Soot Volume Fraction Measurements Research Articles

Simultaneous measurements of temperature, CO2 concentration and soot volume fraction in counterflow diffusion flames using a single mid-infrared laser

Simultaneous measurements of temperature, CO2 concentration and soot volume fraction in counterflow diffusion flames using a single mid-infrared laser

Slight asymmetry induces significant distortion of soot volume fraction measurements in counterflow diffusion flames with diffuse back-illumination imaging.

Counterflow diffusion flame is a favorable platform for fundamental investigation of soot kinetics. A diffuse back-illumination imaging technique for measuring soot volume fractions in these flames was rigorously demonstrated here. It was noticed that the technique is extremely sensitive to slight asymmetry of the flame. Misleading conclusions could be drawn due to the surprisingly large distortion of the measured SVF profile caused by flame tilting, even when the tilting is so slight as to be undetectable through the flame images. To address this issue, the effect of the flame tilting on soot measurements were quantitatively analyzed and a novel procedure was proposed to identify and correct the measurement distortions.

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.

How do the oxygenated functional groups in ether, carbonate and alcohol affect soot formation in Jet A2 diffusion flames?

Four oxygenated fuels: ethanol (EtOH), dimethyl carbonate (DMC), dimethoxymethane (DMM) and polyoxymethylene dimethyl ether (PODE3) were blended with Jet A2 to investigate the sooting behaviour of the fuel mixtures. The smoke point was measured using wick-fed laminar diffusion flames as per the ASTM D1322 standard. The oxygen extended sooting index (OESI) was calculated to determine the sooting tendency of each mixture. Colour-ratio pyrometry and differential mobility spectrometry were used to measure the soot volume fraction (fv) and particle size distribution (PSD). The addition of oxygenated fuels caused a strong reduction in sooting tendency (i.e. OESI) at low blend strengths (5%) and a weaker linear reduction at higher blend strengths (10% and 20%). Each mixture showed a similar reduction at a given mole fraction of oxygenated fuel. The OESI broadly correlated with the soot volume fraction and particle size measurements. Increasing blend strengths resulted in smaller particles at the tip of the flame. The average particle size at the tip was influenced by the oxygen content but not the molecular structure of the oxygenated fuels, whereas the soot volume fraction in the wings was influenced by both the molecular structure of the oxygenated fuels and the oxygen/carbon ratio of the mixture. For the first time, fv and PSD have been reported for flames produced using Jet A2 blends in an ASTM D1322 lamp. The ability to relate data gathered using the ASTM D1322 standard for the sooting behaviour of different mixtures is going to be increasingly important as the aviation industry seeks to switch to sustainable fuels.

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.

Planar Light Extinction Measurement of Soot Volume Fraction in Laminar Counterflow Diffusion Flames

A cost-effective and straightforward light extinction method has been extensively used for measurement of soot volume fraction (SVF) in sooting flames. The traditional pointwise measurement with translation stage suffers from relatively time-consuming operation and low spatial resolution. In the current study, the planar light extinction method is processed by utilizing a CMOS camera to image the combustion field of counterflow diffusion flame (CDF) backlit with the lamp. Collimated and diffuse optical layouts were adopted to explore the feasibility. Investigations of beam-steering effects are presented and discussed through a combination of computational fluid dynamics (CFD) and ray tracing simulations. Measured SVF are compared to the well-validated laser-induced incandescence (LII) measurements. Current measurements show that the diffuse optical layout is feasible and robust to provide accurate and more efficient measurement of the SVF in CDF with superior spatial resolution (21.65 μm).

A calibration-free two-dimensional spectral soot emission platform for temperature and soot measurements

A simple and cost-effective temperature and soot volume fraction measurement technique that does not rely on spectral or absolute irradiance calibration is developed. The technique is based on a three-sensor CMOS camera, where the broadband sensitivity of RGB sensors were narrowed down to representative wavelengths using a tri-bandpass filter. A telecentric imaging lens was used to improve the accuracy of the tomographic deconvolution of the axisymmetrical ethylene laminar diffusion flames stabilized on a co-flow burner. Temperature and soot concentration measurements have been compared with results previously reported from the same burner. Good agreement for temperature measurements and excellent agreement for soot concentration measurements have been shown. Additionally, to demonstrate the dynamic range of the system, two-dimensional temperature and soot volume fraction fields were measured for four cases of dilution corresponding to various soot loadings. As the dilution rate was increased, the peak soot volume fraction was drastically reduced and temperatures were increased in accordance with the reduced dissipative heat losses from broadband soot radiation.

Effects of oxygen on soot formation in laminar co-flow flames of binary mixtures of ethane, DME, and oxygen

Soot volume fractions of binary mixtures of ethane, DME and oxygen were evaluated experimentally in a co-flow burner. Soot volume fractions were measured by diffuse-light line-of-sight attenuation technique. The carbon mass flow rate and the C/H ratio were kept constant for all mixtures, while oxygen was introduced into the fuel stream either as an additive or as intramolecular oxygen in DME or both. Visible flame heights were decreased with increasing oxygen in the fuel stream. Binary mixtures of ethane and DME did not show any synergistic effects on soot formation. The maximum soot volume fraction was decreased from 1.41 ppm in the neat ethane flame to 0.08 ppm in the neat DME flame. On the other hand, oxygen addition to either ethane or DME resulted in an increase in soot concentrations. The maximum soot volume fraction reached to 2.1 ppm and 0.29 ppm in ethane/oxygen and DME/oxygen mixtures, respectively. The peak soot yield initially decreased for all mixtures studied as oxygen was introduced into the fuel tube. A unique transition was observed in DME/oxygen mixtures at an equivalence ratio of 6.8. When more oxygen was added beyond this ratio, the effect of oxygen on soot formation was suddenly reversed, and the peak soot volume fraction and yield were significantly decreased. This sudden reversal was explained as a result of a transition into a more premixed mode of combustion, supported by the flame images as well as chemiluminescence measurements. A distinct two-zone flame structure, that is, an inner rich zone and an outer stoichiometric zone separated by a dark region, was observed for the neat DME flame and DME/oxygen mixture flames. It is argued that as oxygen becomes available from DME decomposition, it produces a premixed region, and the flame chemistry is significantly altered beyond a critical oxygen concentration. Current radially resolved soot volume fraction measurements suggest that the effects of oxygen on soot formation can be quite complex with nonmonotonic behaviour, sudden effect reversals and shifts in soot formation and oxidation zones.

PAHs controlling soot nucleation in 0.101—0.811MPa ethylene counterflow diffusion flames

On the heels of a recent study in an atmospheric pressure ethylene diffusion flame in which the transition from parent fuel molecule to Polycyclic Aromatic Hydrocarbons (PAHs) and, eventually, soot was studied by spatially resolved measurements of PAH concentrations and soot quantities, we extended the focus to diffusion flames with self- similar structure in the 0.101–0.811 MPa pressure range. To that end, we complemented pyrometry based measurements of soot volume fraction with light scattering measurements that, once corrected for beam steering, yielded soot particle size and number concentration profiles. A chemistry model, that had been validated for all species up to 3 ring aromatics in one of the flames investigated at each pressure and up to 4-rings for the flame at atmospheric pressure, was used to compare profiles of chemical species to those of soot quantities. Further analysis yielded the assessment of number nucleation rates of soot and their comparison to the dimerization rates of PAHs. Soot nucleation rate is consistent only with the dimerization of one- and two-ring PAHs, an observation that confirms findings in the atmospheric pressure flame. Changes in pressure and temperature have a progressively larger impact on the concentration of aromatics of increasingly larger molecular weight and, even more so, on soot volume fraction and nucleation rate. A four-fold increase in pressure from 0.101 MPa to 0.405 MPa increases the soot nucleation rate and PAH dimerization rate in flames with constant peak temperature, which is primarily a concentration effect on bimolecular collision rates; a similar but less pronounced effect is observed in the higher (0.405–0.811 MPa) pressure range. Changes in pressure and temperature tend to be progressively more consequential on aromatics of increasing molecular weight and soot.

Soot maturity studies in methane-air diffusion flames at elevated pressures using laser-induced incandescence

Two-dimensional laser-induced incandescence (LII) measurements were used for quantitative soot volume fraction (fv) measurements in methane-air diffusion flames at pressures ranging from 0.1 to 0.5 MPa. Additionally, laser-induced fluorescence (LIF) was used for visualization of polycyclic aromatic hydrocarbons (PAHs) considered as important soot precursors. A heat and mass transfer-based LII model was used for the analysis of experimental LII fluence curves to evaluate soot absorption functions, E(m), at various spatial positions in the flames. Based on previous work, variations in the evaluated E(m) was assumed to be related to soot maturity. Generally, the most mature soot was found at the spatial position of maximum fv along the central axis and at the flame edges. Also, the soot maturity at the position of maximum fv at each pressure was found to increase from the flame at 0.1 MPa to 0.5 MPa. A critical examination of the assumptions and uncertainties in the E(m) analysis was made. The present study emphasizes the limitation of assigning soot a constant E(m) in the overall flame for temperature and soot concentration evaluation using the optical methods spectral soot emission (SSE) and line-of-sight attenuation (LOSA), which are commonly used non-intrusive optical diagnostic techniques in sooting high-pressure flames. The study also demonstrates the critical choice of fluence in quantitative imaging LII measurements of fv when the E(m) of soot spans over large range of values.

Two-dimensional laser-induced incandescence for soot volume fraction measurements: issues in quantification due to laser beam focusing

Two-dimensional laser-induced incandescence (LII) measurements usually involve the use of a cylindrical lens to illuminate the planar region of interest. This creates a varying laser fluence and sheet width in the imaged flame region which could lead to large uncertainties in the quantification of the 2D LII signals into soot volume fraction distributions. To investigate these effects, 2D LII measurements using a wide range of laser pulse energies were performed on a premixed flat ethylene–air flame while employing a cylindrical lens to focus the laser sheet. Using shorter focal length of the focusing lens resulted in larger variation of the LII signal profiles across the flame. A heat – and – mass – transfer - based LII model was also used to simulate the measurements and good agreement was found. The ratio between focal length (FL) and image length (IL) was introduced as a useful parameter for estimating the bias in estimated soot volume fractions across the flame. The general recommendation is to maximize this FL/IL ratio in an experiment, which in practice means the use of a long focal length lens. Furthermore, the best choices of laser fluence and detection gate width are discussed based on results from these simulations.

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.

In-Situ Measurement of Soot Volume Fraction and Temperature in Axisymmetric Soot-Laden Flames Using TR-GSVD Algorithm

To investigate soot formation and oxidation mechanism in combustion processes, determination of soot volume fraction (SVF) and temperature distributions in laminar co-flow axisymmetric soot-laden flames are still crucially important. This study provides a convenient and low-cost solution with a new inversion algorithm for in-situ measurements of SVF and temperature in axisymmetric soot-laden flames. The measurement instrumentation combines an industrial color camera and a tablet computer for in-situ measurements of SVF and temperature in three kinds of experimental flames. Based on the flame image processing, the Tikhonov regularization coupled with a generalized singular value decomposition (TR-GSVD) algorithm is proposed to do the inversion process. Comparing with common inversion algorithms, the TR-GSVD algorithm provides more accurate results. The measurement results show that the reduction in fuel flow of normal diffusion flame would result in higher temperature and lower SVF through the combined effect of radiant heat loss and fuel pyrolysis. The decrease of the equivalent ratio of partially premixed flame leads SVF to increase first and then decrease, and temperature to rise continuously, which is caused by the effect of fuel pyrolysis and the competition of soot formation and oxidation. For inverse diffusion flame, an increase in oxidant flow and a decrease in fuel flow lead to higher SVF and temperature, which is believed to be caused by the reduction of overall equivalence ratio. The detailed explanation is that the enhanced exothermic reactions lead to the temperature rise, and both the temperature and fuel pyrolysis affect soot formation.

Large Eddy Simulation of soot evolution in turbulent reacting flows: Strain-Sensitive Transport Approach for Polycyclic Aromatic Hydrocarbons

Polycyclic Aromatic Hydrocarbons (PAH) are confined to spatially intermittent regions of low scalar dissipation rates due to their slow formation chemistry. The length scales of these regions are on the order of the Kolmogorov scale or smaller, where molecular diffusion dominates over turbulent mixing irrespective of the large-scale turbulent Reynolds number. A strain-sensitivity parameter is proposed to identify such species. A Strain-Sensitive Transport Approach (SSTA) is then developed to model this differential transport in the nonpremixed flamelet equations. Specifically, the strain-sensitive species are modeled with their non-unity molecular effective Lewis numbers, while the remaining species are modeled with unity effective Lewis numbers. An a priori analysis of nonpremixed flamelet solutions reveals that the flame temperature and strain-insensitive species (e.g., major products of combustion, acetylene, etc.) profiles from the proposed SSTA closely match those from the unity effective Lewis number approach, but the mass fractions of strain-sensitive species (e.g., naphthalene) are significantly modified compared to the unity effective Lewis number approach and are consistent with Direct Numerical Simulation (DNS) data. This new SSTA model is implemented within a Large Eddy Simulation (LES) framework, applied to a series of laboratory-scale turbulent nonpremixed sooting jet flames, and validated via comparisons with experimental measurements of temperature and soot volume fraction. Both the unity effective Lewis number approach and SSTA model provide temperature predictions in good agreement with the experimental data, but the non-unity molecular effective Lewis number approach overpredicts the flame length. Compared to the unity effective Lewis number approach, the spatial distribution of soot volume fraction predicted by SSTA is in better agreement with the experimental measurements in terms of the upstream growth of the soot volume fraction and the location of its peak value along the centerline, both of which strongly depend on accurate predictions of the PAH mass fraction. The maximum soot volume fraction is minimally influenced and in good agreement with the experimental measurements. Finally, with this new SSTA model, the influence of global strain rate on turbulent nonpremixed sooting jet flames is analyzed to find that, even though PAH is inversely proportional to the global strain rate, the soot volume fraction may not be inversely proportional to the global strain rate due to an increase in the acetylene surface growth rate coefficient with global strain rate.

Soot formation in laminar flames of ethylene/ammonia

Co-firing NH3 with other fuels is receiving growing interest as a feasible solution to improve its combustion and emission properties. Previous studies mainly focused on NOx emission, and paid less attention to soot formation which is investigated here using a laminar counterflow flame configuration with ethylene fuel (75% by mole) mixed with different proportions of ammonia (between 0 and 25% by mole while the remainder is nitrogen). Soot volume fraction (SVF) was measured using planar laser induced incandescence (PLII). It was found that the addition of ammonia significantly reduced the measured SVF by 4–6% per 1% ammonia addition as compared to the reference flame (25% nitrogen). To rule out temperature effects, the experiments were simulated using Chemkin Pro and it was found that there were negligible differences in temperature between each condition implying that temperature was not responsible for the reduction in SVF. To investigate the chemical effects of ammonia addition, polycyclic aromatic hydrocarbons (PAH) were measured using planar laser induced fluorescence (PLIF) at 4 wavelengths (350 nm, 400 nm, 450 nm, and 500 nm). PLIF intensities at 350 nm is deemed to correlate with PAHs of 2–3 rings and measured profiles at this wavelength were nearly overlapping for all cases. These findings were supported by GC–MS measurements of acetylene and benzene with the latter showing little change in the peak for the cases studied here. At longer wavelengths, PLIF intensities began to show the same trends found for SVF measurements. Additionally, a specific nitrogen detector was used during GC–MS measurements and several nitrogen containing hydrocarbon species were detected with the 25% addition of ammonia. The combined results indicate that the nitrogen containing hydrocarbon species are likely to account for soot reduction, with the precise mechanism yet to be elucidated.

The effects of naphthalene-addition to alkylbenzenes on soot formation

Naphthalene and alkylbenzenes are present in practical transportation fuels. This study investigates the impact of naphthalene addition to alkylbenzenes on soot formation. Naphthalene was added to two kinds of alkylbenzenes, namely, 1,2,4-trimethylbenzene and n-propylbenzene. Because they are isomers, the effect of molecular structure is isolated. The sooting characteristics of naphthalene-added alkylbenzenes are compared to pure alkylbenzenes in laminar coflow flames. The fuel and carbon mass flow rates were kept constant for all cases. The soot volume fraction measurements show that n-propylbenzene is sensitive to naphthalene addition. In contrast, no significant changes in soot volume fraction were observed for the 1,2,4-trimethylbenzene flames. A slight increase in primary particle diameter was observed for both naphthalene-added n-propylbenzene and 1,2,4-trimethylbenzene, suggesting that naphthalene promotes soot surface growth. The calculated number densities show that naphthalene addition promotes soot nucleation for n-propylbenzene but not for 1,2,4-trimethylbenzene. The flames were simulated with the CoFlame code with the CRECK mechanism. The model partially agrees with the experimental results, as the model agrees with the case of 1,2,4-trimethylbenzene but underestimates the effect of naphthalene addition to n-propylbenzene. More understanding of the PAH formation beyond naphthalene is required. In conclusion, the study suggests that the effect of naphthalene addition on soot formation is fuel-type dependent.

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.

The effect of oxygen concentration in the co-flow of laminar ethylene diffusion flames

This paper reports an experimental study of the effects of oxygen concentration (OC) in the oxidant stream on the characteristics of laminar co-flow ethylene diffusion sooting flames. Oxygen concentration was varied from 16.8% to 36.8% (by volume) by either diluting the air co-flow with nitrogen or by adding oxygen. Planar optical measurements were performed, with high spatial resolution, of flame luminosity at 700 nm, soot volume fraction, flame temperature, primary soot particle diameters and OH* chemiluminescence. All measured parameters are found to vary significantly with OC over the investigated range. The soot volume fraction measurements quantify the extent to which soot formation and oxidation are enhanced by increasing OC, revealing that the former effect dominates in the upstream region, while the latter effect dominates the downstream region, close to the flame tip. The changes to flame luminosity, soot volume fraction and flame temperature are strongly correlated with each other, while the primary soot particle diameter correlates well with the soot residence time. In addition, the rate of soot evolution, including its formation and oxidation, correlates with flame temperature along the axis of flames and a second reaction zone emerges in the tip of flames with high OC, as revealed by OH* chemiluminescence. The experimental data are well suited to the development and validation of soot models because of the well-defined boundary conditions, high spatial resolution, systematic nature and combination of the measured scalars.

Influence of water-vapor in oxidizer stream on the sooting behavior for laminar coflow ethylene diffusion flames

The effects of adding water-vapor to the oxidizer stream on soot production in laminar coflow diffusion flames under different oxygen indices (OI) were investigated both experimentally and numerically. A modified coflow Gülder-type burner was employed to produce the laminar ethylene flames for fifteen different conditions of the oxidizer stream from oxygen-deficient (OI 17%) to oxygen-enriched (OI 25%) conditions, and also without and with adding water-vapor into the oxidizer stream up to 10% on mole basis. The measured soot volume fractions were compared with numerical predictions obtained using the CoFlame code and a chemical kinetic mechanism that consists of reaction pathways up to 5-ring PAHs. The sectional soot model used to simulate the soot particles dynamics considers soot nucleation, surface growth, PAH condensation, oxidation, particle coagulation and fragmentation. Fairly good agreement between experimental and numerical results was found, as close as 1% and 5% of difference in the peak soot volume fraction for OI21% with 0% and 10% of water-vapor addition, respectively. Clear trends of increasing the soot volume fraction (peak and also overall) was observed with increasing OI, while a significant reduction was obtained with the addition of the water-vapor. For the cases analyzed, a reduction of the total soot content was up to 60%. The chemical effects were numerically isolated using non-reacting water-vapor and analyzed, contributing especially to the soot oxidation rates. Finally, a study of the main reaction pathways was performed to better understand the chemical effects of water vapor. The results show that water vapor addition affects the concentrations of H and OH radicals and alters the formation and oxidation of soot precursors.

Soot formation and flame structure in swirl-stabilized turbulent non-premixed methane combustion

The relationships between flow field, flame structure, and soot distribution were investigated experimentally in turbulent 4 kW non-premixed swirl flames of methane and air at three different air supply rates. Auto-compensating laser-induced incandescence, particle image velocimetry, OH planar laser induced fluorescence, and OH* chemiluminescence techniques were used to measure soot volume fractions and primary soot particle size, velocity fields, OH, and OH* fields, respectively. While the instantaneous values of soot volume fractions were similar among the flames, the time-averaged soot volume fraction decreased by nearly a factor of three for a 7% increase in the air flow rate due to increased soot intermittency. Peak soot volume fractions occurred between two shear layers caused by the central fuel jet injection, recirculation zone, and swirling air jet interfaces. The decrease in soot towards the air side corresponded to locations with elevated OH, demonstrating its role in soot oxidation. The velocity measurements showed that engulfing the jet through a stronger recirculation rate induced by the higher air velocity enhanced the turbulent mixing between the recirculating fluid and fuel jet, and led to a shorter fuel jet penetration length. The reduction in soot volume fraction with air flow rate therefore is associated with greater turbulent mixing and higher oxidation rate in the recirculation zone. In contrast, the primary soot particle size, whose range was between 30 and 60 nm, did not show a dependence on the air flow rate. These results highlight the sensitivity of soot processes to fluid mechanics under turbulent conditions.