Laser-induced Incandescence Technique 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.

Soot volume fraction measurements in aero-engine model combustor outlet using two-color laser-induced incandescence

Quantitative measurement of Soot Volume Fraction (SVF) is an essential prerequisite for controlling soot particle emissions from aero-engine combustors. As an in-situ and non-intrusive optical diagnostic technique, Laser-Induced Incandescence (LII) has been increasingly applied for soot concentration quantification in various combustion environments such as laminar flame, vehicle exhaust, internal combustion chamber as well as aero-engine combustor. In this work, we experimentally measured the spatial and temporal distribution of SVF using two-color LII technique at the outlet of a single-sector dual-swirl aero-engine model combustor. The effect of inlet pressure and air preheat temperature on the SVF distribution was separately investigated within a pressure range of 241–425 kPa and a temperature range of 292–500 K. The results show that soot production increases with the inlet pressure but generally decreases with the air preheat temperature. Qualitative analysis was provided to explain the above results of parametric studies. The LII experiments were also conducted under 3 designed conditions to evaluate soot emission under practical operations. Particularly, weak soot emission was detected at the outlet under the idle condition. Our experimental results provide a valuable benchmark for evaluating soot emission in the exhaust plume of this aero-engine combustor during practical operations.

Inferring primary particle size distribution and thermal accommodation coefficient of soot aggregate across diverse ambient temperatures using laser-induced incandescence

Laser-induced incandescence (LII) technique has proven to be a powerful tool for non-invasive, in-situ particle sizing of soot aggregates in flames, and the key lies in interpreting LII signals with a heat transfer model. However, the reliability of model-derived primary particle size distribution (PPSD) is sensitive to the uncertainty of thermal accommodation coefficient (TAC), which is typically regarded as a pre-determined parameter in the LII heat transfer model. To overcome this problem, we have proposed a trivariate inversion strategy, i.e., inferring two PPSD parameters and TAC simultaneously from LII signals, which has been theoretically validated at room temperature, but its feasibility in the flame case remains to be investigated. In this study, the trivariate inversion strategy is extensively evaluated over a wide distribution of both ambient temperature and laser fluence. For different cases ranging from room temperature to flame temperature, numerical tests show that the inversion results of all three variables exhibit significantly high accuracy and low uncertainty with proper selection of laser fluence. Furthermore, our study shows that the trivariate inversion strategy can maintain robust performance in ambient temperature fluctuation caused by spatial inhomogeneity of the flame. All these results indicate that this trivariate inversion strategy has a promising application in combustion scenarios.

Simultaneous Inversion of Particle Size Distribution, Thermal Accommodation Coefficient, and Temperature of In-Flame Soot Aggregates Using Laser-Induced Incandescence.

Measuring the size distribution and temperature of high-temperature dispersed particles, particularly in-flame soot, holds paramount importance across various industries. Laser-induced incandescence (LII) stands out as a potent non-contact diagnostic technology for in-flame soot, although its effectiveness is hindered by uncertainties associated with pre-determined thermal properties. To tackle this challenge, our study proposes a multi-parameter inversion strategy-simultaneous inversion of particle size distribution, thermal accommodation coefficient, and initial temperature of in-flame soot aggregates using time-resolved LII signals. Analyzing the responses of different heat transfer sub-models to temperature rise demonstrates the necessity of incorporating sublimation and thermionic emission for accurately reproducing LII signals of high-temperature dispersed particles. Consequently, we selected a particular LII model for the multi-parameter inversion strategy. Our research reveals that LII-based particle sizing is sensitive to biases in the initial temperature of particles (equivalent to the flame temperature), underscoring the need for the proposed multi-parameter inversion strategy. Numerical results obtained at two typical flame temperatures, 1100 K and 1700 K, illustrate that selecting an appropriate laser fluence enables the simultaneous inversion of particle size distribution, thermal accommodation coefficient, and initial particle temperatures of soot aggregates with high accuracy and confidence using the LII technique.

Estimation of soot absorption function via the two separated pulses laser-induced incandescence technique

Estimation of soot absorption function via the two separated pulses laser-induced incandescence technique

Effect of Pressure on Burning and Soot Characteristics of RP-3 Kerosene Droplets under Sub-Atmospheric Pressure.

The burning and soot characteristics of RP-3 kerosene droplets under sub-atmospheric pressure were experimentally investigated in a pressure chamber. The droplet size during combustion was continuously recorded using a high-speed camera, and the burning rate based on the d 2-law was determined. The flame temperature was calculated from ICCD camera spectral data using two-color pyrometry, and the carbon soot volume fraction was measured by the calibrated laser-induced incandescence (LII) technique. Soot particles were also sampled using a thermophoretic deposition probe and characterized using a transmission electron microscope (TEM) for particle size and morphology. The results showed that the droplet burning rate increased monotonically with increasing pressure under sub-atmospheric pressure, and the flame temperature slightly decreased with increasing pressure. Sub-atmospheric pressure environment significantly inhibits the formation of soot particle clusters during the ignition of droplets. The average soot volume fraction in the flame increases approximately with increasing pressure at 0.2-1.0 bar with a power of 2.044 ± 0.066. As the pressure decreases from 1.0 to 0.2 bar, the average soot volume fraction decreases significantly from 11.801 to 0.437 ppm. This is mainly due to the fact that the sub-atmospheric environment not only inhibits the collision growth of soot particles but also promotes the oxidation process of soot particles. The collected soot particles reveal a significant reduction in particle size under sub-atmospheric pressure with average primary soot particles of 48.23, 40.06, 27.096, and 18.718 nm at 1.0, 0.6, 0.4, and 0.3 bar pressures, respectively. The change in the number density of carbon soot particles is not significant, which reveals that the change in the diameter of soot particles under sub-atmospheric pressure plays a key role in the change in the volume fraction of soot.

Effect of maturity on soot volume fraction measurements using the AC-LII technique in a laminar coflow ethylene diffusion flame

The auto-compensating laser-induced incandescence (AC-LII) technique has been frequently used to measure soot volume fraction (fv) in flames. AC-LII relies on an assumption about the spectral variation of soot absorption function E(m,λ) at the two detection wavelengths for soot temperature determination. It has been a common practice to assume a constant E(m,λ) based on the believe that LII measures mature soot only. Since soot maturity in flames is a continuous phenomenon from young to mature, it is unclear how soot maturity affects the AC-LII measured fv. In this study, a correction to the AC-LII measured fv with the conventional hypothesis of mature soot is proposed based on the soot maturity coefficient β and absorption function E(m,λ) in a laminar coflow ethylene diffusion flame. The application of this correction is first demonstrated for synthetic time-resolved LII signals based on the spatial distribution of soot properties in the ethylene diffusion flame predicted by the CoFlame code. A significant decrease of the simulated incandescence signals SLII and soot temperature Teff is observed when the maturity coefficient is increased from 0 for fully mature soot to 1.2 for highly immature soot. Then the proposed correction is applied to AC-LII measured fv in the same flame with the help of spatial distributions of β and ratio of spectrally-resolved E(m,λ) by multiwavelength line-of-sight attenuation and emission (MW-LOSA/EMI) measurements. The uncorrected AC-LII measured fv is in general significantly lower than that measured by MW-LOSA/EMI, which inherently considers soot maturity. The corrected AC-LII fv results are in good agreement, within the experimental error, with those measured by MW-LOSA/EMI. Therefore, the proposed methodology can quantify the effect of soot maturity on AC-LII measured fv and largely eliminate the difference in the measurements of fv by AC-LII and MW-LOSA/EMI.

Measurements of soot and response optimisation of laminar pool and prevaporised jet flames for various oxygenated biofuels

In this study, the generation of soot from the combustion of biodiesels and their blend with fossil diesel are investigated under laminar pool flame and prevaporised diffusion jet flame conditions. Neat biodiesels from eight feedstocks including carotene palm, palm, soy, coconut, rice bran, waste vegetable oil, duck, and goose, alongside two methyl esters of methyl laureate and methyl myristate are blended with fossil diesel at 20% volumetric intervals to form 51 different blends. The soot volume fractions for the combustion of all blends are determined using continuous-wave laser cavity extinction (CW-LCE) calibrated 2D laser-induced incandescence (2D-LII) technique. Results show that biodiesels with higher degree of saturation produce less soot, as reflected in both laminar pool and prevaroprised diffusion jet flames. The experimental data is further analysed using design of experiments (DOE) methodology. The mixture design model (MDM) following the simplex design is used to focus on the soot generating effects arising from the chemical compositions by classifying the chemical components into hydrocarbons and saturated, monounsaturated and polyunsaturated fatty acid methyl esters (FAME). The results from the Cox response trace plot associated with DOE elucidate the effects of the individual chemical group within the fuels. Both laminar flame types for the all 51 biodiesel-diesel blends exhibit the same trends with differing magnitudes. Saturated FAME has the largest singular effect on soot generation followed by hydrocarbons. However, when only neat biodiesels are considered, the laminar pool flame and laminar diffusion jet flames show differing trends for all the chemical groups. Monounsaturated FAME and saturated FAME have the greatest influence on neat biodiesel-generated soot for laminar pool flame and laminar diffusion jet flame, respectively. From the mixture design model, an optimum synthetic biodiesel mixture containing 16.1%, 34.6% and 49.3% of saturated, monounsaturated and polyunsaturated methyl esters is recommended for the lowest soot generation.

Soot Volume Fraction Measurements by Auto-Compensating Laser-Induced Incandescence in Diffusion Flames Generated by Ethylene Pool Fire

The main characteristics of pool fire flames are flame height, air entrainment, pulsation of the flame, formation and properties of soot particles, mass burning rate, radiation feedback to the pool surface, and the amount of pollutants including soot released to the environment. In this type of buoyancy controlled flames, the soot content produced and their subsequent thermal radiation feedback to the pool surface are key to determine the self-sustainability of the flame, their mass burning rate and the heat release rate. The accurate characterization of these flames is an involved task, specially for modelers due to the difficulty of imposing adequate boundary conditions. For this reason, efforts are being made to design experimental campaigns with well-controlled conditions for their reliable repeatability, reproducibility and replicability. In this work, we characterized the production of soot in a surrogate pool fire. This is emulated by a bench-scale porous burner fueled with pure ethylene burning in still air. The flame stability was characterized with high temporal and spatial resolution by using a CMOS camera and a fast photodiode. The results show that the flame exhibit a time-varying propagation behavior with a periodic separation of the reactive zone. Soot volume fraction distributions were measured at nine locations along the flame centerline from 20 to 100 mm above the burner exit using the auto-compensating laser-induced incandescence (AC-LII) technique. The mean, standard deviation and probability density function of soot volume fraction were determined. Soot volume fraction presents an increasing tendency with the height above the burner, in spite of a local decrease at 90 mm which is approximately the position separating the lower and attached portion of the flame from the higher more intermittent one. The results of this work provide a valuable data set for validating soot production models in pool fire configurations.

Study of effects of ammonia addition on soot formation characteristics in n-heptane co-flow laminar diffusion flames

Ammonia is a compelling carbon-free fuel, and co-firing ammonia with conventional hydrocarbon fuels receives growing interest as a feasible solution to mitigate CO2 emission. This study experimentally and numerically investigated the soot formation characteristics of n-heptane co-flow laminar diffusion flames with different contents of ammonia (up to 40% by volume, balanced using argon). The soot volume fraction (SVF) in the flames was quantitatively measured via laser-induced incandescence technique; PAHs and OH radical were qualitatively determined via laser-induced fluorescence technique. Results showed that ammonia addition reduced the SVF, i.e., a peak SVF decrease of 35% at 40% ammonia. Meanwhile, PAH content also decreased with ammonia addition, demonstrating that ammonia would effectively inhibit the formation of PAHs. Thus, the inception and growth of soot decreased. Interestingly, OH radical was reduced with ammonia addition, revealing that ammonia also chemically inhibited soot oxidation. The results confirmed that ammonia reduced the soot loading in the flames due to inhibitory effect on the inception and growth of soot rather than the enhancement on oxidation. The established mechanism effectively captured the variation of different PAHs in the experiments. Numerical analysis showed that the formation of the first aromatic ring benzene was reduced by ammonia as a result of the reduction of C4H5 and R203:C4H5+C2H2 reaction. In addition, the reduced hydrogen abstraction due to decrease in H, coupled with the reduced small PAH, led to the decrease in PAH growth. Nevertheless, a more comprehensive mechanism including the reactions between aromatics and nitrogen species, such as NH, CN, and NOx, is still required in future studies.

Comprehensive characterization of sooting butane jet flames, Part 1: Soot, soot-precursor, and reaction zone

The present work examines the effect of Reynolds number (Re) on n-butane jet flames, ranging from laminar to turbulent regime (Re=2100 to 21,500). n-butane is chosen since it can represent the oxidation of larger paraffins contained in practical fuels. Several quantities are measured, and various data analysis approaches are employed. Soot volume fraction (fv) was measured using a laser-induced incandescence technique, whereas polycyclic aromatic hydrocarbons (PAH) and reaction zone (OH) were imaged using laser-induced fluorescence (LIF) tool. The mean and instantaneous images of OH, PAH, and soot volume fraction are examined. The reaction zone is the least affected by the turbulence, while the PAH-LIF signal is moderately affected. A strong effect of turbulence is observed on the soot concentration and intermittency. Soot and PAH concentration decreased with Re due to reduced residence time, increased strain rate, and likely increase in a scalar dissipation rate. In the turbulent regime, fv decreases with PAH at a sharper rate, possibly due to reduced local time scales associated with turbulent eddies. Soot structure complexity is quantified through a fractal dimension (Df). Df increased with Re; however, in a turbulent regime Df becomes insensitive to Re. A semi-global correlation between fv and sheet thickness is also analyzed. A power-law trend is noted in the fv vs. thickness plot. Coefficients of power-fit are found to be Re-dependent. Additionally, the local correlation between fv and sheet thickness is examined, which revealed a non-monotonic trend. fv peaks at a preferred soot sheet thickness. These analysis-derived insights can contribute towards the understanding of soot-turbulence interaction. Furthermore, the reported database will facilitate the validation of soot chemistry models intended for practical fuels.

Experimental investigation of the flame structure of dilute sprays issuing into a hot and low-oxygen coflow

The combustion of liquid fuels in a hot and low-oxygen environment is commonly encountered in a range of practical situations. To enable investigation of the fundamental combustion processes relating to such applications, liquid fuels were injected into the reaction zone as dilute sprays in this study. Droplets of ethanol, n-heptane, and n-heptane/toluene blends were produced via an ultrasonic nebuliser, and were carried by air through a central jet to a hot coflow of combustion products. The resulting flames were then analysed using four simultaneous laser diagnostic techniques. Planar laser-induced fluorescence (PLIF) was implemented to perform imaging of key intermediate species, including hydroxyl (OH) and formaldehyde (CH2O), while the Mie scattering technique was used to detect the location of droplets. The sooting behaviour of these flames was also investigated, via the laser-induced incandescence (LII) technique. The existence of distinct inner and outer reaction zones is a key feature of all of the flames studied, and this “double flame structure” was found to be related to partial premixing of air and fuel, as well as penetration of droplets into the inner reaction zone. A change in the stabilisation of the inner flame front was observed with variations in fuel type, with a greater likelihood of ignition kernels in the case of the n-heptane and n-heptane/toluene flames, whereas the equivalent ethanol flame displays a bifurcation structure. Variations in the jet Reynolds number and liquid fuel loading were also found to have a notable impact on the distribution and evaporation of droplets, which was in turn found to affect the formation of the double flame structure. Due to the complex coupling between turbulence, chemistry and droplet evaporation in these flames, the accurate prediction of such results a priori is not within the limits of current modelling capabilities. These findings provide a valuable insight to enable future advancements in spray combustion modelling and the design of practical combustion devices.

Soot Formation and Distribution in Coal Jet Flames over a Broad Range of Coal Concentration

The soot formation and distribution in coal jet flames were measured using Laser-Induced Incandescence (LII) technique over a wide range of coal mass concentration (Ccoal). Experiments were conduct...

A forward approach for the validation of soot sizing models using laser-induced incandescence (LII)

While validating the numerical modeling of the primary particle size distribution (PPSD) in sooting flames, a common practice is to compare the numerical results to the corresponding experimental data obtained with the Time-Resolved Laser-Induced Incandescence (TiRe-LII) technique. Since the PPSD is not directly measured by TiRe-LII, but derived with a post-processing procedure, various uncertainties and errors can potentially affect the consistency of such comparison requiring the estimation of many input parameters. On the contrary, nowadays, detailed numerical simulations provide access to a more complete set of data, which can be used to reconstruct the incandescence signal. In this work, a forward approach for the generic validation of numerical models for the PPSD is performed. It is based on the numerical reconstruction of the temporal evolution of the incandescence from the numerical results and its comparison with the measured signal. First, two indexes are proposed to quantify the agreement between the numerically synthesized and the measured signals. Then, the effectiveness of the proposed approach is demonstrated a priori by quantifying the potential errors that can be avoided with this new strategy compared to the classical approach. Finally, the feasibility of the proposed procedure is proven by comparing synthesized signals to the experimental ones available in the literature for a laminar premixed flame. It is shown that the proposed approach can be used for strengthening the analysis on numerical model performances in addition to the classical approach.

Soot primary particle sizing in a n-heptane doped methane/air laminar coflow diffusion flame by planar two-color TiRe-LII and TEM image analysis

Soot primary particle size distribution along the centerline of a laminar coflow methane/air diffusion flame doped with vaporized n-heptane at atmospheric pressure was studied using the planar two-color time-resolved laser-induced incandescence (TiRe-LII) technique and analysis of transmission electron microscope images. An improved thermophoretic probe sampling procedure was used to collect samples of soot particles. The LII signals captured at two wavelength bands in the visible are used to determine the soot effective temperature by two-color pyrometry. The methodology was first validated against the literature data obtained in a laminar coflow ethylene/air diffusion flame. The same methodology is then applied to the n-heptane doped methane flame along the flame centerline. The Sauter and geometric mean diameters of soot primary particles were obtained. Good agreement is found between the soot primary particle size distributions obtained by the two techniques.

Particulate emissions measurements by laser-based techniques in a boiler fueled by wood pellets

This paper describes mainly laser-based techniques applied to a boiler fueled by wood pellets which is a real complicated combustion system. To our knowledge, laser-induced incandescence (LII) and extinction are used to characterize particulate emission at the exhaust for the first time because laser diagnostics are generally applied to laboratory flames, automotive engines and aeroengines. The 30-kW boiler was first characterized by temperature mapping in the combustion chamber and particle diameter analysis by scanning mobility particle sizer at the exhaust and inside the first combustion chamber for a better understanding of the running of this complex combustion system. Non-intrusive laser-based diagnostics including broadband LII, two-color LII and laser extinction excited at 1064 nm have been employed to study the particulate emissions at the exhaust of the boiler during the start-up and the steady-state phases of the boiler. The cross analysis of the experimental results obtained with the different diagnostics shows that particle size distributions, particles volume fractions and the temperature distribution are strongly dependent on the boiler cycle linked to the pellets loading. Laser extinction and LII-based methods provided complementary information about the particulate emissions. The LII technique probes incandescent soot particles formed during the combustion of pellets, while laser extinction detects both soot and non-incandescent particles (such as dust, fly ashes).

Chemical effects of hydrogen addition on soot formation in counterflow diffusion flames: Dependence on fuel type and oxidizer composition

We experimentally studied and kinetically modeled the effects of hydrogen addition on soot formation in methane and ethylene counterflow diffusion flames (CDFs). To isolate the chemical effects of hydrogen in such flames, we also ran a set of experiments on flames of the same base fuels but with the addition of helium. Specifically, we measured the soot volume fractions of the flames using the planar laser-induced incandescence technique. We simulated detailed sooting structures by coupling the gas-phase chemistry with the polycyclic aromatic hydrocarbon (PAH)-based soot model, using a sectional method to resolve the soot particle dynamics. Our experimental and numerical results show that hydrogen chemically inhibits soot formation in ethylene CDFs. While in methane flames, it is interesting to observe that the difference in soot production between the hydrogen- and helium-doped cases became much smaller when the oxygen concentration in the oxidizer stream (XO) was reduced. This suggests that for methane CDFs the chemical soot-inhibiting effects of hydrogen was highly dependent on oxidizer composition (i.e., XO). Explanations are provided through detailed kinetic analysis concerning the effects of hydrogen addition on the growth of PAH, soot inception, and soot surface growth processes. Our results suggest that hydrogen's chemical role in soot formation not only depends on fuel type (ethylene or methane), it also may be sensitive to oxidizer composition.

Laser-induced emission of TiO2 nanoparticles in flame spray synthesis

Oxide nanoparticles are widely studied because of their unique properties, including their crystalline phase, surface area, and porosity, which make them attractive for several applications. These properties are related to the increase in the surface/volume ratio when moving from the bulk to the nanoscale. For this reason, a diagnostic tool capable of monitoring the nanoparticle size and concentration during synthesis is particularly valuable. The laser-induced incandescence technique is widely used to provide such information. This study explored the applicability of this technique to TiO2 nanoparticles in flame spray synthesis. The fluorescence, flame emission, and incandescence signals were investigated. Time-resolved spectral measurements were first carried out on TiO2 nanoparticles deposited on a filter. At low laser fluences, the fluorescence signal of anatase TiO2 nanoparticles was detected. At higher fluences, the incandescence signal appeared. A fluence threshold limit that depended on the matrix effect was observed, above which breakdown phenomena occur. Then, laser-induced incandescence spectral measurements were performed on the flame spray at different heights above the burner and different acquisition delay times. The analysis showed the applicability and challenges in using this diagnostic tool in flame spray synthesis.

Effects of Detection Wavelengths on Soot Volume Fraction Measurements Using the Auto-Compensating LII Technique

ABSTRACT Soot particles in the detection volume in general have different temperatures due to non-uniform laser fluence and particle size distribution. The thermal radiation intensity displays different temperature dependence at different wavelengths. The effective soot temperature inferred from the ratio of laser-induced incandescence (LII) signals in the auto-compensating LII (AC-LII) technique is dependent on the detection wavelengths and affects the measured soot volume fraction. This paper numerically investigates the effects of detection wavelengths on the inferred soot effective temperature and volume fraction under conditions relevant to laminar diffusion flames at atmospheric pressure and for three representative laser fluence distributions. Numerical calculations were conducted using an LII model for a laser pulse of 5.8 ns FWHM and 1064 nm and assuming a lognormal primary particle size distribution and neglecting the aggregation effect. LII signals were modeled at four wavelength bands centered at 420, 560, 680, and 790 nm and the effective soot temperature was derived over three pairs of LII signals, namely [420, 560 nm], [560, 680 nm], and [680, 790 nm]. The two shortest and two longest detection wavelengths respectively result in the highest and lowest effective soot temperature when the laser fluence is non-uniform. The inferred soot volume fraction displays the opposite trend as the effective soot temperature. The effective soot temperature is biased toward the highest values and AC-LII always underestimates the soot volume fraction. The detection wavelengths should be carefully selected to minimize the impact of non-uniform laser fluence and at the same time to maximize the accuracy of effective soot temperature.

Measurement of soot volume fraction and primary particle diameter in oxygen enriched ethylene diffusion flames using the laser-induced incandescence technique

The paper presents an experimental investigation of the effect of oxygen concentration in the oxidizer (O2+N2) on distributions of soot volume fraction (SVF) and flame centerline primary particle size in laminar coflow ethylene diffusion flames. The base flame has an oxygen index (OI, oxygen mole fraction in the oxidizer stream) of 21%. Three additional flames, with OI of 30%, 40% and 50%, are also examined. The fuel flow rate was maintained constant in all the experiments. The two-dimensional laser-induced incandescence (2D-LII) and two-color spectral band measurement based on the response spectrum of R, and G bands of a color 3-CCD camera are applied to measure distributions of SVF. The time resolved laser-induced incandescence (TIRE-LII) is used to measure SVF and primary particle size along the flame centerline at different heights above burner (HAB). The results of SVF are compared among the two-color method, 2D-LII, and light-of-sight attenuation (LOSA) from the literature in the base flame and compared between TIRE-LII and two-color spectral band method in oxygen enriched flames. Good agreements are observed in the measured SVF from different methods. Soot primary particle sizes, measured by TIRE-LII, are compared with those based on transmission electron microscopy (TEM) image analysis from the literature and a fairly good agreement is observed. The results indicate that soot yields increase and the flame becomes shorter and brighter with increasing OI. The primary particle diameter increases firstly and then decreases with the normalized HAB by the visible flame height (hf). At the same HAB/hf, the primary particle diameter generally increases with increasing OI from 21% up to 50%.