Experimental study on flame spread characteristics and heat transfer mechanisms of wire fires in confined spaces

To investigate the flame spread characteristics of wire fires in confined narrow spaces, horizontal flame spread experiments were conducted by varying the opening size of a glass square tube (14 cm × 14 cm, 6 cm × 6 cm, 4 cm × 4 cm, 3 cm × 3 cm, and free space) and the copper core diameter (0.3 mm, 0.6 mm, 0.8 mm). The effects of different confinement levels and wire specifications on the extinction limit, flame morphology, flame spread rate, temperature distribution, and heat transfer mechanisms were systematically analyzed. The extinction limit is governed by the coupled effect of aperture size and copper core diameter. For an opening size ≥ 6 cm × 6 cm, all wires exhibit sustained flame spread over the entire length; at an opening size of 4 cm × 4 cm, only the 0.3 mm wire can maintain flame spread; at an opening size of 3 cm × 3 cm, none of the wires achieve self-sustained propagation. As the opening size decreases, the flame height, flame width, flame spread rate, and flame temperature all exhibit a non-monotonic trend, first increasing and then decreasing, with peak values occurring at 6 cm × 6 cm or 4 cm × 4 cm—a behavior that reflects the competing mechanism between enhanced thermal feedback and oxygen limitation in confined spaces. Based on the thermally thin assumption, a heat transfer model incorporating flame convection, flame radiation, axial copper core conduction, and wall thermal feedback is developed. The model predictions agree well with experimental measurements (within ±10% deviation). The results indicate that thin wires are more sensitive to the confinement level, whereas thick wires are dominated by the heat sink effect. Under strong confinement, restricted air entrainment is the key factor leading to flame extinction. These findings provide a theoretical basis for fire protection design and risk assessment of wire installations in confined spaces such as cable tunnels and utility corridors.