Symposium (International) on Combustion最新文献

Direct numerical simulations conducted in the present study show that a slow fuel gas stream issued between supersonic high-temperature air streams confined in a constant-area channel can mix with air quickly to cause explosive combustion along the following processes: (1) linear flaw instability excitation, (2) eddy formation without shocks, fuel flow acceleration to supersonic speed and enhanced mixing with air, associated with fuel layer meandering, (3) explosive combustion, and (4) thermally choked burnt gas flow. The underlying physics of the supersonic instabilities involved are revealed by interpreting the simulation results in an attempt to find an effective mixing enhancement technique.

The basic flow configuration consists of a confined, plane, double shear/mixing layer flow with forcing fluctuations at the inlet. The difference in velocities between inlet air and fuel streams is supersonic. The reflection condition imposed at the walls serves to disturb acoustically the double shear layer flow in such a way that the walls reflect Mach waves radiated from the inlet disturbance. The most unstable wave excited downstream is skew-symmetric with respect to the centerline, thus leading to the meandering of fuel layer accompanied by Karman-vortex-like eddies. A series of instability excitations couples with the fuel layer meandering in a confined flow region, enhances the exchange of momentum and species between the fuel and air streams, thus accelerating the mixture to a supersonic speed within a short distance prior to explosive combustion. The behavior of the flame front resembles that of lifted turbulent-jet flames. Flame flashback, stationary flame front and flame blowout take place, depending on the inlet condition. Their criteria are provided in terms of the Chapman-Jouguet detonation wave speed.

The demands on current and future aero gas turbine combustors are requiring a greater insight into the role of the injector/done design and manufacturing tolerances. This paper systematically isolates manufacturing tolerances and focuses on hardware design. The target is the structure of the two-phase flow and combustion performance associated with practical injector/dome hardware. A spray injector with two radial inflow swirlers was custom designed to (1) maintain tight tolerances and strict assembly protocol and (2) thereby isolate the sensitivity of performance to hardware design. Although it represents practical hardware, the custom set is a unique modular design that (1) accommodates parametric variation in geometry, (2) retains symmetry, and (3) maintains effective area. Swirl sense and the presence of a venturi were found to be the most influential. The venturi acts as a fuel prefilming surface and constrains the highest fuel mass concentration to an annular ring near the centerline. Coswirl enhances the radial dispersion of the continuous phase, and counterswirl increases the level of mixing that occurs in the downstream region of the mixer. The combined effect of the two parameters (swirl sense and venturi) revealed that the largest drop sizes, which penetrate the continuous phase flow, are formed with coswirl and without venturi. The smallest drop size distributions were found to occur for the counterswirl configuration with venturi. In the case of counterswirl without venturi, the high concentration of fluid mass is found in the center region of the flow. The lean blowout (LBO) equivalence ratio was lower for counterswirl configurations for reasons that involved the coupling of the centerline recirculation zone with the location of high fuel concentration emanating from smaller droplets. In the coswirl configuration, a lack of fuel drops exists in the reaction anchoring region, thereby leading to poor stability characteristics.

We have determined theoretically some critical kinetic parameters in the mechanism of NO_x reburning under flow-reactor conditions. Specifically, using a variety of electronic-structure methods to investigate the potential energy surfaces and the maximum free-energy method of Quack and Troe to determine the resulting rate coefficients, we have deduced the values of k₂ and k₃ for the reactions HCNO+O a3HCO+NO (R2) and HCNO+OH a3HCOH+NO (R3) to be k₂≈7×10¹³ cm³/mole s and k₃≈2×10¹³ cm³/mole s independent of temperature for 300 K<T<2700 K. With such fast reactions converting HCNO to NO, a critical parameter in the reburn mechanism is α(T)=k_1b(T)/k₁(T), the branching fraction of the HCCO+NO reaction, HCCO+NO a3HCNO+CO (R1a) -a3HCN+CO₂ (R1b) -a3NONC+CO (R1c) Again using PES information from a variety of electronic-structure methods (including the QCISD barrier heights of Nguyen et al.), we have used the statistical-theoretical methodology of Miller, Parrish, and Brown to determine α(T)=0.985 exp (−T/1748), valid for 300 K<T<2000 K. Using a value of k₁=k_1a+k_1b+k_1c=2.4×10¹³ cm³/mole s independent of temperature (consistent with experiment), we have determined modified Arrhenius expressions for k_1a and k_1b, k_1a=1.17×10¹¹ T^0.65 cm³/mole s and k_1b=1.45×10¹⁶ T^−0.968 exp(−648/RT) cm³/mole s for 300 K<T<2000 K. Reaction (R1c) never contributes as much as 1% to the total rate coefficient. Our predictions for α(T) disagree with an experimental determination at T=700 K, but they are only slightly smaller than those used in modeling for 1000 K<T<1400 K.

The theoretical analyses and the reburn mechanism are discussed in detail.

Recent studies have shown that organic matter found in fine aerosol or sampled in flames can be only partially speciated, the major part being unidentified. Instrumental limitations of chemical analysis at very high molecular masses and of particle detectors at very low sizes leave unexplored the nanometric size range, where organic molecular clusters might accumulate.

This work reports on the detection of organic extremely fine particles in the exhausts of both diesel and spark-ignited engines, by means of broad-band extinction and scattering spectroscopy in the ultraviolet 190–400 nm band.

The detection techniques rely on a light source, resulting from the laser-induced optical breakdown of air, which features “blackbody” ultraviolet-visible emission, duration of few tens of nanoseconds and tighly confined spot volume.

Samples of internal combustion (IC) engines' emissions have been analyzed in two forms:(a) ordinarily air-diluted exhausts, for extinction measurements and (b) solution/suspension of condensed combustion water, which proved to increase the trapped species concentrations to levels suitable for spectral scattering measurements.

Extinction and scattering spectral data have led to characterize the scatters in terms of: (1) their complex index of refraction in the ultraviolet band 190–450 nm: (2) their average size, in the order of few nonometers and (3) their volume fraction f_v (hundreds of ppm) in the water-trapped exhausts.

The spectral shapes of the extinction coefficient α(λ) in the ultraviolet band have been interpreted in the framework of the solid-state physics, by relating the spatial structures of organic molecular clusters to the value E_g of the optical gap, derived experimentally by the Tauc relationship.

Resulting optical gaps are very low (E_g=0.2 eV) for air-diluted diesel exhausts, involving the presence of soot, as expected, whereas, in all the other cases explored, E_g spans over values greater than 3 eV, associated with carbon-containing nanoparticles.

By burning droplets of benzene in a single-droplet combustor and performing phase-discriminating sampling of the liquid and gas phases of the droplet system, we have found that gas-phase pyrolysis products arise in the liquid phase of the droplet. The experiments are conducted at 1000 K and 21 mol % O₂ in the postcombustion gas from an oxygen-rich premixed methane flame. Disruptive burning, which has not previously been reported for a pure hydrocarbon in normal gravity conditions, is observed at the end of the droplet residence time (∼92 ms). Samples of the liquid phase have been taken at various times throughout the combustion lifetime and analyzed by high-pressure liquid chromatography. Compositional analysis using ultraviolet-visible absorbance spectra of the separated components of the samples reveals a wide variety of pure polycyclic aromatic hydrocarbons (PAH), substituted PAH, and cyclopenta-fused PAH. In addition, recent synthesis of new reference standards has enabled identification of cyclopenta-fused PAH—cyclopent[hi]acephenanthrylene, cyclopenta[cd]fluoranthene, and dicyclopenta[cd, jk]pyrene—which have never before been identified as benzene products. Because the droplet remains relatively cold (∼350 K) with respect to the gas phase in the oxygen-deficient zone between the droplet and the flame (∼2000 K), we conclude that these compounds are gas-phase pyrolysis products that are obsorbed into the droplet, rather than products of reactions within the droplet. These heavier species may play a role in observed terminal disruptive burning events by acting as additional droplet components that promote multicomponent effects. Analysis of species concentrations over time reveals the dominance of both ring rupture pyrolysis products such as phenylacetylene, triacetylene, and acenaphthylene, and biaryl pyrolysis products such as biphenyl. These four products in particular represent 70% of the identified mass of absorbed pyrolysis products, which accounts for up to 5% of the droplet mass at the end of its lifetime.

This paper considers ignition and combustion of small (50–100 μm) single particles of magnesium and 50-50 magnesium-aluminum alloy in the atmosphere of carbon dioxide or its mixtures with argon. This investigation is of interest for both basic combustion science and applications to rocket engines, including those using Martian CO₂ as an oxidizer, An experimental setup with an electrodynamic levitator inside a high-pressure chamber was employed. A CO₂ laser was used for heating to ignition of the particles. The laser was switched off after ignition. The experiments were conducted with the oxidizer at room temperature over the range of pressures from 0.1 to 2 MPa. Effects of the CO₂ concentration and pressure on the critical ignition conditions, ignition delay times, and burning times have been determined for Mg particles. The results clearly indicate that ignition of Mg in CO₂ is controlled by chemical kinetics and that its combustion is controlled by diffusion in gas phase. Quantitative disagreement of the observed critical ignition pressures with previous experimental data on ignition of Mg disks in CO₂ is explained by the differences in heat-loss mechanisms. The measured values of the burning rate correlate well with previous experimental results on combustion of 2-mm particles and with a quasi-steady model of Mg particle burning in CO₂. In contrast to pure Mg and Al, particles of Mg−Al alloy did not ignite in CO₂ under the present experimental conditions.

The bimolecular reaction HO+CO ixH+CO₂ involves the intermediate formation of HOCO. As a consequence, the rate coefficient shows a complicated temperature and pressure dependence. An optimized E- and J-resolved rigid activated complex RRKM theory, with simplified E- and J-resolved pressure-dependent collision efficiencies, fits the available experimental data and allows for extrapolations to unexplored conditions. Experiments between 80 and 2370 K, between 10⁻³ and 10³ bar in the bath gas He, and below 1 bar in Ar, N₂, CF₄ SF₆, and H₂O at 298 K, serve as the database. A limiting low-pressure rate constant for HO removal of k_o=[1.0×10¹³ exp(−8050 K/T)+9.0×10¹¹ exp(−2300 K/T)+1.01×10¹¹ exp (−30 K/T)] cm³ mol⁻¹ s⁻¹ and a limiting high-pressure rate constant of k_∞=[1.23×10¹⁵ exp (−7520 K/T)+1.1×10¹³ exp(−1850 K/T)+8.0×10¹¹ exp(−120 K/T] cm³ mol⁻¹ s⁻¹ will reproduce the results. The pressure dependence of the rate coefficient as a function of the temperature is represented for the bath gases He, Ar, N₂, CF₄, SF₆, and H₂O. Rate coefficients for HOCO formation and HOCO dissociation are also given.

An experimental and theoretical investigation of CH and CN radical formation and destruction in a low-pressure 13.3-hPa (10 Torr) premixed stoichiometric CH₄/O₂ flame seeded with NO is presented. Relative concentration profiles of CH and CN are measured by linear unsaturated laser-induced fluorescence (LIF). An absolute calibration of the relative profiles is obtained by Rayleigh scattering. A computational study is performed to identify key uncertainties in the formation and destruction chemistry of the CH and CN radicals. It is shown that the reaction of the CH radical with molecular oxygen is of particular importance in the present flame. Prevailing uncertainties in the reactions of ³CH₂ with hydrogen atoms and molecular oxygen are also discussed. The present quantitative measurements of the CN radical also indicate that further attention should be given to the formation and oxidation chemistry of HCN. Nevertheless, computational results are encouraging and reasonable agreement is obtained for both the CH and CN radicals. It is further shown that the effects on CH concentration levels of introducing NO dopants may be reproduced. Comparisons of absolute concentration profiles of CH₃, OH, and CH radicals as well as NO are also made with computed results obtained using GRI Mech. 2.11 and a reaction mechanism developed by Warnatz. The computations highlight significant differences in reaction paths and rate selection. The major areas of uncertainty are outlined and tentative recommendations are made in relation to the key reaction paths.