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 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.

Autoignitions of n-pentane and 1-pentene are studied by rapid compression between 600 and 900 K at high pressure. Both hydrocarbons show a two-stage ignition and a negative temperature coefficient region (NTC). However, 1-pentene is less reactive. Ignition temperature limit is 50 K higher; cool flames and NTC are weaker and confined to a narrower temperature range. Chemical analyses are performed on the reacting mixture for fuel consumption and cyclic ethers. n-Pentane and 1-pentene give very different distribution patterns for cyclic ethers. 2-Methyltetrahydrofuran dominates the n-pentane pattern, whereas propyloxirane is by far the major cyclic ether formed by 1-pentene. Detailed mechanisms based on a common skeleton scheme are developed and used to simulate the experiments. They are validated for ignition delay times, cool flame intensities, and cyclic ether distributions. Good results are obtained for 1-pentene only if (1) direct addition channels of OH and HO₂ to the double bond are included and (2) if a higher rate constant for the decomposition of the hydroperoxyalkyl radicals into cyclic ethers is used when this radical is formed by direct HO₂ addition instead of isomerization of alkylperoxy radicals. The sensitivity analysis of the low-temperature scheme for 1-pentene points out that the total ignition delay time is dependent upon the competition between the decomposition channels of hydroperoxyalkyl radical into the branching sequence and into alkenes. The cool flame delay time is less sensitive but depends mainly upon the decomposition rate of unsaturated ketohydroperoxides.

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 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.

The formation of polycyclic aromatic hydrocarbons (PAH) and soot has been investigated in atmospheric pressure, laminar, methane/oxygen/argon premixed flames as a function of mixture equivalence ratio. Mole fraction profiles of major products, trace aromatic, substituted aromatic, and PAH were quantified by direct gas chromatography/mass spectrometry. In addition, soot-particle diameters, number densities, and volume fractions were determined using classical light scattering. The dependencies of flame species on equivalence ration, using the expression X_i^max=A_iⁿⁱ were also determined; results reveal that the sensitivity parameter n for stable aliphatic species exhibit the following rank order: c-C₅H₆ (4.2)>C₃H₄ (3.9)>C₄H₆ (3.3)>C₄H₄ (2.8)>C₄H₂ (2.2)>C₂H₂ (1.8). For aromatic species, the values of n were in the following order: phenylacetylene (10.8)>benzene (10.2)>indene (7.5)>toluene (5.5). In comparison, PAH species were extremely sensitive to flame equivalence ratios, with the following n values: acenaphthylene (14.2)>fluoranthene (14.1)>pyrene (13.2)>anthracene (13.1)>phenanthrene (11.7)>naphthalene (10.8). Similarly, the sensitivity of soot volume fraction to equivalence ratio was about 13. These results suggest that PAH and soot formation are closely related and their levels are not strongly influenced by the levels of acetylene present in the flame under the conditions investigated.

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 this work, autoignition-delay times of liquid fuel sprays for flow situations similar to those in premixing ducts are calculated. An intensive parameter study was conducted to identify the influence of the evaporating spray on autoignition delay. The parameter variation covers duct conditions relevant to gas turbines. Three monodisperse sprays with droplet sizes of 10,50, and 100 μm and two sprays with Rossin-Rammler droplet size distribution are investigated. A full 3-D Navier-Stokes code is used for the prediction of the turbulent flow. It is coupled to a code based on a Lagrangian formulation for the prediction of the motion and evaporation of the droplets. The evolution of the chemical kinetics is predicted with the CHEMKIN package for n-heptane, which is selected as fuel. A detailed n-heptane low-temperature mechanism including 168 species and 904 reactions describes the chemical kinetics.

For initial temperatures inside the negative temperature coefficient region (NTC), the only spray parameter influencing autoignition delay is the spray evaporation time. If the initial temperature is on the lower boundary of the NTC region, the strong temperature dependence of autoignition in this region leads to a substantially longer autoignition delay due to the cooling of the gas phase caused by evaporation. A delaying effect of evaporation time is only present if the evaporation time is higher than the first induction time. Generally, the safety margin between autoignition and the end of evaporation is enhanced by utilization of a spray with small droplets and a narrow droplet size distribution. Also, a minimum autoignition delay for lean conditions at Φ=0.5 is identified.

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.