| Chinese aerospace industry has achieved a breakthrough over the early21st century, which provided very important theoretical and technical support for the next stage of development of aerospace technology, designing and manufacturing of aerospace with our own intellectual property rights, development and utilization of space resources, as well as carrying out space exploration and even deep space exploration. In order to ensure the safety of space missions, we should not only learn from the advanced foreign technology, but also develop our own standards and specifications of fire safety in space, which requires quantities of theoretical and experimental data as support database.Electrical wires which is likely to start a fire due to overload, short circuit and other poor contact are widely applied for circuit board and electronic devices in aircraft and space vehicle utilization, in which flame spread behavior is one of the key fundamental points as received focused attention in recent years. Therefore, it’s significant in theory and practice to conduct flammability test and flame spread behavior of electrical wire.Flame spread behavior over electrical wire in both microgravity and normal gravity has been widely investigated in recent years focused on flame spread rate, flame shape, flame temperature and heat flux, as well as limiting oxygen concentration and limiting oxygen index, where no inclination effect needs to be considered. Researchers investigated the effect of different insulation and core metal, wire (core) sizes, external flow speeds, overloaded current, external radiation and oxygen concentrations (LOC) on flame spread behavior. Based on investigation, the author here found that it’s necessary to concern the effect of inclination angle and multi-parameter coupling with it on flame spread behavior, which is the purpose of this paper.Experiments were conducted to study the effects of inclination angle on flame spread over electric wires and its limiting oxygen concentration and the correlation of LOC between the normal gravity and microgravity in this paper, which mainly focused on flame shape, flame spread rate and limiting oxygen concentration. The dynamic process of flame spread is recorded by different CCD camera videos. The characteristic parameters of flame shape, as well as the change of flame front position with time, are obtained by image post-processing. The limiting oxygen concentration are obtain from a monitor by a special software. Firstly, experiments were conducted to reveal the flame spread rate (FSR)[both upward (concurrently) and downward (opposed)] over electric wire with high thermal conductivity metal core at different inclination angles. Polyethylene (PE) insulated copper (Cu) wires with inner core diameter (dc) of0.30mm,0.50mm0.80mm and insulation thickness (δp) of0.15mm,0.30mm are studied with inclination angles ranged from-90°to+90°. Their behaviors are examined in both naturally normal (Hefei city with altitude of50m;100kPa) and a reduced (Lhasa city with altitude of3650m;64kPa) ambient pressure atmosphere. Results show that with increase in inclination angles from-90°to90°, the FSR first decreases and then increases ("U" trend) with its value being lowest at nearly horizontal condition (0°) in both pressures, which is quite different from what we normally know for other materials with low thermal conductivity. Two characteristic lengths, the flame base width (Wj) and the pyrolysis zone length (Lp), are found to account for this special variation behavior with their variation trend with inclination angle consistent with that of FSR. A simplified heat balance analysis concerning core thermal conduction effect is performed to calculate the FSR in relation to these two characteristic lengths, thermal conductivity of the metal core as well as the effective convection heating of the wire by the flame base. The calculated FSR are shown to be in fairly good agreement with the measured values at different inclination angles for different inner core (wire) diameters in both ambient pressures.Same sample as above with inner core diameter (dc) of0.80mm and insulation thickness (δp) of0.15mm is studied with inclination angles ranged from-75°to+75°to reveal the effect of different ambient pressure (100kPa-40kPa) on flame spread rate in hypobaric chamber. Results show that flame sizes decrease with the decreasing of ambient pressure and the flame tip becomes more smoothing in downward spread. Moreover, the flame spread rate increases with increasing of ambient pressure. The flame base width decreases with increasing of ambient pressure in upward spread. However, the flame spread rate shows different trend in both two spread type, which increases monotonously with ambient pressure when inclination angle is from-75°to45°and decreases monotonously with ambient pressure when inclination angle is large. Based on two characteristic length, the flame base width (Wf) and the pyrolysis zone length (Lp), we found that burning rate and heating length of naked Cu-core are accounted for the variation of flame spread rate respectively.In the end, this paper reveals experimentally the extinction Limiting Oxygen Concentration (LOC) of flame spreading over electric wire at different inclination angles (0°-+75°) with different forced external opposed air flow (0-250mm/s) in normal gravity and quantifies its difference from those in microgravity. Nichrome (NiCr)-core wire with inner core diameter (dc) of0.50and polyethylene (PE) insulation thickness (δp) of0.15mm are studied here. The difference between LOC in normal gravity and those in microgravity is quantified and correlated with inclination angle and opposing flow speed. It is found that:(1) for a given inclination angle (except the horizontal condition,0°), the LOC first increases, then decreases and finally increases again with opposing flow speed, the first up-limit turning point of this LOC variation curve corresponds to a critical opposing flow speed resulting in change of flame spread pattern from concurrent to opposed, and this critical opposing flow speed is found to well correlated with the buoyancy-induced flow speed component in the wire direction at the inclination angle using the flame base width along the wire as characteristic length;(2) LOC is generally relative higher in normal gravity than that in microgravity and their difference decreases with increase in opposing flow speed, in nearly a liner trend at relative higher opposing flow speed (over100mm/s); and (3) the decrease rate of LOC difference between normal gravity and microgravity with increase in opposing flow (their correlated linear slope) is also found to well correlated with gravity acceleration component in the wire direction [gsinθ]. An empirical formula is finally found for extending the LOC measured in normal gravity (at different inclination angle and opposing flow speed in the tested range of this work) to that in microgravity. |
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