Overall Kinetics of Hot Gas Ignition

Abstract

Overall rate expressions have been developed to describe the ignition of nearly stagnant fuel-air mixtures in a vertical jet of hot, inert gas. Kinetic parameters, frequency factor A, activation energy E, fuel power C and oxygen power D, were computed to match experimental ignition results in the rate expression where (C + D) = total order of reaction M_f = molecular weight of fuel M_o = molecular weight of oxygen P = total pressure (1.0 atomspheres in this study) R = gas constant T = absolute temperature of hot gas ignition measured at jet base °K W_f = fuel mass rate of consumption gm/cm³ sec Y_f = fuel mole fraction Y_o = oxygen mole fraction Because the current hot gas ignition system is similar to a dilute batch or plug flow reactor at constant density, a simplifying correlation of where u_cl , = center line velocity of hot gas z = ignition distance from jet base at constant composition yielded a straight line with a slope of −E/R. This correlation proved to be valid for all ignition experiments run with five hot gas diameters ranging from 0.476 to 1.588 cm and all computations with reasonable values of the kinetic parameters. Using literature estimates of 1.1, 1.7 and 1.8 for (C + D) yielded E values from this correlation of 50,430, 44,930 and 41,460 cal/mole for methane, ethane and ethylene respectively. A and C were computed on the basis of complete reaction to CO₂ and H₂O in the preignition zone. To account for the observed maximum rate with a stoichiometric mixture, fits were made with C being positive for lean mixtures and negative for rich ones. An overall fit with a C of zero was made to stoichiometric experiments with A's of 2.9 × 10⁷, 4.7 × 10¹⁰ and 1.4 × 10¹⁰ for methane, ethane and ethylene respectively.