Symposium (International) on Combustion最新文献

The effects of small amounts of dimethyl ether (DME), NO, and NO₂ on the autoignition and oxidation chemistry of methane, with an without small amounts of ethane present, were experimentally studied in a flow reactor at pressures and temperatures similar to those found in spark- and compression-ignition engines (under autoignition conditions). The reactions were studied at pressures from 10 to 18 atm, temperatures from 800 to 1060 K, and equivalence ratios from 0.5 to 2.0. It is found that 1% DME addition is as effective in stimulating the autoignition and oxidative behavior of methane as 3% C₂H₆ addition, and that NO_x at even ppm levels is more effective than hydrocarbon additives. For the same reaction time and temperatures below 1200 K, addition of small amounts of NO_x lowered the temperature at which reaction becomes significant by more than 200 K. Chemical kinetic mechanisms in the literature for the interactions of methane, ethane, and NO_x do not predict the reported observations well. The most significant rate-controlling reactions for CH₄ autoignition is found to be CH₃+HO₂=CH₃O+OH. Good agreement, with and without NO_x perturbations can be obtained by modifying the rate constants of three reactions (CH₃+HO₂=CH₃O+OH; CH₃+HO₂=CH₄+O₂: CH₂O+HO₂=HCO+H₂O₂) and by adding the reaction CH₃+NO₂=CH₃O+NO to the GRI-Mech v2.11 mechanism. These modifications do not significantly affect predictions for shock tube, flame, and other results used in developing GRI-Mech v2.11. Results strongly suggest that exhaust gas residuals and/or exhaust gas recirculation can have as profound an effect as natural gas contaminants on the apparent octane and cetane behavior.

This paper presents a theoretical investigation of combustion instabilities in low NO_X gas turbines (LNGT) that burn fuel in a lean premixed mode. It is shown that these instabilities may be caused by interactions of combustor pressure oscillations with the reactants' supply rates, producing equivalence ratio perturbations in the inlet duct. These perturbations are convected by the mean flow to the combustor where they produce large-amplitude heat-release oscillations that drive combustor pressure oscillations. It is shown in this study that in contrast to earlier analyses, which assumed a uniform instantaneous heat release throughout the flame region, the heat release within the flame may exhibit strong spatial dependence that can significantly affect the combustor stability. The proposed instability mechanism is incorporated into a model that is used to predict LNGT stability limits. The model results show that LNGT are highly prone to combustion instabilities, especially under lean operating conditions, and that the regions of instability can be approximately described in terms of a ratio of the reactants' convective time from the fuel injector to the combustor and the period of the oscillations (with some modifications that account for the structure of the combustion region). Significantly, the developed model's predictions are in good agreement with available experimental data, strongly suggesting that the proposed mechanism and the developed model properly account for the essential physics of the problem.

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 C₂ 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 (C₂H₂, 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.

The present paper is devoted to the study of the effects of nonunity Lewis numbers on triple-flame propagation in nonuniform mixtures. For definiteness, the case of a strained reactive mixing layer is considered. The fuel and oxidizer that are fed to the mixing layer are allowed to have different initial temperatures. Specifically, we examine how the triple flames encountered in this context are influenced by (a) the transverse gradients in the temperature and composition of the fresh reactive mixture and (b) by differential-diffusion effects.

The analysis is carried for a single irreversible reaction with a large activation energy and using the thermo-diffusive model. Analytical expressions describing the flame shape, the local burning speed, and the propagation velocity of the triple flame are obtained. In particular, it is found that the Lewis numbers affect the propagation of the triple flame in a way similar to that obtained in the studies of stretched premixed flames. For example, the flame curvature determined by the transverse gradients in the frozen mixing layer leads to flame-front velocities that grow with decreasing values of the Lewis numbers.

Interactions between acoustic waves and the transient combustion response of a double-base homogeneous propellant in a rocket motor have been analyzed numerically. The analysis extends the previous work on gas-phase flame dynamics to include the coupling with condensed-phase processes. Consequently, a more complete description of propellant combustion response to imposed acoustic oscillations can be obtained. Emphasis is placed on the near-surface flame-zone physiochemistry and its coupling with unsteady propellant burning in an oscillatory environment. The formulation treats complete conservation equations and the finite-rate chemical kinetics in both the gas-phase and subsurface regions. The instantaneous propellant burning rate is predicted as part of the solution. Various distinct features of unsteady heat release arising from propellant combustion response in a motor with forced oscillations are studied systematically. As in the pure gas-phase dynamics of the previous case, the dynamic behavior of the luminous flame plays a decisive role in determining the motor stability characteristics. However, the propellant combustion response may qualitatively modify the temporal evolution of heat-release distribution in the luminous flame and as a result exerts a significant influence on the global stability behavior. The primary flame structure adjacent to the propellant surface is usually little affected by flow oscillation. This may be attributed to the large thermal inertial of the condensed phase, which tends to restrain the temperature variation in the near-surface zone in the present study of laminar flows. The situation with a turbulent flow may be drastically different, as turbulence may penetrate directly into the, primary flame and substantially change the local flame dynamics and transport phenomena.

The extinction of stretched premixed flames under the influence of dilute fuel sprays is studied using activation energy asymptotics. A completely prevaporized mode and a partially prevaporized mode of flame propagation are identified. Three parameters for flame extinction in the analysis consist of the mass fraction of liquid fuel and the initial droplet size of the spray, indicating the internal heat loss and heat gain for rich and lean sprays, respectively, and the flow stretch coupled with Lewis number (Le) that intensifies and weakens the burning strength of the Le<1 and Le>1 flame, respectively. The study presents sample calculations on lean ethanol-spray flames (Le>1), rich ethanol-spray flames (Le>1), and rich methanol-spray flames (Le<1). Results show that the burning intensity of a spray flame with Le>1 is weakened by the flow stretch: however, it is enhanced (further reduced) when the lean (rich) spray has a larger amount of liquid fuel loading or a smaller initial droplet size. It is generally found that the external heat loss associated with the flow stretch dominates the trend for flame extinction. The coupling effects of flow stretch and internal heat gain result in that there exists flame flashback instead of flame extinction for rich methanol-spray flames (Le<1).

Self-ignition and flame propagation properties of silane combustion systems have been studied through computer simulations using a database of kinetic and thermodynamic information that is consistent with current understanding of the elementary processes. These new inputs include the mechanism for chain branching through the SiH₃ radical, rate constants for the reactions of HO₂ with silane and its breakdown products, and the reaction of SiO with oxygen. Over the entire temperature range, the simulations show two distinct mechanisms. At low temperatures, the kinetics of SiH₃ is controlling, whereas at high temperatures, SiH₂ chemistry is of key importance. The results demonstrate that the upper explosion limit and ignition at room temperature and 1 bar can be described by the same set of reactions. With the new database, many of the experimental observations can be reproduced, and predictions are made regarding dependencies on process parameters. These include the critical conditions for chain ignition, the dependence of the critical pressure on the ratio of silane and oxygen concentration, and the temperature dependence of the critical ratio of silane to oxygen concentration. A scenario for low-temperature ignition is presented. At high temperatures, the importance of condensation processes for accurate prediction of flame velocities is clear. For very lean flames, the maximum reaction rate occurs at the lower temperature region of the flame zone.