Analysis of heat transfer during flame spread over energized-wire under high currents

Abstract

Electrical wire is one of the primary causes of electrical fires because of short circuiting or overload current. Experiments were carried out to characterize the influence of high currents on flame spread and dripping behaviors over wires. Two samples of polyethylene (PE)-insulated copper wires with different sizes (dc/do = 0.9 mm/3.0 mm for wire #1 and 1.3 mm/4.2 mm for wire #2) were electrified with current, ranging from 10 A to 50 A, to conduct experimental study. Results show that the dripping becomes faster as the current increases and the continuous dripping occurs under high current (I > 40 A for wire #1 and I > 30 A for wire #2), causing the flame to be narrower and shorter. The heat transfer by different passages (radiation, convection, conduction through core and the joule heat) were quantitatively analyzed. The heat feedback from core to the insulation (q˙cp) were the dominant heat transfer mode during flame spread over energized wire. Heat required for flame spread (q˙req) is mainly supplied from the core, nearly 80%, including the joule heat (q˙joule) and the heat conduction through the core from the burning zone (q˙cc). The proportions q˙cp/q˙req decreased with the increasing current, and has a sharp decent to 70% or 50% at I = 50 A or 35 A for wire #1 or wire #2, respectively. In addition, the proportion of q˙cp to the sum of q˙joule and q˙cc drops obviously under high current, which means the dominated role of core heat feedback in heat transfer mode is weakened under high current. A flame spread model for energized wire based on the quantitative analysis of controlling heat transfer mechanism was proposed, and it predicted the flame spread rate well within error of ±20%. The findings of the presented study will be essential for simulation study of flame spread behavior over energized wires with high current and may guide the design of future electric fire safety.