On the Reignition

Abstract

There are little theoretical works on the reignition of water-extinguished wood. Usui³⁾⁴⁾ performed the experiments on the reignition time and obtained relations among the char-depth, water content and environmental temperature.
The author rearranged his data and drew an interesting simple conclusion.
By assuming the reignition time, τ (min), is the time required for total evaporation of the soaked water, the following theoretical equation must hold:
Lx = φσ (T ⁴ − T₀⁴) · τ
where,
L  :   heat required for evaporation of water (≈ 620 kcal/kgH₂O)
x  :   water content (kg/m²)
φ  :   overall heat absorption coefficient
σ  :   Stephan Boltzman coefficient (kcal/m² · °K⁴ · min)
T  :   environmental temperature (°K)
T₀ :   surface temperature of the wet extinguished wood (≈ 373°k)
because T ⁴ » T₀⁴ , τ is rewritten simply as,
τ = (L /φσT ⁴) x (min)
Using φ ≈ 0.7, the time calculated agreed well with Usui's experimental results.