Asymptotic structure and extinction of COH₂ diffusion flames with reduced kinetic mechanisms

Models are considered for diffusion flames involving a fuel consisting of mixtures of carbon monoxide, hydrogen, and nitrogen and an oxidizer consisting of mixtures of oxygen and nitrogen. Initially the kinetic scheme is reduced systematically to the two-step mechanism CO + H₂O ⇄ CO₂ + H₂, 2H₂ + O₂ ⇄ 2H₂O, the water-gas shift, and hydrogen oxidation. In a model for low strain rates there is a broad region of water-gas equilibrium, bounded on the fuel-lean side by a thin zone of hydrogen oxidation and on the fuel-rich side by a thin zone of sudden water-gas freezing caused by chain-carrier removal. Structures of each of these thin zones are analyzed with the aid of asymptotic methods, and it is shown that a three-step mechanism is needed to obtain a reasonably accurate description of the water-gas freezing. In these analyses simplified transport descriptions are employed in which only the Lewis number of hydrogen is allowed to differ from unity. In the model for high strain rates the oxidation of carbon monoxide is nearly frozen while hydrogen oxidation occurs in a broad zone throughout which the reaction rate is approximated as constant. Many predictions of these simplified models are found to agree surprisingly well with results of full numerical integrations for flame structure and extinction.