14 research outputs found

    Effects of Additives on the Non-Premixed Ignition of Ethylene in Air

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    The ignition characteristics of heated C_2H_4 counterflowing against heated air were numerically investigated in the presence of additives such as NO, F_2, and H_2. C_2H_4 and air temperatures were chosen to resemble conditions relevant to high-Mach number, air-breathing propulsion. The numerical simulations were conducted along the stagnation streamline of the counterflow and included detailed descriptions of chemical kinetics and molecular transport. It was found that addition of NO at concentrations of about 10,000 ppm (1%), results in a substantial increase of the ignition strain rate, from 300 /s to values up to 32,000/s. This ignition promotion is caused by enhanced radical production that is initiated through the interaction between NO and HO_2. A further increase in the NO amount leads to reduced improvements. Small additions of F_2 and H_2 were also found to promote ignition, but to lesser extent compared to NO. Results also show that with the addition of F_2 in the presence of NO, ignition promotion is further enhanced, and for F_2 and NO concentrations larger than 25,000 ppm, the system becomes hypergolic. The present investigations suggest that the use of C_2H_4, NO, and F_2 may permit ignition at conditions of relevance to SCRAMJET’s

    A Comparative Numerical Study of Premixed and Non-Premixed Ethylene Flames

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    Detailed numerical simulations of premixed and non-premixed C_2H_4/air flames were conducted, using six available kinetic mechanisms. The results help assess differences between these mechanisms and are of interest to proposed hydrocarbon-fueled SCRAMJET concepts, in which C_2H_4 can be expected to be a major component of the thermally cracked fuel. For premixed flames, laminar flame speeds were calculated and compared with available experimental data. For non-premixed flames, ignition/extinction Z-curves were calculated for conditions of relevance to proposed SCRAMJET concepts. Results revealed a large variance in predictions of the kinetic mechanisms examined. Differences in laminar flame speeds as high as factors of 2.5 were found. For the conditions investigated, computed ignition and extinction strain rates for non-premixed flames differed by factors as high as 300 and 3, respectively. This indicates that while there are differences in high-temperature kinetics that control flame propagation and extinction, discrepancies in low-temperature kinetics that control ignition can be even more significant. Sensitivity- and species-consumption analyses indicate uncertainties in fuel kinetics and, most importantly, on the oxidation of C_2H_3 and the production of CH_2CHO, whose kinetics are not well known and can crucially affect production of the important H radicals. These findings stress the need for experimental data in premixed and non-premixed configurations that can be used to assess these phenomena and provide the basis for a comprehensive validation

    Imaging through turbulence with a quadrature-phase optical interferometer

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    We present an improved technique for imaging through turbulence at visible wavelengths using a rotation shearing pupil-plane interferometer, intended for astronomical and terrestrial imaging applications. While previous astronomical rotation shearing interferometers have made only visibility modulus measurements, this interferometer makes four simultaneous measurements on each interferometric baseline, with phase differences of π/2 between each measurement, allowing complex visibility measurements (modulus and phase) across the entire input pupil in a single exposure. This technique offers excellent wavefront resolution, allowing operation at visible wavelengths on large apertures, is potentially immune to amplitude fluctuations (scintillation), and may offer superior calibration capabilities to other imaging techniques. The interferometer has been tested in the laboratory under weakly aberrating conditions and at Palomar Observatory under ordinary astronomical observing conditions. This research is based partly on observations obtained at the Hale Telescope

    Effects of Additives on the Non-Premixed Ignition of Ethylene in Air

    Get PDF
    The ignition characteristics of heated C_2H_4 counterflowing against heated air were numerically investigated in the presence of additives such as NO, F_2, and H_2. C_2H_4 and air temperatures were chosen to resemble conditions relevant to high-Mach number, air-breathing propulsion. The numerical simulations were conducted along the stagnation streamline of the counterflow and included detailed descriptions of chemical kinetics and molecular transport. It was found that addition of NO at concentrations of about 10,000 ppm (1%), results in a substantial increase of the ignition strain rate, from 300 /s to values up to 32,000/s. This ignition promotion is caused by enhanced radical production that is initiated through the interaction between NO and HO_2. A further increase in the NO amount leads to reduced improvements. Small additions of F_2 and H_2 were also found to promote ignition, but to lesser extent compared to NO. Results also show that with the addition of F_2 in the presence of NO, ignition promotion is further enhanced, and for F_2 and NO concentrations larger than 25,000 ppm, the system becomes hypergolic. The present investigations suggest that the use of C_2H_4, NO, and F_2 may permit ignition at conditions of relevance to SCRAMJET’s

    A Comparative Numerical Study of Premixed and Non-Premixed Ethylene Flames

    Get PDF
    Detailed numerical simulations of premixed and non-premixed C_2H_4/air flames were conducted, using six available kinetic mechanisms. The results help assess differences between these mechanisms and are of interest to proposed hydrocarbon-fueled SCRAMJET concepts, in which C_2H_4 can be expected to be a major component of the thermally cracked fuel. For premixed flames, laminar flame speeds were calculated and compared with available experimental data. For non-premixed flames, ignition/extinction Z-curves were calculated for conditions of relevance to proposed SCRAMJET concepts. Results revealed a large variance in predictions of the kinetic mechanisms examined. Differences in laminar flame speeds as high as factors of 2.5 were found. For the conditions investigated, computed ignition and extinction strain rates for non-premixed flames differed by factors as high as 300 and 3, respectively. This indicates that while there are differences in high-temperature kinetics that control flame propagation and extinction, discrepancies in low-temperature kinetics that control ignition can be even more significant. Sensitivity- and species-consumption analyses indicate uncertainties in fuel kinetics and, most importantly, on the oxidation of C_2H_3 and the production of CH_2CHO, whose kinetics are not well known and can crucially affect production of the important H radicals. These findings stress the need for experimental data in premixed and non-premixed configurations that can be used to assess these phenomena and provide the basis for a comprehensive validation

    Non-premixed hydrocarbon ignition at high strain rates

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    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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