A Phenomenological Model for Smoke-Point and Soot Formation in Laminar Flames

Aglobal soot formation model is proposed, validated and applied based on the postulate that soot formation in laminar diffusion flames is controlled by gaseous reaction processes and not the available soot surface area. Although this model was not derived from detailed soot formation mechanisms, it is consistent with such mechanisms if the active sites for soot growth are so many that soot growth depends on the availability of gaseous species (e.g., C2H2 ). Global soot formation models in laminar diffusion models based on a surface growth process can not be reconciled with experiments. For the present model, a soot formation time in axisymmetric laminar diffusion flames below the smoke-point is shown to beproportional to the smoke-point height. Moreover the present model implies that smoke-point heights are proportional to pressure in agreement with experiments. Predictions of the model for flow. dilution and pressure effects on soot concentrations agree with experiments. both in laminar jet flow (2-D or axisymmetric) and opposed flow diffusion flames. The effects of radiant cooling on the flame and the flow structure are also briefly addressed in this work.