Experimental study of downward flame spread and extinction over inclined electrical wire under horizontal wind

Abstract

Flame spread and extinction behavior are significantly influenced by both fuel inclination and wind velocity, however, the coupling effect of two has not been well quantified yet. In this paper, downward flame spread and extinction over inclined electrical wire under horizontal wind is investigated. Polyethylene(PE)-insulated wires (0.5 mm/0.15 mm in core diameter/insulation thickness) of two representative core materials (Copper/Nickel-Chrome: high/low conductivity) are used as samples. Flame geometrical characteristics (flame length Lf, pyrolysis length Lp, gas-phase thermal length Lg), as well as the flame spread rate (FSR) are quantified. Results show that as the horizontal wind velocity increases, FSR demonstrates a non-monotonic trend before extinction, where four regimes are identified based on different heat transfer controlling mechanisms. On the other hand, FSR of Cu-core wire and NiCr-core wire show different dependence on inclination angle. FSR reaches the maximum when the flame is pushed by the horizontal wind to be parallel to wire, which can be explained by a balance of horizontal wind and buoyancy-induced flow (i.e. inclination). A quantified model based on the three characteristic lengths (Lf,Lp,Lg) and a proposed mixed-convective coefficient hmix combining the effect of inclination angle and wind velocity is established. The proposed model well represents the experimental FSR and interprets the controlling heat transfer mechanism. Moreover, the extinction limit is represented by a heat loss factor Rloss as a function of strain rate a, showing an enhanced quenching effect and a weakened blow-off effect with increased inclination angle. The flame with Cu-core is more difficult to extinguish at small inclination but becomes easier at large inclination than that with NiCr-core, indicating a transition of inner core role from “heat source” to “heat sink”. This work provides essential knowledge on flame spread and extinction mechanism over inclined fuel under wind.