A numerical model for flame spread over polymeric surfaces is constructed.
The dependence of the flame spread rate on phase change and thermal propert
ies is investigated by varying three non-dimensional parameters, (S) over b
ar St, (k) over bar (t), and (C) over bar (Pt). Quantitative comparisons in
dicate that the numerical model provides excellent agreement to an analytic
al formula in the cases of variable latent heat of the phase change, variab
le liquid thermal capacity, and variable thermal conductivity. However, the
deRis formula yields a constant spread rate higher than the numerical resu
lt and is independent of phase change. Qualitatively, with the increase of
Sr, or with the decrease of (k) over bar (t) or C-Pt, the flame spread rate
increases. In addition, (k) over bar (t) is the strongest determinant of t
he influence of the thickness of the liquid region. The mechanisms of flame
spread at the steady state are interpreted by applying an energy balance p
rinciple for the control volume upstream of the flame leading edge. It is f
ound that a ratio between the total heat applied to the condensed material
upstream of the flame leading edge and the spread rate reveals the physical
mechanisms that control the preheating of the condensed material to the ig
nition temperature. The dependence of flame structure on St, (k) over bar (
t), and (C) over bar (Pt) is studied. It is found that with the increase of
St or (k) over bar, or with the decrease of (C) over bar (Pt), the size of
the flame increases. These results indicate that flame size dependence fol
lows the magnitude of the spread rate when the properties of the condensed
material are variable. (C) 2001 by The Combustion Institute.