Dripping and Fire Extinction Limits of Thin Wire: Effect of Pressure and Oxygen

ABSTRACT

Fire safety is a significant concern in aviation as well as space travel and settlement, where reduced pressure and raised oxygen concentration are used. Electrical wire has been identified as a major fire hazard in these special applications. Besides the flame spread in wire insulation, the molten insulation surrounded by a flame can drop due to gravity. Such flame dripping in wire fire represents an important and different type of fire risk but has been ignored. To better evaluate the aircraft fire risk, we conduct laboratory experiments with thin nichrome and copper wire samples under various ambient oxygen concentration and pressure. For the first time, we find two important limits for wire fire, the upper dripping limit and the lower extinction limit, as a function of pressure and oxygen concentration. Between these two limits, both the spread of flame and dripping accompanied by flame occur, defining the worst fire-wire scenario. In the “normoxic” atmosphere (i.e., oxygen partial pressure of 21 kPa), dripping occurs to the copper wire below 70 kPa, but never to the low-conductivity nichrome wire. The mass of drip is controlled by the force balance between gravity, surface tension, and inertia forces, and it is insensitive to the wire size and core material while changes with the ambient condition. At the extinction limit, the wire core changes from a heat source to a heat sink as the oxygen concentration is decreased.

KEYWORDS:

• Thin wire
• normoxic
• combustion limit
• flame spread
• drip mass

Symbols

A	=	cross-section area (mm2)
B	=	mass transfer number
Bo	=	Bond number (-)
c	=	specific heat (kJ/kg/K)
d	=	diameter (mm)
D	=	diameter of drip (mm)
g	=	gravity acceleration (m/s2)
h	=	convection coefficient (W/m2-K)
H	=	enthalpy (MJ/kg)
$\mathrm{\Delta }H$	=	heat of reaction (MJ/kg)
k	=	thermal conductivity (W/m-K)
l	=	length (m)
m	=	mass (g)
$\dot{m}$	=	mass-loss rate (mg/s)
$M_{dr}$	=	mass of one drip (mg)
Nu	=	Nusselt number (-)
${\dot{q}}^{{ }^{\prime \prime }}$	=	heat flux (kW/m2)
T	=	temperature (°C)
Vf	=	flame spread rate (mm/s)
We	=	Weber number (-)
Y	=	mass fraction (-)

Greeks

$\sigma$	=	surface tension (Pa)
$\delta$	=	thickness (mm)
$\rho$	=	density (kg/m3)
$\lambda$	=	thermal conductivity (W/m-K)
$\mu$	=	dynamic viscosity (pa.s)
$\varphi$	=	equivalence ratio (-)

subscripts

a	=	ambient
b	=	burning
c	=	core
conv	=	convection
dr	=	dripping
f	=	flame
g	=	gas
m	=	melting
o	=	outer
O2	=	oxygen
p	=	polymer or preheat
py	=	pyrolysis
PE	=	polyethylene

Additional information

Funding

This work was supported by the National Natural Science Foundation of China [51323010,51576186,51636008];National Key R&D Program [2016YFC0801504];Key Research Program of the CAS [QYZDB-SSW-JSC029];Fundamental Research Funds for the Central Universities [WK2320000034,WK2320000036].