Symposium (International) on Combustion最新文献

An innovative 2-D numerical model of composite propellant combustion is proposed. It takes into acount the detailed description of the propellant topology, five chemical reactions, gas molecular diffusion, and heat transfer in the gas phase. Partial differential equations are numerically solved introducing a topological matrix. The propellant surface is determined by scanning the topological matrix and performing a series of logical tests. The bidimensional profiles of the temperature and molar fractions in the spatial domain are obtained. The average surface temperatures are also evaluated in both binder and oxidizer regions, as well as the linear burning rate. The model can predict the time evolution of the composite for different propellant topologies in agreement with experimental observations of the propellant surface reported in the literature. The propellant topology, the pressure, the oxidizer-binder mass ratio, and the characteristic dimension play a large role on surface temperatures and linear burning rate. They increase with pressure and decrease, with asymptotic tendency, with increase of both mass ratio and characteristic size. High burning rates are predicted for topologies that enhance the mixing betwen binder and oxidizer in particular when fine spherical particles of the oxidizer are dispersed within a binder matrix.

An experimental and numerical study is performed to elucidate the structure and mechanisms of extinction of non-premixed n-heptane flames. Experiments are conducted on flames stabilized between two connterflowing streams. The fuel stream is a mixture of prevaporized n-heptane and nitrogen, and the oxidizer stream is a mixture of air and nitrogen. Concentration profiles of C₇H₁₆, O₂, N₂, CO₂ CO, H₂, CH₄, C₂H₂, C₂H₄, C₂H₆, C₃H₄, C₃H₆, C₃H₈, C₄-hydrocarbons, C₅-hydrocarbons and C₆-hydrocarbons are measured. The measurements are made by removing gas samples from the flame using a quartz microprobe and analyzing the samples using gas chromatographs. The identity of the species is established using a mass selective detector. Temperature profiles are measured using a thermocouple. In addition, critical conditions of extinction are measured, giving the mass fraction of reactants as a function of the strain rate. Numerical calculations are performed using detailed chemistry to determine the flame structure and critical conditions of extinction at conditions identical to those used in the experiments. Calculated and measured flame structures are found to agree reasonably well: however, a small shift is observed between the calculated and measured temperature and concentration profiles. In general, the measured profiles are broader than the calculated profiles. At given values of the mass fraction of oxygen in the oxidizer stream, the calculated strain rates at extinction are noticeably higher than those measured. Experiments are also performed on non-premixed flames stabilized in the counterflowing configuration over a liquid pool of n-heptane. Critical conditions of extinction are measured. Numerical calculations are performed at conditions used in these experiments, and critical conditions of extinction are obtained. At given values of the mass fraction of oxygen in the oxidizer stream, the calculated strain rates at extinction are noticeably higher than those measured. The differences between the calculated and measured strain rates at extinction for the liquid pool flame are higher than for n-heptane-vapor flames.

Quantitative CN, OH, and OH rotational temperature, and velocity two-dimensional profiles have been imaged using planar laser-induced fluorescence (PLIF) imaging or particle-imaging velocimetry (PIV) during ignition and deradiative extinguishment of cyclotrimethylene-trimitramine, (RDX) at three different heat flux levels. Relative NO₂ and NO profiles were also imaged with PLIF. Decomposition products, such as NO and NO₂ were formed early in the laser heating process and the gas plume moved, away from the surface. At a later time, ignition occurred in the gas phase, as evidenced by radical buildup such as CN and OH. This often showed as a spherical ignition kernel away from the surface. The flame then transitioned rapidly to a thin flame sheet that moved toward the surface. With longer heating times, laser-supported quasi-steady-state-deflagration develops as the flame sheet again moves somewhat further from the surface. This data can be used as an aide in the development of fully time-dependent RDX combustion models. The ignition and deradiative extinguishment data will help validate these time-accurate models, which can then be used to study combustion instability.

Active instability suppression using periodic liquid-fuel injection was demonstrated in a dump combustor. The controller fuel, which made up 12%–30% of the total heat release, was pulsed directly into the combustion chamber, and the injection timing was adjusted with respect to the combustor pressure signal. Because the injection timing determined the degree of interaction between pulsed fuel sprays and periodic large-scale flow features, it significantly affected the spatial distribution of fuel droplets inside the combustion chamber. Simple closed-loop control of the pulsed injection timing was applied to two different cases that developed natural instabilities. In the first case, the instability frequency was unchanged at the onset of the closed-loop control, and this fact allowed up to 15 dB reduction in the sound pressure level. A detailed investigation showed that the pressure oscillation amplitude reached the minimum value when the start of the pulsed fuel injection was synchronized with the inlet vortex shedding process. In the second case, the same controller was applied to a higher output combustor, where the injection timing affected not only the oscillation amplitude but also the instability frequency. For the high output case, the controller was able to suppress the oscillations initially, but it could not maintain the suppressed amplitude, resulting in unsteady modulation of the oscillation amplitude and frequency. The intermittent loss of control was linked to the frequency-dependent phase shift, associated with an electronic band-pass filter. The present results open up the possibility of utilizing direct pulsed liquid-fuel injection for active combustion control in propulsion devices, but they also show the limitation of a simple phase-delay approach in completely suppressing the natural oscillations under certain conditions.

The thermal decomposition of indene was studied behind reflected shock waves in a pressurized driver single-pulse shock tube over the temperature range 1150–1900 K and densities of ≈3×10⁻⁵ mol/cm³. GC analyses of post-shock mixtures revealed the presence of the following decomposition products, given in order of increasing molecular weight: CH₄, C₂H₂, CH₂=C=CH₂, CH₃C≡CH, C₄H₂, C₆H₆, C₆H₅−CH₃, C₆H₅−C≡CH, and also naphthalene and its structural isomer, probably 1-methylene-1H-indene. Small or trace quantities of C₂H₄, C₄H₄, C₅H₆, C₅H₅−C≡CH, and C₆H₄ were also found in the postshock mixtures. A kinetic scheme based on cyclopentadiene decomposition pathway alone cannot account for the observed product distribution. It can be accounted for if H-atom attachment to the π bond in the five-membered ring followed by consecutive decomposition of the formed indanyl radical is assumed in addition to the indenyl channel. A reaction scheme with the two pathways containing 50 species and 74 elementary reactions reproduces very well the experimental product distribution. In this paper, we show the reaction scheme, the results of computer simulation, and sensitivity analysis. Differences and similarities in the reaction patterns of cyclopentadiene and indence are discussed.

The dissociation of SiCl₄ was studied behind reflected shock waves. Atomic resonance absorption spectroscopy (ARAS) was applied for time-resolved measurements of Cl- and Si-atoms in gas mixtures containing 0.4 to 4 ppm SiCl₄ highly diluted in argon. The signals obtained were kinetically evaluated by computer simulations based on a simplified reaction mechanism. Rate coefficients for the reactions$\begin{array}{c}{SiCl}_4+Ar\stackrel{k_1}{\rightleftharpoons }{SiCl}_3+Cl+Ar(R1)\\ SiCl+Ar\stackrel{k_4}{\rightleftharpoons }Si+Cl+Ar(R4)\\ k_1=6.9\times {10}^{16}exp(-37760K/T){cm}^3{mol}^{-1}{s}^{-1}\\ k_4=1.4x{10}^{40}{T}^{-7.0}exp(-49315K/T){cm}^3{mol}^{-1}{s}^{-1}\end{array}$

Thermochemical evolution of a droplet suddenly exposed to a hot environment is studied to assess the transient characteristics of the ignition, flame bifurcation and scavenging combustion, transition of premixed flame to nonpremixed combustion, and the ultimate burnout of an isolated droplet. Canonical theory of droplet gasification gives a general criteria of ignition and serves to identify all the gasification submechanisms of an arbitrary geometry in a stationary or convective environment. The theory is used in conjunction with numerical analysis for prediction of the transient flow-field structures and the gasification rates of all the submechanisms of gasification. The results reveal that the ignition transience exhibits flame bifurcation in a broad range of the environmental temperature, which lies between 930 K and 1700 K, for an n-heptane droplet with the simulated reaction rate model. At temperature higher than 1700 K, flame splitting does not occur. There are, in general, seven gasification submechanisms for a droplet: however, the net gasification rate during the ignition, flame bifurcation, and scavenging combustion is primarily controlled by the exothermic reaction and thermal energy accumulation, each of which has the effective gasification rate of nearly 10 to 40 times of that of the conventional Godsave-Spalding gasification rate: whereas the other submechanisms contribute at nearly the same order of magnitude of the Godsave-Spalding gasification rate. The droplet combustion is also classified into fully evolved and partially evolved combustion depending on the state of the droplet at the burnout. The predicted ignition delay time for n-heptane droplets in the size range of 600–2200μm are in good qualitative agreement with available experimental data. Areas of future research are also discussed.

In recent years, concerns over the impact of internal combustion engine hydrocarbon emissions on the environment have prompted tighter regulation on allowable emission lebels. While much work has been done on reducing hydrocarbon emissions after they have entered the exhaust stream, less direct monitoring of the emission sources has been performed. An optically accessible four-stroke internal combustion engine was used to investigate how fuel composition and engine operating conditions affect hydrocarbon emissions. Various crevices, ranging in size from 1 to 2 mm, were simulated by drilling holes into a flat wall built into the head of the engine. Emissions from the residual fuel ejected by these crevices were directly monitored using planar laser-induced fluorescence (PLIF) from iso-octane/n-heptane fuel blends doped with 3-pentanone. The fluorescence was imaged at various times during the engine cycle and found to be extremely dependent on crevice size, engine load, and fuel reactivity, Under most normal load conditions, the largest crevice showed evidence of significant flame penetration, while flame penetration into the smaller crevices was found to vary with engine load. The results for the quench diameter were in good agreement with a simple crevice flame-quenching model. Fuels with lower octane ratings were shown to enhance flame penetration due to their increased reactivity.

A Monte Carlo (MC) model for the simulation of chemical reactions of a surface coupled to a reactive gas phase with correct real-time dependence is presented. To avoid the shortcomings of the mean-field (MF) approximation, a lattice-gas model is used to describe the kinetics on the reactive surface. The local environment of an adsorbate molecule is included explicitly. The corresponding master equation is solved by simulation of stochastic processes with a Monte Carlo method. The model includes a description for the adsorption and desorption equilibria of CO and molecular oxygen on platinum as well as the surface reaction of CO with O atoms. The focus is on the influence of adsorbate-adsorbate interactions, which cannot be accounted for correctly in the mean-field model. The transport processes in the gas phase are described by a molecular multicomponent transport model. The governing equations are formulated for the geometry of a stagnation-point flow onto the catalytically active plate. Gas phase and reactive surface are treated separately. The coupling between both parts of the system is realized by choosing time steps small enough to limit the change of the mass fractions at the heterogeneous boundary within a defined range. Under this assumption, the mass fraction in the gas phase at the first grid point above the surface can be regarded as constant for the Monto Carlo simulation. After each time step, the mass fractions at the boundary are updated according to the fluxes resulting from the events during the Monte Carlo step. This is followed by a time step for the gas phase generating new initial values for the subsequent Monte Carlo step. Between each Monte Carlo step, the spatial distribution of the adsorbed species is preserved.

Catalytic ignition of hydrogen-oxygen on platinum at atmospheric pressure is studied experimentally. The hydrogen-oxygen mixtures are diluted with nitrogen to prevent the homogeneous ignition. The parameters are relative hydrogen concentration and dilution ratio, a mole fraction of nitrogen. The surface temperature is measured with an R-type thermocouple. The time histories of surface temperature show an inflection point, and the catalytic ignition temperature is defined as the temperature at the inflection point. As a result, the ignition temperature is increased with the dilution ratio and shows a minimum at a certain relative hydrogen concentration. The reciprocal ignition temperature is proportional to the natural log of total reactant mole fraction, and the proportional constants are almost the same in all relative hydrogen concentration.

If the catalytic ignition has occurred, an abrupt transition from a kinetically controlled system to one controlled by mass transport is produced. It is approximated that the surface conditions could be evaluated in a mass transport controlled system as the time needed to change the systems is very short and that the gaseous compositions are the same before the ignition has occurred as the surface reaction rate is much faster than the velocity of the mass transport. To analyze the relation between the ignition temperature and the reactant concentrations simply, an overall reaction model with an Arrhenius expression is used, and the orders of reaction of the adsorbed H and O are one in the expression. The expression includes the desorption of the excess of the adsorbed H.

The expression explained the dependence of the ignition temperature on the dilution ratio quantitatively and denoted a minimum ignition temperature at a certain relative hydrogen concentration as shown in the experimental results.