Symposium (International) on Combustion最新文献

Active instability suppression using periodic liquid-fuel injection was demonstrated in a dump combustor. The controller fuel, which made up 12%–30% of the total heat release, was pulsed directly into the combustion chamber, and the injection timing was adjusted with respect to the combustor pressure signal. Because the injection timing determined the degree of interaction between pulsed fuel sprays and periodic large-scale flow features, it significantly affected the spatial distribution of fuel droplets inside the combustion chamber. Simple closed-loop control of the pulsed injection timing was applied to two different cases that developed natural instabilities. In the first case, the instability frequency was unchanged at the onset of the closed-loop control, and this fact allowed up to 15 dB reduction in the sound pressure level. A detailed investigation showed that the pressure oscillation amplitude reached the minimum value when the start of the pulsed fuel injection was synchronized with the inlet vortex shedding process. In the second case, the same controller was applied to a higher output combustor, where the injection timing affected not only the oscillation amplitude but also the instability frequency. For the high output case, the controller was able to suppress the oscillations initially, but it could not maintain the suppressed amplitude, resulting in unsteady modulation of the oscillation amplitude and frequency. The intermittent loss of control was linked to the frequency-dependent phase shift, associated with an electronic band-pass filter. The present results open up the possibility of utilizing direct pulsed liquid-fuel injection for active combustion control in propulsion devices, but they also show the limitation of a simple phase-delay approach in completely suppressing the natural oscillations under certain conditions.

The transformation of chromium was studied numerically and experimentally in a simple laminar premixed hydrogen-air flame. Chromium was added to the flame as a vapor of chromium hexacarbonyl that provided a source of zero-oxidation-state pure elemental chromium through its rapid pyrolysis. The flame was operated over a range of equivalence ratios. The metal aerosol and vapors that were created in the flame were sampled at different heights with a dilution sampling probe. Aerosol was collected on a filter and vapors in a liquid nitrogen trap. Analysis of the samples showed an initial increase in the amount of hexavalent chromium with increasing distance from the burner, followed by a drop to about 5% far from the burner. The modeling of detailed chromium kinetics showed a similar behavior. The observations highlighted the importance of finite-rate kinetics in controlling the ultimate state of the metal oxide. Aerosol samples were analyzed with an electrical mobility analyzer and condensation nuclei counter. Mean diameters were of the order of 40 nm. The numerical model of the chromium aerosol showed qualitatively good agreement between measured and predicted aerosol size distributions.

The formation of fullerenes is though to be a molecular weight growth process similar to the formation of polycyclic aromatic hydrocarbon (PAH) and soot in flames, although little is known about the specific mechanisms involved. The goal of this study is to investigate possible fullerences formation pathways. This work measures for the First time concentration profiles of fullerences C₆₀, C₇₀, C₇₆, C₇₈, and C₈₄, PAHs (up to 300 amu), and unidentified PAHs in the mass range between PAHs and soot in a heavily fullerene-forming premixed benzene/oxygen flame operated at the following conditions: fuel equivalence ratio, 2.4 (atomic C/O ratio, 0.96): cold gas velocity, 25 cm/s: pressure, 5.33 kPa: and fraction of argon in fuel mixture, 10 mol%. Two regions of fullerenes formation are identified in this flame. The First formation region occurs early in the flame simultaneously with PAH consumption. The rate of PAH consumption is more than large enough to account for the obsered rate of fullerenes formation, so this formation region may involve reactions of PAH. The Second region, which accounts for most of the fullerenes mass produced in the flame, occurs later in the flame and is more consistent with stepwise acetylene addtion to fullerene precursors. In both regions, fullerenes consumption that may involve reactions between fullerenes and soot is observed. In addition, measurements indicate that the different PAHs grow roughly simultaneously, reach a peak concentration, and decay together in a consumption process that occurs at the same time as a rapid rise in soot mass concentration early in the flame. This behavior is consistent with the major soot formation being from PAH and occurring along with the minor fullerenes formation in this flame.

The combustion of coal volatiles produced by rapid pyrolysis was studied using a two-stage reactor consisting of a fluidized-bed reactor coupled to a tubular-flow reactor. Volatiles were generated in the fluidized-bed reactor under high heating rates and at 600°C such that the major volatile species produced were tars. The freshly generated tars were subsequently oxidized in the tubular-flow reactor at 900 and 1000°C. Fourier transform infrared (FTIR) analysis showed that, with an increase in oxygen concentration, the recovered tars exhibited and increased in the carbonyl, C=O, functionality. The position of the C=O peak and the presence of absorbances in the aromatic C−H out-of-plane deformation region in the FTIR spectra and GC/MS identification demonstrate that polycyclic aromatic ketones and aldehydes are significant oxygenated polycyclic aromatic hydrocarbons (OPAH) products from coal volatiles combustion. The results indicate that combustion processes are primarily responsible for OPAH formation. HNCO yield was found to increase rapidly with the addition of small amounts of oxygen. The results show that HCN oxidation is not primarily responsible for HNCO formation: reactions of other N-containing species are likely sources. The observation of HNCO suggests that previous measurements of NH₃ in coal combustion probably represent the sum of NH₃ and HNCO yields. The presence of hydrocarbon species (gases and tars) has a significant effect on fuel-N conversion. The experimental results clearly demonstrated that NO production increased significantly onee the concentration of hydrocarbons decreased.

Fuel distribution measurements in the cylinder of a port-injected, spark ignition (SI) engine are presented with a discussion of the effects of injection timing, swirl, and engine speed. Planar laser-induced exciplex fluorescence (PLIEF) was used to study the liquid-and vapor-phase fuel distributions. Optical access to the cylinder was provided by placing a fused silica cylinder between the head and block of a production engine and using a Bowditch-type piston extension. Separate measurements were made with fluorescent tracers indicative of the light and heavy components of gasoline. A blend of four pure fuels mixed in proportions to give a distillation curve similar to that of gasoline was used for all measurements to achieve consistency in the base fuel as well as similarity with the vaporization characteristics of gasoline. These measurements indicate that the vapor-phase fuel distribution is strongly affected by the presence of liquid fuel in the cylinder, the amount of which varies with injection timing. The liquid that enters the cylinder is composed mostly of the heavy components of the fuel. Variations in the amount of swirl indicate that the vapor-phase fuel distribution also is affected by the bulk velocity field. Engine speed, over the range from 200 to 1200 rpm, did not exert a significant influence on the fuel distribution.

The shock tube technique has been used to investigate high-temperature reactions of C₅-species, which appear to play an important role in polycyclic aromatic hydrocarbon (PAH) and soot formation. The unimolecular decomposition of cyclopentadiene (C₅H₆) has been studied behind reflected shock waves. The temperature ranged from 1260 to 1600 K at pressures between 0.7 and 5.6 bar. Initial cyclopentadiene concentrations ranged from 0.5 to 120 ppm, diluted in argon. Atomic Resonance Absorption Spectrometry (ARAS) was used to record the temporal concentration profiles of H-atoms during the pyrolysis of cyclopentadiene under very low concentration conditions. Absorption spectrometry and optical multichannel analyzer for the detection of acetylene during C₅H₆ pyrolysis were applied. For the main channel of cyclopentadiene decomposition C₅H₆→C₅H-c+H (R1) a revised rate expression of k₁=4.0×10¹⁴×exp(−38760/T) s⁻¹ was deduced, after reevaluation of the previous experiments with an improved experimental calibration curve. For evaluating the decay rate of the cyclopentadienyl radical C₅H₅-c→C₂H₂+C₃H₃ (R3) PUMP2 level calculations were performed. The results were validated by means of the measured C₂H₂-and H-absorption profiles. Theory and experiments presented in this work verify quantitatively the decomposition process of the C₅H₅-c radical.

In the 50 years since its introduction, the gas-turbine engine has become an essential component of our global society. One need only look at the nearest airport to realize its dominance of air transportation. It has also become a significant element of the power-generation industry. In the last decade, power-generating combined-cycle powerplants have increased in thermal efficiency to about 60%, while NO_x emissions have been reduced by an order of magnitude, to below 9 ppm (dry, at 15% O₂) in some cases. This paper reviews the ongoing transition from science to the needed technologies: new modes of combustion have been introduced in gas turbines, including lean premixed combustion, reheat and axially staged combustion, catalytic combustion, and rich-lean combustion: high-efficiency low emissions performance is being extended to nonpremium fuels such as coal gas and crude oil: new materials such as superalloys thermal harrier coatings, and ceramics have been incorporated into designs: and improved theories greatly dependent on advanced laser-based diagnostics of flame structure have led to design tools of increasing scope. Future challenges—such as viable propulsion for supersonic transports, powerplants fueled byrenewable resources, and extension of gas turbines to micropower applications—can be met only through further progress in the underlying aerothermal and materials sciences.

Simultaneous two-dimensional CH-LIF/Rayleigh measurements were carried out in highly stretched turbulent premixed stoichiometric methane-air flames. These flames fall into the thin reaction zones regime. In this regime, the Kolmogorov scale is smaller than the preheat zone thickness of a laminar flame, but it is larger than the reaction zone thickness. Therefore, small eddies can penetrate into the preheat zone but not into the reaction zone. These small eddies widen the preheat zone by turbulent mixing.

In the present flames at Karlovitz numbers Ka of 23 and 91, thin reaction zones with relatively thick turbulent preheat zones are being observed. The thickness of the preheat zone is expected to scale with the mixing length scale l_m, which is the thickness of an eddy within the inertial range that has a turnover time equal to the flame time.

The temperature/CH images are presented at different axial locations together with line profiles through the reaction zone to illustrate the different structures. In addition probability density functions (PDFs), of temperature conditioned on CH are presented.

The present data show that the temperature in the thin reaction zones' regime at the early positions in the highly stretched flames is relatively low. This is attributed to heat loss to the burner. In addition, in highly stretched flames at the borderline of this regime, local extinction has sometimes been observed, which could be due to entrainment of small eddies into the reaction zone.

Experimental and detailed chemical kinetic modeling work has been performed to investigate the role of hydrocarbon oxidation in NO-NO₂ conversion. An atmospheric pressure., quartz flow reactor was used to examine the dependence of NO oxidation to NO₂ by hydrocarbon type, reaction temperature, and residence time. The five hydrocarbons examined were methane, ethylene, ethane, propene, and propane. In the experiment, probe measurement of the species concentrations was performed in the flow reactor using a mixture of NO(20 ppm)/air/hydrocarbon(50 ppm) at residence times from 0.16 to 1.46 s and temperatures from 600 to 1100 K. In the chemical kinetic calculation, the time evolution of NO, NO₂, hydrocarbons, and reaction intermediates were evaluated for a series of the hydrocarbons and the temperatures. The chemical mechanism consisted of 639 reversible reactions and 126 species.

Experimental results indicate that, in general, ethylene and propane effectively oxidize NO to NO₂ while methane is less effective. The calculation indicates the important chemical kinetic features that control NO-NO₂ conversion for each hydrocarbon type. The dependence of NO-NO₂ conversion with hydrocarbon type and temperature is qualitatively reproduced by the calculation. The calculation indicates that all five hydrocarbons oxidize NO to NO₂ predominantly through NO+HO₂ ahNO₂+OH and that the contribution of oxidation by RO₂ and HORO₂ is minor. Highest effectiveness comes from hydrocarbons that produce reactive radicals (i.e., OH, O atom) that promote hydrocarbon oxidation and lead to additional HO₂ production. On the other hand, if hydrocarbons produce radicals, such as methyl and allyl, which resist oxidation by O₂, then these radicals tend to reduce NO₂ to NO. Experimental results show that the effectiveness of hydrocarbons varies appreciably with temperature and only within the low-temperature range. Propane shows the greatest NO-NO₂ conversion for the lowest temperatures. This ability is primarily due to the hydroperoxy-propyl plus O₂ reactions as indicated by the sensitivity analysis results.

A three-dimensional quantum mechanical scattering study of the NH(X³Σ⁻)+NO reaction was carried out in the framework of the nonreactive infinite-order sudden approximation (IOSA) using a global HNNO potential energy surface able to describe branching into the H+N₂O and N₂+OH reaction product channels. Negative imaginary potentials (NIPs) were employed to decouple the product channels, avoiding the need to explicitly treat the Schrödinger equation in the product channels and hence eliminating the complicated transformation between the various sets of Jacobi coordinate systems. Absolute integral reactive cross sections were calculated for different translational energies in the range 0.05–0.50 eV and were used to determine the thermal rate coefficient k(T) in the temperature range 300≤T≤5000 K. Comparison with experimental overall rate measurements indicates that the potential energy surface used in the present quantum scattering calculations has a too narrow cone of acceptance leading to a calculated room-temperature rate constant that is about a factor of 10 lower than the experimental ones. Good agreement between theoretical and recent experimental rate constants was obtained in the temperature range T=1200–2000 K. Comparison of the theoretical results with experimental data obtained at T>2000 K indicates that the inclusion of the NNH+O reaction product channel into the global HNNO potential energy surface is necessary in order to accurately describe the measured temperature dependence of the overall NH(X³Σ⁻)+NO rate constant in the high-temperature region 2000<T<5000 K.