The authors discuss the development and application of 2D imaging methods for the study of fuel-air mixing and the in-situ measurement of soot in flames. Fuel imaging is based on planar laser-induced fluorescence (PLIF) with pulsed UV lasers. Fuel concns. may be estimated from the fluorescence of common fuel components or through the addition of a fluorescent species or seed. PLIF of fuel is shown to be applicable to both premixed and non-premixed combustion. The choice of fluorescent seed and the role of interfering species, such as combustion derived polyarom. hydrocarbons (PAH) and laser-induced incandescence from soot, are discussed. The application of the method is illustrated with an example of an isothermal mixing study in a gas turbine combustor sector rig and measurements in a simple flame. The results demonstrate that PLIF can achieve high spatial resolution, 0.4 mm, in a combustor with dimensions of order of 400 mm. The dynamic range of the measured intensities exceeds 1500 with typical signal-to-noise ratios of better than 100:1. A major source of interference in PLIF studies of non-premixed flames is laser-induced incandescence (LII) from soot particles. The authors discuss the role of LII both as an interference in PLIF imaging and as an imaging method for soot volume fraction. A detailed anal. of the physics of LII is presented on a theor. model developed. Results from the model and supporting exptl. data are presented. LII images from a highly turbulent sooting flame indicate that soot structures, probably in the form of thin sheets as small as 100 mm across, are formed through vortex mixing.
Raman spectra of annealed carbon soot reveal strong structural changes. Downshifts of the graphite-like phonon bands to lower energies after annealing are suggested to be related to strained or curved graphitic planes. The effect of curvature on the energy of the in-plane optical phonon mode is quantitatively estimated by applying the semi-empirical interatomic Tersoff potential. A weighted average curvature corresponding to a bond bending of 2.1[o] is deduced for spherical shells with 20.6 Å radius. These findings are consistent with high-resolution electron microscopy images which reveal closed-shell carbon particles in the same size range
A technique was developed that allows the determination of the temperature dependence of the refractive indices of carbonaceous materials from ellipsometric intensity measurements on bulk samples. The refractive indices of the carbonaceous samples pyrolytic graphite, amorphous carbon and flame soot were determined over the temperature range 25-600 degree C and the spectral region 400-700 nm. For all three samples it was found that the inferred refractive index shows insignificant variation with temperature for this range of temperature and wavelength. These results differ by 30 percent or more from the predictions of the Drude Lorentz dispersion model which has been used extensively to predict the variation of the optical properties of carbonaceous particulates. A new set of dispersion constants is presented that accurately predict the indices in the temperature range 25-600 degree C and in the wavelength range 400-700 nm. (Author abstract).
It is shown that the Abel inversion, onion-peeling, and filtered backprojection methods can be
intercompared without assumptions about the object being deconvolved. If the projection data are taken
at equally spaced radial positions, the deconvolved field is given by weighted sums of the projections
divided by the data spacing. The weighting factors are independent of the data spacing. All the methods
are remarkably similar and have Abelian behavior: the field at a radial location is primarily determined by
the weighted differences of a few projections around the radial position. Onion-peeling and an Abel
inversion using two-point interpolation are similar. When the Shepp-Logan filtered backprojection
method is reduced to one dimension, it is essentially identical to an Abel inversion using three-point
interpolation. The weighting factors directly determine the relative noise performance: the three-point
Abel inversion is the best, while onion peeling is the worst with approximately twice the noise. Based on
ease of calculation, robustness, and noise, the three-point Abel inversion is recommended.
The mass density normalized absorption and total scattering coefficients have been measured using in situ
techniques at selected wavelengths from the visible to 1 cm for soot generated by the open combustion of
diesel fuel. Particle morphologies are complex although similar to those of soots of other hydrocarbons and
methods of generation. An ellipsoidal model has been applied as an approximation to the often multiconnected,
chainlike aerosol and then compared with the measured results. The experimental results show an
approximate (lambda)-1 dependence over more than five decades of wavelength data. There is only general
agreement with the simplified calculations in this feature as well as in the magnitude.
This article proposes a new method for the evaluation of the dispersion of the optical properties of absorbing submicronic aerosols, starting from the simultaneous measurements of the scattering and extinction coefficients in the near UV and visible. The experiments were on premixed flat flames at atmospheric pressure with such nonaromatic fuels as CH4, C2H4, and C2H2 and different C-O ratios and flow rates. A quantitative determination of the special behavior of the real and imaginary parts of the complex refractive index of soot was obtained in conditions where the molecular contribution could be neglected and where the particles were not agglomerated and behaved as Rayleigh scatterers. (Edited author abstract).
Through numerical calculations we have investigated the possibility of developing soot diagnostics based on laser heating of the soot particles. Two strategies, one using the laser-modulated incandescence of the particles, and the other using direct detection of the evaporated C2 molecules, were examined. Both strategies can yield size distribution and volume fraction information provided the laser wavelength is near the graphite absorption band at 260 nm; otherwise, only volume fractions can be obtained.