Aerodynamic, unsteady, kinetic and heat loss effects on the dynamics and structure of weakly-burning flames

The first objective of the program is to introduce the meritorious counterflow methodology in microgravity in order to quantify the steady and unsteady characteristics of weakly-burning premixed and diffusion flames for a wide variety of conditions including elevated pressures. Subsequently, through detailed modeling and comparisons with the experimental data, to provide physical insight into the elementary mechanisms controlling the flame response. The configuration offers good control over the parameters of interest and can be modelled closely. The knowledge which will be gained from the counterflow flames will be subsequently used to analyze near-limit phenomena related to other configurations by conducting detailed numerical simulations including multidimensional ones. Among the problems to be analyzed are the downward and upward propagation of near-limit flames in tubes and phenomena observed in spherical and cylindrical geometries

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

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

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

A Numerical Approach to Determining Flammability Limits of Hydrocarbon Process Fluids

PresentationIntrinsic to the computer modeling of explosions and fires is the concept of flammability limits. Conventionally, the term “flammability limit” is defined loosely as the concentration limits beyond which flame propagation is no longer possible. More formally, a fundamental flammability limit is defined as the mixture concentration at which a steady, laminar, one- dimensional, planar, and adiabatic flame fails to propagate. Fundamental flammability limits are reached when the heat release from chemical reactions becomes comparable to the radiative heat loss from the flame. The difficulty in predicting these fundamental limits, a priori, for a given combustible mixture is the dependence of the flammability limit on chemical kinetics. In this study we present a computational methodology for the determination of a mixture’s fuel lean and fuel rich flammability limits. Numerical calculations were performed using a modified version of the CHEMKIN PREMIX flame code. This code has been modified to allow for the capturing of the singular behavior around the turning point and allowing, thus, the accurate determination of a mixture’s flammability limits. The present methodology has been extensively validated to determine the flammability limits of single component and binary fuel mixtures. These validations are presented. Real hydrocarbon process fluids are complex mixtures that consist of hundreds of species spanning a wide range of molecular weights and chemical classes. The surrogate fuel approach, whereby the kinetics of the complex mixture is modeled using a few individual components, is now applied to determine the flammability limits of real hydrocarbons

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

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

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

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

Experimental and Modeling Studies of the Combustion Characteristics of Conventional and Alternative Jet Fuels. Final Report

The objectives of this project have been to develop a comprehensive set of fundamental data regarding the combustion behavior of jet fuels and appropriately associated model fuels. Based on the fundamental study results, an auxiliary objective was to identify differentiating characteristics of molecular fuel components that can be used to explain different fuel behavior and that may ultimately be used in the planning and design of optimal fuel-production processes. The fuels studied in this project were Fischer-Tropsch (F-T) fuels and biomass-derived jet fuels that meet certain specifications of currently used jet propulsion applications. Prior to this project, there were no systematic experimental flame data available for such fuels. One of the key goals has been to generate such data, and to use this data in developing and verifying effective kinetic models. The models have then been reduced through automated means to enable multidimensional simulation of the combustion characteristics of such fuels in real combustors. Such reliable kinetic models, validated against fundamental data derived from laminar flames using idealized flow models, are key to the development and design of optimal combustors and fuels. The models provide direct information about the relative contribution of different molecular constituents to the fuel performance and can be used to assess both combustion and emissions characteristics

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 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)