Downward flame spreading over electric wire under various oxygen concentrations

The downward flame spread over laboratory electric wire under various oxygen concentrations has been investigated experimentally to improve our knowledge of electric-wire combustion. Two kinds of electrical wire (low-density-polyethylene (LDPE)-insulated copper (Cu) and nickel-chrome (NiCr)) are used in this study. The oxygen concentration of the mixture stream (O2 and N2) in the test section is varied between 15 and 41 vol%. Opposed-flow velocity in the test section is fixed at 15 cm/s. For NiCr wire, the flame spread rate (Vf) and flame length (Lf) monotonically increase with oxygen concentration. For Cu wire, both Vf and Lf show non-monotonic behavior against oxygen concentration. Most interestingly, Vf decreases with oxygen concentration increase in the 25–31% range. Theoretical analysis shows two regimes of variation of Vf with oxygen concentration: the “temperature-dependent regime (TDR)” and the “negative-oxygen-dependent regime (NOR)”. The non-monotonic behavior of Vf against oxygen concentration for Cu can be explained by the controlling mechanism behind TDR and NOR. However, experimental results show one more regime above 31% oxygen concentration that cannot be explained by the theory proposed in this work, namely the “soot-generation-dependent regime (SGR)”; here, radiation from the flame and soot deposit plays a dominant role in flame spread.