An asymptotic solution of flame spreading velocity along a vertical combustible solid is derived on the basis of previous experimental relations on the heat transfer from the flame to the surface. From the comparison of this with the steady-state solution, it was found that spontaneous flame spreading velocity starting from arbitorary initial conditions falls within relatively narrow range. Measurements of flame spread along vertical PMMA slabs with and without external radiation were conducted to verify the solution, which have revealed considerable acceleration of flame spread due to external radiation. Application of the theory to the evaluation of this influence has resulted in 30~40% overestimate of flame spreading velocity. This error is attributed to the higher pyrolysis temperature in the experiments than reference value and the dripping of molten fuel which was not considered in the model.
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