The Transition Mechanism Of Sub-atmospheric Flame Spread Over Thermally Thin Material From Extinction Limits To Power Law Region

More and more researchers begin to pay their attention to the sub-atmospheric fire safety,such as plateau and aero spacecraft.Flame spread over solid surfaces in low air pressure is a basic issue in fire safety.The development of flame spread influences the initial growth of fire and the final fire scale.Investigating the sub-atmospheric flame spread behaviors over solid surfaces is helpful to the low air pressure environment's fire prevention and controlling.A thorough literature review has indicated that there are significant limitations in studies on the mechanism of sub-atmospheric flame spread rate over thermally thin material from extinction limits to power law region.The paper is chosen as the typical combustible solid and the sub-atmospheric chamber as well as flame spread experimental system is also designed and built.Experiments are carried out with various pressures and oxygen concentrations.In these experiments,some typical features(flame spread rate,flame image,flame radiation and flame temperature.)of flame spread over paper samples are measured.The mechanism of flame spread phenomenon is investigated based on the experiment data as well as the theories of heat transfer and combustion.Firstly,the flammability map depending on pressure and oxygen concentration is obtained.The flame spread rate,flame thickness as well as flame length with limited oxygen concentration generally decrease with the air pressure,where there is a marked increase near 30kpa.Secondly,the influences of pressure and oxygen concentration on flame spread are studied respectively.It is found that when pressure above 25 kPa,the flame spread rate begins to increase linearly,the inner blue flame turns to be yellow while flame becomes more-luminous and the flame shape changes from ellipse to triangle shape,and the flame radiation increases significantly.Finally,based on the experimental results and data of literature,the mechanism of sub-atmospheric flame spread rate over paper from extinction limits to power law region is studied.By analyzing the thermal boundary thickness and heat feedback,the transient boundary of 25 kPa is proposed and a global mode for sub-atmospheric flame spread rate over horizontal thin paper sample is also built.