| Flame spread of combustible solids is a very common phenomenon in the real fire scenario,which has always been the top priority of fire safety research,so it has attracted a large number of researchers to carry out experimental and theoretical research.The results show that flame spread can occur only if there is some type of communication between the burning region and the nonburning fuel.The critical problem in ascertaining mechanisms of fire spread is to establish the character of this communication.It is necessary to analyze the heat and mass transfer of gas and solid phase in the process of flame spread,and clarify the combustion mechanism as well as the flame spread behavior of combustible solids.This is a significant work for the research of fire science,and can provide strong technical support for the design of fire protection system,fire protection materials,fire detection and emergency rescue in real life.The flame spread will be accelerated and the combustion area will be expanded when the solid combustible materials are ignited and there is external concurrent flow in the fire scenario,which would give rise to more casualties and property losses.However,previous theoretical models and experimental studies mostly focus on the flame spread under the condition of opposed flow.Meanwhile,the theoretical and experimental research on the concurrent flame spread is relatively inadequate,which deserve futher study.Moreover,the size and placement of the combustible solids material are various in real life(spread configuration,inclination angles et.al.),which would affect the heat transfer from the pyrolysis zone to the preheating zone,and play a significantly important role in the spread and development of the flame.The influence of external environment wind,geometric structure of combustible solids and placement method on concurrent flame spread behavior are rarely studied.Meanwhile,in order to reveal the control mechanism of concurrent flame spread regarding combustible solids,the polymethyl methacrylate(PMMA)was utilized to carry out experiments of solid flame spread,which aims at investigating the influence of material thickness,and inclination angle on the concurrent flame spread process with different spread configrations(ceiling;floor;lateral)under the action of environmental wind.The research on flame spread behavior in this paper mainly has three aspects:The influence of ambient wind speed and sample thickness on the concurrent flame spread of ceiling configration;Secondly,experimental study on flame spread behavior of thermal thin PMMA with different inclined angles under concurrent airflow;Lastly,the experimental research on the lateral and horizontal flame spreading behavior under the action of environmental wind.The main content shown as below:(1)The influence of concurrent airflow and sample thickness on the concurrent flame spread of ceiling fire was studied.The experiments were carried out in a combustion wind tunnel,which concentrate on investigating the flame spread behavior of solid materials in the ceiling configration under the action of different wind velocities and sample thicknesses.The characteristic parameters such as flame spread rate(FSR),pyrolysis zone length and average heat flow in pyrolysis zone were measured.The results showed that the FSR increased with the increasing concurrent wind velocity and also presented that the FSR decreased rapidly first and then tended to be slowly with the increase of sample thickness.The length of pyrolysis zone increased monotonically with the increase of thickness.Meanwhile,it increased monotonously with the increase of wind velocity for small thickness samples,but decreased a little bit first and then increases for large thickness samples.With the increase of air velocity,the average heat flux increased with the increasing wind velocity,but decreased with the increase of thickness.A simplified model of material surface heat exchange in the pyrolysis region was established based on the theoretical analysis of energy conservation and heat transfer.The internal heat loss of solid phase in the pyrolysis zone was calculated by utilizing the experimental data,and it was revealed that the heat loss of solid phase would become significant with the increase of sample thickness.(2)The coupling effect of horizontal wind and inclination angle(0-90°)on the concurrent flame spread behavior of thermal thin PMMA was studied.The results showed that the FSR increases linearly with the wind speed,and there was a competition mechanism between the buoyancy acceleration effect caused by inclination angle and the cooling effect of ambient wind.For horizontal flame spread,MLR of unit area increased first and then tended to be steady with the increase of wind speed.The MLR decreased first and then tended to be stable with the increase of wind speed when the fuel inclined.The proportion of radiant heat flux in preheating zone and heat flux distribution in pyrolysis zone were analyzed based on this.The study found that the flame length has a linear relationship with the pyrolysis length,and the heat release rate per unit width increased exponentially with the pyrolysis length.Futhermore,the empirical relationship between the dimensionless flame length and the dimensionless heat release rate was obtained.At the same time,it was observed that the flame standoff distance increased first with the pyrolysis length,and then tended to be stable.The spatial heat flux distribution in front of the burning surface was obtained by dimensionless method.Finally,the prediction model of flame spread rate coupled with different inclination angles and wind velocities was given based on quintiere's theory.(3)The difference and mechanism of flame spread behavior and combustion characteristics for lateral and horizontal solid(thermal-thick PMMA)flame spread under the effect of different wind velocity were analyzed,including the characteristic parameters of flame spread velocity,pyrolysis length,and angle of pyrolysis end and pyrolysis front,flame boundary layer thickness,material thickness and so on.The results showed that the process of lateral concurrent flame spread has transition stage and steady spread stage under the action of environmental wind.TThe difference between lateral and horizontal flame spread under the action of environmental wind is found and characterized The horizontal flame spread speed increased linearly with the wind velocity under the action of ambient wind,while the lateral flame spread speed was approximately unchanged with the wind velocity.The heat transfer for the lateral flame spread increased with the increase of ambient wind velocity.Meawhile,the heat loss would also increase due to the cooling effect of ambient wind velocity,so that there will be extinction at high wind velocity.The characteristics of heat transfer in the pyrolysis region was analyzed based on the calculation of experimental data.Futhermore,the mechanism of the effect of ambient wind on the burning rate was revealed. |
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