Effects of one-side confinement and current on heat transfer mechanisms over single electrical wire flame spread behaviors

The electrical wire fire often occurs in the confined space, making the flame spread behavior complex and the risk increase. In this paper, experimental and theoretical study on the flame spread over three types of polyethylene (PE) electrical wires under different side-confined distances (0 similar to 40 mm) and currents (0 similar to 60 A) was carried out. The ratios of copper diameter to entire wire diameter were 6mm/10 mm, 8mm/12 mm, and 6mm/12 mm for type I, type II and type III, respectively. The results show that, with the increase of s, the flame width and height, and flame spread rate will firstly increase to the maximum peaks at the position of s = 5 mm with attachment wall effect, and then gradually decrease to the values of the unconfined conditions. And with the increase of current, these parameters will gradually increase to the largest at I = 40 A and then decrease for I = 60 A, as the current effect changes from preheating to dripping acceleration. The firm correlation between preheating length delta(ph) and pressure difference Delta P is built, which demonstrates the smallest preheating length is at s = 5 mm with the maximum promotion of heat transfer by gypsum board. On the other hand, the generated Joule heat by the current will enhance the length of preheating, making it proportional to current. Meanwhile, it is found that, the dripping frequency influenced by the dripping mass rate, will increase with the current. The heat transfer components including of the solid conduction heat flux (q(joukte) and and (q) over dot(c)''), flame heat flux ((q) over dot(vf)'' and (q) over dot(rf)'') and gypsum board heat flux (q) over dot(rg)" and (q) over dot(vg)'') are quantitatively analyzed. Correspondingly, a heat transfer model over flame spread is established, which can well predict the flame spread rate. As the diameter of the wire is relatively small, the controlling heat transfer is the flame convective heat flux (q) over dot(vf)'' in this research. In addition, it is illustrated that, the heat flux of gypsum board (q) over dot(vg)" and (q) over dot(rg)'' will decrease with the increase of copper core diameter from type I of 6 mm to type II of 8 mm.While the solid conduction heat flux of copper (q) over dot(cg)'' +(q) over dot(joule)'' will decrease with the increase of thickness of PE from type I of 2 mm to type III of 3 mm. These findings can well give the understanding of heat transfer of flame spread over electrical wires with the effects of sidewall and current.