Fluorescence Of Polycyclic Aromatic Hydrocarbons Research Articles

Anomalous Fluorescence Quenching in Fluorous Solvent-Added Media.

Due to the inherent high electronegativity of fluorine, perfluorocarbons have the potential to exhibit unusual characteristics. Fluorous solvents, in this context, may afford an anomalous solubilizing behavior compared to their hydrocarbon analogues. Addition of perfluorodecalin (PFD) to n-hexane results in unusual fluorescence quenching of polycyclic aromatic hydrocarbons (PAHs) by the quencher nitromethane. As more viscous PFD is added to less viscous n-hexane, the dynamic viscosity (η) of the media increases. The bimolecular quenching rate constants (kq) of the PAHs, instead of decreasing, increase as PFD is added to n-hexane until an equimolar mixture composition is obtained; kq exhibits an expected decrease only in the PFD-rich region of the mixture. The expected decrease in kq in the hydrocarbon analogue decalin added to n-hexane is observed across all compositions. It is proposed that highly electronegative fluorines on PFD stabilize the partial positive charge (δ+) that develops on excited PAHs during electron/charge transfer to the quencher nitromethane, facilitating quenching in the process. In the PFD-rich region, however, increased η starts to dominate the quenching, resulting in the expected decrease in kq. A monotonic decrease in kq is observed in the PFD-added n-hexane system for fluoranthene quenching by triethylamine (TEA) as TEA acts as the electron/charge donor and a partial negative charge (δ-) develops on excited fluoranthene during the quenching process. No such stabilization by PFD is observed when nitrobenzene is employed as the quencher. This is attributed to the significantly higher quenching (KS > 104 M-1) of PAH fluorescence by nitrobenzene due to the presence of aromatic π-π interactions between PAH and nitrobenzene, further facilitated by the higher electron affinity of nitrobenzene as compared to nitromethane. The role of fluorous media in facilitating electron/charge transfer processes is clearly established.

Nitrogen Effects Endowed by Doping Electron-Withdrawing Nitrogen Atoms into Polycyclic Aromatic Hydrocarbon Fluorescence Emitters.

Unveiling innovative mechanisms to design new highly efficient fluorescent materials and, thereby, fabricate high-performance organic light-emitting diodes (OLEDs) is a concerted endeavor in both academic and industrial circles. Polycyclic aromatic hydrocarbons (PAHs) have been widely used as fluorescent emitters in blue OLEDs, but device performances are far from satisfactory. In response, we propose the concept of "nitrogen effects" endowed by doping electron-withdrawing nitrogen atoms into PAH fluorescence emitters. The presence of the n orbital on the imine nitrogen is conducive to promoting electron coupling, which leads to increased molar absorptivity and an accelerated radiative decay rate of emitters, thereby facilitating the Förster energy transfer (FET) process in the OLEDs. Additionally, electronically withdrawing nitrogen atoms enhances host-guest interactions, thereby positively affecting the FET process and the horizontal orientation factor of the emitting layer. To validate the "nitrogen effects" concept, cobalt-catalyzed multiple C-H annulation has been utilized to incorporate alkynes into the imine-based frameworks, which enables various imine-embedded PAH (IE-PAH) fluorescence emitters. The cyclization demonstrates notable regioselectivity, thereby offering a practical tool to precisely introduce peripheral groups at desired positions with bulky alkyl units positioned adjacent to the nitrogen atoms, which were previously beyond reach through the Friedel-Crafts reaction. Blue OLEDs fabricated with IE-PAHs exhibit outstanding performance with a maximum external quantum efficiency (EQEmax) of 32.7%. This achievement sets a groundbreaking record for conventional blue PAH-based fluorescent emitters, which have an EQEmax of 24.0%.

Role of the Equivalence Ratio on Soot Formation in a Perfectly Premixed Turbulent Swirled Flame: A Combined Experimental and Large Eddy Simulations Study

Abstract The understanding of processes that govern soot production in aero-engines is fundamental for the design of new combustion systems with low environmental impact. Many combustors, more specifically those used in aero-engines, feature rich flame regions typically exploited in the so-called rich-quench-lean (RQL) technology. Thus, it is important to consider rich turbulent flames operating in the premixed mode. To this purpose, a model scale swirled combustor, called EM2Soot, was designed at the EM2C laboratory to analyze soot production under perfectly premixed rich conditions. In this work, the effect of the equivalence ratio on soot production in this burner is experimentally characterized and numerically simulated. Measurements of Planar Laser Induced Fluorescence of polycyclic aromatic hydrocarbons (PLIF-PAH) were performed to examine soot precursors presence, whereas soot volume fraction is measured with planar laser-induced incandescence (LII). Large Eddy Simulations (LES) are carried out using models already established in literature. By considering a range of equivalence ratios, the soot volume fraction from the experiments was found to reach a maximum near 1.8, whereas a lower level of soot volume fraction was measured for lower and for higher equivalence ratios. The large eddy simulations are found to be in qualitative agreement with experimental data in terms of polycyclic aromatic hydrocarbons (PAHs) and soot location. The soot volume fractions fv are notably overestimated with respect to the LII measurements. However, the numerical results correctly retrieve a reduction of soot production for the highest considered equivalence ratio value and can be used to explain the experimental behavior.

Comparative analysis of fluorescent methods selectivity for ecotoxicants environmental monitoring

A comparative analysis of classical and synchronous fluorescent methods selectivity for ecotoxicants determination of polycyclic aromatic hydrocarbons (PAHs) was carried out. PAHs sorption preconcentration from aqueous cetyltrimethylammonium bromide micellar solutions and spectra recording on a viscose matrix were used. In the case of complex environmental samples, the scope of fluorimetric analysis can be expanded by registering synchronous fluorescence spectra obtained by simultaneously scanning the excitation and emission wavelengths with a constant shift: λem - λex = Δλ = const. The composition of PAHs mixture (phenanthrene, anthracene, fluorene, pyrene) was experimentally evaluated by classical fluorescence with selective excitation and synchronous fluorescence. It was concluded that the use of synchronous spectra makes it possible to improve the selectivity of the PAHs fluorescence analysis in a mixture. These methods can be successfully applied to identify individual PAHs in monitoring of environment water bodies.

PAH formation characteristics in hydrogen-enriched non-premixed hydrocarbon flames

The utilisation of hydrogen with conventional hydrocarbons offers an excellent opportunity to decarbonise current energy systems without significant hardware upgrades. However, this presents fresh scientific challenges, one of which is the difficulty in effective control of pollutant soot emissions due to complex reaction kinetics of hydrogen enriched flames. This paper focuses on polycyclic aromatic hydrocarbons (PAHs), which are the building blocks of soot and responsible for its carcinogenicity. Detailed understanding of the effect of H2 on the underlying processes of PAH formation and growth is important for the development of effective strategies to curtail PAH formation and hence, reduce soot emissions from combustion systems.In this study, an experimental methodology was employed to analyse PAH formation and growth characteristics of laminar inverse diffusion flames of various hydrocarbon fuels (alkanes and alkenes) enriched with H2 using simultaneous planar laser induced fluorescence (PLIF) imaging of PAHs and hydroxyl radicals (OH). OH PLIF was used to indicate peak temperature locations in the flame (flame front), while PAH PLIF was used to determine PAH formation characteristics. Methane (CH4) was also separately added to the same hydrocarbon fuels to study effects of carbon-bound hydrogen addition, in comparison to H2 addition. It was observed that only the addition of H2 to CH4 showed significant variation in the magnitude of PAH reduction levels as the length along the flame front, Lf increased. The results also showed that while the addition of H2 was more effective in reducing the rate of PAH fluorescence signal increase (indicative of concentration growth) when compared to CH4 addition, both fuels showed two distinct regions in the PAH growth curve; a steep growth region followed by a slower growth region. This is potentially indicative of the self-limiting nature of PAH formation and growth. The study concluded that the growth rate of PAHs lies within a narrow band irrespective of the fuel bonding, molecular structure and the H:C ratio of the fuel mixtures tested.

Bioaccessibility of Polycyclic Aromatic Hydrocarbons (PAHs) in Grilled Meat: The Effects of Meat Doneness and Fat Content.

Exposure to polycyclic aromatic hydrocarbons (PAHs) through diet is gaining concern due to the risk it poses to human health. This study evaluated the bioaccessibility of PAHs contained in charcoal-grilled beef and chicken in different segments of the gastrointestinal tract (GIT) with regard to the degree of doneness and fat content of the meats. The levels of 15 PAHs in the grilled meat samples and bioaccessible fractions were determined using high-performance liquid chromatography (HPLC) equipped with PAH column, and UV and fluorescence detectors. Total PAHs were found in beef (30.73 ng/g) and chicken (70.93 ng/g) before its digestion, and different PAHs’ bioaccessibility were observed in the different segments of GIT, with the highest in the stomach followed by the small intestine, despite the relatively higher bioaccessibility of individual PAHs in grilled beef as compared to those in grilled chicken. Additionally, the PAHs’ bioaccessibility increased with the increase in the degree of doneness. Positive linear correlation was observed for the PAHs’ bioaccessibility and the fat contents of grilled meat. Overall, this study highlights the influence of meat doneness (cooking time) and fat contents on the bioaccessibility and bioaccumulation of PAHs.

OH, PAH, and sooting imaging in piloted liquid-spray flames of diesel and diesel surrogate

Comparison of the flame structure, polycyclic aromatic hydrocarbons (PAH), and soot formation characteristics of commercial diesel and a diesel surrogate is investigated using planar laser-induced fluorescence (PLIF) of OH and PAH at 283 nm, and laser-induced incandescence (LII) of soot at 1064 nm. The diesel and diesel surrogate flames present a similar overall appearance, with a “dark core” region of fine droplets exiting the injector and a light blue outer flame near the base. Hot soot regions appear downstream, indicated by bright yellowish flame luminosity. Comparison of simultaneously acquired single-laser-shot PLIF images of OH and PAH indicates that smaller PAH reside mostly in the central regime in the ~20–60% height of the flame, sandwiched between the flame fronts. In contrast, larger PAH and soot particles are formed further downstream. Soot-LII signals are present only in the upper ~40% height of the flame for both the fuels. The distribution of mean PAH-PLIF and soot-LII signals along the flame height is also found to be similar for both fuels, except diesel is shown to produce approximately 1.5× more soot than the diesel surrogate. Based on the analysis of probability distributions, the maximum OH-PLIF signals in both fuels have similar radial and axial spatial distributions, while maximum PAH-PLIF signals occur at a higher axial location for the diesel surrogate. However, for the diesel surrogate, maximum soot-LII signals are frequently appeared at lower heights. Therefore, exploration of more appropriate diesel surrogates may be necessary to closely mimic the PAH and soot formation behaviors of diesel fuel. The present study shows that the combined optical diagnostic techniques and the statistical analysis can be used for studying commercial diesel and diesel surrogates, and thus establish a robust framework for detailed studies of chemical kinetics, combustion dynamics, and soot formation in liquid hydrocarbon spray combustion.

Study on fluorescence interaction between humic acid and PAHs based on two-dimensional correlation spectroscopy

Polycyclic aromatic hydrocarbons (PAHs) pollution is an increasingly serious environmental problem, and the effective and rapid detection of PAHs is in urgent need for environment management and remediation. At present, fluorescence technology is the preferred method for fast in-situ dynamic detection of PAHs. However, the impact of humic acid on PAHs fluorescence has become a bottleneck for accurate detection of PAHs using this technology. In this work, 2D correlation analysis is used to study the interaction between the fluorescence spectroscopy of humic acid and benzo[ghi]perylene (BGP), a typical molecule in the PAHs family. First of all, the fluorescence characteristics of BGP and humic acid were analyzed and feature peaks were identified separately. Meanwhile, the fluorescence spectra of humic acids at various concentrations excited at different wavelengths were obtained to analyze the variation of fluorescence intensity with concentration for different groups of humic acid. Secondly, the conventional one-dimensional fluorescence spectra of the mixed samples of different concentrations of humic acids with fixed BGP concentration were studied to explore the effect of humic acid on the fluorescence of BGP. Finally, 2D correlation fluorescence spectra of the mixed samples of humic acid and BGP were studied with excitation wavelength as the external perturbation. The results showed that humic acid had a significant effect on the fluorescence intensity of BGP, and indicated that high concentration of humic acid could lead to fluorescence quenching of BGP. And the fluorescence characteristic information of humic acid with lower concentration could be extracted from sliced asynchronous spectrum at the signature peak of BGP. This study demonstrated a novel approach for the separation of fluorescence intensities of humic acid from PAHs, providing corrected PAHs spectra for quantitative analysis of the mixture.

DEVELOPING A SENSOR SYSTEM FOR IDENTIFICATION OF POLYCYCLIC AROMATIC HYDROCARBONS AS ENVIRONMENTAL TOXICANTS IN AQUEOUS AND PROTEIN MEDIA

Aim: to investigate the possibility of identifying toxic polycyclic aromatic hydrocarbons (PAHs) in aqueous and protein media using a fluorescent sensor system based on serum albumins. Methods: The studies were conducted using fluorescent methods, in particular, by quenching the intrinsic fluorescence of proteins, recording the fluorescence of PAHs and using the pyrene luminescent probe. Results: It was experimentally found that when PAHs were introduced into serum albumin solutions, a general decrease in the fluorescence intensity of proteins was observed, indicating PAHs' binding to protein macromolecules and, probably, the formation of a non-fluorescent serum protein-PAH complex. The analysis of the fluorescence spectra of PAHs in serum albumins was carried out. The dependences of changes in the intensity of the fluorescence spectra maxima of PAHs in albumins with an increase in the concentration of PAHs were linear, which makes it possible to use these systems for analytical purposes to determine toxic PAHs in protein and aqueous media. Conclusions: The results of the luminescent study on the interaction of PAHs with protein molecules can be used for monitoring of environmental toxicants in various media, as well as for the development of the methods for early diagnosis of diseases associated with the effect of various agents on the proteins.

A Glider-Compatible Optical Sensor for the Detection of Polycyclic Aromatic Hydrocarbons in the Marine Environment

This study presents the \emph{MiniFluo-UV}, an ocean glider-compatible fluorescence sensor that targets the detection of polycyclic aromatic hydrocarbons (PAHs) in the marine environment. Two MiniFluos can be installed on a glider, each equipped with two optical channels (one PAH is measured per channel). This setup allows the measurement of up to 4 different fluorescent PAHs: Naphthalene, Phenanthrene, Fluorene and Pyrene. Laboratory tests on oil products (Maya crude oil and Diesel fuel) as well as on marine samples near industrial areas (urban harbor and offshore installations) revealed that the concentration of the four PAHs targeted accounted for 62-97\% of the total PAH concentration found in samples ($\sum$16 PAHs determined by standard international protocols). Laboratory tests also revealed that for marine applications, the calibration on Water Accommodated Fraction (WAF) of crude oil is more appropriate than the one on pure standards (STD). This is because PAH fluorescence is constituted in large part of alkylated compounds that are not considered with STD calibration. Results from three glider deployments with increasing levels of complexity (a laboratory trial, a field mission in non-autonomous mode and a fully autonomous mission) are also presented. During field deployments, the MiniFluo-glider package was able to detect concentration gradients from offshore marine waters towards the head of a Mediterranean harbor ($\rm <80\,ng\,L^{-1}$) as well as hydrocarbon patches at the surface waters of an oil and gas exploitation field in the North Sea ($\rm <200\,ng\,L^{-1}$, mainly Naphthalene). It is suggested that using only the WAF calibration, the concentration derived with the MiniFluo agrees within one order of magnitude with the concentration determined by Gas Chromatography coupled with Mass Spectrometry (overestimation by a factor 7 on average). These performances can be improved if the calibration is made with a WAF with PAH proportions similar to the one find in the environment. Finally, it is shown that the use of \emph{in situ} calibration on water samples collected during the glider deployment, when possible, gives the best results.

Study of polycyclic aromatic hydrocarbons (PAHs) in hydrogen-enriched methane diffusion flames

Polycyclic aromatic hydrocarbons (PAHs) are the carcinogenic components of soot. Detailed understanding of PAH formation characteristics is required for development of effective strategies to curtail PAH formation and reduce soot in combustion devices. This study presents an experimental methodology to analyse PAH formation characteristics of a non-premixed methane-air flame with and without hydrogen (H2) addition, using simultaneous planar laser induced fluorescence (PLIF) imaging of PAH and hydroxyl radical (OH). OH PLIF was used to represent peak temperature regions in the flame front. One-dimensional, opposed-jet laminar non-premixed flame simulations were also carried out for the same fuel mixture conditions. This work describes comparison of trends from both sets of studies. PAH fluorescence intensity values were observed to increase with increasing height above burner, however this rate of increase reduced with H2 addition. This observed rate of change in PAH fluorescence (that is, PAH growth characteristics) is indicative of the sooting potential of the fuel mixture. PAH fluorescence from experiments and PAH concentration from simulation show strong reduction with increase in H2 addition. The percentage reduction in PAH fluorescence signal with H2 addition closer to the burner tip was of a similar magnitude to that observed with flame simulations. The reduction in PAH with H2 addition could be attributed to the reduction in acetylene and propargyl concentrations, and reduced H-abstraction rates, which reduced the availability of active sites for PAH growth. The proposed experimental methodology for PAH measurements can be readily applied to any fuel mixtures.

Measurement and extrapolation modeling of PAH laser-induced fluorescence spectra at elevated temperatures

The laser-induced fluorescence technique is widely used in the measurement of polycyclic aromatic hydrocarbons (PAHs) in sooting flames. One of the main limitations is the absence of the PAH fluorescence spectra at elevated temperatures. In this study, fluorescence spectra and photophysical properties of five typical PAHs were experimentally studied in the temperature range of 673–1373 K in an optical cell. The experimental results indicated that the fluorescence spectra of PAHs were greatly sensitive to PAH structures and were likely to shift to the red and became broader as temperature increases. Further, we also observed that absorption cross sections of PAHs increased linearly as a function of temperature. Fluorescence quantum yields, which were calculated using integral fluorescence intensities and absorption cross sections, decreased monotonically with increasing temperature. However, the descent gradients of different PAHs were quite different, e.g., naphthalene, fluoranthene and fluorene were more sensitive to temperature, and their fluorescence production was much lower at elevated temperature compared with phenanthrene and pyrene. In order to investigate the fluorescence quantum yields at higher temperatures (up to 2000 K), which cannot be measured in the optical cell, a multistep decay model was established and optimized based on experimental results. At temperatures between 1373 and 2000 K, the extrapolation results indicated that fluorescence quantum yields of phenanthrene and pyrene would be two orders of magnitude higher than those of naphthalene, fluorene and fluoranthene. This contributed to explaining that the PAH fluorescence signals emitted from phenanthrene and pyrene were stronger than those emitted from naphthalene in flames, although the concentrations of phenanthrene and pyrene were much lower than that of naphthalene in flames.

Distribution of polycyclic aromatic hydrocarbons (PAHs) from legacy spills at an Alaskan Arctic site underlain by permafrost

Widespread polycyclic aromatic hydrocarbon (PAH) contamination resulting from petroleum spills at military and industrial sites along the Arctic Ocean Coast is a major issue in the Alaskan Arctic. Polycyclic aromatic hydrocarbons (PAHs) consist of one or more fused aromatic rings and comprise a major portion of petroleum products like fuels. PAHs are commonly found in a variety of geomedia and are considered environmentally persistent, toxic, and carcinogenic. In the present study, the subsurface distribution of PAHs was investigated at a former U.S. Department of the Navy site in northern Alaska using laser-induced fluorescence coupled with ultraviolet optical screening tool (LIF-UVOST). The former Naval Arctic Research Laboratory (NARL) study site has historical presence just outside the city of Utqiaġvik, Alaska (formerly Barrow) and has a long history of petroleum spills since the early 1950s primarily at the Airstrip and Powerhouse sites. To determine the extent of PAH accumulation in the subsurface a total of 143 vertical soil profiles were characterized using LIF-UVOST during September 2015 (87 at the Airstrip site and 56 at the Powerhouse site). We also compared the intensities of different wavelengths (350 versus 500 nm) to estimate spill source and we found a substantial accumulation of PAHs in the surface and subsurface at both sites and vertical distribution of PAHs was primarily heterogeneous, varying as a function of probing locations and soil depth. Both sites were found to have no correlation between depth of the maximum fluorescence signal and depth of the probing refusal depth (top of permafrost). Using LIF-UVOST for the direct chemical sensing of PAHs proved to be a useful tool and revealed unique signature PAH fluorescence responses when comparing 350 and 500 nm intensities between the Airstrip and Powerhouse sites, likely attributable to the original source of the spill. Results from this project can be directly used for other potential studies, including planning remediation strategies and investigating local bodies of water for PAH transport.

Maternal polycyclic aromatic hydrocarbon (PAH) transfer and effects on offspring of copepods exposed to dispersed oil with and without oil droplets

ABSTRACTCopepods of the genus Calanus have the potential for accumulating lipophilic oil components due to their high lipid content and found to filter and ingest oil droplets during exposure. As female copepods produce eggs at the expense of lipid storage, there is a concern for transfer of lipophilic contaminants to offspring. To assess the potential for maternal transfer of oil components, ovigerous female copepods (Calanus finmarchicus) were exposed to filtered and unfiltered oil dispersions for 4 days, collected and eggs maintained in clean seawater and hatching and gene expression examined in hatched nauplii. Oil droplet exposure contributed to polycyclic aromatic hydrocarbon (PAH) uptake in dispersion-treated adult copepods, as displayed through PAH body residue analyses and fluorescence microscopy. Applying the latter methodology, transfer of heavy PAH from copepod mothers to offspring were detected Subtle effects were observed in offspring as evidenced by a temporal reduction in hatching success appear to be occurring only when mothers were exposed to the unfiltered oil dispersions. Offspring reared in clean water through to late naupliar stages were collected for RNA extraction and preparation of libraries for high-throughput transcriptome sequencing. Differentially expressed genes were identified through pairwise comparisons between treatments. Among these, several expressed genes have known roles in responses to chemical stress including xenobiotic metabolism enzymes, antioxidants, chaperones, and components of the inflammatory response. While gene expression results suggest a transgenerational activation of stress responses, the increase in relatively small number of differentially expressed genes suggests a minor long-term effect on offspring following maternal exposure.

Biomonitoring method for the determination of polycyclic aromatic hydrocarbons in hair by online in-tube solid-phase microextraction coupled with high performance liquid chromatography and fluorescence detection.

Polycyclic aromatic hydrocarbons (PAHs) are formed from the incomplete combustion or pyrolysis of organic matter during industrial processing and various human activities, but human exposure to PAHs has not yet been elucidated in detail. To assess long-term exposure to PAHs, we developed a simple and sensitive method for measuring PAHs in hair by online in-tube solid-phase microextraction using a CP-Sil 19CB capillary column as an extraction device, followed by high-performance liquid chromatography using a Zorbax Eclipse PAH column and fluorescence detection. Seventeen PAHs could be analyzed simultaneously, with good linearity from 20 to 1000pg/mL each as determined using stable isotope-labeled PAH internal standards. The detection limits of PAHs were 0.5-20.4pg/mL. PAHs in human hair samples were extracted by ultrasonication in 50mM NaOH in methanol, and successfully analyzed without any interference peaks, with good recovery rates above 70% in spiked hair samples. Using this method, we evaluated the suitability of using hair PAHs as biomarkers for long-term exposure.

Visualization of soot inception in turbulent pressurized flames by simultaneous measurement of laser-induced fluorescence of polycyclic aromatic hydrocarbons and laser-induced incandescence, and correlation to OH distributions

Distributions of polycyclic aromatic hydrocarbons (PAH) and their correlation with soot formation were studied in ethylene–air swirl flames stabilized in a gas turbine model combustor at increased pressure. The combustor can be operated with secondary air injection to study the influence of soot oxidation. We employed PAH laser-induced fluorescence using UV excitation simultaneously with IR-excited laser-induced incandescence to identify soot. PAH signatures typically appear discontinuous unlike OH, yet similar to soot but exhibit more uniform intensity and larger size. The correlation of both diagnostics allowed identification of a wide range of soot formation progress, including isolated soot or PAH, as well as PAH transitioning into soot. The occurrence of soot, PAH and OH and their spatial variations are strongly dependent on the properties of the flow field. In the bottom part of the inner recirculation zone and for the reference case, a rich flame with additional oxidation air, soot levels are relatively high, while PAH intensities in this region are minimal. This correlates well with high temperatures in this region published recently, which are unfavorable for soot formation as the precursors, PAH, decompose. Consequently, soot presence here is attributed to transport. In contrast to OH and soot distributions which change significantly upon addition of secondary air downstream of the primary combustion zone, PAH distributions for both cases look relatively similar. This is attributed to a downstream consumption of PAH by different processes. Without oxidation air, PAH completely transform into soot, while additional oxidation air leads to their oxidation.

Fluorescence Spectra of Polycyclic Aromatic Hydrocarbons and Soot Concentration in Partially Premixed Flames of Diesel Surrogate Containing Oxygenated Additives

Partially premixed laminar flames were formed using our purpose-built burner. The soot reduction mechanism of blends of diesel and oxygenated fuel was explored. The mixture of toluene and n-heptane(volume ratio, 20:80)(T20) was used as a diesel surrogate. Methanol, ethanol, n-butanol, methyl butyrate, and2,5-dimethylfuran(DMF) were blended with T20, whilst retaining a 4% oxygen content. Laser-induced fluorescence(LIF) was used to obtain spatial fluorescence spectra of polycyclic aromatic hydrocarbons(PAHs)in partially premixed co-flow flames. Laser-induced incandescence(LII) was used to measure soot concentration(volume fraction). The formation and growth of PAHs in flames varied with the fuel blend. Four-ring aromatics(A4) exhibited similar formation and oxidation to soot, so A4 was suitable for estimating soot formation and oxidation. With oxygenated additives, the content of toluene is reduced in T20 fuel, which is the major reason for the reduction of PAH fluorescence spectral intensity and soot concentration. The contribution of different oxygenated additives to PAH formation also affected soot reduction. The PAH-LIF spectral intensity and soot concentration of n-butanol/T20 blends were lower than those of fuels containing methanol, ethanol, methyl butyrate, and DMF. Therefore, n- butanol more effectively reduced PAHs and soot emission during the

In-cylinder soot precursor growth in a low-temperature combustion diesel engine: Laser-induced fluorescence of polycyclic aromatic hydrocarbons

The growth of poly-cyclic aromatic hydrocarbon (PAH) soot precursors are observed using a two-laser technique combining laser-induced fluorescence (LIF) of PAH with laser-induced incandescence (LII) of soot in a diesel engine under low-temperature combustion (LTC) conditions. The broad mixture distributions and slowed chemical kinetics of LTC “stretch out” soot-formation processes in both space and time, thereby facilitating their study. Imaging PAH–LIF from pulsed-laser excitation at three discrete wavelengths (266, 532, and 633nm) reveals the temporal growth of PAH molecules, while soot-LII from a 1064-nm pulsed laser indicates inception to soot. The distribution of PAH–LIF also grows spatially within the combustion chamber before soot-LII is first detected. The PAH–LIF signals have broad spectra, much like LII, but typically with spectral profile that is inconsistent with laser-heated soot. Quantitative natural-emission spectroscopy also shows a broad emission spectrum, presumably from PAH chemiluminescence, temporally coinciding with of the PAH–LIF.

Time-resolved spatial patterns and interactions of soot, PAH and OH in a turbulent diffusion flame

Soot control raises important fundamental issues and industrial challenges, which require a comprehensive understanding of processes governing its formation, interactions and destruction in turbulent flames. A physical insight of the soot space–time evolution in a turbulent diffusion flame is reported in this article by combining three simultaneous high sampling rate imaging diagnostics operating at a frame rate of 10kHz: light scattering from soot particles, planar laser induced fluorescence (PLIF) of the OH radical, a marker of the flame region, and planar laser induced fluorescence of Polycyclic Aromatic Hydrocarbons (PAHs), classically identified as soot precursors. Images issued from these diagnostics provide a spatially resolved information on the production of soot, its interaction with the turbulent flow and its link with the flame surface and with soot precursors regions. It is shown in particular that soot pockets are highly distorted by turbulent eddies forming a characteristic layered pattern. A statistical analysis is also proposed to analyze such high-speed imaging results. Information on soot–OH–PAH correlation deduced from the high speed imaging could be employed to verify the adequacy of models devised to represent soot dynamics in direct or large eddy simulations of turbulent flames.

Planar Laser-Induced Fluorescence Fuel Imaging During Gas-Turbine Relight

This experimental study investigates the influence of fuel distribution on ignition outcome during high-altitude relight of a gas turbine. Planar laser-induced fluorescence is used to image fuel inside a lean direct-injection combustor under realistic conditions. A novel apparatus is developed to permit planar laser-induced fluorescence imaging, in which large quantities of poorly atomized fuel impinges on the internal surfaces of the combustor. Results reveal high variability in atomization quality. In the absence of flame, small droplets are confined to areas of recirculating flow, whereas large droplets impact on the walls. All fuel is introduced through a pilot air-blast atomizer close to the injector centerline. However, comparatively little fuel is apparent near the igniter tip because the outside swirlers of the fuel injector create a fast-moving stream of fuel-free air that flows directly below the upper combustor wall. The droplet size and fuel concentration in the main recirculation zone do not differ radically at test conditions with markedly different fuel-to-air ratios, suggesting that turbulent straining is a more important factor than equivalence ratio in the failure of ignition when the airflow rate is high. In the presence of flame, medium-sized burning droplets are observed close to the injector centerline. Flame interference resulting from fluorescence of polycyclic aromatic hydrocarbons is apparent, but small, suggesting that kerosene planar laser-induced fluorescence is a useful tool for the analysis of all stages of altitude relight.