Numerical and asymptotic studies of the structure of premixed iso-octane flames

Abstract

Numerical calculations and rate-ratio asymptotic analysis are performed to obtain the structure and burning velocities of premixed iso-octane flames. The numerical calculations employ a detailed chemicalkinetic mechanism comprising 967 elementary reactions, a skeletal chemical-kinetic mechanism comprising 47 elementary reactions, and a reduced chemical-kinetic mechanism for lean to stoichiometric conditions made up of six overall reactions among nine species including the hydrogen radical. The values of burning velocities calculated numerically using the detailed, skeletal, and reduced chemical-kinetic mechanisms are found to agree well with each other as well as with previous measurements. The asymptotic structure of stoichiometric and lean flames is analyzed using a reduced chemical-kinetic mechanism comprising five overall reactions. This mechanism is deduced from the reduced chemical kinetic mechanism employed in the numerical calculations after introducing steady-state approximation for the hydrogen radical. In the analysis, the flame structure is presumed to consist of three zones—a preheat zone of thickness of order unity, a thin reaction zone, and a postflame zone. In the reaction zone, the chemical reactions are presumed to take place in three layers—an inner layer, a C3H4-consumption layer, and a H2-CO oxidation layer. Within the inner layer, the fuel iso-octane is consumed in a thin sublayer and i-C4H8 is formed, which subsequently reacts with radicals to form the intermediate hydrocarbon compound C3H4. This intermediate hydrocarbon is consumed in the C3H4-consumption layer. In the inner layer and the C3H4-consumption layer, H2 and CO are formed. Most of the final products CO2 and H2O are formed in the H2-CO oxidation layer. In this layer, H2 is presumed to be in steady state everywhere except in a thin sublayer called the H2-consumption layer. The burning velocities calculated using the results of the asymptotic analysis are found to agree reasonably well with those calculated numerically using the detailed, skeletal, and reduced chemical-kinetic mechanisms and with previous measurements.