Studies on strained non-premixed flames affected by flame curvature and preferential diffusion

Experimental and numerical studies are made of H 2 -N 2 -air laminar counterflow diffusion flames strained by steadily impinging micro fuel jet or air jet from the fuel side or the air side. Direct observation and two-dimensional temperature measurements by laser Rayleigh scattering method show that the flame strained by the micro fuel jet from the fuel side (flame 1) is easily quenched locally, but the flame strained from the air side (flame 2) hardly extinguishes and the flame temperature tends to increase. Numerical computations, taking into account detailed chemical kinetics and multicomponent diffusion, predict well the characteristics of the experimental findings noted above for the strained flames 1 and 2. They reveal the factors dominating the flame characteristics; that is, (1) H 2 of the fuel component is diluted by nitrogen compared to the original fuel near the central axis at stoichiometric condition in flame 1 with micro fuel jet but is concentrated in flame 2 with micro air jet due to the preferential diffusion among species in relation to the flame curvature; and (2) significant excess enthalpy is induced at the off-axis region in the lean and stoichiometric region due to the nonunity Lewis number effect. It is also noted that the maximum flame temperature cannot be rationalized by the local stretch rate but changes widely depending on the flame configuration.