Laser-Induced Incandescence and Complementary Diagnostics for Flame Soot Characterization

This work has been aimed at developing and applying laser-induced incandescence (LII) for flame soot characterization. The basic principle of LII is rapid heating of the soot particles to temperatures of 3500-4000 K by short laser pulses. Thereby the intensity of the soot incandescence is increased. By detection of this increased incandescence and analysis of the detected signal, the volume fraction, particle size and optical properties of the soot can be evaluated. Additionally, both optical and probing techniques have been utilized in combination with LII for studies of specific soot properties. LII has been applied for characterization of different laboratory flames of interest for soot studies. The soot distribution in flat premixed ethylene/air flames on McKenna burners was found to deviate somewhat from the predicted one-dimensional behavior, where no variation is supposed to be seen radially. A variation was also seen depending on the choice of co-flow gas. Additionally, partially premixed flames burning vaporized n-heptane and n-decane and diluted flat unstrained CH4/O2 diffusion flames were characterized in terms of soot volume fraction distributions. As the ageing process of soot particles can be followed as height above burner (HAB) in flat premixed flames, these have been utilized as measurement targets for studies of soot formation. Significant differences have been found between newly formed nascent soot and more mature soot. The LII signal response of newly nucleated nascent soot particles in low-sooting flames was found to deviate from what is commonly seen. Instead of displaying an S-shaped fluence curve (signal vs. laser energy) the fluence curve of the nascent soot showed an almost linear trend. Even though these results are challenging to interpret, they show potential for LII as a diagnostic technique for investigations of these newly nucleated soot particles. Evaluation of the absorption function, E(m), showed a significant increase with soot maturity, approaching a nearly constant value of ~0.35 for mature soot. A similar trend was found when combining LII and elastic light scattering for measuring the sublimation threshold i.e. the onset of sublimation. The evaluated sublimation threshold temperature was found to increase with maturity and reach an essentially constant temperature at ~3400 K for mature soot. When studying processes affecting the decay time of time-resolved LII signals, an increasing level of aggregation of the soot particles was found to increase the decay time. A plausible explanation is an aggregate shielding effect, effectively decreasing the heat conduction rate of the soot. Additionally, by combination of LII and rotational coherent anti-Stokes Raman spectroscopy, a local gas heating effect could be measured. The gas temperature was found to increase ~100 K in a flame with 4 ppm of soot when heating the soot by ~2000 K, effectively increasing the decay time of the LII signal. If not accounting for effects increasing the decay time of LII signals in e.g. soot particle size evaluations, this will lead to an over prediction of the sizes

Large expert-curated database for benchmarking document similarity detection in biomedical literature search

Document recommendation systems for locating relevant literature have mostly relied on methods developed a decade ago. This is largely due to the lack of a large offline gold-standard benchmark of relevant documents that cover a variety of research fields such that newly developed literature search techniques can be compared, improved and translated into practice. To overcome this bottleneck, we have established the RElevant LIterature SearcH consortium consisting of more than 1500 scientists from 84 countries, who have collectively annotated the relevance of over 180 000 PubMed-listed articles with regard to their respective seed (input) article/s. The majority of annotations were contributed by highly experienced, original authors of the seed articles. The collected data cover 76% of all unique PubMed Medical Subject Headings descriptors. No systematic biases were observed across different experience levels, research fields or time spent on annotations. More importantly, annotations of the same document pairs contributed by different scientists were highly concordant. We further show that the three representative baseline methods used to generate recommended articles for evaluation (Okapi Best Matching 25, Term Frequency-Inverse Document Frequency and PubMed Related Articles) had similar overall performances. Additionally, we found that these methods each tend to produce distinct collections of recommended articles, suggesting that a hybrid method may be required to completely capture all relevant articles. The established database server located at https://relishdb.ict.griffith.edu.au is freely available for the downloading of annotation data and the blind testing of new methods. We expect that this benchmark will be useful for stimulating the development of new powerful techniques for title and title/abstract-based search engines for relevant articles in biomedical research.Peer reviewe

Influence of potassium chloride and other metal salts on soot formation studied using imaging LII and ELS, and TEM techniques

An experimental investigation has been performed where the influence of metal salts on soot formation has been studied. By combining two-dimensional laser-induced incandescence (LII) and elastic light scattering (ELS), two-dimensional information could be obtained on soot properties in the flames. For these studies, seven metal salts (NaCl, MgCl2, AlCl3, KCl, CaCl2, FeCl3 and ZnCl2) were dissolved in water and aspirated into a premixed ethylene/air flame. At lower flame heights, in the soot inception region, the LII signal (representing soot volume fraction) was marginally affected by all additives, whereas the ELS signal strongly decreased with increasing additive concentration for the alkali salts. At higher heights, in the soot growth region, the soot volume fractions were lowered for the addition of potassium, calcium and sodium chloride, in order of significance. Some of the salts (MgCl2, AlCl3 and FeCl3) resulted in negligible influence on LII signals and slightly higher ELS signals throughout the flames, and we relate the increased ELS signals to salt particles propagating through the flame. Main focus in our study was on the addition of potassium chloride for which several parameters were investigated. For example, soot primary particle sizes were evaluated using combined LII and ELS, showing decreasing particle sizes for increasing concentrations of potassium, in reasonable agreement with particle sizes evaluated using transmission electron microscopy. Also, CARS thermometry showed slightly higher flame temperature, ∼30 K, for the potassium-seeded flame compared to the reference flame

Soot sublimation studies in a premixed flat flame using laser-induced incandescence (LII) and elastic light scattering (ELS)

Laser-induced incandescence (LII) as a diagnostic technique is based on rapid heating of soot particles to temperatures of several thousand Kelvin and subsequent detection of the thermal radiation from the laser-heated particles. At such high temperatures, soot sublimation effects must be considered when estimating uncertainties in LII measurements. In this work we have investigated the use of various laser fluences in LII using a Nd:YAG laser at 1,064 nm. Using another Nd:YAG laser at 532 nm, the elastic light scattering (ELS) signal from soot particles heated by the 1,064-nm laser was monitored. This approach makes it possible to determine at which fluence of the LII laser soot sublimation starts to become visible as a decrease in the ELS signal. By performing the measurements at various heights in a premixed sooting flat ethylene/air flame, the fluence threshold above which the ELS signal decreased was found to be higher at the lower flame heights corresponding to younger, smaller and less aggregated particles. The results from this work indicate that the different fluence thresholds for sublimation may be explained by a lower absorption function E(m) for the younger soot particles

Influence of soot aggregate structure on particle sizing using laser-induced incandescence: importance of bridging between primary particles

Soot aggregates formed in combustion processes are often described as clusters of carbonaceous particles in random fractal structures. For theoretical studies of the physical properties of such aggregates, they have often been modelled as spherical primary particles in point contact. However, transmission electron microscopy (TEM) images show that the primary particles are more connected than in a single point; there is a certain amount of bridging between the primary particles. Particle sizing using the diagnostic technique laser-induced incandescence (LII) is crucially dependent on the heat conduction rate from the aggregate to the ambient gas, which depends on the amount of bridging. In this work, aggregates with bridging are modelled using overlapping spheres, and it is shown how such aggregates can be built to fulfil specific fractal parameters. Aggregates with and without bridging are constructed numerically, and it is investigated how the bridging influences the heat conduction rate in the free-molecular regime. The calculated heat conduction rates are then used in an LII model to show how LII particle sizing is influenced by different amounts of bridging. For realistic amounts of bridging (), the primary particle diameters were overestimated by up to 9 % if bridging was not taken into account

Soot Formation in Unstrained Diffusion Flames

The formation of soot particles has been investigated in CH4/O-2 diffusion flames using a unique burner design, which allows the creation of a nearly unstrained planar reaction sheet. Spatially resolved soot volume fractions were obtained using laser-induced incandescence. These soot measurements and the sooting limits were obtained as a function of bulk flow across the flame and mixture strength. Samples were collected using thermophoretic sampling and analyzed using electron microscopy, revealing a broad range of microstructures including particles with unusually large primary diameters and carbon nanotubes. A theoretical model is presented, which confirms that under certain conditions the 1D nature of the flow field of the burner and the strong adverse temperature gradient on the fuel side of the flame result in the soot particles being held in place by thermophoretic forces and allowed to grow for very long time periods. Some of these so-called super aggregates reached sizes of tens of microns and became visible to the naked eye in the soot layer

Optical soot characterization using two-color laser-induced incandescence (2C-LII) in the soot growth region of a premixed flat flame

The soot growth region of a premixed one-dimensional ethylene/air flame has been investigated using two-color laser-induced incandescence (2C-LII) with focus on optical soot properties. From the 2C-LII technique, primary soot particle sizes were deduced together with the temperature from pyrometry of the laser-heated particles, while the gas temperatures were obtained from pure rotational CARS nitrogen thermometry. Soot particle sizes were also measured from thermophoretically sampled soot particles analyzed by transmission electron microscopy (TEM), and the results showed growing isolated primary soot particles up to a height of 10 mm, after which strong soot aggregation occurred and the increase in soot primary particle size ceased. The measured data was used in the evaluation of the soot absorption function, E(m), and a strong increase by a factor of two was observed from the lowest heights to the highest for assumed constant values of soot density and specific heat at all heights. By comparing the 2C-LII particle sizes with the sizes obtained from TEM, differences were observed. Part of the explanation is that the 2C-LII evaluation did not take aggregation into account, but it is additionally speculated that the thermal accommodation coefficient decreases with height above burner. These results are interesting in view of morphological and composition changes of the soot during the growth process and have implications for the use of 2C-LII as a soot diagnostic technique. (C) 2010 The Combustion Institute. Published by Elsevier Inc. All rights reserved

Influence of potassium and iron chloride on the early stages of soot formation studied using imaging LII/ELS and TEM techniques

Laser-induced incandescence (LII), elastic light scattering (ELS) and extinction were used to investigate the early stages of soot formation when seeding metal salts at various concentrations into a premixed ethylene/air flame on a PerkinElmer burner. The investigation contained seven salts, where we in this work mainly focus on two of these salts, potassium chloride (KCl) and iron chloride (FeCl3), as these for mature soot resulted in the largest reduction and highest increase in soot concentration, respectively. The optical measurements show that the potassium chloride significantly reduces the build-up of larger soot precursors and also decreases the sizes of the primary soot particles both at the nucleation stage as well as for more mature soot later in the formation process. This is also confirmed by analysis of transmission electron microscopy (TEM) images, which clearly show smaller primary soot particles for the potassium chloride-seeded flame. The iron chloride-seeded flames were found to have marginal influence on the primary particle sizes based on the optical data, although somewhat more irregular shapes could be observed in TEM images

Evolution of properties for aging soot in premixed flat flames studied by laser-induced incandescence and elastic light scattering

A detailed study has been made of soot growth in two premixed flat ethylene/air flames, at I broken vertical bar = 2.1 and I broken vertical bar = 2.3, where focus has been on following the change in optical properties from nascent to more mature soot, and the importance of these properties for laser-induced incandescence (LII). A combination of two-color LII (2C-LII) and elastic light scattering was utilized for studies of soot absorption and sublimation for a range of laser fluences in a pump-probe experiment, and the experimental results were compared with LII model predictions. Both flames show similar trends, indicating that the soot becomes less transparent during the growth process until some level of maturity is reached at higher flame heights, where the measured properties reach almost constant values. A sublimation fluence threshold of similar to 0.14 J/cm(2) (at 1064 nm for a flame temperature around 1700 K) was evaluated for mature soot, corresponding to a sublimation temperature of similar to 3400 K. Soot peak temperatures from 2C-LII were evaluated both using a constant E(m) and a wavelength dependence for E(m) extracted from extinction measurements, leading to a discussion on how the sublimation temperature relates to the maturity of soot