Effect of external and internal heating on the flame spread and phase change of thin polyethylene tubes

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.