Mun Young Choi

Simultaneous laser-induced emission of soot and polycyclic aromatic hydrocarbons within a gas-jet diffusion flame

Abstract Simultaneous images of laser-induced fluorescence (LIF) due to polycyclic aromatic hydrocarbons (PAHs) and laser-induced incandescence (LH) visualization of soot concentrations are presented for laminar gas-jet diffusion flames. Spatially integrated measurements reveal similar spectral emission for LIF and LII, but vastly different time scales associated with radiative decay. Comparison of spatially resolved images using either 266-nm or 1064-nm excitation light reveals distinct regions of molecular fluorescence and soot incandescence. Consideration of photophysical properties of PAHs suggests that the fluorescence wavelength distribution is dependent on the size of the PAH. Using different detection spectral bands, spatially resolved regions attributed to different-sized PAHs are presented. The spatial distribution of PAH size is consistent with the putative growth mechanisms of PAHs. In the region between the LIF due to PAHs and LII due to soot, a dark zone is observed that is attributed to the presence of soot precursor particles. Current understanding of soot formation indicates that through both physical and/or chemical condensation, large PAHs react first to form soot precursor particles prior to the formation of soot particles. Transmission electron microscopy analysis of thermophoretically collected material from within this dark region confirmed the presence of soot precursor particles 2 to 5 nm in diameter.

Pulsed laser heating of soot: morphological changes

Abstract Pulsed laser heating of carbon soot is used here to achieve heating in excess of 1011xa0Kxa0s−1, cooling rates on the order of 109xa0s−1 and total integrated times at elevated temperature of ca 1 milliseconds. To resolve detailed morphological changes induced in the soot by pulsed high intensity laser light, high resolution transmission electron microscopy and selected area electron diffraction are used to examine the laser-heated soot. Partial graphitization, formation of hollow particles and the average number of graphitic planes formed within soot primary particles upon high intensity pulsed laser heating are explained via a thermal annealing mechanism. This mechanism is discussed in the context of the initial physical structure of the soot.

The effects of rapid heating of soot: Implications when using laser-induced incandescence for soot diagnostics

Recent experimental efforts have exploited the high temporal and spatial resolution of laser-induced incandescence (LII) as both a qualitative and quantitative measure of soot volume fraction. As a relatively new diagnostic technique, issues remain as to appropriate excitation laser intensities and the potential intrusive characteristics of LII. The high temperatures to which the soot is heated may accelerate heterogeneous reactions between the soot and flame gases. Vaporization of soot by high energy pulsed laser light has been theoretically modeled and experimentally observed. Potential physical and/or chemical changes in the laser-heated soot raises the question of how the LII signal depends upon these changes as well as the inferred soot volume fraction. Thus the authors investigated the effects of high energy pulsed laser light on the soot particles. The results caution use of laser-induced incandescence without careful consideration of excitation laser intensity and possible variation in soot composition at different measurement locations.

MEASUREMENT OF THE MASS SPECIFIC EXTINCTION COEFFICIENT FOR ACETYLENE AND ETHENE SMOKE USING THE LARGE AGGLOMERATE OPTICS FACILITY

The mass specific extinction coefficient, σ s of smoke produced from acetylene and ethene fuel burned under laminar and turbulent conditions was measured using the Large Agglomerate Optics Facility. Key design features that enable a threefold reduction in the uncertainty compared with previous measurements include a 10-times longer pathlength, less than 0.05% drift in the light intensity ratio steady-state smoke generation and dilution, accurate flow calibration, and more precise filter weight measurements. The measurements of σ s are consistent with previous results obtained for smoke from a variety of fuels for both small- and large-scale fires. Specifically, the σ s , of 7.80 m 2 /g for acetylene smoke produced by a turbulent flame using the new apparatus, is in excellent agreement with 7.82 m 2 /g as reported by Choi et al. [3] for the same fuel. However, these values are significantly larger than the value of 4.5 m 2 /g obtained from the study of Wu et al. [7] for acetylene smoke from turbulent flames. The reliability of the present experimental measurements is supported by an absolute calibration using an aerosol comprised of particles of known size, density, and refractive index. The measured values of σ s , in this study appear to be inconsistent with the values of the refractive index of smoke widely used in the combustion community. Measurements of the specific extinction coefficient for acetylene and ethene smoke indicate that σ s depends on fuel type but displays little dependence on flame conditions (laminar or turbulent cases). For ethene smoke, the average specific extinction coefficient is 12% higher than for acetylene smoke. The larger σ s may be due to a beam-shielding effect that is dependent on the primary particle size and the number of spheres comprising an agglomerate.

Calibration and correction of laser-induced incandescence for soot volume fraction measurements

A procedure for accurately calibrating and correcting the projected laser-induced incandescence (LII) intensity measurements for axisymmetric flames is presented. The corrected LII intensity and soot volume fractions along the measurement axis (which are unknowns) are calculated using the projected LII distribution. This technique does not require calibration using an independent measurement of soot volume fraction. Instead, a single line-of-sight extinction measurement is needed to obtain the calibration factor and corrections for the projected LII intensities. This technique is demonstratcd to provide accurate soot concentrations for mediums producing significant trapping of the LII signal.

The Effects of Initial Diameter on Sooting and Burning Behavior of Isolated Droplets under Microgravity Conditions

Abstract The influence of initial droplet diameter on the sooting and burning behavior of isolated droplets under microgravity conditions was investigated by measuring soot volume fraction, ƒv, soot mass, m s, soot and burning rate. Theƒ v, and m s, soot were measured using a full-field light extinction and tomographic inversion technique. The experiments were conducted at the NASA-Lewis 2·2 second droptower in Cleveland, OH. The ƒ v, and m s, soot measurements represent the first quantitative assessment of the influence of initial diameter on the sooting behavior in microgravity conditions. Results indicate that ƒ v(which provides information regarding the local magnitude of sooting) and m s, soot (which is related to overall sooting magnitude) are sensitive to the initial droplet size. It is believed that the spatial extent of the flame which is proportional to the droplet size, influences the residence time for fuel vapor transport. Thus, larger droplets will provide longer residence time for fuel pyro...

Sizes, Graphitic Structures and Fractal Geometry of Light-Duty Diesel Engine Particulates

The particulate matter of a light-duty diesel engine was characterized in its morphology, sizes, internal microstructures, and fractal geometry. A thermophoretic sampling system was employed to collect particulates directly from the exhaust manifold of a 1.7-liter turbocharged common-rail direct-injection diesel engine. The particulate samples collected at various engine-operating conditions were then analyzed by using a high-resolution transmission electron microscope (TEM) and an image processing/data acquisition system. Results showed that mean primary particle diameters (dp), and radii of gyration (Rg), ranged from 19.4 nm to 32.5 nm and 77.4 nm to 134.1 nm, respectively, through the entire engine-operating conditions of 675 rpm (idling) to 4000 rpm and 0% to 100% loads. It was also revealed that the other important parameters sensitive to the particulate formation, such as exhaust-gas recirculation (EGR) rate, equivalence ratio, and temperature, affected particle sizes significantly. Bigger primary particles were measured at higher EGR rates, higher equivalence ratios (fuel-rich), and lower exhaust temperatures. Fractal dimensions (D{sup f}) were measured at a range of 1.5 - 1.7, which are smaller than those measured for heavy-duty direct-injection diesel engine particulates in our previous study. This finding implies that the light-duty diesel engine used in this study produces more stretched chain-likemorexa0» shape particles, while the heavy-duty diesel engine emits more spherical particles. The microstructures of diesel particulates were observed at high TEM magnifications and further analyzed by a Raman spectroscope. Raman spectra revealed an atomic structure of the particulates produced at high engine loads, which is similar to that of typical graphite.«xa0less

Measurement of soot optical properties in the near-infrared spectrum

Abstract The dimensionless extinction constant, K e , was measured using the NIST Large Agglomerate Optics Facility (LAOF) for soot produced from acetylene and ethene flames. Measurements were performed simultaneously using light sources at 632.8 and 856 nm. The experiments at 856 nm represent the longest wavelength for which accurate extinction measurements have been performed for soot. The mean values of present measurements of Ke at 632.8 nm for the acetylene and ethene soot are 8.12 ± 0.59 and 9.65 ± 0.54, respectively. For acetylene, the mean value of K e measured at 856 nm was 8.83 ± 0.69, whereas the mean value for ethene at the same wavelength was 9.35 ± 0.51. The reduction in discrepancy for the fuels between 632.8 and 856 nm may be related to beam shielding effects. As in the case of 632.8 nm, the measured K e values for 856 nm are significantly larger than values calculated using traditional methods. The present measurements provide a more reliable value of K e for use in optical-based soot diagnostics.

Investigation of sooting in microgravity droplet combustion

This investigation describes experiments performed at the 2.2-s microgravity facility at NASA-Lewis Research Center using n -heptane droplets burning in atmospheric pressure air. The transient soot distributions within the region bounded by the droplet surface and the flame were measured using a full-field light-extinction technique and subsequent tomographic inversion using Abel transforms. It has been speculated that under microgravity conditions, the absence of buoyancy and the effects of thermophoresis create a situation in which high concentrations of soot accumulate into a soot cloud. This study presents the first quantitative measurements of the degree of sooting for microgravity droplet combustion. Results indicate that the soot concentrations for microgravity heptane droplet flames (with maximum soot volume fractions ≈60 ppm) are significantly higher than corresponding values that are reported for normal-gravity flames (which are typically ∼1 ppm). Since the accumulated soot represents incomplete combustion and can also modify the heat-transfer mechanism by altering the local temperature distributions within the fuel-rich region, sooting effects can significantly influence the burning behavior under microgravity conditions. Experiments were also performed to asses the droplet-size-dependent effects on the sooting behavior. Initial experiments using 1.0 and 1.75 mm initial diameter droplets indicate that while the distribution of soot volume fractions are comparable for the two cases (for the observation times that were available), the ratio of the instantaneous mass of soot contained within the fuel-rich region for the 1.75-mm droplet compared to the 1.0-mm droplet was more than a factor of 3. This ratio is also expected to increase for longer observations.

The burning of large n-heptane droplets in microgravity

Experimental results are presented on the burning and sooting behavior of large n -heptane droplets in air at atmospheric pressure under microgravity conditions. The experiments were performed at the Japanese Microgravity Center (JAMIC) 10 s dropshaft in Hokkaido, Japan. Soot volume fraction, burning rate, flame standoff, and luminosity were measured for droplets of 2.6 mm and 2.9 mm in initial diameter. These are the largest droplets for which soot volume fraction measurements have ever been performed. Previous measurements of soot volume fractions for n -heptane droplets, confined to smaller droplet sizes of less than 1.8 mm, indicated that maximum soot volume fraction increased monotonically with initial droplet size. The new results demonstrate for the first time that sooting tendency is reduced for large droplets as it has been speculated previously but never confirmed experimentally. The lower soot volume fractions for the larger droplets were also accompanied by higher burning rates. The observed phenomenon is believed to be caused by the dimensional influence on radiative heat losses from the flame. Numerical calculations confirm that soot radiation affects the droplet burning behavior.