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Concurrent Flame Spread and Blow-Off Over Horizontal Thin Electrical Wires

Abstract

Electrical wires with the flammable polymer insulation and the metal core are responsible for many fire accidents in buildings, nuclear power plants, aircraft, and spacecraft. For the first time, this work studies the horizontal flame spread and blow off under the concurrent airflows over the thin copper-core wires. Four wires of about 1-mm diameter with different insulation thicknesses and core diameters are tested in a horizontal wind tunnel. Results show that as the concurrent airflow velocity increases, the flame spread rate first quickly increases to a maximum value, and then slightly decreases until blow-off at about 2 m/s. Heat transfer analysis shows that the preheating from flame and core have different dependences on the airflow. The flame spread is slower for a larger copper core diameter because the heat-sink effect of the core is highlighted by increasing the thermal inertia. We found a critical Froude number of 3.6 which helps determine the critical concurrent airflow velocity when the fastest flame spread is achieved. This study not only provides valuable information about the worst scenario of wire fires but also advances the fundamental understanding of the concurrent flame spread mechanism over thin fuels.

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Abbreviations

c:

Specific heat (kJ/kg/K)

d :

Diameter (mm)

Da :

Damkohler number (–)

E :

Activation energy (kJ/mol)

Fr :

Froude number (–)

g :

Garvity acceleration (m/s2)

Gr :

Grashof number (–)

h :

Convection coefficient (W/m2 K)

H 0 :

Flame height without airflow (m)

k :

Slope (–)

L :

Heating length (m)

Nu :

Nusselt number (–)

\( \dot{q}\prime \prime \) :

Heat flux (kW/m2)

R :

Universal gas constant (J/mol K)

Re :

Reynolds number (–)

V f :

Flame-spread rate (mm/s)

t :

Time (s)

T :

Temperature (°C)

U :

Airflow velocity (m/s)

Z :

Pre-exponential factor (s−1)

α :

Thermal diffusivity (m2/s)

δ :

Thickness (mm)

ρ :

Density (kg/m3)

λ :

Thermal conductivity (W/m K)

* :

Critical

∞:

Ambient

b :

Burning

c :

Core

ex :

Extinction

f :

Flame

F :

Fuel

g :

Gas

o :

Outer

O :

Oxygen

p :

Plastic insulation

py :

Pyrolysis

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Acknowledgements

This work was supported jointly by the Key project of NSFC (No. 51636008), Newton Advanced Fellowship (No. NA140102), Key Research Program of Frontier CAS (No. QYZDB-SSW-JSC029) and Fok Ying Tong Education Foundation (No. 151056). YL also thanks for the support from CSC.

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Correspondence to Xinyan Huang or Longhua Hu.

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Lu, Y., Huang, X., Hu, L. et al. Concurrent Flame Spread and Blow-Off Over Horizontal Thin Electrical Wires. Fire Technol 55, 193–209 (2019). https://doi.org/10.1007/s10694-018-0785-0

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Keywords

  • Horizontal concurrent flow
  • Electrical wire
  • Copper core
  • Froude number