Experimental Study On Pyrolysis And Ignition Of FEP Insulated Wire Under Sub-Atmospheric Conditions

Due to the use of a large number of wires and cables in the spacecraft,the electrical fires caused by the short circuit and overload must be considered in the normal operation of the spacecraft.First of all,the occurrence of electrical fires affects not only the normal function of the spacecraft,but also the astronauts' living environment.Secondly,since the interior of the spacecraft is a microgravity environment,the natural convection is weakened,so the burning of the wires can not be simply explained by the ignition mechanism got on the ground.Finally,the oxygen concentration in the cabin is higher than 21%,which will have an important impact on the combustion of materials.With the continuous development of space science and technology,the era of "space economy" has arrived.At the stage of this era,the fire safety of manned spacecraft becomes critical to the completion of the space mission and the development of China's space strategy.Hence,the ignition of the wire is a key problem which should be paied high attention to.In most of previous experimental studies,PE,PVC and other ordinary wires have been chosen as the object of study,but FEP,ETFE and other high-temperature wires,which are widely used in space,have seldom been chosen,so the wire coated with FEP insulation were mainly studied.The low pressure experiment system is composed of four parts:(1)JRC-500 low pressure chamber,providing a low pressure environment;(2)Experimental control system,including pressure,oxygen concentration and current control devices;(3)Image capture system,recording flame images and the entire experimental process;(4)Carrying platform for the wire.The experimental materials used in this paper are NiCr-FEP wires,and the NiCr-PE wires are mainly used for comparative analysis.The temperature and humidity of the low pressure chamber is 20 ? and 40%,respectively.9 pressure conditions(20 kPa-100 kPa,10 kPa interval)and 3 oxygen concentration conditions(21%,30%,40%)were conducted.The overload current is fixed at 13 A t.The entire experimental process is carried out in the darkroom to avoid the effect of external light sources on the fire image.Through the analysis of experimental results,the following conclusions are drawn in this paper.The pyrolysis process of FEP wire with an overloaded current can be divided into two parts.The first part is the external pyrolysis of the insulating layer,in which the shape of the pyrolysis gas changes with the environmental pressure,and the rising rate of pyrolysis gas increases with the increase of environmental pressure,and the effect of oxygen concentration is not obvious.The other is the internal pyrolysis of the insulating layer,in which the pyrolysis gas generates in the inner of wire insulation and accumulates to form a bubble,and the width of the bubble increases with the increase of pressure,while the height decreases with increasing pressure.When the bubble reaches to the maximum size,it will broke and burst pyrolysis gas,then the phenomenon of fire followed.The bursting position with highest probability is in the middle of the wire.According to the fire characteristics,the bursting fire can be classified into symmetrical bursting fire and asymmetric bursting fire.The flame shape of asymmetric bursting fire looks like a solar prominence,and the fire bursts at one side and then ignites the pyrolysis gas at another side.In contrast,the shape of symmetrical bursting fire is always symmetrical from the moment of ignition to its maximum flame.For the study of the ignition time,the ignition time is divided into two parts,the bubble formation time and the induction time.The bubble formation time is defined as the time interval from the moment,when the experimental circuit is energized,to the moment when the pyrolysis bubble reaches its maximum size.The induction time is defined as the time interval from the bubble bursting to ignition.Through the analysis of the experimental results,the bubble formation time increases with decreasing environmental pressure,and the influence of the oxygen concentration is not significant.The induction time increases with decreasing environmental pressure,and the increase of oxygen concentration also reduces the induction time.Since the ignition time is the sum of the above two times,it decreases as the pressure and the oxygen concentration increase.