Effects of water vapor addition on soot formation in laminar ethylene counterflow flames and comparison with CO2 addition

Abstract

A numerical investigation of H2O addition effects on soot formation was performed in laminar counterflow diffusion flames under an oxygen enriched atmosphere. The general flame structure, the chemical effects of H2O addition and a global comparison between CO2 and H2O addition are presented. Water vapor was added either on the fuel or in the oxidizer side, considering the same amount of that species at the flame sheet. Dilution effects were supressed by keeping the C2H4 and O2 concentrations constant for all flames while N2 is replaced by the added species. The reactive flow was modelled assuming a one­dimensional approximation, soot was modelled by the discrete sectional method and a detailed kinetic mechanism was employed. It was observed that H2O addition suppresses soot nucleation and surface growth by decreasing H molar fraction through reaction H2O +H ↽− −⇀ OH +H2, while soot surface oxidation is enhanced mostly by reaction H2 O + O ↽− −⇀ OH + OH, which increases OH molar fraction. Addition of H2O on the fuel side was more effective in inhibiting soot formation than H2O addition on the oxidizer side. Water vapor and CO2 additions were compared in different addition levels, which showed that H2O is a more effective soot suppressor.