Burning Behavior And Smoke Particles Characterizations Of FEP Wire In Microgravity And Forced Airflow Conditions

Since the spacecraft were full of various wires directly exposed to the air in the spacecraft or hidden in the electrical appliances, fires aboard spacecraft were mostly caused by overheated wires or electrical components in the past. Unlike terrestrial electrical fires, electrical fires aboard spacecraft have their own characteristics. First, spacecraft fluorine materials, such as Tezel (ETFE) or fluorinated ethylene propylene (FEP), which could not be ignited unless the temperature is higher than 200℃, are widely used in the insulation of electric wires in spacecraft instead of the polyvinyl chloride (PVC), which is commonly used in terrestrial environment. Second, without gravity, there is not buoyant plume in the fire aboard spacecraft. Third, to ensure the breath space crew, forced convection exists in space capsule with typical imposed air flow velocity of about 0.2-0.3 m/s; it means that the exposed wire’s burning behavior would be influenced by this.Since the smoke fire detectors mainly designed to use in normal gravity, and the smoke generated in spacecraft are quite different, so there are a lot of false alarm in last 20 years over spacecrafts. To insure the safety of astronaut and spacecraft, it is necessary to study the smokes generated in typical spacecraft environment, provide basic data to support the improving design of capsule smoke detectors. This paper designed an experiment to collecte smoke particles generated in typical capsule environment, analyzed the experimental results and then carried on light scattering simulation analysis.First, an experiment platform was developed for high temperature wire’s combustion and smoke catching in forced convection environment, and the drop tower of Beijing microgravity laboratory was used to provide microgravity environment. Accordingly, the typical capsule environment was simulated to experiment. In the process, special attention was paid to the impact of gravity level and air flow to the burning process of wire, because the change of the burning process would directly affect the shape of the generated smoke particles. Effects of gravity level and air flow velocity on parameters was analyzed, such as break time, ignition time, burning time for high temperature wires, and flame height, flame temperature and brightness, soot volume fraction. The experimental results show that except for the wire broken time and burning time mainly influenced by the internal conductor temperature and the insulation material properties, nearly not affected by gravity or the air flow velocity, other parameters were all influenced by gravity and air velocity to different degrees.Then electron microscopy (SEM) and Energy Disperse Spectroscopy (EDS) were used to analysis the collected smoke particles samples. The impact of environments on smoke particles was studied in multiple aspect, such as the primary particle size, gyration radius, fractal characteristics and chemical composition, and a reasonable explanation was given by the relevant theory of combustion and aerosol dynamics. The results show that when there is no forced convection, the primary particle size and gyration radius of smoke particles generated in microgravity are much bigger compared to smokes generated in normal gravity, and for most of smoke particles gravity level has no impact in fractal dimension, the fractal dimension keeps 1.79 unchanged, but the fractal leading factor is decreased obviously. It is worth noting that there was another kind of particles with a much large volume, whose fractal dimension is 2.6. In addition, primary particle size and gyration radius decrease with the increasing air flow velocity, when the coflow velocity reaches to 0.3 m/s, the gyration radius nearly equals to the smoke generated in normal gravity and still environment, and the size of primary particle could be even smaller than that Fractal dimension affected by coflow will become smaller compared to still environment, at the same time the leading factor will increase with coflow velocity.Finally, based on the most representative sample of smoke particles collected, use light scattering model to simulate the light scattering characteristics of smoke particles generated in typical environment of space capsule, and then analyze the difference of light scattering characteristics between microgravity and normal gravity conditions. Results show that the scattered light intensity of particles generated in microgravity and still environment can increase 300% more than that of the normal gravity at some location in the forward scattering area. But with increasing air velocity, the scattering intensity gradually decreased. In addition, F33/F11 and F34/F11 also have obvious changes in different scattering angle range.