An analysis of laminar, free-convection, diffusion flames burning on a vertical fuel surface is presented. The study is directed towards determining the effects of inert additives on the boundary layer structure and burning rates. Results are presented for several classes of inert additives using polymethyl methacrylate burning in air as the base system. The use of a fuel-phase additive to release noncombustible gases is modeled by varying the parameter YFT, the mass fraction of fuel in the gaseous products of pyrolysis. The effects of an additive which promotes char formation is modeled by varying the heat of pyrolysis per unit mass of gaseous pyrolysate released for combustion. The effects of a spray of water droplets on a wall fire is modeled by a combination of the release of a noncombustible gas and a surface heat sink which results from evaporation of water droplets at the fuel surface. The water spray calculations predict an extinction limit which is within a factor of two of experimental results.

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