Laser Diagnostics in Combustion - Elastic Scattering and Picosecond Laser-Induced Fluorescence

Elastic scattering and the Lorenz-Mie (LM) theory in particular is used for the characterization of sub-micron- and micron-sized droplets of organic fuels in sprays and aerosols. Calculations on the Lorenz- Mie theory show that backward-sideward scattered visible radiation can be used for unambiguous detection of ensembles of homogeneous droplets of organic substances with diameters around 1 micrometer (size parameter between 2 and 6). A backward feature in the polarization ratio appears with a value considerably higher than one, on the opposite to the case of the rainbow observed for larger droplets. A comparison between measurements and LM calculations showed that a large amount of droplets in aerosols and well-atomized sprays were smaller than one micrometer in diameter. The LM theory was also used to characterize different size groups in a burning spray. A 3 – D technique based on a picosecond laser and a streak camera was demonstrated for measurements of fast and turbulent biphase flows. The entire 3 – D information was obtained within a time-span of less than 15 nanoseconds. A 2 – D technique for lifetime measurements based on a picosecond laser and a streak camera has been demonstrated on static objects. An analysis indicates that the technique may be applied to measurements of lifetimes around or below one picosecond employing femtosecond lasers and femtosecond streak-cameras. The technique may in princple be used to study dynamic systems when two detectors are used. Fluorescence lifetime measurements on hydrogen and oxygen atoms in flames at atmospheric pressure demonstrate the need of lasers with suiting spectral properties such as jitter and linewidth and the need of detectors with high sensitivity in the near IR in the case of oxygen atoms. The fluorescence lifetimes of gas phase acetone and 3-pentanone at 266 nm excitation wavelength have been measured for mixtures with nitrogen and air at temperatures between 323 and 723 K and pressures between 0.01 and 10 bar. The effective lifetimes generally decrease with temperature and increase with pressure and in different ways for the two substances which gives implications of their use to combustion related studies. The fluorescence emission from gas phase aromatic hydrocarbons has been studied at elevated temperatures and at atmospheric pressure for different concentrations of oxygen (0-10%). Picosecond laser radiation at 266 nm was used for the excitation. The lifetimes for most of the substances decreased between one and two orders of magnitude when the temperature was increased between 400 and 1200 K. The dependence of the lifetimes on temperature and oxygen concentration indicate that the relaxation dynamics is determined by sequences and the density of vibrational states. The results give also some implications to combustion related diagnostics

Exploring the formation of carbon-based molecules, clusters and particles by in situ detection of scattered X-ray radiation

The angular pattern of scattered X-ray radiation (1.2-2.5 Angstrom) from flames has been measured and analyzed. The results are compared with calculations on different molecules, such as polyaromatic hydrocarbons, fullerenes and other spherical structures. Measurements have also been performed on soot condensed on a cooling plate inserted inside a flame. Size- and shape-dependent structures are observed in the measured patterns from flames as well Lis soot measurements. The agreement between calculations and measurements allow the determination of characteristic sizes of the different Structures. We have identified aromatic and graphitic structures and features related to sub-nanometer spherical structures. (C) 2004 Elsevier B.V. All rights reserved

Neutron imaging of heterogeneous solid fuels under heating

reportSolid-fuel pyrolysis, gasification, and combustion continue to play an important role in the industrial and utility sectors. Behavior of heterogeneous fuels such as biomass (e.g., wood and grasses), municipal solid waste (MSW), and coal is less understood on the micro than the macro scales, particularly in relation to computational models which can assist reactor scale-up design and optimization. One limitation is understanding the complex processes of fuels as they degrade under heating, with counterflowing heat and mass fluxes

Raman spectroscopy for characterizing porous carbon

Raman spectroscopy is performed to disclose structural properties of porous carbon samples including lignocellulosic biomass. Analysis of spectral signatures of carbon, around 1350 cm-1 and 1580 cm-1, indicates higher porosity for the biomass sample

Two-dimensional surface temperature measurements of burning materials

A new technique for two-dimensional temperature measurements of burning surfaces is presented. Laser-induced phosphorescence from a thermographic phosphor material applied to a surface of investigation was measured with a fast framing camera. The phosphor was excited by the 4th harmomic from a pulsed Nd:YAG operating at 10 Hz. The phosphorescence images measured by eight consecutively gated CCD detectors enable pixel-by-pixel lifetime evaluation of the phosphorescence by interpolating an exponential decay curve to the counts of the corresponding pixel positions of the sequential CCD images. The temperature at each pixel position was evaluated using a calibration procedure of temperature against lifetime. These measurement procedures were used for surface temperature measurements of the evolution of flame spread on low-density fiber boards. The results from experiments showed the possibility of measuring surface temperature during all phases of the flame spread. The total time window used for each two-dimensional temperature measurement was 800 mus to obtain high accuracy and precision at high temperatures, 680-780 K, temperatures characteristic of burning surfaces. The best precision, better than +/-5 K, was obtained at these temperatures. In this region, evaluation by the lifetime method shows a higher sensitivity to temperature than what can be expected from methods based on spectral line intensities. The results of the experiments were in accordance with those reported from previous one-point measurements. In the low-temperature region close to room temperature, the accuracy deteriorated considerably. The results obtained from the two-dimensional imaging experiments are presented and discussed

Finger-like carbon-based nanostructures produced by combustion of flour-based sticks (spaghetti)

Biomass is becoming particularly important as a starting material for advanced carbon structures. In this study, we found interesting nanostructures on the surface of burnt spaghetti using scanning electron microscopy, transmission electron microscopy (TEM), and energy dispersive X-ray spectroscopy (EDX) for analysis. The structures were elongated and finger-like, with evidence that the tubes have shell and core components. The shell was carbon that included amorphous and layered graphene structures. EDX showed enriched potassium and phosphorous in the core and at the tip of the tubes. The results indicate that tube formation depends on phase separation of polar/ionic and nonpolar moieties when water is produced in the biomass from the pyrolysis/combustion. The tube growth is most probably due to the raising pressure of vapor that cannot escape through the carbon film that is formed at the surface of the stick from flame heat. This process resembles glass blowing or volcanic activity, where the carbon acts as the glass or earth’s crust, respectively. These observations suggest that new interesting tubular nanostructures with different properties on the inside and outside can be produced in a relatively simple way, utilizing processes of combustion of starch-rich biomass materials

X-ray scattering studies of the generation of carbon nanoparticles in flames and their transition from gas phase to condensed phase

The dynamics of particle formation were investigated for a diffusion-like ethylene flame containing a cooling metal plate on which particles condensed. Small-angle and wide-angle X-ray scattering techniques were combined to follow size- and structural changes of the soot particles using a new detector. The high dynamic range allowed continuous monitoring from the gas to the condensed phase of the gradually growing layer depositing on the plate. Different stages in the process of particle formation were observed. In addition the sub-nanometer structure of the deposited material changed rapidly as the flame was turned off. The detection system and methodology presented are of general interest since they can be further developed and applied to the study of reactive systems for materials processing in gas- and condensed phases. (c) 2009 Elsevier Ltd. All rights reserved