Studies On Effect Of Environmental Wind Velocity And Wire Separation Distance On Flame Spread Behaviors

Fire safety is one of the most sensitive issues in man-made space missions.Accidental arc,short circuit or overheating may ignite the insulation layer of combustible wires and cause fire,resulting in a serious safety hazard.In particular,in a closed environment,such as a manned spacecraft,there is no escape space,in previous space missions,there were several accidents,which due to short circuits or local overheating wires.First,from the national strategic level,China is now vigorously developing aerospace technology,such as "Shijian"series of recoverable satellites,space stations and large transport aircraft,etc.From the point of view of domestic and foreign aerospace practices,it is also true that there have been aerospace accidents caused by short circuit of electric wires,such as the air crash of Swiss airlines flight 111 in 1998 and the Apollo 1 space accident in the United States.In terms of aerospace fire research,China's basic theory,prevention and control technology and product performance are seriously insufficient,while European and American countries have strict technical monopoly on China.Therefore,we must develop the aerospace fire research vigorously,and reverses our country technology backwardness situation.Once the wire is ignited,with external airflow,flame spread rapidly along the wire,producing more smoke and toxic gas,causing greater damage;In real life,a large number of wires were used and wire installation layout is also ever-changing,different wire installation also has an important impact on occurrence and development of wire fire.At present,there are few studies on the influence of external wind velocity and wire separation distance on wire fire.The research on wire fire in this paper mainly has four aspects:Evolution characteristics of ignition temperature and ignition delay time of wire ignition under ambient wind;Secondly,study on flame spreading and extinguishing behavior of horizontal electrical wire under horizontal concurrent wind;Thirdly,study on the flame spreading and extinguishing behavior of inclined wire under horizontal wind;Finally,study on interaction and heat transfer mechanism in the flame spreading process of parallel twin wires with different separation distance.The specific research content is as follows:Firstly,the characteristics of ignition temperature and ignition delay time under different wind speeds were studied.By combining experimental research and theoretical analysis,In order to carry out the relevant research in this paper,the author developed a small combustion wind tunnel and established a test device for wire ignition under ambient wind,five typical wires were used for conducting wire ignition experiments,characteristic parameters(wire ignition temperature and wire ignition delay time)of wire ignition process were measured,At the same time,the influence of the environmental wind velocity on the wire ignition behavior was analyzed.Combined with the knowledge of heat and mass transfer,the ignition temperature evolution rule of wire surface with different environmental wind velocity was studied.The prediction model of ignition delay time was established,and heat transfer law of the inner core of the wire during the ignition process under ambient wind is analyzed.Secondly,the flame spreading and extinguishing behaviors of the horizontal electrical wire under the horizontal wind were studied.In order to study the influence of concurrent wind velocity,the experiment was carried out in wind tunnel,which provided uniform and stable wind velocity for concurrent spreading of the horizontal electrical wire experiment.Four typical types of aviation wire(with different insulation thickness and inner core diameter)were used in the experiment,and the influence of geometric structure on the flame spreading and extinguishing behavior over wires in different concurrent wind velocity was investigated,the flame spread rate was measured,a flame spreading model based on heat transfer analysis was established,and the relationship between the maximum flame spread rate and the Froude number(Fr)was analyzed.Through the experiment,it was found that the concurrent wind had a significant promoting effect on the flame spread rate,revealing the heat transfer rule of the flame spreading over the wire under horizontal downwind.The flame blow-off mechanism was analyzed by using the Damkohler number and the heat loss of the solid phase.Thirdly,the flame spreading and extinguishing behavior of inclined electrical wire under horizontal wind were studied.Based on previous studies,it was found that the concurrent wind had a significant effect on the flame spreading,but the interaction between the wire inclination and the horizontal wind speed was not well understood.In order to study the interaction of wind speed and wire inclination angle,wire flame spreading and blow-out experiment,with the horizontal wind from 0(static)to 2.5 m/s,inclination angle from the horizontal(0°)to vertical wires(90°),were carried out.In this paper,the flame spread rate and geometrical characteristics are quantized as functions of wind speed and wire inclination.The results show that with the increase of the horizontal wind velocity,the flame spread rate along the wire at each inclination angle,first increases to the maximum value,and then decreases slightly until the flame is blown off.The variation of flame spread acceleration point with inclination angle and the flame shape at the maximum rate of fire propagation were studied.The interaction between fuel inclination and wind speed is quantified by using Froude Number.In addition,with the increase of wire inclination,the blow-off wind velocity decreases,and the critical strain rate are used to analyze the flame blow-off mechanism.Last,flame spread behavior characteristics of horizontal twin wires was studied,and the experiment was carried out using self-designed twin wires experimental device.The characteristic length of flame(flame height,flame base width)was measured experimentally,at the same time,the flame spread rate and average mass loss rate were compared,and the influence of different wire separation distance on flame spread was analyzed with on the mass loss and energy transfer.Combined with the characteristic length of flame in the process of flame spreading over twin wires,based on the analysis of convection and radiant heat feedback,the transition of interaction of flame spreading over wires with different wire separation distance were divided into five stages,which revealed rules of flame interaction and mutual heat transfer.