Effect of transverse flow on flame spread and extinction over polyethylene-insulated wires

Abstract

The spread of flames over solid combustible surface in a transverse flow (perpendicular to the flame spread direction) is common in real fire scenarios, but has not been well quantified yet. In this paper, we systematically investigate the effect of transverse flow on flame spread and extinction over electrical wire insulation by using eight types of polyethylene (PE)-insulated wire of various configurations (core diameter; insulation thickness) and two representative core material (Copper: high-conductivity, Nickel-chrome: low-conductivity). The experiment is conducted in a combustion tunnel providing a uniform transverse flow. Results demonstrate that the flame spread rate (FSR) shows non-monotonic behavior with increasing transverse flow velocity until extinction, where four regimes can be identified with different controlling mechanisms. Flame spread faster over Copper (Cu)-core wires than that over Nickel-chrome (NiCr)-core wires, but is easier to extinguish in high flow region, revealing simultaneous dual effect of the “heat source” and “heat sink” due to wire core conduction. A theoretical model is developed based on two characteristic lengths (flame base width Wf and average gas-phase thermal length Lg¯), and a proposed mixed-flow velocity Vmix coupling transverse flow velocity and buoyancy-induced flow velocity. The model well predicts the experimental FSR. Moreover, an extinction limit is proposed in terms of heat loss factor Rloss as a function of strain rate am, where the coupling effects of gas-phase blow-off and condensed-phase quenching are well integrated. The present work facilitates understanding of flame spread mechanism in mixed-transverse flow condition over cylindrical fuels not only on the electric wires, but also on the branches in wildland fires.