Impact of CO2, N2 or Ar diluted in air on the length and lifting behavior of a laminar diffusion flame

Air-side dilution on laminar methane/air diffusion flames in the buoyant regime was extensively studied. CO2, important for EGR and fire safety, was chosen as a diluent. It was complemented by N2 and Ar. Flame stability, its external aspect and inner structure were quantified by complementary experiments and simulations. Two flame-response categories were highlighted, depending on whether mechanisms relied on several effects or mainly on pure dilution. In the former case, physico-chemical properties of diluents were essential. The greatest influence was detected with CO2, the least one with Ar. No critical value of the initial molar O2-concentration (XO2,i) was found to control quantities involved in stability (lifting limit, standoff distance of the attached flame), soot formation and flame-tip opening. It was found a critical flow-rate ratio from which flame-liftoff always occurred, crucial to describe lifting process induced by dilution. The reaction, naturally weak at the flame tip, led to an apparent open-tip with diluents. However, simulation showed the reaction, even weakened, persisted. On the contrary, pure dilution controlled flame length behavior. When XO2,i was decreased, length increased according to a unique evolution whatever the diluents. To measure flame length, several techniques were applied: OH Planar Laser Induced Fluorescence (Lf,OH), soot-Laser Induced Incandescence (Lf,LII), direct flame emission (Lf,luminous and Lf,peak). Length Lf,luminous was unable to correctly characterize flame lengthening. Lf,peak was the closest to the calculated stoichiometric length. So, the method giving Lf,peak appeared as the easiest and the most accurate one. Comparison between our data and correlations found in literature indicated Roper’s correlation was the most efficient to predict length evolution.