Symposium (International) on Combustion最新文献

Combustion instabilities often occur in the liquid-fueled ramjet combustors using bluff body or sudden expansion for stabilization. From a practical point of view, the most severe oscillations are at the 100–500-Hz range. This low-frequency rumble is generally characterized by longitudinal acoustic oscillations. It has been shown that combustion oscillations can be stabilized by controlled periodic addition of secondary fuel, usually in the form of premixed gas fuel. The main idea of the present study is to use the air from the oscillating flow of the unstable combustor for the atomization and distribution of part of the main liquid fuel to obtain the required oscillating phase-shifted heat addition for stabilization. The effervescent spray injection, at relatively low operating pressure, was investigated as a model for pressure-dependent atomizer. A special laser light sheet system was used to obtain an integral indication of spray oscillation. Phase Doppler anemometry measurements were performed to determine the droplet velocity and droplet diameter oscillations with the different phase shifts respect to the oscillations of the atomizing air. A special diagnostic system based on the chemiluminescence of CH radicals is used for direct determination of heat addition oscillations.

The measurements revealed very fast response of the combined effervescent atomizer and flame-holding configuration. Response time between pressure perturbations and the heat release, of the order of 1–3 ms, as well as the limited spatial distribution of the control heat release was achieved. These characteristics proved the system's ability to serve as a passive control unit for suppressing low-frequency combustion oscillations in unstable combustors.

We report on recent investigations of the role of inorganic mineral matter on the evolution of char structure during carbon burnout. Char samples collected in a carefully controlled, laminar flame-supported entrained flow reactor have been characterized using a number of microscopy tools. Observations of the inorganic structure of chars produced at a variety of combustion conditions are coupled with in situ particlesizing pyrometry measurements of the char particle population with an eye toward identifying the mechanism of mineral interaction and its effects on carbon burnout kinetics during pulverized coal char combustion. No evidence of a macroscopic ash film which has been hypothesized to retard char oxidation kinetics, was found on the chars. High-resolution electron microscopy, however, shows a surprising amount of inorganic mineral in solid solution within the carbonaceous matrix. This intimate mixing of organic and inorganic constituents may affect reactivity by both blocking oxygen access to active carbon sites and influencing the microscopic carbon structure that evolves during combustion.

The use of hydrocarbons for fullerenes synthesis has been described in the literature. In this study, the formation of fullerenes C₆₀, C₆₀O, C₇₀, and C₇₀O is investigated in premixed benzene-oxygen flames operated under a pressure of 7.5 kPa by systematically varying two physicochemical parameters: the initial velocity of the fresh gas mixture (at 298 K) at the burner, which is varied between 40 and 50 cm s⁻¹, and the atomic C/O ratio, which is varied from 0.7 to 1.15. The objective of running each flame at different sets of conditions is to assess the sensitivity of reaching an optimum in the process of fullerenes production in flames.

A higher production rate of fullerenes C_n under different conditions is achieved at an optimal level of C/O ratio of 1.05 and 45 cm s⁻¹ of gas velocity. In addition, the highest production rate of fullerenes is 786.7 mg/h, and the highest yield of carbon transformed to fullerenes, obtained is 0.22%. Flame synthesis of fullerenes would seem to offer potential for large-scale production. Different patterns for the production of fullerene and fullerenes oxide are obtained. This result seems to challenge the notion of complexity of the combustion, which accompanies the formation of these carbon molecules in flames. The mass spectrometer shows that heavy fullerenes containing more than 150 atoms are present in the production process. Peaks of PAH at m/e smaller than 500 amu suggest that the reaction of combining the polycyclic aromatic hydrocarbons in burned gases may play an important role in the formation of fullerenes in burned gas flames.

In this work, the laser-induced fluorescence (LIF) technique is used to detect minor species (CCl, NO, and CH) in premixed stoichiometric methane-air flames seeded with monochloromethane or dichloromethane. Quenching data are extracted from time-resolved fluorescence lifetime measurements for all the excited species. First quenching measurements of CCl under flame conditions are reported. It is shown that LIF measurements are strongly perturbed by the presence of background emissions issued from the radiative relaxation of photolytic fragments (HCl^*, CCl^*, CH^*, and C₂^*) formed upon laser excitation. The parent molecules that are partly responsible for these emissions are C₂H₃Cl (for HCl^*, CH^*) and CHCl₂ (for CCl^*).

Profiles of both photolytic fragments and species directly measured by LIF are used to study the influence of CH₃Cl and CH₂Cl₂ addition on CCl and NO formation in methane-air flames. CCl radical is found to be formed in the reaction zone of the flames. The reaction path leading to CCl appears to be dependent on the nature of the chlorinated hydrocarbon (CHC) seeded in the flame. The suggested reaction paths may preferentially involve the contribution of CHCl₂ in case of CH₂Cl₂ degradation and CH₂Cl in case of CH₃Cl degradation. An important increase of NO in presence of CHC is pointed out for the first time. The NO formation in flames containing CHC appears to occur in the reaction zone of the flames, and [NO] is found to be constant in the burned gases: This suggests a predominance of the prompt-NO mechanism in this kind of flame as confirmed experimentally by the observed [CH] increase. Reaction paths involving the degradation of CHCs, particularly CHCl₂, should largely contribute to the formation of CH in flames seeded with CHCs.

The behavior of integral curves has been theoretically investigated in the problem of combustion wave propagation through a heterogeneous model system. The effect of radiation heat transfer on the steadystate (possibly nonstationary) combustion modes has been considered. At sufficiently high but limited spaces between the plates in the system with quasi-homogeneous temperature distribution, the effect of radiation heat transfer can be significant at low retardation by the growing product layer, and the combustion velocity can be much increased by the decrease in the characteristic time of radiation. At the same parameters of heat transfer in the mode of high retardation, the effect of radiation is much weaker and the increase in the combustion velocity is negligibly small if the chemical reaction maximum appears at low temperatures and indexes of the initial substance conversion.

Quantitative CN, OH, and OH rotational temperature, and velocity two-dimensional profiles have been imaged using planar laser-induced fluorescence (PLIF) imaging or particle-imaging velocimetry (PIV) during ignition and deradiative extinguishment of cyclotrimethylene-trimitramine, (RDX) at three different heat flux levels. Relative NO₂ and NO profiles were also imaged with PLIF. Decomposition products, such as NO and NO₂ were formed early in the laser heating process and the gas plume moved, away from the surface. At a later time, ignition occurred in the gas phase, as evidenced by radical buildup such as CN and OH. This often showed as a spherical ignition kernel away from the surface. The flame then transitioned rapidly to a thin flame sheet that moved toward the surface. With longer heating times, laser-supported quasi-steady-state-deflagration develops as the flame sheet again moves somewhat further from the surface. This data can be used as an aide in the development of fully time-dependent RDX combustion models. The ignition and deradiative extinguishment data will help validate these time-accurate models, which can then be used to study combustion instability.

Ash deposits reduce heat transfer rates to furnace walls, superheater tubes, and other heat transfer surfaces in coal-fired power plants. The effective thermal conductivity of a porous ash deposit is one important parameter for determining the magnitude of this reduction. In this paper, we report in situ, time-resolved measurements of the effective thermal conductivity of ash deposits formed under conditions that closely replicate those found in the convective pass of a commercial boiler. Experiments were conducted using an Illinois #6 coal and a blend of Illinois #6 coal and wheat straw to determine the thermal conductivity of highly porous, unsintered deposits and to examine the influence of the initial stages of sintering on these deposits. For deposits formed while firing both fuels the measured thermal conductivity of loose, unsintered deposits is 0.15 W/(m K), almost a factor of three greater than that of air under these conditions. The initial stages of deposit sintering and densification are accompanied by a substantial increase in deposit thermal conductivity. Subsequent sintering continues to densify the deposit but has little effect on deposit thermal conductivity. These trends correspond to anticipated effects of sintering on the development of a layered deposit structure and on particle contact efficiency. Measured values of thermal conductivity are also observed to lie between rational theoretical bounds based on deposit porosity and structure.

Studies of flame-vortex interactions are quite valuable in the analysis of turbulent combustion. As turbulence may be viewed as a collection of vortices with different scales and intensities, the interaction of isolated vortical structures with flames defines the elementary process by which turbulence acts on flames. Experiments and interpretation are thus simplified because the unperturbed flame and the incoming vortex may be controlled with precision. We here investigate the influence of vortex velocity (directly related to its induced strain rate) and of global mixture ratio on the extinction limits. Three vortex types with different velocities interact with a non-premixed diluted hydrogen-air flame. The global mixture ratio of this flame has been varied between 0.5 and 1.2. Four different kinds of interaction are described, and the limits of the connected-flame regime, relevant for flamelet modeling, are identified. The growth of the flame surface during the interaction is also examined, showing very different effects depending on vortex velocity and global mixture ratio. The increase in flame surface area is maximum for slow vortices and intermediate values of the mixture ratio. The main features of the interaction and the relative importance of the increase in flame surface are then explained in the light of characteristic times and extinction strain rates obtained by asymptotic analysis. The extinction of the flame front is finally examined using direct numerical simulations of flame-vortex interactions, including complex chemistry, detailed thermodynamics, and multicomponent diffusion velocities. The relative importance of the strain rate acting on the flame front and of mixing effects is assessed, proving that unmixedness is not responsible for the extinction.

Laser-induced incandescence (LII), a technique that determines relative soot volume fraction, requires calibration to achieve quantitative results. Although not spatially resolved, cavity ring-down (CRD), an absorption-based method, provides an integreated meausre of f_v along the line-of-sight. Here, spatially resolved LII signals from soot within a methane/air diffusion flame are calibrated using CRD, which avoids extrapolation required of less sensitive methods in current use. Comparison of CRD with traditional light extinction and path-integrated LII verifies its accuracy for f_v determination. Using CRD, quantification of LII for parts per billion (ppb) f_v levels is demonstrated. Experimental tests demonstrate the accuracy of CRD for a single laser-pulse to be better than ±5% for measurement of ppb soot volume fraction levels over a 1-cm pathlength. Using calibrated detector characteristics and a predetermined f_v level, the absolute LII signal level within a detection bandwidth of 405–415 nm produced by a laser fluence of 0.25 J/cm² at 1064 nm within a laminar ethylene/air diffusion flame was calculated. This value is 5×10⁵ photons/srnm per ppm of soot, collected over a 10-ns interval centered at the peak of the LII signal. Comparison of LII with CRD reveals that CRD may be used to advantage in applications where spatially resolved information is not necessary and/or achieving high geometric collection efficiency is impractical LII's chief advantages are the spatially resolved f_v visualization and geometric versatility.