Scale Modeling of Flame Spread Over PE-Coated Electric Wires

Abstract

A universal correlation between the flame spread rate and the flame length formed along the electric wire was studied experimentally based on scale modeling concept. In the first place, we studied the burning behavior of research-graded wire (i.e., controlled wire, polyethylene-coated metal thin rod) in order to examine the precise effect of the total pressure (30–100 kPa), the core material (nickel chrome, iron, copper), and the scale (e.g., diameter, coating thickness, etc.) on the spread rate. It turned out that the flame shape was not the only primary factor involved in determining the spread rate, implying that the heat transfer process in solid phase is essential to consider. The simplest 1-D heat transfer model along the core was introduced, and two kinds of non-dimensional groups (i.e., Peclet number (Pe) and one to describe the radial direction of heat transfer process: Λ) were found necessary to preserve the similarity. By introducing two length scales to represent the processes in gas and solid phases, all measured data were found to have collapsed into the single line in Pe-Λ plane, suggesting that flame spread behavior would be predictable based on their correlation. This correlation curve is justified with the spread data obtained using practical electric wire and cables (with/without sheath), confirming that scale modeling of flame spreading over the electric wire was successful.