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.

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.

A diode-laser sensor system based on absorption spectroscopy techniques has been developed to measure CO₂, H₂O, and temperature nonintrusively in high-temperature combustion environments. An external-cavity diode laser operating near 2.0 μm was used to scan over selected CO₂ [(12°1)–(00°0) band] and H₂O transitions [(011)–(000), (021)–(010) bands] near 1.996 and 1.992 μm for measurements of CO₂ and H₂O concentration and gas temperature. Gas temperature was determined from the ratio of integrated line intensities. Species concentration was determined from the integrated line intensity and the measured temperature. The system was applied to measure temperature and species concentrations in the combustion region of a premixed C₂H₄-air flat-flame burner operating at fuel-lean conditions. The laser-based temperature measurements were in agreement with values determined using a (type S) thermocouple to within 3%. In addition, the measured CO₂ and H₂O concentrations agreed to within 6% and 3%, respectively, with calculated equilibrium values at measured temperatures. The minimum CO₂ detectivity was 200 ppm (for =0.51, 1470 K, a l-m path length, 200-Hz detection bandwidth). These results represent the first in situ combustion measurements of CO₂ concentration using room-temperature near-IR diode lasers. Furthermore, the results demonstrate the utility of diode-laser absorption sensors, operating near 2.0 μm, as attractive diagnostic tools for in situ combustion measurements of temperature and the concentrations of CO₂ and H₂O.

The influence of coal type on the evolution characteristics of alkali metal compounds, especially sodium compounds, on the supposition of pressurized fluidized-bed coal combustion was elucidated experimentally in this study by using a rapidly heated electrical batch reactor. The evolution fraction of sodium was evaluated quantitatively by analyzing the sodium content in the burnt particles. Water and ammonium acetate extractions were carried out to classify the form of sodium compounds in the raw coals, and the ion components in the water-extracted solution were also analyzed by ion chromatography. Additionally, the relation between the existing locations of sodium and other elements at the cross section in the particle of raw coals was analyzed by an energy-dispersive X-ray (EDX) system and was quantified by means of the cross-correlation method between the locations of two elements.

The results show that the evolution characteristics of sodium are influenced by the coal composition/structure. The water-soluble sodium was the largest fraction in all of the coals tested. Most of the sodium evolved was classified as water-soluble sodium. From the results of the cation and anion components in the water-extracted solution, the sodium in the coals with sodium chloride as a major sodium compound was evolved more easily than that in other coals. The distributions of sodium, silicon, and aluminum contributed to the evolution characteristics of sodium. The coals with high cross-correlation coefficients between sodium and silicon/aluminum had a low evolution fraction of sodium.

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.

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.

A fuel-rich, nonsooting (C/O=0.773) premixed laminar propene flame at 50 mbar was investigated by combining laser techniques and molecular beam sampling mass spectrometry (MBMS) to contribute to the understanding of the regime between stoichiometric and sooting flames, where models have difficulties in predicting important flame features. As a quantity of paramount influence, the temperature profile was measured by laser-induced fluorescence (LIF) of the OH radical: also, the burnt gas temperature was determined from the Stokes/anti-Stokes Raman intensities of CO and H₂. In addition, absolute OH concentrations were obtained using LIF.

The profiles of several hydrocarbons including radical species were measured by MBMS with particular attention to intermediate species, which were discussed as precursors of the first aromatic ring. Quantitative data were obtained for a variety of stable compounds, and semiquantitative profiles were determined for several other species, thus providing a broad database for modeling studies. The analysis of the data shows that C₆H₆ formation via acetylene plays only a minor role under our flame conditions, whereas significant contributions from the propargyl recombination are noted. Furthermore, additional reaction sequences via C₆H_x (x>6) species should be considered in this flame for the formation of C₆H₆ an aromatic compounds: here, the recombination of allyl and propargyl addition to propene seem to be important steps. It was concluded that fuel-specific aspects need to be considered in the formation of higher hydrocarbons in general and of the first aromatic ring in particular.

Crossed-plane tomography, a new laser tomographic technique, is used to directly measure for the first time the instantaneous flamelet surface orientation in premixed turbulent flames. Simultaneous orthogonal tomographic images provide planar data that can be used to calculated the flamelet surface normal, N. The beam from a pulsed Nd:YAG laser is split and rendered into two orthogonal sheets that intersect along a horizontal line, z. above the burner face. The flame boundaries in the planes of the two laser sheets are recorded simultaneously using digital cameras. The technique relies on the use, of polarization of the laser illumination sheets to block, in the view of one plane, the light scattered from the other plane so that the flame boundary and boundary tangent vector can be evaluated without interference, from the other sheet, N is determined from the images by taking the cross-product of the flame-boundary tangent vectors at those points where the boundaries intersectz. The technique is evaluated by measurements on a laminar flame perturbed by a two-dimensional von Kármán vortex street. Data for six turbulent flame conditions are presented and discussed. The probability density function (PDF) of azimuthal angles of N with respect to rotation about the mean normal, <N>, is found to be uniform for all turbulent flames studied, while the surface-weighted PDF of the corresponding polar angles can be fit to the form P_s()=A exp[−(/σ)²] in all cases studied. The mean inverse direction cosine of N with respect to z is calculated, and the burning rate integral, B_T, is estimated, B_T results are compared with data obtained by an independent method and are found to be in good agreement with those data.

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”.

A new concept of coupling combustion and metal hydride heat conversion by means of superadiabatic thermal waves in a porous medium is suggested and investigated analytically and numerically. As an example of the concept implementation, the heat conversion cycle in a two-channel porous medium with metal hydride pairs incorporated in it is considered. The heat conversion is initiated by a superadiabatic combustion of extremely lean gaseous mixture and develops as a series of thermal waves induced by positive and negative heat sources corresponding to charge and discharge stages of metal hydride cycle. It is shown that the thermal effectiveness of heat conversion in a porous medium is provided by heat recirculation mechanism, similar to the superadiabatic effect of the lean fuel combustion. To analyze the concept and its effectiveness, two methods have been employed: (1) analytical study of possible quasi-steady-state structures of the coupled thermal waves and (2) numerical modeling of their dynamic interaction. It is demonstrated that the minimal temperature in the superadiabatic refrigerating wave and the maximal temperature in a combustion wave are attained when both of these waves move synchronously with the free thermal wave generated by convective heat transfer in the porous medium, that is, under the thermal resonance conditions. Modeling has revealed that the superadiabatic combustion wave with the maximal temperature ≈1000 °C (the adiabatic effect of the lean mixture is 100 K) can induce the refrigerating wave with the temperature down to −100 °C (the adiabatic effect of hydrogen phase transition in the porous medium is −20 K).