Symposium (International) on Combustion最新文献

Automotive engine knock limits the maximum operating compression ratio and ultimate thermodynamic efficiency of spark-ignition (SI) engines. In compression-ignition (CI) or diesel cycle engines, the premixed burn phase, which occurs shortly after injection, determines the time it takes for autoignition to occur. In order to improve engine efficiency and to recommend more efficient, cleaner-burning alternative fuels, we must understand the chemical kinetic processes that lead to autoignition in both SI and CI engines. These engines burn large molecular-weight blended fuels, a class to which the primary reference fuels (PRF) n-heptane and iso-octane belong. In this study, experiments were performed under enginelike conditions in a high-pressure flow reactor using both the pure PRF fuels and their mixtures in the temperature range 550–880 K and at 12.5 atm pressure. These experiments not only provide information on the reactivity of each fuel but also identify the major intermediate products formed during the oxidation process. A detailed chemical kinetic mechanism is used to simulate these experiments, and comparisons of experimentally measured and model predicted profiles for O₂, CO, CO₂, H₂O and temperature rise are presented. Intermediates identified in the flow reactor are compared with those present in the computations, and the kinetic pathways leading to their formation are discussed. In addition, autoignition delay times measured in a shock tube over the temperature range 690–1220 K and at 40 atm pressure were simulated. Good agreement between experiment and simulation was obtained for both the pure fuels and their mixtures. Finally, quantitative values of major intermediates measured in the exhaust gas of a cooperative fuels research engine operating under motored engine conditions are presented together with those predicted by the detailed model.

This paper develops and uses a computation model to explore the transient ignition dynamics of catalytic combustion in a stagnation-flow configuration. The analysis considers the elementary heterogeneous chemistry associated with catalytic behavior at the surface. It also considers gas-dynamic effects in the boundary layer, including temporal and spatial pressure variations. The gas-dynamic effects are included through the axial momentum equation, which has been neglected in previous analyses of unsteady stagnation flows. In addition to the physical interpretation of ignition transients, the paper presents a mathematical and computational analysis and comparison of the constant-pressure and compressible stagnation-flow equations. The constant-pressure equations, as commonly formulated and used, are a system of differential-algebraic equations (DAE) that have an index greater than two. This high-index behavior is responsible for severe numerical difficulties in regions of fast transients or stringent numerical error control. This paper relaxes the constant-pressure assumption using a compressible-flow formulation, which extends the range of physical validity and reduces the index of the transient stagnation-flow problem while preserving stagnation-flow “similarity”.

Ten and 20 wt % LaMnO₃ perovskites supported on La₂O₃-stabilized γ-Al₂O₃ were studied for catalytic combustion of methane. A comparison with Mn₃O₄ and Mn₃O₄-Al₂O₃ spinel oxides was also drawn. The catalysts were characterized by microanalysis, X-ray diffraction (XRD), temperature-programmed reduction (TPR), and O₂ temperature-programmed desorption (TPD) techniques. Catalytic activity tests were carried out in a fixed-bed reactor at T=300–800°C, space velocity = 40000 h⁻¹, CH₄ concentration = 0.4% v/v, O₂ concentration = 10% v/v. Both XRD and microanalysis indicated a uniform dispersion of the perovskite phase. The structure of γ-alumina was retained after the treatment at 800°C, the treatment at 1100°C led to the transition to the θ and β phases. TPR measurements suggested the presence of a fraction of Mn⁴⁺ in supported perovskites. The possible interaction of manganese with alumina, which stabilizes Mn²⁺, led to the reduction of the initial average oxidation state of manganese with the perovskite content and the temperature of treatment. O₂ desorption in TPD measurements was significant from spinel oxides, whereas negligible from supported perovskites. Supported perovskites gave complete CH₄ conversion within 650°C with 100% selectivity to CO₂. The activation energy value, evaluated from a methane first-order rate equation, suggested the occurrence of the same reaction mechanism of unsupported LaMnO₃. The preexponential factors of the catalysts treated at 800 °C were proportional to the perovskite content, in agreement with a monolayer model. Samples treated at 1100°C showed the same activity not depending on the perovskite content, suggesting that only a fraction of manganese in the 20 wt % LaMnO₃ is available for the reaction. This was related to the stabilization of a fraction of Mn²⁺, probably not involved in the reaction. Spinel oxides catalyze the reaction at lower temperature, giving complete conversion within 600°C with 100% selectivity to CO₂. The activation energy was lower than that of supported perovskites. A correlation with the ability to desorb O₂ was hypothesized.

The oxidation of propyne was studied experimentally in an atmospheric-pressure flow reactor and in laminar premixed flames. Species profiles were obtained for propyne oxidation experiments conducted in the Princeton turbulent flow reactor (PTFR) in the intermediate- to high-temperature range (∼1170 K) for lean, stoichiometric, and rich conditions. Laminar flame speeds of propyne/(18% O₂ in N₂) mixtures were determined, over an extensive range of equivalence ratios, at room temperature and atmospheric pressure, using the counterflow twin flame configuration. A detailed chemical kinetic model of high-temperature propyne oxidation, consisting of 437 reactions and 69 species, was developed. It is shown that this kinetic model predicts reasonably well the flow-reactor and flame-speed data determined in this study and the shock tube ignition data available in the literature. The remaining uncertainties in the reaction kinetics of propyne oxidation are discussed.

Two measurement techniques capable of monitoring droplet sizes and number density in optically thick sprays are presented: both techniques use infrared probe beams in order to minimize the attenuation from the high number density of droplets in the spray. The first technique relies on multiple wavelength extinction from coaxial beams (wavelengths 1.06 and 9.27 μm). This method provides a line-of-sight measurement of the Sauter mean diameter for the spray. The second technique uses forward scattering from a 9.27-μm beam and 90° scattering from a 1.06-μm beam to produce size, again Sauter mean diameter, at specific locations within the spray. Simultaneous application of the two techniques to the same region of the spray has been used to cross-validate the measurements: agreement on droplet size is excellent and well within the predicted error levels. In addition to providing details of the diagnostic technique, this paper discusses potential sources of error for the measurement, namely, detector noise and calibration, size distribution effects, multiple-scattering and beam-steering considerations, droplet sphericity, optical thickness effects and a correction for optical thickness, and the effect of size distribution widths. Results for the example spray used in this work, a pressure-atomized single-hole diesel injector, indicate droplet diameters of 3 μm at 25 mm from the injector tip along the spray axis on the spray centerline, compared with 4 and 7 μm at radii 2 and 3 mm from the centerline, respectively. The diagnostic shows great promise for providing detailed information on the structure and temporal character of diesel-type sprays in a region that is relatively unexplored: the optically thick zone near the injector orifice.

A comparison between the dynamic and scalar timescales in turbulent premixed flames is presented. Methane-air turbulent Bunsen-type flames are used in this study. They are all lean flames and basically in the flamelet regime. Laser Doppler velocimetry (LDV), Mie scattering and Rayleigh scattering techniques allowed the determination of dynamic (both conditional and nonconditional) and scalar (density) timescales. For both dynamic and scalar temporal scales, we used the autocorrelation function of the LDV signal and the Rayleigh scattering signal, respectively. The main results of the paper indicate that there are significant differences between the conditional (in the reactants) dynamic timescales and those evaluated from nonconditional velocity measurements. The nonconditional velocity timescales are much closer to the scalar timescales, which indicates that they are strongly influenced by the flame front dynamics. Therefore, to evaluate the turbulent premixed flame regimes, only turbulence parameters based on conditional measurement are meaningful, as the nonconditional ones incorporate part of the flame response to the turbulence structure in the reactants. Comparisons between scalar and dynamic timescales, as well as between integral and dissipation timescales, are made. They can be useful in turbulent premixed combustion models for the mean reaction term. In general, the results presented here indicate a complex effect of the u′/S_L, ratio, which seems to depend on the way this parameter is varied.

A fuel-rich, near-sooting, low-pressure, premixed haphthalene/oxygen/argon flame has been analyzed for condensible raction intermediates, mainly mono- and polycyclic aromatic hydrocarbons (PAHs), and for free radicals. Samples were taken through a nozzle beam that was expanded and condensed together with the radical scavenger dimethyldisulfide in a cold trap. Product separation was done with gas chromatography (GC) equipped with a mass spectrometer and a special detector for radical scavenging products. More than 310 different nonradicalic compounds and 23 mainly aromatic radicals could be detected. Among them were partially oxidized aromatics such as 1- and 2-naphthol phenol, benzofuran, inden-1-one, and benzaldehyde. Aromatic radicals with an even number of C and σ-radicals, for example, phenyl and two naphthyls: those with an odd number are π-radicals, such as indenyl and cycloheptatrienyl, the latter being detected for the first time in a flame. An important initiation of growth reactions toward larger PAHs is the formation of various biaryls by the reaction of aromatic radicals with the fuel molecule and aromatic degradation products from it. Biaryls are themselves highly reactive intermediates that first undergo intramolecular cyclodehydrogenations, if they posses four-or five-sided carbon bays, and then grow to highly condensed PAHs by addition of acetylene.

To examine certain aspects of coal tar composition, we have pyrolyzed acid-washed Yallourn brown coal under nitrogen at temperatures of 600 to 1000°C in a fluidized-bed reactor. Analysis of the product tar by reverse-phase high-performance liquid chromatography with diode-array ultraviolet-visible absorption detection reveals that the tars are composed of a large number of polycyclic aromatic compounds, many of which are polycyclic aromatic hydrocarbons (PAH) with peripherally fused cyclopenta rings (CP-PAH). Among PAH, CP-PAH are of particular interest because of their proneness to oxidation in the en vironment, their relatively high biological activity, and their postulated role in soot formation. Of the 10 CP-PAH identified in our tar samples, 4 of the most abundant are acenaphthylene (C₁₂H₈), acephenanthrylene and aceanthrylene (C₁₆H₁₀), and cyclopental [cd]pyrene (C₁₈H₁₀)—all of which have been detected previously in products of coal pyrolysis and/or combustion. The recent synthesis of several new CP-PAH reference standards, however, has enabled us to also identify, in the brown coal tars, six additional CP-PAH-cyclopent[hi]acephenanthrylene and cyclopenta[cd]fluoran thene (C₁₈H₁₀), dicyclopenta[cd, mn]pyrene and dicyclopental[cd, jk]pyrene (C₂₀H₁₀), benzo[ghi]cyclopenta[cd]perylene (C₂₄H₁₂), and cyclopenta[bc]coronene (C₂₆H₁₂)—none of which has ever before been identified in coal products. The mass fractions of individual CP-PAH span a range of four orders of magnitude—from 0.000062 for cyclopenta[bc]coronene to 0.265 for acenaphthylene in the 1000°C tar smaple. Accounting for approximately one-third of the mass of the tar produced at 1000°C, the CP-PAH yields show a monotonic increase with pyrolysis temperature—confirming that the CP-PAH are not primary products of coal devolatilization but instead result from secondary pyrolytic reactions in the gas phase. Possible reaction mechanisms are explored.

In order to better understand the combustion behavior of spray flames, simultaneous measurments of droplet cluster visualization using laser tomography and local OH chemiluminescence and CH-band emission using a newly develped optical probe system named the Multi-color Integrated Cassegrain Receiving Optics (MICRO) are applied to a premixed-spray flame. Time-series planar images of droplet clusters and their transient structures during combustion are examined using an Ar-ion laser and a high-speed digital CCD camera. By observing the droplet clusters and local chemiluminescence simultaneously in the premixed-spray flame, it is confirmed that some portions of the spray stream disappear very rapidly due to preferential flame propagation, while other portions of the spray stream survive over a long period to form droplet clusters, disappearing gradually from their outermost portions, which seems similar to a diffusion flame. The disappearance speed of individual droplet clusters in the premixed-spray flame, instead of a conventional evaporation rate of a single droplet, is defined and calculated by processing the obtained droplet-cluster planar images. The disappearance speed for rapid preferential flame propagation through easy-to-burn regions in the upstream region of the flame is about 2.5 m/s. On the other hand, the disappearance speed when droplet clusters burn dominated by a diffusion combustion mode in the downstream region of the flame is approximately 0.45 m/s.

An extensive investigation has been carried out on the thermal behavior of biomass pyrolysis oil derived from different feedstocks (mixed hardwood, pine, and poplar). Experiments were performed on streams of monodispersed droplets (50–100 μm) and on large suspended droplets (300–600 μm).

The small droplets were generated by a single-droplet generator and injected in a drop-tube furnace, the temperature of which ranged from 300 to 850°C. Light-scattering methods have been used to follow the process undergone by the pyrolysis oil droplets. The solid material, residual of the droplet heating and vaporization processes, has been examined by means of optical and scanning electron microscopy. Two different morphologies of residual particles have been observed at the exit of the furnace: (a) more compact, mechanically resistant spheres, with typical diameter of 10–40 μm, and (b) fragile, glasslike cenospheres with thin walls and menisci, with diameter of 100–200 μm.

The large droplets were suspended at the exit of the furnace, using both a quartz fiber and an exposed junction thermocouple. The furnace temperature was varied between 400 and 1200°C. The phenomena, occurring during the vaporization and burning of each droplet, were followed by coupling the thermocouple measurement to high-speed visualization. The imaging was performed by means of fast digital video recording (50–250 frames/s) and high-speed cinematography (400–1000 frames/s). Temperature-time curves of the burning droplets show a stepped behavior, with two zones at constant temperature, at ∼100 and ∼450°C. The first step is due to the vaporization of water. The second plateau corresponds to the heating of heavy compounds due to selective vaporization and liquid-phase pyrolysis. Swelling, shrinking, and microexplosions with ejection of matter characterize this phase. The combustion of the droplets starts with an enveloping blue flame. The flame then develops a yellow tail of increasing size. After the flame extinction, the remaining char particle burns without residual.