Chemical kinetic factors of hydrocarbon oxidation are examined in a variety of ignition problems. Ignition is related to the presence of a dominant chain-branching reaction mechanism that can drive a chemical system to completion in a very short period of time. Ignition in laboratory environments is studied for problems including shock tubes and rapid compression machines. Modeling of the laboratory systems is used to develop kinetic models that can be used to analyze ignition in practical systems. Two major chain-branching regimes are identified, one consisting of high temperature ignition with a chain branching reaction mechanism based on the reaction between atomic hydrogen with molecular oxygen, and the second based on an intermediate temperature thermal decomposition of hydrogen peroxide. Kinetic models are then used to describe ignition in practical combustion environments, including detonations and pulse combustors for high temperature ignition and engine knock and diesel ignition for intermediate temperature ignition. The final example of ignition in a practical environment is homogeneous charge, compression ignition (HCCI), which is shown to be a problem dominated by the kinetics of intermediate temperature hydrocarbon ignition. Model results show why high hydrocarbon and CO emissions are inevitable in HCCI combustion. The conclusion of this study is that the kinetics of hydrocarbon ignition are actually quite simple, since only one or two elementary reactions are dominant. However, many combustion factors can influence these two major reactions, and these are the features that vary from one practical system to another.

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Chemical kinetics of hydrocarbon ignition in practical combustion systems

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In this study, we extend the results of previous combined numerical and experimental investigations of an axisymmetric laminar diffusion flame in which difference Raman spectroscopy, laser-induced fluorescence (LIF), and a multidimensional flame model were used to generate profiles of the temperature and major and minor species. A procedure is outlined by which the number densities of ground-state CH (X(sup 2)II) excited-state CH (A(sup 2)Delta, denoted CH*), and excited-state OH (A(sup 2)Sigma, denoted OH*) are measured and modeled. CH* and OH* number densities are deconvoluted from line-of-sight flame-emission measurements. Ground-state CH is measured using linear LIF. The computations are done with GRI Mech 2.11 as well as an alternate hydrocarbon mechanism. In both cases, additional reactions for the production and consumption of CH* and OH* are added from recent kinetic studies. Collisional quenching and spontaneous emission are responsible for the de-excitation of the excited-state radicals. As with our previous investigations, GRI Mech 2.11 continues to produce very good agreement with the overall flame length observed in the experiments, while significantly under predicting the flame lift-off height. The alternate kinetic scheme is much more accurate in predicting lift-off height but overpredicts the over-all flame length. Ground-state CH profiles predicted with GRI Mech 2.11 are in excellent agreement with the corresponding measurements, regarding both spatial distribution and absolute concentration (measured at 4 ppm) of the CH radical. Calculations of the excited-state species show reasonable agreement with the measurements as far as spatial distribution and overall characteristics are concerned. For OH*, the measured peak mole fraction, 1.3 x 10(exp -8), compared well with computed peaks, while the measured peak level for CH*, 2 x 10(exp -9), was severely underpredicted by both kinetic schemes, indicating that the formation and destruction kinetics associated with excited-state species in flames require further research.

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Experimental and computational study of CH, CH*, and OH* in an axisymmetric laminar diffusion flame

In this study, we extend the results of previous combined numerical/experimental investigations of an axisymmetric laminar diffusion flame in which spontaneous Raman spectroscopy, laser-induced fluorescence, and a multidimensional flame model were used to generate profiles of the temperature and major and minor species. We discuss issues related to the computation and measurement of NO in an unconfined laminar flame in which a cylindrical fuel stream is surrounded by a co-flowing oxidizer jet. Computationally, the governing conservation equations of mass, momentum, species, and energy were solved with detailed transport and finite-rate chemistry submodels to predict the velocity, species mass fractions, and temperature fields as functions of the two independent spatial coordinates. A discrete solution was obtained on a two-dimensional grid by employing Newton's method with adaptive mesh refinement. The computations were performed in both serial and parallel using up to 32 processors of an IBM SP2. Experimentally, NO radical concentrations were measured using laser-induced fluorescence. The associated results of the computational study clearly indicate that the prompt production path is the dominant formation route of NO. Thermal NO and the N 2 O submechanism play minor roles in total NO production. Excellent agreement was obtained for the structural features of the computed and measured NO. The peak NO mole fractions agreed to within 30% of the experimental value. Additional quantitative CH computations and measurements are essential for further study of this problem.

Computational and experimental study of no in an axisymmetric laminar diffusion flame

Forced, time-varying flames are laminar systems that help bridge the gap between laminar and turbulent combustion. In this study, we investigate computationally and experimentally the structure of an acoustically forced, axisymmetric laminar methane-air diffusion flame, in which a cylindrical fuel jet is surrounded by a coflowing oxidizer jet. The flame is forced by imposing a sinusoidal modulation on the steady fuel flow rate. Rayleigh scattering and spontaneous Raman scattering of the fuel are used to generate the temperature profile. Particle image velocimetry (PIV) is used to measure the fuel tube exit velocity over a cycle of the forcing modulation. CH flame emission measurements have been done to predict the excitedstate CH (CH * ) levels. Computationally, we solve the transient equations for the conservation of total mass, momentum, energy, and species mass with detailed transport and finite-rate C 2 chemistry submodels to predict the pressure, velocity, temperature, and species concentrations as a function of the two independent spatial coordinates and time. The governing equations are written in primitive variables. Implicit finite differences are used to discretize the governing equations and the boundary conditions on a nonstaggered, noniumiform grid. Modified damped Newton's method nested with a Bi-CGSTAB iteration is utilized to solve the resulting system of equations. Results of the study include a detailed description of the fluid dynamic-thermochemical structure of the flame at a 20-Hz frequency. A comparison of experimentally determined and calculated temperature profiles and CH * levels agree well. Calculated mole fractions of species indicative of soot production (C 2 H 2 , CO) are compared against those levels in the corresponding steady flame and are observed to increase in peak concentration values and spatial extent. Analysis of acetylene production rates reveals additional significant production in the downstream region of the flame at certain times during the flame's cyclic history.

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Computational and experimental study of a forced, timevarying, axisymmetric, laminar diffusion flame

Detailed numerical calculations, which are supported experimentally, are used to investigate the sensitivity of flame structure to fuel premixedness and overall strain rate. The study covers a wide range of fuel premixedness—from =1.3 to =2.0, plus a pure diffusion flame—over a wide range of strain rates—from moderate to near extinction—in methane-air vs. air counterflow flames. The flames are observed to change drastically in structure and character—from a single, merged flame in the vicinity of the stagnation plane to a double flame consisting of a premixed-type, fuel-side flame and a stagnation-region diffusion flame—as the fuel stream equivalence ratio is perturbed slightly below ≈1.5–1.4. Accordingly, the mode and amount of NO x formation changes severely. This duality in flame structure is further discerned by monitoring the relative locations of CH and OH profiles, as these are indicators of specific flame chemistry. The exact value of the “changeover” equivalence ratio depends upon the flame's strain rate, and in fact, flames close enough to extinction remain as a merged flame structure even at the lowest equivalence ratio. The maximum fuel-side velocity gradient is shown to be an extremely sensitive and sharp indicator of flame character, being completely insensitive to fuel-stream equivalence ratio above certain strain-dependent values, but varying sharply with equivalence ratio below these values. Other parameters, such as the width of the temperature or product species profiles, are shown to be indicators of flame structure, also, but are not nearly as sharply responsive as the fuel-side velocity gradient. These results could have important implications for design criteria for commercial burners, as well as for applications to the prediction of turbulent flame structure, including suppression and extinction.

Michael A. Tanoff

Papers

Experimental and computational study of CH, CH, and OH in an axisymmetric laminar diffusion flame

Computational and experimental study of no in an axisymmetric laminar diffusion flame

Computational and experimental study of a forced, timevarying, axisymmetric, laminar diffusion flame

The sensitive structure of partially premixed methane-air vs. air counterflow flames

Computational and experimental studies of laser-induced thermal ignition in premixed ethylene-oxidizer mixtures