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 CH_4 and C_2H_4. A fuel temperature of T_(fuel)=950 K was employed throughout. At higher air temperatures (T_(air)=1200 K), addition of small amounts of CH_4 to C_2H_4 molerately inhibits C_2H_4 ignition, while at lower T_(air)=1050 K, CH_4 promotes ignition. Large amounts of CH_4, however, inhibit C_2H_4 ignition at all T_(air)s. Ignition promotion was also investigated through the independent addtion of H_2 and F_2 in the reactant streams. H_2 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 σign≅4000 s^(−1), as compared to σ_(ign) ≅ 250 s^(−1) for pure C_2H_4, can be attained with such additives at lower temperatures (T_(air)=1050 K).

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