Laser Extinction Measurement Research Articles

Experimental evaluation of dimensionless extinction coefficient of soot in co-flow laminar diffusion flames at elevated pressures

Dimensionless extinction coefficient (Ke) is one of the most important optical parameters of soot since it affects the accuracy of laser extinction measurement of soot volume fraction. In literature, Ke values have been measured for soot from laminar and turbulent flames with different fuels at atmospheric pressure. However, relevant studies about the measurement of Ke values in flames at elevated pressures are still lacking. In this study, a simultaneous gravity sampling and laser extinction system (GSLE) capable of operating in a combustion chamber was proposed to evaluate the value of Ke in laminar diffusion flames at elevated pressures. Methane and two straight-chain alkanes (n-heptane and n-dodecane) were selected as fuels for comparative studies at 0.1–0.8 MPa and 0.1–0.5 MPa, respectively. The morphology properties of soot samples were analyzed quantitatively by transmission electron microscopy (TEM). The results show the Ke values of in-flame soot for methane, n-heptane, and n-dodecane range from 6.1 to 9.7, 6.5 to 10.7, and 7.4 to 10.9 at the corresponding experimental pressures, respectively. The Ke values of in-flame soot for all three fuels increase with pressure. The above results conclude that using a constant Ke value measured at atmospheric pressure may result in larger errors in the measurement of soot volume fraction in flames at elevated pressure. The quantitative analysis of TEM images shows that the significant variation of soot morphology parameters with pressure is one of the main factors for the variation of Ke value with pressure.

Combination of laser interferometric and laser extinction methods for precise thickness measurement of liquid film between coalescing twin air bubbles

A combined laser extinction and laser interferometric measurement system was developed for the measurement of the change in thickness of a liquid film formed between colliding twin bubbles. The liquid film thickness during the bubble coalescence process is measured based on the bubble approach velocity and the contact duration. The proposed method affords a ten-fold spatial resolution (∼6 μm) of the liquid film thickness distribution compared to the current laser extinction measurement method. Through the use of the proposed method, it was found that the liquid film in the vicinity of its thinnest area immediately before coalescence was wedge-shaped, with the minimum film thickness immediately before coalescence being 0.3–1.3 μm, which is ∼0.6-μm thinner than the value determined by the current laser extinction measurement method. Greater precision of the proposed method is enabled by its improved spatial resolution.

Modification of sooting tendency by magnetic effects

Experimental investigations assess for the first time the influence of the gradient of the square of the magnetic flux density (∇(B2)) on the soot production in a laminar axisymmetric non-premixed flame. The steady non-smoking ethylene flame is established in a coflowing mixture, composed of oxygen and nitrogen, over a Santoro type burner. This burner is located in an electromagnet. The flame experiences different magnitudes of upward ∇(B2), ranging from 0 to 18.2T2/m, as well as different oxygen contents of the coflow, ranging from 21% to 50% in volume. Soot volume fraction is mapped in the flame by Laser Extinction Measurement technique. Increasing the magnitude of the ∇(B2) allows for the modification of soot production in the flame. This modification is enhanced by increasing oxygen content, as oxygen exhibits a relatively high paramagnetic susceptibility. Furthermore, the aforementioned modification is shown to enable the shift among similar soot concentration profiles in the flame, just as the variation of oxygen content can do. Consequently, modification of the inner flame structure by magnetic effects could contribute to the control of oxyfuel combustion.

The evolution of soot morphology in a laminar coflow diffusion flame of a surrogate for Jet A-1

An experimental study is performed to investigate the evolution of soot morphology in an atmospheric pressure laminar coflow diffusion flame of a three-component surrogate for Jet A-1. The laser extinction measurement method and the rapid thermocouple insertion technique are used to obtain soot volume fraction profiles and temperature profiles, respectively. Thermophoretic sampling followed by transmission electron microscopy and atomic force microscopy is used to study the morphology of soot particles at different locations inside the flame. Soot formation on the centerline appears to be different from conventional models. Liquid-like particles, which are transparent at the wavelength of 623nm, are formed and grow up to a volume equivalent diameter of dp=60nm at temperatures below T=1500K. When the temperature exceeds 1500K, transition of the transparent particles to the mature agglomerated particles happens immediately, i.e. in less than 12ms. The volume of the liquid-like particles just before the start of their transformation to solid is about five times larger than the volume of mature primary particles. This significant size difference suggests that a large liquid-like particle does not transform into a single primary particle. In addition, multiple dark nuclei are observed in the liquid-like particles prior to carbonization. The significant size discrepancy and the presence of multiple dark nuclei may indicate that primary particle formation and agglomeration on the centerline happen inside the liquid-like particles. In contrast to the centerline, on another streamline with a significantly different temperature history, soot particles form from relatively small liquid-like particles. These particles have the same size as mature primary particles. Carbonization happens early on the streamline. A single dark nucleus grows inside each liquid-like particle and primary particles agglomerate after carbonization is completed. Most of the currently used computational soot models consider a single evolution process for all of the streamlines inside the flame which may not be an accurate assumption. This study shows that soot evolution processes may be different across the flame and are a function of temperature and the concentration of specific species inside the flame.

An Experimental Comparison of the Sooting Behavior of Synthetic Jet Fuels

An experimental study is conducted for the laminar, atmospheric pressure, sooting, coflow diffusion flames of prevaporized Jet A-1 and four synthetic jet fuels to compare their sooting characteristics and flame structures. Soot volume fraction, species concentration, and temperature profiles are measured, using the laser extinction measurement method, gas chromatography, and fine wire thermocouples, respectively. To evaluate the sooting tendency of the fuels, their smoke point heights are measured using the ASTM D1322 method and compared. The synthetic jet fuels under investigation are (1) Fully Synthetic Jet Fuel (FSJF), which is a coal-to-liquids (CtL) kerosene plus some coal tar derived material from Sasol, (2) Fischer–Tropsch Synthetic Paraffinic Kerosene (FT-SPK), which is a gas-to-liquids (GtL) product from Shell, (3) SPK plus naphthenic cut (50% by volume), and (4) SPK plus hexanol (20% by volume). The threshold sooting index (TSI) of the proposed surrogate mixtures for the fuels are calculated and...

A numerical and experimental study of a laminar sooting coflow Jet-A1 diffusion flame

As the drive towards a better understanding of airline fuel combustion, and its associated emission characteristics continues, there is a need for fundamental numerical and experimental jet fuel studies. In the present work, a numerical and experimental study is conducted for a complex blended liquid fuel, Jet-A1, in an atmospheric pressure, laminar sooting coflow diffusion flame. The numerical model uses a surrogate mixture, comprising 69% n-decane, 20% n-propylbenzene, and 11% n-propylcyclohexane (by mole). The combustion chemistry and soot formation are solved using a detailed chemical kinetic mechanism with 304 species and 2265 reactions, detailed transport, and a sectional soot model including soot nucleation, heterogeneous surface growth and oxidation, soot aggregate coagulation and fragmentation, and PAH surface condensation. The problem is intractable by serial processing; therefore, distributed-memory parallelization is used, employing 192 CPUs. Experimentally, soot volume fraction and gaseous species concentration profiles are determined by a Laser Extinction Measurement method and Gas Chromatography, respectively, in a coflow diffusion flame of vaporized Jet-A1. These data are used to validate the model. Centerline species concentrations are satisfactorily reproduced by the model. The order of magnitude of the peak soot volume fraction is well predicted without calibrating any of the model constants to the experimental data, but discrepancies remain between numerical and experimental results on the radial locations of the peaks and the centerline soot concentration levels.

Soot volume fraction and morphology of conventional and surrogate jet fuel sprays at 1000-K and 6.7-MPa ambient conditions

This paper presents soot processes of a blend of 23% m-xylene and 77% n-dodecane, which has been selected by several working groups as a surrogate for jet fuel. Fuel sprays were injected into high-temperature, high-pressure ambient conditions that are representative of practical engine combustion. Simultaneous laser extinction ( KL) measurement and planar laser-induced incandescence imaging were performed to derive the in situ soot volume fraction. Also, soot particles were extracted from different positions within the reacting jet by means of a thermophoretic probe, and analyzed using transmission electron microscopy (TEM) to clarify the soot structure and its correlation with the measured soot volume fraction. The same measurements were repeated for the conventional jet fuel to understand the overall performance of the selected surrogate fuel. The soot volume fraction results show that, at fixed ambient conditions, the surrogate fuel produces more soot than the conventional jet fuel. The TEM images show that the soot aggregates are more agglomerated, which may not be easily eliminated by in-cylinder oxidation. The total number of primary particles and the mean primary particle size are higher for the surrogate fuel, consistent with the soot volume fraction trend. Considering that there is similar lift-off length between fuels, the differences in soot level and morphology are caused by molecular structure effects, such as a higher aromatic content. The quantitative soot database obtained from the present study offers data for the validation of soot kinetic models, particularly at high temperature and pressure conditions where little fundamental data exist.

Spray characterization in high pressure environment using optical line patternator

For the quantitative measurement in an optically dense spray, the intensity of the attenuated signal should be corrected. Therefore, the optical line patternator was applied to get the original distribution of the dense spray injected from a swirl injector at high ambient pressure up to 4.0 MPa. The optical line patternator is a combined technique of laser extinction measurement and image processing for the spray characterization. The spray was scanned with the laser beam and the line image of Mie scattering was captured simultaneously in the path of each laser beam by using a CCD camera. A photo-diode was used to obtain the transmission data that was the amount of the incident laser beam passing through the spray region. The distribution of the attenuation coefficients in the spray was obtained by processing the transmission data and Mie-scattering distribution data by an algebraic reconstruction technique. From the distribution of attenuation coefficients, we can obtain the accurate surface distribution from the Mie-scattering signal. Because the optical line patternator uses a laser beam instead of a laser sheet to scan the spray, the effect of multiple scattering, due to the increased number density of droplets in a high pressure environment is reduced significantly. The optical line patternator is suitable for investigating the characteristics of a relatively large spray under high pressure environments such as liquid rocket engines.

Study on the Effect of Iron on PM10 Formation and Design of a Particle-Generating System Using a Cocentric Diffusion Burner Flame

Iron pentacarbonyl was added to a cocentric diffusion burner flame burning a mixture of acetylene and ethylene in a co-flowing stream of air. Samples of aerosols and gaseous species were collected within the flames and above the flames with filters and a sampling bottle, and soot volume fraction through the flame was calculated with laser light extinction measurements. Aerosol was isokinetically collected in the inhalation chamber to measure particle concentration and size distribution. Laser extinction measurement showed that iron (Fe) gave an effect on soot formation process and scanning electron microscopy of the aerosol sample showed that soot particle size for the Fe-doped flame was relatively smaller than that of non-Fe-doped flame. Transmission electron microscopy results indicated that Fe species were separated from the soot at the downstream flame. Particles of the soot and Fe mixture could be generated continuously, and the concentration was kept constant under a given experimental condition using the cocentric diffusion flame burner. The mass loading variation for each target concentration (i.e., 100, 200, and 400 μg/m3) in the inhalation chamber was less than ±5% during 10 hr. This particle-generating burner system could be used effectively for a bioassay test to evaluate risk assessment for particulate matter with aerodynamic diameter less than 10 μm (PM10).