Symposium (International) on Combustion最新文献

Suspension firing of sawdust and sanderdust fuels is often used in the wood product's industry to raise steam and provide a heat source for drying and curing operations. The unusually high alkali content of these fuels can give rise to a number of problems that affect the operation of downstream plant systems. The research reported here focuses on the physical and chemical properties of the ash generated by these unique and important biomass fuels and uses this information to identify the mechanisms that control mineral-to-ash transformations.

Four sanderdust and sawdust fuels, typical of those used to fire industrial-scale suspension burners, are fired in a laboratory-scale tunnel furnace. Size distribution, morphology, and size versus composition are obtained for particles between 0.0075 and 10 μm by combining a cascade impactor and an electrical aerosol analyzer (EAA). Each of the fuels showed a dominant mode of calcium-rich skeletal particles of size ≥8.3 μm that are the residue from char burnout. A second, minor mode that seems to be the result of fragmentation appears at 1–3 μm. This consists of fluxed particles that, while still predominantly calcium, also contain Fe, Al, Mn, and Si. Another minor mode at 0.4 μm also appears to be the result of fragmentation. Much of the alkali mineral matter becomes submicron aerosol via the vaporization, condensation, coagulation mechanism. This large yield of aerosol (of the order of 30% of the total ash mass) appears as chlorides in high chlorine fuels and as sulfates and carbonates otherwise. In general, only a small fraction of the alkali metals are captured by the residual ash, and no metals other than Na and K are generally detected with the aerosol. The aerosol size varies between 0.01 and 0.1 μm depending on experimental conditions.

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.

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).

The structure of failed one-dimensional detonations is derived using a high activation energy analysis. On a time scale longer than the chemical time based upon the von Neumann temperature, high activation energy one-dimensional detonations break down into a weaker shock, a contact surface separating hot burned gases from a colder, unburned mixture, and an expansion wave. While this leading order solution is unaffected by chemistry, hence self-similar, the perturbation, accounting for the chemistry, which depends upon chemical times, is not. By accounting for the chemistry, the perturbation problem determines the delay until reignition of the detonation occurs. This problem is almost identical to the problem of initiation in the region between a shock and contact surface, which is created by the collision of two shock waves. The main difference between that problem and the current analysis is that the downstream boundary condition now consists of radiating acoustics into hot burned products, at the location of the surface discontinuity. When the Newtonian limit is applied, that is, for a ratio of the specific heats approaching unity, the hot spot at which reignition occurs approaches the location of the contact surface. The time and length to reignition are then found to vary exponentially with the activation energy of the mixture. However, the Newtonian limit is not a very realistic model, because it makes the interval between a Mach number of 1/√γ and 1 disappear: in this range of Mach numbers, adding energy to a steady flow lowers the temperature, hence the reaction rate.

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.

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”.

The nonideal behavior of condensed explosives with metal particle loading has been studied by many researchers. These previous studies have shown that the explosives behavior is different than that predicted by equilibrium codes (generally lower detonation velocities and pressures are observed or measured), although under many circumstances an increase in performance of metallized explosives is observed. To investigate these phenomena, an unsteady, one-dimensional model is presented that simulates the buildup toward steady detonation of an organic explosive (HMX) containing dispersed aluminum (A1) particles. Heat liberated by secondary oxidation reactions of A1 with the products of the initial decomposition of the explosive is modeled, and parametric studies are presented in which the delay time and the rate of the A1 reactions are varied. Endothermic processes are also considered. Results indicate that induction delay for the A1 particles, combined with endothermic processes, alter the structure of the reaction zones and produce a secondary shock wave that never reaches the detonation wave front. The results of the model are shown to be in agreement with the observed nonideal behaviors of metallized explosives. Future work with the model will include the interaction of the predicted reaction zone structure with a compressible media.

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.