Role of wire core in extinction of opposed flame spread over thin electric wires

Abstract

Theoretical and experimental works have been carried out to clarify the effect of core material on the extinction characteristics of the flame spreading over electric wires. Additionally, an attempt has been made to explain previous experimental results (Takahashi et al., 2013 [1]) which found that Limiting Oxygen Concentration (LOC) of copper (Cu) wire is higher than that of nickel-chrome (NiCr) wire. A theoretical model is developed to discuss the heat loss mechanism in the unburned zone ahead of the gas-phase preheat zone and its validity is confirmed by measured temperature profiles along Cu, iron (Fe), and NiCr wires insulated by low-density polyethylene (LDPE). The theoretical analysis reveals that the flame spread rate is a crucial value to assess the extinction characteristics of the flame spreading over electric wires because it controls heat loss in the unburned zone. The reduction of the flame spread rate increases heat loss in the unburned zone and induces the quenching extinction as a result of extended thermal diffusion length along the electric wire. It is also found that a highly conductive wire increases heat loss rather than poorly conductive wire even under the same flame spread rate. Such characteristics are well described by a newly introduced parameter, ηf, which is a dimensionless flame spread rate derived by the present study. By using ηf, previous experimental findings [1] are successfully explained.