Simplified Reaction Mechanisms for the Oxidation of Hydrocarbon Fuels in Flames

Simplified reaction mechanisms for the oxidation of hydrocarbon fuels have been examined using a numerical laminar flame model. The types of mechanisms studied include one and two global reaction steps as well as quasi-global mechanisms. Reaction rate parameters were varied in order to provide the best agreement between computed and experimentally observed flame speeds in selected mixtures of fuel and air. The influences of the various reaction rate parameters on the laminar flame properties have been identified, and a simple procedure to determine the best values for the reaction rate parameters is demonstrated. Fuels studied include n-paraffins from methane to n-decane, some methyl-substituted n-paraffins, acetylene, and representative olefin, alcohol and aromatic hydrocarbons. Results show that the often-employed choice of simultaneous first order fuel and oxidizer dependence for global rate expressions cannot yield the correct dependence of flame speed on equivalence ratio or pressure and cannot correctly predict the rich flammability limit. However, the best choice of rate parameters suitably reproduces rich and lean flammability limits as well as the dependence of the flame speed on pressure and equivalence ratio for all of the fuels examined. Two-step and quasi-global approaches also yield information on flame temperature and burned gas composition. However, none of the simplified mechanisms studied accurately describes the chemical structure of the flame itself.