Horizontal And Inclined Flame Spread Behavior Over Electrical Wire With Different Thermal Conductivity Of Inner Core Under Sub-atmospheric Pressures

Developing manned space technology and further developing and utilizing space resources are among the important strategic objectives of our country.In recent years,China's aerospace industry has made breakthrough progress and made great contributions to the national economy and national defense construction.Aviation wires are densely packed between the electronic devices in the aviation spacecraft,which is likely to cause fire due to faults.Therefore,it is of great practical significance to carry out research on the flammability safety test and flame spread behavior of aviation wires.Flame spread over electrical wire is different from the flame spread over general flammable solid surface,because the thermal conductivity of the metal core is much larger than that of common combustible solid materials.The heat transferred to the preheating zone through conduction of the metal core as an additional heat transfer path will greatly affect the flame spread characteristics over electrical wire.Therefore,the wires with different thermal conductivity cores are selected in this paper,focusing on the flame spread behavior of high thermal conductivity copper wire.Meanwhile,through investigations of previous studies,it is found that the inclination of the wire is very important in the flammability test.And the actual application scenarios of electrical wire mostly under low-pressure environments have a significant influence on the flame shape.In summary,it is necessary to carry out research on the flame spread behavior of horizontal and inclined electrical wires with different thermal conductivity metal core under low pressure.Polyethylene(PE)insulated Cu and NiCr core electrical wires were studied with ambient pressure ranged from 100 kPa to 40 kPa and inclination angles ranged from-750°to+75°.The experiment was carried out in a closed low-pressure chamber,using a self-designed test platform for ignition and flame spread of electrical wire with variable inclination.The present study revealed experimentally flame spread behavior over high-conductivity copper(Cu)core electrical wire in sub-atmospheric pressures,and compared those with relative low-conductivity nickel-chrome(NiCr)core electrical wire.Results showed that the blue flame height(or the 6"flame standoff distance"from the bottom edge of the high temperature yellow flame region to wire surface),and the flame base width both increased as the pressure decreased.The flame spread rate(FSR)of NiCr-core electrical wire just increased;however,that of Cu-core electrical wire first decreased then increased as the ambient pressure decreased.The critical pressure for the turning of FSR variation of Cu-core wire with pressure was larger as the core diameter was smaller or the insulation was thicker.The "wire-driven" and"flame-driven"heat transfer mechanisms in controlling the flame spread behavior over electrical wires with metal core were well verified by the experimental results shown in the present study.The relative importance of "wire-driven"heat transfer mechanism decreased,while that of "flame-driven" increased,as the ambient pressure is reduced.NiCr-core wire is controlled by "flame-driven" heat transfer mechanism,so its FlSR always increases with pressure reduction;however,for Cu-core,FSR is controlled by the balance of "wire-driven" and "flame-driven".The evolutions of"wire-driven" and "flame-driven"heat transfer mechanisms are further revealed and quantified in supporting flame spread with pressure decreasing.The study of the flame spread behavior of Cu-core wire under low pressure condition shows that the flame spread rate is the smallest when the wire is placed horizontally.At upward spread or downward spread,the greater the inclination of the wire,the higher FSR will be.The law of change is roughly the letter "U" type.The flame shape and the instantaneous flame base width of slightly inclined wire are affected by the distribution of molten PE into a periodic fluctuation state.The transient displacement characteristics of the flame front and the flame tail are quantified.It is found that as the pressure increases,the fluctuation of the time history curve of the flame front(downward spread)or the flame tail(upward spread)decreases.And the flame shape tends to be stable,the amplitude and frequency of the periodic fluctuation of the instantaneous flame base width curve are also reduced.