Non-premixed hydrocarbon ignition at high strain rates

Abstract

We report on the results of numerical-simulation investigations of ignition characteristics of hydrocarbon-fuel blends expected from thermal cracking of typical jet fuels, at conditions relevant to high-Mach-number, air-breathing propulsion. A two-point-continuation method was employed, with a detailed description of molecular transport and chemical kinetics, focusing on the effects of fuel composition, reactant temperature, additives, and imposed strain rate. It captured the entire S-curve that describes the processes of vigorous burning extinction, and ignition. The results demonstrate that ignition of such fuel blends is dominated by the synergistic behavior of CH₄ and C₂H₄. A fuel temperature of T_fuel=950 K was employed throughout. At higher air temperatures (T_air=1200 K), addition of small amounts of CH₄ to C₂H₄ molerately inhibits C₂H₄ ignition, while at lower T_air=1050 K, CH₄ promotes ignition. Large amounts of CH₄, however, inhibit C₂H₄ ignition at all T_airs. Ignition promotion was also investigated through the independent addtion of H₂ and F₂ in the reactant streams. H₂ addition (e.g., 2–10%) produces a two-stage ignition and sustains higher ignition strain rates. Small amounts of F₂ (1%) result in F-radical production, contributing to efficient fuel consumption, enhancing ignition characteristics. Ignition strain rates of σ_ign≅4000 s⁻¹, as compared to σ_ign≅250 s⁻¹ for pure C₂H₄, can be attained with such additives at lower temperatures (T_air=1050 K).