Non-premixed hydrocarbon ignition at high strain rates

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 moderately inhibits C2H4 ignition, while at lower T-air = 1050 K, CH4 promotes ignition. Large amounts of CH,I however, inhibit C2H4 ignition at all T(air)s. Ignition promotion was also investigated through the independent addition of H-2 and F-2 in the reactant streams. Hg addition (e.g., 2-10%) produces a two-stage ignition and sustains higher ignition strain rates. Small amounts of F-2 (1%) result in F-radical production, contributing to efficient fuel consumption, enhancing ignition characteristics. Ignition strain rates of s(ign) similar or equal to 4000 s(-1), as compared to s(ign) similar or equal to 250 s(-1) for pure C2H4, can be attained with such additives at lower temperatures (T-air = 1050 K).