To ex plore the flame spread mech a nisms over the solid fuel sheets, down ward flame spread over ver ti cal polymethylmethacrylate sheets with thick nesses from 1.75 to 5.75 mm have been ex am ined in the qui es cent en vi ron ment. The de pend ence of the flame spread rate on the thick ness of sheets is ob tained by one-di men sional heat trans fer model. An equa tion for the flame spread rate based on the ther mal prop er - ties and the thick ness of the sheet by scale up method is de rived from this model. Dur ing com bus tion, tem per a ture within the gas and solid phases is mea sured by a fine ther mo cou ple. The py rol y sis tem per a ture, the length of the py rol y sis zone, the length of the pre heat ing zone, and the flame tem per a ture are de ter mined from the ex per i men tal data. Math e mat i cal anal y sis has yielded re al is tic re sults. This model pro vides a use ful for mula to pre dict the rate of flame spread over any thin solid fuel.

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