Flame spread studies over solid fuel has gained importance in recent years and has been an
ongoing research for many investigators because of its far-reaching fire safety applications
both in normal gravity and in microgravity environment for human endeavours in space.
Flammability of solid fuel depends on several factors including those in solid fuel, in gas
phase and related to geometry. While factors in gas phase has been studied and understood
well, the factors related to solid fuel degradation/pyrolysis, heat and mass transport inside the
solid are not understood well as this part is difficult to understand even experimentally. A
2D/axisymmetric model of opposed-flow flame spread over planar and cylindrical solid fuel
has been developed and solved numerically. The gas-phase combustion model includes the
full Navier-Stokes momentum equations along with conservation equations of mass, energy
and species. The solid phase model consists of continuity and energy equations whose
solution provides boundary condition for the gas phase. The gas-phase reaction is represented
by a one-step, second order finite rate Arrhenius kinetics and the solid fuel pyrolysis is
approximated by a one-step, first-order decomposition obeying Arrhenius law. The flame
spread model is then used to study flame spread phenomena in non-charring, charring planar
and cylindrical fuel in normal gravity and microgravity environment. In case of cylindrical
fuel, even for a thin fuel, it was found that temperature gradient exists across its thickness
because of high flame spread rate indicating that the fuel behaves like a thermally thick fuel
so assuming a thin cylindrical fuel to be thermally thin fuel may not be valid. For a fuel to be
thermally thin, temperature should remain constant across the thickness of fuel. Studies on
the temperature variation inside thin flat fuel revealed that flat fuel exhibits thermally thin
behaviour at most conditions considered in this study (normal gravity, microgravity, different
inflow velocities, and different oxygen concentrations). Study on the effect of fuel thickness
on flame spread rate reveals that flame spread rate increases rapidly as fuel becomes thinner
in both normal gravity and microgravity environment for both cylindrical and planar fuels. In
microgravity environment, it was found that for cylindrical fuel, sustained flame could not be
achieved when fuel is thicker and at no inflow velocity condition but with some forced flow
sustained flame could be achieved even for thicker fuels. It was also found that fuels are more
flammable in microgravity environment than in normal gravity environment for most
conditions except at low inflow velocities for flat and thick cylindrical fuel. From the
comparison of flammability boundaries for different thickness in normal and microgravity, it
was also revealed that in normal gravity there is little difference in extinction limit for
different thickness of fuel but in microgravity environment, thinner fuels are more flammable
at low inflow velocities while at higher inflow velocities, there is little difference in
extinction limit for different thickness of fuel.