Symposium (International) on Combustion最新文献

The vorticity generation and flame distortion by the shock propagation through a cylindrical H₂/air flame are investigated numerically with detailed chemistry. it is shown that flame distortion and the appearance of the second shock waves greatly affects the total circulation. For strong shock wave, the results show that the flame distortion and interaction of the second shock waves yield subscale vortices. A comparison between the present prediction and the theory reveals that the predicted total circulation is lower than that given by theory. The results also show that the shock flame interaction results in significant distortion and break-up of the flame. The effects of shock strength on the flame distortion, the length of flame front, and the mass burning velocity are examined. The results show that both the total mass burning velocity and the length of the flame front increase dramatically with the shock Mach number. The mean local burning velocity is obtained by normalizing the total burning velocity with the length of the flame front. A good agreement between the mean local burning velocity and the burning velocity of laminar H₂/air flame is obtained. It is concluded that the flame distortion induced by the shock flame interaction is very close to the laminar flamelet regime.

Sulfur transformation during pyrolysis of a high sulfur low-rank coal from South Australia has been studied. Three experimental techniques covering a wide range of conditions, namely, temperature-programmed pyrolysis, fixed-bed pyrolysis, and fluidized-bed pyrolysis, have been employed to investigate the effect of pyrolysis conditions on the interactions between different forms of sulfur and mechanisms of sulfur evolution during coal pyrolysis. Both chemical analysis method following Australian Standards and SEM with an energy dispersive X-ray detector are used for sulfur analysis of the char. The results reveal that sulfur evolution is a net result of organic and inorganic sulfur decomposition and interaction. The presence and conversion of inorganic sulfur into complex organic sulfur compounds provide a major mechanism for sulfur retention in char during pyrolysis. The difference in heating rates in the different pyrolysis experiments does not change the natural of the sulfur transformations but affects the extent to which they occur. Faster heating rates do not necessarily imply greater sulfur evolution due to formation of complex organic sulfur. Coal samples pretreated by acid washing and Ca and Na ion exchange are also used to examine the role of inorganic matter in sulfur transformation. At low temperatures (<400°C) acid washing shows little effect on sulfur retention, but at higher temperatures, sulfur retention is greatly reduced. While Na ion exchange enhances sulfur retention compared to the acid washed coal particularly at high temperatures (>400°C), Ca ion exchange shows the opposite trend. An increase in sulfide formation in the Ca ion-exchanged coal at high temperatures is observed, indicating that organic sulfur decomposition is enhanced in the presence of Ca. The effect on sulfur retaining of potential reactions involving Ca ions with sulfur may be offset by the catalyzing influence of Ca ions on organic sulfur decomposition.

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.

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).

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.

Local heat release rate represents one of the most interesting experimental observables in the study of unsteady reacting flows. The direct measure of burning or heat release rate as a field variable is not possible. Numerous experimental investigations have relied on inferring this type of information as well as flame-front topology from indirect measures that are presumed to be correlated. A recent study has brought into question many of the commonly used flame-front marker and burning-rate diagnostics. This same study found that the concentration of formyl radical offers the best possibility for measuring flame burning rate. However, primarily due to low concentrations, the fluorescence signal level from formyl is too weak to employ this diagnostic for single-pulse measurements of turbulent-reacting flows.

In this paper, we describe and demonstrate a new fluorescence-based reaction-front imaging diagnostic suitable for single-shot applications. The measurement is based on taking the pixel-by-pixel product of OH and CH₂O planar laser-induced fluorescence (PLIF) images to yield an image closely related to a reaction rate. The spectroscopic and collisional processes affecting the measured signals are discussed, and the foundation of the diagnostic, as based on laminar and unsteady flame calculations, is presented. We report the results of applying this diagnostic to the study of a laminar premixed flame subject to an interaction with an isolated line-vortex pair.

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.

Ignition delay times for monomethylhydrazine (MMH or CH₃NHNH₂)/O₂/Ar gaseous mixtures have been modeled by a reaction scheme containing 70 species and 373 equilibrated elementary reactions. For many reactions, the rate constants had to be estimated or adjusted because rate constants are available for only a few reactions. The basis for a comparison with reality is measurements of ignition delay times behind reflected shock waves performed over a range of temperatures pressures, and composition. Good agreement between measured and calculated ignition delay times was obtained for lean and less dilute mixtures. A least-squares analysis of the computed ignition delays provides power dependences of the concentrations and an activation energy very similar with those obtained experimentally. The relative importance of the different reactions has been clearly shown by performing different sensitivity analyses. The reaction begins with the scission of the N-N bond in MMH, but elimination reactions from MMH also have to be considered, especially at high initial temperature. The NH₂ radicals formed react with MMH to give CH₃NNH₂ radicals and products. These radicals react with O₂ to produce methyldiazene (CH₃N=NH) and HO₂ radicals. These two species appear to be very important for this specific chemistry. In particular, HO₂ radicals react with MMH to give CH₃NNH₂ radicals and H₂O₂, this last species playing a major role through its thermal decomposition that produces hydroxyl radicals.