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Spontaneous ignition of electrical wire caused by the Joule heat generated in core due to overloading or a short circuit is common, and is significantly influenced by wind velocity, but has been very limited addressed yet. This paper, for the first time, presents an experimental investigation in spontaneous ignition of overloaded wire under transverse wind (perpendicular to wire). Polyethylene (PE)-insulated wires with a NiCr core diameter of 0.8 mm and the insulation layer thickness of 0.3 mm and 0.15 mm are employed as samples. It can be found that with the increase of transverse wind velocity, the ignition delay time demonstrates a non-monotonic tendency (firstly decreases then increases, and decreases again, finally increases) until ignition limit, where four regimes are clarified according to different controlling mechanisms. The ignition delay time declines with the increase of electrical current. One of the most noteworthy findings is that the ignition delay time declines as the insulation thickness increases, which shows an opposite trend with the results of piloted ignition for thermally thin materials. In order to distinguish the ignition and non-ignition zones, an ignitability map considering both the gas phase kinetic effects and the solid phase heat loss effect is established based on two non-dimensional parameters [Damköhler number (Da) and R loss ]. A theoretical ignition model combining the effects of pyrolysis time, mixing time and chemical time is established, which well interprets the variation trend of ignition delay time and verifies the significant role of the chemical time in spontaneous ignition. This work provides essential knowledge on spontaneous ignition of overloaded electrical wires under practical transverse wind. [ABSTRACT FROM AUTHOR]

This paper investigated experimentally the interaction behaviors of flames spreading over two parallel adjacent electrical wires at various separation distances. Experiments were carried out with thin electrical wires of copper inner core and polyethylene (PE) insulation. The separation distances between the two parallel electrical wires (their core center distance) were ranged from 1 mm (flames merge) up to 20 mm (flames completely separate with behavior identical to that of one single electrical wire) to reveal the transition of the flame behaviors with increasing separation distance. The flame base width along the wire (W f), flame height (H f), average mass loss rate (AMLR, m ˙) and flame spread rate (FSR, V f) were measured to quantify this transition. Results showed that the flame height, the AMLR and the FSR all increased first and then decreased with increasing of separation distance. The FSR and AMLR were maximized at the same separation distance during the transition of flames separating from its base to complete separation with increasing separation distance. Five regimes were identified for the variation transition of flame spread rate with increasing separation distance, which can be interpreted physically by the change of convection and radiation feedback with separation distance. The maximum FSR of the wire-pair, which occurred at the separation of about 8∼10 times of the wire outer diameter, was around 12% ∼22% greater than that over one single wire. Such FSR increment was more remarkable if the wire size was larger. This experimental study provides fundamental knowledge, quantitative data and physical understanding of the phenomenon of flame spreading interactions over two parallel adjacent electrical wires. [ABSTRACT FROM AUTHOR]

• Concurrent flame spread behaviors over glass fiber reinforced composites were explored. • GFUPR-ATH/MH has a higher flame spread rate than GFUPR under external airflow and radiation. • A theoretical analysis of concurrent flame spread over composites was performed. • Decomposition of ATH/MH may cause a larger total flame heat feedback in preheat zone. This paper reports an experimental investigation of concurrent flame spread over composites, for the first time, with combined flame retardants and external radiation factors. Glass fiber reinforced unsaturated polyester resins without flame retardants (GFUPR) as well as those containing aluminum hydroxide (ATH) and magnesium hydroxide (MH), which were utilized widely in modern high-speed train, were systematically explored. Infrared measurement was employed to track the displacement of pyrolysis front. Because of the dilution of H 2 O generated during the decomposition of ATH and MH, the flame temperature (T f) of GFUPR-ATH/MH is thought to be lower than that of GFUPR. Nonetheless, the most noteworthy finding is that the flame spread rate, flame preheat length and flame height for GFUPR-ATH/MH could be larger than that of GFUPR if concurrent airflow velocity exceeds 0.5 m/s under high external radiation. To interpret this unexpected while practically important phenomena, theoretical analysis based on the heat/mass transfer theory is established to explore the general mechanism. It is concluded that in the transition from natural-convection to forced-convection, the different in the flame convection in burning zone between the sample with high T f (i.e. GFUPR) and the sample with low T f (i.e. GFUPR-ATH/MH), will decrease rapidly. It is primarily because the sample with low T f is earlier to enter in the region where forced-convection is controlled as the airflow velocity increases. The critical parameter (Fr 2 Re)-1 = 1 could be used to characterize this transition. As a result, in forced-convection region, the role of thermal conductivity of the fuel-gas mixture (k mix) may emerge, that the sample with high k mix but low T f (i.e. GFUPR-ATH/MH) may impart more heat to burning zone than the sample with low k mix but high T f (i.e. GFUPR), generating more combustible gases in the burning zone hence larger flame heat feedback to preheat zone. Moreover, this effect will be reinforced by the larger flame preheat length of GFUPR-ATH/MH, due to the temperature dependency of molecular diffusivity. This mechanism analysis is well corroborated by the experimental results. This new finding may facilitate understanding of the effect of flame retardants on the heat/mass transfer mechanism in flame spread under concurrent airflow and external radiation. [ABSTRACT FROM AUTHOR]