Effects of confined distance near floor and wire size on electrical wire flame spread behaviors based on heat transfer

Abstract

This paper aims to investigate the confined distance near the floor(0∼28 mm) and wire size (the ratios of copper core diameter to entire wire diameter are: 6mm/8 mm,6mm/10 mm,8mm/12 mm and 6mm/12 mm for type Ⅰ, type Ⅱ, type Ⅲ and type Ⅳ, respectively) on the flame spread over polyethylene (PE) wires. It is indicated that, when the confined distance is relatively small, the extinction occurs for all types.The typical parameters of flame shape including of flame width, flame height and flame area, flame spread rate and mass loss rate with the increase of confined distance s can be separated into continuous growth stage and stable fluctuation stage. At the continuous growth stage, the flame area shows an exponential relationship with s as: $A\sim s^{\frac{5}{2}}$. And at the stable fluctuation stage, the flame width is larger than that at the unconfined condition accounting for a large portion.While, the flame height is always smaller than that in the unconfined case.In order to explicitly describe the heat transfer, the upward large main flame and downward small flame are firstly introduced in this paper. Correspondingly, the heat flux feedback of components to the preheating zone is established with the upward main flame flux ${\dot{q}}_{f\left(up\right)}^{″}$ [includes of ${\dot{q}}_{vf\left(up\right)}^{″}+{\dot{q}}_{rf\left(up\right)}^{″}$], the downward small flame heat flux ${\dot{q}}_{f\left(down\right)}^{″}$ [ includes of ${\dot{q}}_{vf\left(down\right)}^{″}+{\dot{q}}_{rf\left(down\right)}^{″}$], the conductive heat flux ${\dot{q}}_c^{″}$ and the gypsum board heat flux ${\dot{q}}_g^{″}$. With the increase of s, ${\dot{q}}_{f\left(down\right)}^{″}$ shows an increasing and then decreasing trend, making it take a second role during heat flux feedback. Meanwhile, the analysis demonstrates that for the larger copper core (type Ⅲ) and the smaller of PE thickness (type Ⅰ), the ratio of heat flux of $\frac{{\dot{q}}_{f\left(down\right)}^{″}}{{\dot{q}}_{f\left(up\right)}^{″}+{\dot{q}}_{f\left(down\right)}^{″}}$ will be increased, which will enhance the heat transfer effect of downward small flame.