摘要:

Simultaneous transient emission/absorption measurements at five wavelengths in strongly radiating, optically thick, turbulent diffusion flames burning acetylene in air were completed. The data were processed to obtain CO2, mole fractions, temperatures and soot volume fractions spatially resolved to the estimated local integral length scale of mixture fraction fluctuations. Temperatures and soot volume fractions based on emission intensities showed strong negative correlation due to radiative cooling effects. Probability density functions of soot volume fractions conditioned on CO2, mole fractions showed similarities with position. However, probable effects of negligible diffusivity of soot particles were observed. Probability density functions of soot volume fractions conditioned on both CO2, concentrations and temperature illustrate the important role of radiative heal transfer in determining the flame structure. A multivariate stochastic analysis resulted in good predictions of radiation intensities.

ISSN:0010-2202    

DOI:10.1080/00102209108951774

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

摘要:

A situation in which a dilffusion flame reaches an end at some position in a medium of non-premixed reactants is studied. The mixing of reactants that takes place ahead of the diffusion flame leads to the formation of a “triple-flame”, a structure which consists of a fuel-rich premixed flame, a fuel-lean premixed flame, and a diffusion flame that starts where the two premixed flames meet. An important property of such an end-point is its ability to propagate. The limits of low heat release, unit Lewis number and large Zeldovich number are considered. The structure of the triple-flame and the unique relationship between propagation speed and transverse mixture Traction gradient are computed numerically. For the range of values considered here, the end of the diffusion flame is shown to extend itself at a rate that can be substantially reduced, but that remains positive as the gradient of the mixture fraction is increased.

ISSN:0010-2202    

DOI:10.1080/00102209108951775

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

摘要:

Results are presented from an experimental programme of studies on the influence of water sprays on flame propagation. Two experimental configurations were used with differing length-to-diameter ratios. The present results suggest that the mitigating effect of a water spray is enhanced for flames propagating in vessels with large length-to-diameter ratio. The effectiveness of the sprays is further increased if the flame is subjected to greater acceleration before it is incident on the spray. Evaporation alone is insufficient to account for the resulting flame quenching and the mitigating effect of the spray is attributed to increased heat and mass transfer in the combustion zone as a result of a break-up of the initial spray in the induced flow field ahead of the flame front. The time scales over which quenching is observed are consistent with an existing model for shear-layer induced droplet break-up, but the gasdynamic conditions are such that other break-up mechanisms cannot be discounted.

ISSN:0010-2202    

DOI:10.1080/00102209108951776

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

摘要:

CHEMACT is a computer code that uses the QRRK treatment of chemical activation reactions to estimate apparent rate constants for the various channels that can result in addition, recombination and insertion reactions. The working equations are developed, the necessary input file is described, and sources of required input data are discussed. Three applications of the code to representative examples of reactions relevant to combustion are presented. These include C2H5+ O2, H + C6H5Cl, and CH3+ OH. These reactions illustrate some of the expected behavior with different types of chemical activation and demonstrate the necessity of explicitly accounting for the effects of chemical activation in high temperature gas-phase reactions. Methods by which these results can be included in detailed kinetic mechanisms are discussed.

ISSN:0010-2202    

DOI:10.1080/00102209108951777

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

摘要:

There is now convincing theoretical and experimental evidence, assembled and discussed here, for the remarkable insensitivity of the orientation-averaged thermophoretic properties of aggregated particles to aggregate size and structure (morphology), as well as the nature of gas/surface scattering. Indeed, theoretical consideration of straight chains, and uniformly “packed” quasi-spherical agglomerates, as well as recent experimental data on soot aggregate transport in/from laminar flames at atmospheric pressure, indicates that the orientation-averaged thermophoretic diffusivity, (αTD)n, of an aggregate containing N primary particles is usually within about 8% of the (αTD)i,-value for a single “primary” sphere in the free-molecule regime and within about 21 % in the continuum limit. Among other things, this implies that, especially in the free-molecule regime, thermophoretically-dominated transport rates can be adequately predicted without a detailed knowledge of the size and morphology (-distribution) of the aggregated particles or the nature of gas/particle surface scattering, which is definitely not the case for particle transport by Brownian diffusion or inertial drift (see, e.g., Rosner, 1991). This result also implies that thermophoretic particle sampling from “low pressure” flames (Dobbins and Megaridis, 1987) does not itself introduce a significant bias in the relative populations of various sampled aggregate sizes and morphologies. As a corollary, local gas temperature and sool volume fraction estimates based on mass transfer rates- or thermocouple response-methods (Eisner and Rosner, 1985, 1986) will be negligibly influenced by the inevitably uncertain sizes and morphologies of the prevailing soot agglomerates. Since the optical properties (e.g., effective cross-sections for light-scattering and extinction) of soot aggregates are now known to be size- and structure-.?™v»'w (Mackowski. 1987, 1988, Dobbins, Santoro, and Semerjian, 1991. Dobbins and Megaridis, 1991)) we anticipate that the drag vs. thermal force “compensation” effects that produce (αTD)-values insensitive to aggregate si:e (N), and structure, and the nature of gas molecide/surface scattering will find important R&D applications for many systems in which agglomeration occurs. It is also concluded that thermophoretic means would not be useful to rapidly separate various asymmetric particle morphologies unless orientation-averaging is suppressed, perhaps using external fields (E, B).

ISSN:0010-2202    

DOI:10.1080/00102209108951778

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

摘要:

Diffusion flames of methane, ethane, ethene, acetylene, propane, allene 1-butene, 1,3-butadiene and benzene in coflowing normal atmospheric air were investigated by analyzing samples of malerials collected on fine probes inserted into the flames. To focus attention on early slages of soot formation, small flames (heights 2cm to 4cm) on a burner 1.6cm in diameter were studied, and the more heavily sooting fuels were diluted with nitrogen (up to a nitrogen mole fraction of 90% for acetylene). Radiation-corrected thermocouples were employed to measure temperatures in the regions of earliest soot deposits, and the distances from the point of earliest deposit to that of the first visible soot emissions of radiation were also recorded. Scanning electron micrography (SEM) and X-ray photoelectric spectroscopy (ESCA) were applied lo the samples to investigate changes in soot morphology with increasing axial distance. The results showed the earliest deposils to be highly fuel-specific but later deposits to be quite similar for all fuels. For methane and benzene the first deposils appeared to be smooth and light brown under an oplical microscope, unique and subjectively polymer-like by SEM, and essentially aliphatic by ESCA. Later deposits for these two fuels, and all deposits for the other fuels, were black and initially shiny bul becoming progressively duller and rougher with increasing height as seen under an optical microscope, mainly sool-like agglomerates of spheres by SEM, and virtually completely aromatic by ESCA. Contrary to results of many earlier studies, these observations suggest a large effect of the initial fuel on the early history of soot production in diffusion flames.

ISSN:0010-2202    

DOI:10.1080/00102209108951779

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

摘要:

The flame front dynamics of a hydrogen jet diffusion flame interacting with co-flow grid-turbulence have been investigated using the laser induced Mie Scattering from Oil Droplets (MSOD) technique. The flame influence on the velocity field was also determined by two-component LDA, The experimental conditions were designed to minimize the generation of shear turbulence between the fuel and air flows. Both single point statistical data and tomographic high speed movies of MSOD were obtained. As shown by the tomographic movies, the flamelets arc characterized by wrinkles comparable to the size of the turbulence integral scale. The probability density functions of the single point data show the intermittent nature of the flamelets. The velocity fluctuations, however, were not influenced by the flamelet fluctuation. The shear layer formed between the co-flow and room air is found to have influenced the flamelet dynamics and the velocity fluctuations at positions away from the jet exit.

ISSN:0010-2202    

DOI:10.1080/00102209108951780

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

摘要:

This paper reporls char burnout data which supplement previously published gas phase data for Ihe large scale pulverized-coal-fired cylindrical combustor at Imperial College. The data demonstrate the effects of: excess air ratio, the ratio of the primary to secondary momenta and the secondary swirl number. Great care has been exercised to ensure axial symmetrical flow conditions in order that the data set may be of particular value to mathematical modellers. It emerges that the quality of the burnout in the near burner region, is primarily influenced by the particles residence time in the burner inner recirculation zone.

ISSN:0010-2202    

DOI:10.1080/00102209108951781

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

ISSN:0010-2202    

DOI:10.1080/00102209108951782

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

摘要:

Flame propagation through a periodic system or time independent eddies is studied. It is shown that at sufficiently low amplitudes of the velocity spatial variation, the effective speed of a passive flame has a global dependence on the underlying flow. However, at high amplitudes, the effective flame speed is determined by the local features of the flow. Simultaneously, the shape of the flame undergoes a transition from a smooth configuration to a cusped one.

ISSN:0010-2202    

DOI:10.1080/00102209108951783

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor