Planar Laser-induced Incandescence Research Articles

Effects of carbon dioxide addition on soot dynamics in ethylene/air inverse diffusion flames: An experimental and computational analysis

The effects of carbon dioxide (CO2) addition to ethylene (C2H4)/air inverse diffusion flames (IDFs) to the air stream on soot formation characteristics are investigated with the addition ratio of 0–13.64 %. The planar laser-induced fluorescence (PLIF) and planar laser-induced incandescence (PLII) techniques, in conjunction with CoFlame and Chemkin code simulations were utilized to assess the distributions of Polycyclic Aromatic Hydrocarbons (PAHs) and Soot Volume Fraction (SVF). The findings indicate that increasing CO2 addition results in a gradual decrease in the mole fraction of hydroxyl (OH) radicals and flame temperature, accompanied by a reduction of approximately 15 % in the reaction zone height in experimental observations and 19 % in simulations. The inhibition of soot formation is evident through a consistent decline in the normalized total SVF, a decrease in the peak volume fraction of radial soot distribution, and reduced total SVFs observed across different flame sections at varying heights. In the meanwhile, increasing the CO2 doping ratio significantly reduces the peak signal intensity of PAHs, particularly affecting high molecular weight PAHs (A3-A4, A2-A3) with reductions of up to 75.5 %. Furthermore, reductions are noted in the rates of soot inception and subsequent surface growth, accompanied by an upward displacement of the initial inception and growth location. The condensation of PAHs controls the soot surface growth. The thermal and chemical effects of CO2 were differentiated by employing the virtual substance FCO2. The results suggest that the thermal effect of CO2 lowers flame temperature, reduces combustion intensity, and consequently inhibits soot nucleation. The chemical effect of CO2 competes for H radicals through the reverse reaction of CO + OH ≤> CO2+H. This process suppresses the formation and growth of PAHs, consequently leading to a reduction in soot production.

Optical investigation on lubricant combustion characteristics under the ammonia-methane premixed environment

The combustion process is an indispensable energy conversion step in modern industry and energy production. Especially in high-performance engines and energy conversion systems, understanding and controlling the combustion process is critical to improving efficiency and reducing environmental pollution. As a key component of mechanical systems, the combustion characteristics of lubricants at high temperatures and pressures have a direct impact on system performance and emissions. This study investigated the combustion characteristics of lubricant films in an ammonia-methane premixed environment using Planar Laser-Induced Fluorescence (PLIF) and Planar Laser-Induced Incandescence (PLII). It focused on the effects of varying ammonia concentrations on the emission of soot and polycyclic aromatic hydrocarbons (PAHs) in the constant volume combustion vessel. Experiments varied the ammonia-to-methane molar ratios, initial oil film thickness, and base oil type to study their influence on the lubricant combustion characteristics. Results indicated that ammonia significantly reduced soot formation by altering the dynamics of NH* and OH* and impacting PAH distribution. For the lubricating oil soot formation factors, the increase of the initial mean oil film thickness was more obvious for the promotion of soot formation, the increase of the unsaturated hydrocarbon content in the oil film also promoted the generation of carbon soot to a certain extent, and the initial flame energy ratio had the least influence on the promotion of soot formation. Overall, the study contributed to the understanding of the formation mechanism of soot caused by lubricating oil in zero-carbon and low-carbon blended atmospheres and provided a new insight into the interaction between nitrogen-containing zero-carbon fuels and large-carbon-number hydrocarbons, which is of great significance for controlling soot emissions from engines under zero-carbon fuel blending.

Planar laser-induced incandescence for the study of soot production in a multi-sector RQL Jet A combustor

Understanding the production of non-volatile particulate matter (nvPM), which is composed primarily of soot, is critical not only for reducing emissions but also for improving engine performance. While there has been significant prior work studying the fundamentals of soot formation, there is significantly less work that investigates soot formation with realistic aeroengine geometries, injectors, and fuels in high pressure conditions. In this work, soot production in a three-sector rich-quench-lean (RQL) aeroengine combustor is studied with Jet A fuel. Global equivalence ratios ranging of 0.10 to 0.20 and pressures ranging from 2.7 to 6.9 bar absolute (40 to 100 psia) are tested. In order to characterize in-situ soot production near the fuel injectors, two-dimensional laser-induced incandescence is utilized to estimate single-shot and average soot volume fractions. Time-resolved laser-induced incandescence is then used to create single camera and single laser-shot incandescence decay time images in order to infer how soot particle sizes evolve. Results show a significant increase in soot production at higher global equivalence ratios and higher pressures. Incandescence decay times, however, do not change significantly over the same range of conditions. These measurements can not only help understand soot distributions in practical RQL systems but also help improve future aeroengine combustor designs.

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.

Soot and PAH formation in laminar diffusion flames of RP-3 jet kerosene and its surrogates at preheat temperature

For investigating the effects of preheat temperature on the soot/PAH formation of RP-3 kerosene surrogates, planar laser-induced incandescence (PLII), planar laser-induced fluorescence of PAH (PAH-PLIF) and chemical kinetic simulation are conducted in several laminar diffusion flames. The goal is to provide a surrogate that matches well with the soot formation characteristics of RP-3 kerosene and to investigate the mechanism behind the temperature sensitivity of different surrogates. Firstly, surrogate S1 (30.3% n-hexadecane, 39.8% iso-cetane, 29.8% o-xylene, in mole fraction) is formulated to obtain a surrogate with good reproducibility of soot formation characteristics corresponding to the RP-3 jet kerosene. Compared with another reported surrogate S2, S1 presents a higher soot yield due to its higher PAH formation and a stronger soot formation propensity. With elevated preheat temperature, soot yield of all the studied flames gets higher, and the temperature sensitivities for soot formation of S1 and S2 are shown with little difference. By further analyzing the PAH results experimentally and numerically, the temperature sensitivity for soot yield of S2 is more closely related to the temperature sensitivity of PAH precursors, while the subsequent processes after soot inception may be more important to the temperature dependence of S1.

A Comprehensive Review of Optical Systems for Soot Volume Fraction Measurements in Co-Flow Laminar Flames: Laser-Induced Incandescence (LII) and Laser Extinction Method (LEM)

ABSTRACT Laser-Induced Incandescence (LII) has been widely acknowledged as a highly effective method for measuring soot in co-flow laminar flames. Conversely, quantitative assessments of soot volume fraction (SVF) are achievable through the Laser Extinction Method (LEM). This comprehensive review centers on the optical systems and experimental parameter configurations employed in LII and LEM measurements. Subsequently, several pivotal parameters specific to LII and LEM, such as laser fluence and detection wavelength, are individually discussed. Furthermore, this review showcases current experimental arrangements utilized for LEM measurements. In addition, recommendations are provided for the execution of planar LII and LEM measurements. Lastly, a summarized analysis and discussion pertain to an optical system capable of jointly measuring LII and LEM.

Effects of hydrogen on PAH and soot formation in laminar diffusion flames of RP-3 jet kerosene and its surrogate

PAH and soot are harmful substances that can be produced in any type of combustion equipment including aircraft engines. The co-combustion of hydrogen and jet fuel has been applied in aero-engine combustors and large-scale hydrogen addition is a promising solution for reducing the consumption of fossil fuels in the aviation industry. However, it remains unclear how H2 influences the soot and PAH formation characteristics of jet kerosene. In this study, to investigate the impact of H2 on soot and PAH formation, planar laser-induced incandescence (PLII), planar laser-induced fluorescence of PAH (PAH-PLIF) and chemical kinetic simulation were conducted for the laminar diffusion flames of RP-3 jet kerosene and its surrogate S1 with different H2 doping rates. It is found that the introduction of H2 leads to the increased soot formation. However, the promotion effect of H2 on the PAH formation weakens as the number of PAH rings increases, and the formation of A4 is significantly inhibited. But the rapidly increase of benzene and alkynes in the H2-doped kerosene flame may ultimately lead to the promotion of soot formation. Furthermore, the changes in direct synthesis reactions and PAH=>PAH- jointly affect the converse changes in A1 and A4 formation. These findings will contribute to the development of the soot model and soot/PAH-reduction strategy for the co-combustion of jet fuels and hydrogen.

Planar time-resolved laser-induced incandescence for pressurized premixed Jet-A combustion

Planar time-resolved laser-induced incandescence for pressurized premixed Jet-A combustion

Soot structure and flow characteristics in turbulent non-premixed methane flames stabilised on a bluff-body

The soot properties of methane in turbulent regimes are not well characterised but are highly desirable. Methane is the main constituent of natural gas that is broadly used in many industrial combustors. Investigation of turbulent methane flames under well-defined boundary conditions is therefore useful for interpreting soot formation in practical burners and can be used for further model development. This study presents a joint experimental and numerical study of a series of turbulent non-premixed bluff-body flames fuelled with pure methane for three values of the momentum flux ratio of fuel jet to co-flowing air. Soot volume fraction (SVF) and flowfield are measured simultaneously using planar laser-induced incandescence (P-LII) and 2D-polarised particle image velocimetry (P-PIV). Additionally, time-averaged temperature, mixture fraction, OH and C2H2 concentrations are estimated numerically using RANS models. The global flame structure for all three flames features a recirculation zone with a double-vortex structure, a jet-propagating zone, and a neck zone connecting the two regions. The soot distribution within the recirculation zone shows clear distinct features, which is attributed to the mean mixture fraction distribution in this zone. Increasing the momentum flux ratio shifts the location of the mean stoichiometric mixture fraction to the rich inner vortex core, leading to a distinct peak of the total integrated soot in the inner vortex of the recirculation zone that is not observed in other cases. Also, it is deduced that the soot inception starts earlier in the recirculation zone for the flame with the highest momentum flux ratio and in the jet zone for the other two flames. Much higher soot concentration and lower intermittency are found with ethylene-based flames stabilised on the same burner and with the same operating conditions. In addition, the study has generated a database of soot and flowfield results, which can be helpful for future model validations.

Effect of preheating air temperature on sooting tendency in laminar co-flow diffusion flame of n-heptane

This work investigates the effect of preheating air temperature on the sooting tendency in the laminar co-flow diffusion flames of n-heptane. A modified co-flow burner was employed to produce steady n-heptane laminar diffusion flames with the preheating temperatures of co-flow air varying from 344 K to 588 K. The planar laser-induced incandescence (LII) calibrated by the line-of-sight attenuation (LOSA) was performed to quantitatively measure the soot volume fraction and the soot yield in flames and an ICCD camera was used to capture the flame luminosity images. The results reveal that the visible flame height is decreased with the preheating co-flow air temperature, and it is hardly affected by the air velocity with a constant air temperature. As the co-flow air temperature increases, the soot inception and oxidation and the peak soot concentration initiate at the lower positions in flames, and the soot loading in flames has a remarkable increasing trend by assessing the evolution of the soot volume fraction and the soot yield. However, the enhancement rate of total soot loading in flames with the co-flow air temperature below the initial fuel temperature is lower than that for the co-flow air temperature above the initial fuel temperature. In addition, the temperature sensitivity of soot loading is assessed by the maximum soot volume fraction, which is in accordance with those of the alkane in previous research. According to the response of the maximum soot yield to the co-flow air temperature and adiabatic flame temperature, the temperature sensitivity of sooting tendency in n-heptane flames is further confirmed.

Influence of primary reference fuels on isobaric combustion in a heavy-duty optical diesel engine

Previous studies on isobaric combustion achieved using n-heptane/diesel fuel have shown the potential for higher engine efficiency compared to conventional diesel combustion. This was due to the lower heat transfer losses thanks to the lower rate of heat release and in-cylinder temperature, along with exhaust losses when coupled with a split cycle concept. However, the close-coupled injections implemented in isobaric combustion led to spray-flame interactions resulting in higher soot emissions. Potential ways to reduce the soot emissions are either employing multiple injectors, which helps to avoid the spray-flame interaction, or, as investigated in this study, burning higher octane or low reactivity fuels to increase the pre-combustion mixing time. This study investigated the effect of fuels on isobaric combustion in both all-metal and optical configurations of a heavy-duty optical diesel engine. Isobaric combustion was achieved using multiple injections from a single central injector; the engine performance, exhaust emissions, particle size distributions, and in-cylinder flame and soot distributions have been compared for three different primary reference fuels (PRFs) i.e., PRF0 (n-heptane), PRF50 (volumetric mixture of 50 % n-heptane and 50 % isooctane), and PRF100 (isooctane). High-speed combustion luminosity, OH* chemiluminescence, and planar laser-induced incandescence of soot (soot-PLII) have been performed at a fixed air-excess ratio and peak cylinder pressure for all three fuels. The results demonstrated that PRF100 being the least reactive fuel out of the three tested fuels led to longer ignition delay resulting in more premixed combustion. PRF100 resulted in 4 % higher gross indicated efficiency, 48.8 % lower heat transfer losses, 17.6 % lower exhaust losses, and 87.4 % lower soot emission than PRF0; however, isobaric combustion using PRF100 was difficult to achieve. PRF50 is a promising candidate for isobaric combustion with slightly lower gross indicated efficiency than PRF100 but lower uHC/CO/soot/NOx emissions. Overall, PDI of Soot-PLII indicates initial soot is formed near the nozzle holes, and grow along the flame direction, until they hit the bowl-wall and flow along it due to flame-wall interactions. The soot PDI values are comparatively lower as we move axially away from the top of the cylinder.

In-flame soot structure of a jet fuel with 24% aromatics in a small-bore optical compression-ignition engine

This study applies a multi-location soot sampling method in a small-bore optical compression ignition engine to show in-flame particle evolution of a surrogate jet fuel with 24% aromatics (AR24) and a conventional diesel fuel. Four soot sampling probes are installed on the piston-bowl wall with 60° spacing angles for simultaneous sampling from the same firing cycle. These sampling locations represent a jet-wall impingement point (JW), an up-swirl point (US), a down-swirl point 1 (DS1) and a down-swirl point 2 (DS2) along the sooting flame path identified by high-speed soot luminosity images and planar laser induced incandescence (PLII) images. The sampled soot particles are examined with transmission electron microscope (TEM) and the images are post processed for the analysis of morphology. The results showed both fuels produce a large amount of small soot aggregates at JW point due to fuel-rich mixtures while AR24 soot aggregate number count is much less than diesel, indicating less soot formation for AR24. They grow as the flame travels along the bowl wall through aggregation, which is more significant at DS1 point than that of US point. Simultaneously, some small soot aggregates disappear due to oxidation, which is more significant at US point. As the sooting flame reaches DS2 point, only large aggregates with compact structures survived intense oxidation. The soot sampling performed for high resolution TEM and internal structure analysis of AR24 showed that the core-shell particle structure becomes clearer and the carbon-layer fringe-to-fringe gap decreases during the flame penetration, indicating the soot oxidation continued. From the results, the in-flame soot evolution is characterised by high formation of small aggregates at JW point, simultaneous aggregation and oxidation at US and DS1 point with the latter more prone to aggregation, and significant oxidation at DS2 point.

Relationships between Soot and the Local Instantaneous Strain Rate in Turbulent Non-Premixed Bluff-Body Flames

In this paper, the simultaneous measurements of the local velocity using polarized particle image velocimetry (P-PIV) and soot volume fraction, SVF, using planar laser-induced incandescence (LII) are reported for turbulent non-premixed ethylene–nitrogen bluff-body flames. The measured data were used to assess the correlation between soot volume fraction and the shear-strain rate (S), both mean and instantaneous, in different regions of the bluff-body flames. In the recirculation zone, an exponential function was found to reasonably describe the relationship between total SVRZ and S. It was also observed that the strain rate associated with the maximum SVF in the recirculation zone and the jet region are ≲1000 s–1, consistent with the soot inception region of simple jet flames. The joint PDFs show that the SVF is not well correlated with the local instantaneous strain rate in either the recirculation zone or the jet region of these flames. Qualitatively, the correlation of coefficient, R2, between the local instantaneous SVF and inverse of the strain rate (1/S) reveals a low to weak correlation, 0.3 < R2 < 0.6, in the recirculation zone and the jet region, which is consistent with the joint PDFs. This suggests that the timescales for SVF are significantly greater than those driving local fluctuations. In addition, a high-fidelity experimental dataset has been provided for model development and validations.

Study Of Soot Aggregate Formation And Oxidation In A Swirled Stratified Premixed Ethylene/Air Flame

Soot particles are one of the main causes of today's pollution because of their negative contribution to global warming and human health. Aviation is one of the domains dealing with soot reduction as the new engine concepts are developed to reduce fuel consumption and global emissions. Despite the fact that most combustion devices used for air transportation operate at high pressure (e.g., aircraft gas turbines up to 40 bar), our understanding of soot formation and oxidation in such conditions is not yet at an appropriate level, as there is still a fundamental lack of experimental data and corresponding predictive models in the literature. Thus, the objective of the current study is to evaluate soot formation and oxidation processes in stratified, swirled, premixed ethylene/air flames examined with a variety of laser diagnostics designed to simultaneously measure soot particle and soot precursor 2D-distributions, as well as the flame structure and the aerodynamic field. For that, the SIRIUS burner was selected because of its ability to produce flames with topologies similar to those encountered in aircraft combustors. Soot particle distributions are measured by Planar Laser-induced incandescence (PLII) diagnostic. The flame structure is obtained by detecting the hydroxyl radicals (OH) with Planar laser-induced fluorescence (PLIF). A second PLIF diagnostic is also used to investigate the production of polycyclic aromatic hydrocarbons (PAH) with the detection of two benzene rings molecules which are recognized as good precursors of soot nucleation and growth. Finally, the particle Image Velocimetry (PIV) diagnostic is used for measuring the velocity distributions. These laser diagnostics are coupled together in order to obtain cross-correlations between several scalar parameters playing a determining role in the soot formation/consumption processes. The experimental results collected at atmospheric pressure are reviewed and critically assessed. A scenario describing the link between the soot inception, growth, aggregation and oxidation processes is proposed by analyzing velocity, OH, PAHs and soot distributions. In particular, the data reveal the presence of distinct regions for these processes. Incipient soot production zone is strongly function of specific local conditions of velocity, PAH concentration, and strain rate encountered at the interface of the internal recirculation zone and the fuel/air jet. The central part of the inner recirculation zone in which large structures move at low velocities provides suitable conditions for the aggregation of nascent soot particles while an oxidation region located in the upper zone of the internal recirculation zone favors the consumption of soot.

The effect of preheating temperature on PAH/soot formation in methane/air co-flow flames at elevated pressure

Preheating technology is widely used to improve the emission or efficiency of combustors, such as diesel engines and gasifiers operating at high pressures. Soot is an unwanted by-product, but its formation is unavoidable in high-pressure diffusion combustion. In this study, the effect of preheating temperature on polycyclic aromatic hydrocarbon (PAH) and soot formation in methane/air co-flow flames was comprehensively investigated in the pressure range of 1–5 bar. The temperature of inlet gas ranges from 295 K to 573 K. The soot, PAH, and OH* concentrations were obtained using planar laser-induced incandescence, planar laser induced fluorescence, and chemiluminescence techniques, respectively. The experimental results reveal that soot and PAH formation is greatly enhanced at higher pressure or with a higher preheating temperature of inlet gas. At a fixed preheating temperature, the peak/integrated soot volume fraction follows a power law with pressure. As pressure increases, the enhancement of soot formation by preheating temperature is suppressed. As the preheating temperature is raised from 295 K to 573 K, the integrated soot volume fraction is increased by 33.7 times at 1.5 bar, but the difference narrows to 2.3 times at 5 bar. OH* signal increases with preheating temperature at 1 bar, but the difference becomes indistinguishable at higher pressure. Further, the experimental results were utilized to evaluate the soot modeling, PAH and soot formation under experimental conditions are examined using four different kinetic mechanisms. While the soot trend along different pressure and preheating temperature is qualitatively captured, the quantitative predictions vary depending on the mechanisms. Specifically, KAUST and Narayanaswamy-Blanquart-Pitsch (NBP) mechanisms overpredict the soot volume fraction while DRL and Appel-Bockhorn-Frenklach (ABF) mechanisms underpredicts the soot volume fraction. In terms of the PAH spatial distribution, only DRL and ABF mechanisms show the ability to capture the experimental observations, that is the peak PAH appears in the flame centerline. The reaction pathway analysis indicates both fuel-pyrolysis chemistry and PAH growth chemistry should be accounted for the discrepancy.

A layer-peeling method for signal trapping correction in planar LII measurements of statistically steady flames

This work presents a layer-peeling (LP) algorithm to correct the signal trapping effect in planar laser-induced incandescence (LII) measurements of soot volume fraction. The method is based on measurements of LII signals captured by an intensified CCD camera at a series of parallel planes across a diffusion flame. A method based on presumed function (PF) of soot volume fraction is also proposed for comparison. The presented methods are numerically tested based on synthetic LII signals emitted from a simulated axisymmetric laminar diffusion flame using the CoFlame code. Numerical results showed that the LP method is able to correct the signal trapping effect, even for fairly large optical thicknesses and in a wide range of detection wavelengths. The correction decreases the relative errors induced by neglecting the trapping effect considerably. The signal trapping effect correction is less important for the determination of integrated soot quantities such as radially integrated soot volume fraction or total soot loading. Planar LII measurements were carried out and calibrated in order to test the method experimentally in a coflow flame. The LP, PF and a simplified analytical (SA) model were compared. The results indicate that the differences in soot volume fraction of 1 ppm or about 15% are obtained in zones of maximum soot loading of 6.5 ppm when the trapping effect is accounted for. Also, the LP and SA methods were found computationally efficient and accurate compared to the PF method. Although the study was performed in a canonical laminar axisymmetric flame, the proposed method can be applied to any statistically steady 3D flame.

Application of planar auto-compensating laser-induced incandescence to low-sooting turbulent flames and investigation of the detection gate width effect

Laser-induced incandescence (LII) is an optical technique that is widely used for the in situ measurement of the soot volume fraction in flames. The low intensity of LII signals in low-sooting turbulent flames means that long camera acquisition times are needed to achieve sufficiently high signal-to-noise ratios. The direct application of auto-compensating LII (AC-LII) to infer the soot volume fraction from the measured LII signals can lead to large errors due to the significant decrease in the temperature of the soot particles over the duration of the camera gate. In order to reduce the measurement errors from AC-LII, we propose an improved approach in which the cooling of soot particles during the camera acquisition of LII signals is considered. The proposed methodology is applied to planar AC-LII measurements for the determination of soot volume fraction and primary particle diameter in a low-sooting turbulent flame, with the help of the open source LII software LIISim. This study represents the first application of planar AC-LII to a turbulent flame. A sensitivity analysis is also conducted in order to determine the main factors affecting the uncertainty of the proposed approach.Copyright © 2021 American Association for Aerosol Research

Effect of the jet fuel cetane number on combustion in a small-bore compression-ignition engine

Three custom-made jet fuels with cetane numbers 30, 40 and 51 are investigated in an optically accessible small-bore compression-ignition engine. The low-temperature and high-temperature reactions are visualised using planar laser-induced fluorescence imaging of formaldehyde (HCHO-PLIF) and hydroxyl radicals (OH-PLIF) and is complemented with high-speed, line-of-sight integrated signal imaging of cool-flame and OH* chemiluminescence. For soot measurements, planar laser-induced incandescence (soot-PLII) imaging is performed. The cool-flame chemiluminescence and HCHO-PLIF images show that the low-temperature reaction develops quicker and covers a larger area in the combustion chamber for higher cetane number fuels. Also, the OH* chemiluminescence and OH-PLIF images indicate the transition from low-temperature reaction to high-temperature reaction occurs faster in both spatial distribution/concentration and temporal evolution, indicating the predominant effect of higher fuel reactivity despite lower charge premixing. The results obtained for single injection conditions are extended to pilot-main injection conditions in an attempt to further reduce the charge premixing to the level that its impact becomes measurable. Indeed, the significantly reduced charge premixing condition induces lower HCHO-PLIF and OH-PLIF for CN50 than those of CN40 while the soot-PLII is most intense. The CN30 exhibits the lowest soot-PLII than the other two fuels due to the enhanced charge premixing but the OH-PLIF signals are weaker due to the lower fuel reactivity. This study successfully identifies the important trade-off characteristics between fuel reactivity and charge premixing among the tested fuels and given injection conditions, CN40 shows the most optimised performance with the strong high-temperature reaction and low remaining in-cylinder soot.

Generating planar distributions of soot particles from luminosity images in turbulent flames using deep learning

We report a computational method based on deep learning (DL) to generate planar distributions of soot particles in turbulent flames from line-of-sight luminosity images. A conditional generative adversarial network (C-GAN) was trained using flame luminosity and planar laser-induced incandescence (LII) images simultaneously recorded in a turbulent sooting flame with an exit Reynolds number of 15,000. Such a training built up the underlying relationship between the two types of images i.e., a predictive model which was then used to predict LII images from luminosity images and the accuracy was assessed using four different methods. Results show that the model is effective and capable of generating LII images with acceptable prediction accuracies of around 0.75. The model was also found to be applicable over a range of heights in the flames, as well as for the flames with a range of exit Reynolds numbers spanning from 8000 to 20,000. Besides, the probability density function (PDF) of LII signals in different flames can also be predicated using the model. This work, for the first time, demonstrates the feasibility of predicting planar signals from corresponding line-of-sight signals from turbulent flames, which potentially offers a much simpler optical arrangement for a modest trade-off in accuracy.

Correction of laser-induced incandescence signal trapping in soot measurement in a microgravity boundary layer laminar diffusion flame

This work presents a novel method to auto-correct the trapping effect of laser-induced incandescence (LII) signals inside the flame to obtain soot volume fraction distributions in laminar axisymmetric (2D) and three-dimensional (3D) flames using planar LII. The development of the proposed model is described with an estimation of the propagated uncertainties. The proposed model is firstly numerically validated in a canonical 2D coflow Santoro flame using synthetic LII signals and soot properties predicted by the CoFlame code. Secondly, experimental measurements of LII were carried out in the same 2D coflow Santoro flame and the LII signals were converted to soot volume fraction using the proposed model. Simultaneous LII and line-of-sight attenuation measurements were conducted to obtain the calibration factor. The proposed model automatically took into account the signal trapping effect in the conversion of the detected LII signal to soot volume fraction. Finally, the validated methodology is applied to a 3D laminar boundary layer diffusion flame established in microgravity. The significant differences in the measured soot volume fraction distributions with and without considering signal trapping, particularly in the 3D zone of the flame, demonstrate the importance of considering signal trapping to LII measurements of soot volume fraction in this flame. The model developed in this work can be readily applied in planar LII measurements of soot in any flame configuration as long as it is steady or statistically steady, to allow measurements to be performed at different positions inside a flame, such as for pool-fires or pulsating flames.