The flame spread over combustible materials is often affected by the fire thermal radiation and convection and the heat exchange with adjacent objects, which are especially complex on melting thermoplastics. This work chooses polyethylene (PE) tubes with a 2-mm thin wall to study the flame-spread behaviors under three heating conditions, (a) hot inner boundary, (b) hot ambient, and (c) external radiation. The tubes could simulate the insulation of electrical wires, and the inner boundary was controlled by flowing oil through at a constant temperature. Results show that just above the fuel molten point, the flame-spread rate unexpectedly decreases with the increasing environmental temperature, because the conductive cooling changed to convective cooling of molten PE. A thin layer of fuel can remain after the flame spread, and as the boundary temperature increases, the remaining PE decreases while the dripping mass increases. Under intense heating, burning behaviors eventually become similar regardless of the heating scenario. This work helps understand the flame spread and phase change of thermoplastic fires, particularly wires and cables, under various heating scenarios of realistic fire events.

火焰散布在可燃材料上时，通常会受到火的热辐射和对流以及与相邻物体的热交换的影响，这在熔化热塑性塑料时尤其复杂。这项工作选择了壁厚2毫米的聚乙烯（PE）管，以研究三种加热条件下的火焰传播行为：（a）高温内部边界，（b）高温环境和（c）外部辐射。这些管可以模拟电线的绝缘，并且内部边界是通过使油在恒定温度下流过来控制的。结果表明，在正好高于燃料熔点的情况下，火焰蔓延速率出乎意料地随着环境温度的升高而降低，这是因为传导性冷却变成了熔融PE的对流冷却。火焰蔓延后会残留一层薄薄的燃料，随着边界温度的升高，剩余的PE减少，而滴落质量增加。在强烈加热下，无论加热情况如何，燃烧行为最终都会变得相似。这项工作有助于理解在现实火灾事件的各种加热情况下，热塑性火种（尤其是电线和电缆）的火焰传播和相变。