To explore the flame spread mechanisms over the solid fuel sheets, downward flame spread over vertical PMMA sheets with thicknesses from 1.75-5.75 mm have been examined in the quiescent environment. The dependence of flame spread rate on the thickness of sheets is obtained by a simple one-dimensional heat transfer model. An equation for the flame spread rates based on the thermal properties and the thickness of the sheet by scale up method is derived from this model. During combustion, temperature within the gas and solid phases is measured by a fine thermocouple. The pyrolysis temperature, the length of the pyrolysis zone, the flame temperature, and thermal diffusivity of the solid are determined from the experimental data. Mathematical analysis has yielded realistic results. Although constant flame spread rates are not typical of fire environment, the model provides a useful formula to predict the rate of flame spread over any solid fuel sheet thickness.

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