Structure of Laminar Opposed-flow Diffusion Flames With CO/H₂/N₂ Fuel

Abstract

The detailed structure of laminar opposed-flow diffusion flames with a fuel composition of 40% CO, 30% H₂, and 30% N₂ and an oxidizer composition of 79% N₂ and 21% O₂ has been calculated over a wide range of stretch conditions (α = 0.1-5000s^-1) using 32 elcmenlal chemical reactions and realistic transport properties. Previously, when compared to experimental measurements at χ = 70, 180, and 330s-', this model was found to predict peak flame temperatures within 150K. widths of temperature profiles to 20%, and positions of temperature maxima to 0.1 cm. The present paper presents calculations over a wider range of stretch (χ = 0.1-5000s^-1) and quantifies the effect of stretch on fundamental flame processes such as preferential diffusion. overlap of fuel and air. superequilibrium radical formation. deviations from partial equilibrium, and extinction. Preferential diffusion of H₂ leads to superadiabatic temperatures in the α = 0.1 s^-1 flame. Pronounced nonequilibrium effects occur even in the α = 0.1 s^-1 flame and increase markedly in flames with higher stretch. Water gas equilibria occur only when the temperatures are larger than 1800 K, with deviations greater than a factor of 20 in low temperitlure. lean flame zones. Results are compared to previous calculations on stretched flames with H₂ and CH₄ fucls. The complexity of the finite rate chemical processes shown here in the laminar opposed-flow CO/H₂/N₂ diffusion flames suggests that the assumptions of full or partial equilibrium used in previous models of turbulent CO/H₂/N₂ jet diffusion flames are inadequate. The present results provide the basis for turbulent combustion models using the laminar flamclet approach which may give a more realistic description of turbulence-chemistry interactions.