金属芯热传导效应对电缆线水平火蔓延的影响

The influence of heat conduction effect of the metal core on horizontal wire flame spread

为研究金属芯热传导效应对导线水平火蔓延的影响,建立水平火蔓延速率预测模型,开展了不同热导率金属芯导线的火蔓延试验研究,从火焰形态、质量损失速率及火蔓延速率三个角度,分析了不同热导率及不同直径对电缆导线火蔓延的影响。研究结果表明,导线护套的质量损失速率随金属芯直径的增加先增加后减小。由于铜芯的热传导率较不锈钢芯大,铜芯导线预热区积累的热量较多,其质量损失速率相比不锈钢芯较大。铜芯、不锈钢芯直径分别为3.5、4.5 mm时,其质量损失速率分别达到极大值0.0071、0.0068 g/s。金属芯直径较小时,随着直径的增加,金属芯向已燃区的热传导热量增强,进而导致火蔓延速率下降;金属芯直径大于3.5 mm时,仅有金属芯下部的绝缘护套参与燃烧蔓延,相比于金属芯绝缘护套整体燃烧蔓延而言,下部的绝缘护套燃烧较快,进而导致火蔓延速率短暂上升;随着金属芯直径进一步增加,金属冷却作用显著增强,导致火蔓延速率再次下降。此外,在火焰完全覆盖导线表面时,火蔓延速率理论计算值与试验评估值吻合较好,表明推导的水平火蔓延速率理论模型适应性较好。

To investigate the heat conduction effect of the metal core on horizontal wire flame spread,this paper established the predicted model for the horizontal wire flame spread rate,and then performed the wire flame spread experiments for different thermal conductivity.In terms of the flame contour,mass loss rate(MLR)and flame spread rate,the effect of the metal core diam‑eters and the thermal conductivity for different metal core on the wire fire were examined.With the metal core diameter increasing,the MLR firstly increases to a higher value and then decreases.Due to the higher thermal conductivity for the copper wire,a large amount of heat conduction was accumulated in the preheated area,and the MLR is higher than these of the stainless steel wire.The bigger MLR of 0.0071 g/s is obtained for the copper inner diameter of 3.5 mm while the bigger MLR is 0.0068 g/s for the stainless steel inner diameter of 4.5 mm.Also,compared to the stainless steel wire,the mass loss rate and the flame spread rate for the copper wire attained the bigger value.For the smaller mental core diameter,the heat conduction of the mental core from the pyrolysis area to the preheated area is relatively larger with the insulating sheath diameter increasing,and thus the flame spread velocity gradually decreases.For the mental core diameter bigger than 3.5 mm,only the insulating sheath below the metal core gets involved in the combustion and thus the larger mass loss rate is obtained,further causing a bigger flame spread rate.With the mental core diameter further increasing,the effect of heat sink for the mental core is strengthened,making the flame spread rate decrease again.Moreover,when the flame fully cover the wire,the measured flame spread velocity form the experiment is in a good agreement with the predicted velocity from the model,further verifying the adaptability of the predicted model.