Effect of n-dodecane decomposition on its fundamental flame properties

Abstract

The effect of fuel decomposition on fundamental flame properties was investigated computationally for atmospheric-pressure n-dodecane/air mixtures. The fuel decomposition was modeled under isobaric and adiabatic conditions for initial temperatures of 1100, 1200 and 1300 K, and equivalence ratios of 0.7, 1.0, and 1.4. For various extents of n-dodecane oxidative thermal decomposition, the combustion characteristic of mixtures of the resulting products with air were investigated by keeping the total enthalpy constant and equal to that of the n-dodecane/air mixture. The endothermic n-dodecane decomposition was found, to a large extent, to be decoupled from the subsequent oxidation of the attendant products that include largely hydrogen, ethylene, methane, and other small alkenes. The mass burning rates in freely propagating flames were found to increase with an increase in the extent of n-dodecane decomposition, but the change is limited to 15%, which occurs in the highest extent of decomposition. On the other hand, the extinction strain rate of decomposed, lean to stoichiometric mixtures increases notably compared to the corresponding un-decomposed fuel–air mixtures. Sensitivity analyses of mass burning rates and extinction strain rates to kinetics and binary diffusion coefficients reveal that the laminar flame speed is primarily sensitive to key heat release and radical branching reactions, and as such fuel decomposition has a small effect on the mass burning rate. On the other hand, the extinction strain rate of the fuel-lean mixtures is sensitive to the diffusivity of the fuel, and for this reason, fuel decomposition removes the difficulties associated with the transport of the large fuel molecules into the flame zone.