to explore in depth the combustion and the ceiling jet flame characteristics for different fire source shapes.Based on the experimental research on the square and rectangular oil pool fires attaching the sidewall,with decreasing effective ceiling height,it is shown that the mass loss rate of fuel increases and the combustion time decreases.It is because that due to the restriction effect from the sidewall and ceiling,the volume of fire-induced air entrainment reduces resulting in less heat loss,and the heat radiation from the ceiling jet flame,hot smoke gas and sidewall enhances,which causes the fire burning rate increases.For rectangular pools,the mass loss rates with the long side against the sidewall is greater than that with the short side against the sidewall.With the decrease of the distance between fire source and ceiling the confinement degree becomes more obvious.An equation predicting the mass loss rate per unit area is developed which considers a dimensionless effective ceiling height and the ratio of pan edges as well as which side of rectangular pan is attached to the sidewall.Based on the experimental results and some empirical models,a correlation is established between the dimensionless longitudinal length of ceiling jet flame and the dimensionless heat release rate.The correlation takes into account the comprehensive effects of heat release rate,effective ceiling height,pan size and layout.The maximum ceiling jet temperature for sidewall fires in a tunnel with different effective ceiling height are also analyzed.When the fire source is closer to the ceiling,the ceiling temperature is higher and the high temperature zone is expanding.For square fires,the measured maximum ceiling jet temperatures are higher than those predicted by the formula of Li and Ingason and less than the values obtained by the mirror theory.It suggests the sidewall fires have higher the maximum ceiling jet temperatures compared to the fires far away from the sidewall and the mirror model would significantly over-predict the maximum ceiling jet temperature values.Rectangular pool fires with long side against the sidewall have higher maximum ceiling temperature than that with the short side against the sidewall.The dimensionless maximum ceiling jet temperature rise for flame intermittently and constantly hitting the ceiling can be expressed in a similar way as McCaffrey's model suggests.Thus a formula for calculating the maximum ceiling jet temperature considering the effects of the heat release rate of fire source,effective ceiling height,the aspect ratio of pan edges and pan layout way is proposed.In order to study the effect of the tunnel structure size on the combustion characteristics of sidewall fires and ceiling temperature distribution,a series of small-scale experiments were conducted for sidewall fires under adjustable tunnel width.Results show that in the experimental-set width range,the changes for the fire source burning rate,ceiling jet flame length and ceiling temperatures near the fire source are insignificant.When the fire source is closer to the ceiling,the burning rate is greater due to the enhanced heat radiation from the hot gas under the ceiling and the heated ceiling and the sidewall near the fire source.The ceiling temperatures in the downstream of the fire source increase with the decreasing of the channel width.When the tunnel width decreases to 0.75 and 0.5 m,at the positions with a certain distance longitudinally and transversely away from the fire source,the temperatures below the ceiling increase more obviously,and at the transverse position,the temperatures below the ceiling are more significantly affected by the tunnel width compared with the longitudinal direction.Then we study the sidewall effects on the combustion and ceiling jet characteristic for linear oil pool fires.For the larger line pool fires with the flame directly impinging the ceiling,through small-scale experiments,the tunnel sidewall effects on the fire source burning rate,ceiling flame length,ceiling temperature distribution and the radiant heat feedback on the floor are analyzed.Results show that for line fires with long side against the sidewall,because of the limitation of sidewall on flame entrained air,the flame is attached to the vertical sidewall and the ceiling flame needs to extend a long distance to complete the combustion so that the flame length is greatly increased.The ceiling jet flame,as well as the flame heated wall and ceiling increase the heat feedback to the fuel.However,for line fires,when the long side is flush with the sidewall,the mass loss rate is less than the condition with fire located at the centerline of the tunnel.When the short side is flush with the sidewall,the mass loss rate is greater.This is due to the comprehensive effect by the sidewall limited entrainment and enhanced radiation.For the ceiling flame length and the radiation distribution at the floor,it is fit using mirror model assumption for line sidewall fires.The ceiling buoyant plume temperature distribution is not in conformity with the mirror model hypothesis.The correlation of ceiling temperature distribution,the relation between the ceiling flame length and the fire source power of per unit length,as well as the correlation between the received radiation heat at the floor and the dimensionless flame length are established.Along with the rapid development of transportation and petrochemical industry,in the process of fuel transport and use,oil spill occurs frequently.Among tunnel fires,there are quite a number of accidents due to leakage of transport tank car or vehicle engine.Usually,Tunnels exists a certain slope due to the construction need and geographical restrictions,and drains channel are often set on both sides.Leakage of fuel oil will flow with the tunnel slope or channel,and this would expand the fir scope and ignite combustible materials surrounded.Currently the study of liquid fuel flame spread focus on open space.This study first through small-scale experiments to study the flame spread rate over a narrow pool located,respectively,in the center of tunnel and flush with the sidewall,analyzes the influence of the tunnel sidewall for different thickness of fuel.Results show that the sidewall effect makes the spread rate increases for thin fuel layers,and decreases for thick fuel layers.To rule out the effect from the pool edge above the fuel layer,the fuel was slick with water layer to keep the oil at the fuel surface at the constant height,the result also suggests that the rule for sidewall effect is consistent.In spite of the one dimensional flame spread stage with low flame height,the mass loss rate and radiation values are small.The sidewall can still affect the fire spread by changing the flame shape.In addition,the sidewall exerts a significant effect on the mass loss and radiation values at the combustion stage for the different fuel thicknesses.Then in view of the large area leakage of fuel,the flame radiation effects on two-dimensional fire spread is studied.Through an one-meter diameter round pan in the open space,the experimental results show that for thin oil layer the flame radiation can dominates the heat transfer in short distance,while for thick oil layer the flame radiation need to increase the fuel surface temperature rise by long distance heating zone to become the main mode of heat transfer.Therefore,it can speculate that in the tunnel due to the existence of the ceiling and sidewall,the influence of radiation will enhance.