Experimental Study On The Law Of Flashover In Low Atmospheric Pressure Confined Space

Flashover is an important sign that the fire in confined space develops from local to global.As a typical confined space,aircraft cargo compartment has low internal atmospheric pressure,poor ventilation conditions,many kinds of combustibles and large fire load.In case of fire,combustible smoke will rapidly accumulate at the top and develop to flashover in a short time,which will seriously threaten the life and property safety of the personnel on the aircraft.Moreover,there are a large number of high altitude airports in China.The fire behavior,buoyancy and flame structure under high altitude and low pressure environment are different from those under normal pressure,and the critical conditions for flashover will also be different.Therefore,it is of great significance to carry out experimental research on the flashover law in low pressure confined space.In this thesis,the 1/4 volume FAA standard aircraft cargo cabin is used to independently design and build a confined space flashover experimental platform.N-heptane,aviation kerosene and anhydrous ethanol were selected as the ignition sources,and the single wall corrugated box was ignited as the criterion for flashover.A series of confined space flashover experiments were carried out at two atmospheric pressures of Guanghan（96 k Pa）and Kangding Airport（60 k Pa）.The average temperature of the hot smoke layer in the confined space,the radiant heat flux on the floor,the mass loss rate of the fire source,the heat release rate of the fire source and the smoke concentration were measured and calculated,and the fire development process was recorded.It is found that the critical condition for flashover initiation at low pressure is higher.The critical condition for flashover under 96 k Pa atmospheric pressure is that the average temperature of the hot smoke layer reaches 454℃,and the radiant heat flux received by the floor reaches 14.4 kw/m².At 60 k Pa atmospheric pressure,the above two values are 551℃and19.6 kw/m² respectively.The flashover of confined space fire can be divided into four stages:incubation stage,outbreak stage,complete development stage and extinction stage.The increase of the size of the oil pan will increase the average temperature of the hot smoke layer and the radiant heat flux on the floor,and improve the possibility of flashover and the intensity of flashover.After reaching the critical oil pan size required to initiate flashover,continue to increase the oil pan size,and the flashover time will be advanced.By fitting the critical oil pan size required to initiate flashover with the thermophysical parameters of the fire source,it is found that the critical oil pan size required to initiate flashover is inversely proportional to the combustion heat of the fire source and is directly proportional to the evaporation heat of the fire source.The occurrence of flashover will cause the thermal feedback received by the fire source to rise suddenly,so the heat release rate of the fire source will change abruptly and increase rapidly in a short time.With the further intensification of combustion,the fire source will be heated to the overall boiling state.At this time,bubbles will be continuously generated at the solid-liquid interface at the bottom of the oil pan,which is the so-called nucleate boiling.This phenomenon causes the fire source to splash out of the oil pan,and then the heat release rate of the fire source reaches the peak.The occurrence of flashover will consume O₂ faster and produce higher concentrations of CO₂ and CO.The amount of O₂ consumed by pool fire during combustion is related to the hydrocarbon content in the fire source.The higher the content of hydrocarbon in the fire source,the more O₂ is consumed in the combustion process,and the more CO₂ and CO are produced.Based on the classical nonlinear dynamic model of flashover,the calculation parameters of the nonlinear dynamic model such as the size of the confined space,the size of the opening,the parameters of the fire source,the fluid parameters,the smoke parameters,the thermal conductivity and the thermophysical parameters of the fire source are coupled with the low pressure.The calculation results of the prediction model at 60 k Pa atmospheric pressure are solved and compared with the experimental results.It is found that the modified nonlinear dynamic model can accurately predict the occurrence of flashover.