Laminar Coflow Ethylene Diffusion Flames Research Articles

Physical and Chemical Effects of Ammonia on Gas Emissions and Soot Formation in Laminar Coflow Ethylene Diffusion Flames

Physical and Chemical Effects of Ammonia on Gas Emissions and Soot Formation in Laminar Coflow Ethylene Diffusion Flames

On the mechanisms affecting soot production in oxygen-depleted buoyant flames

The objective of this article is to apply a detailed polycyclic aromatic hydrocarbons (PAH)-based soot production model to investigate the effects of oxygen depletion on the overall soot production processes in laminar coflow ethylene diffusion flames. This task is accomplished by simulating the flames, studied experimentally by Sun et al. (Combust. Flame 211:96–111, 2020), burning under oxygen concentration in the oxidizer down to 16.8% and characterized by a constant volumetric stoichiometric air to fuel ratio to keep the residence time unchanged. This configuration allows isolating the effects of reducing oxygen on both soot formation and oxidation processes and avoiding the complex interactions between soot production and turbulence. Model predictions are in reasonable agreement with the experiments in terms of temperature, soot volume fraction, and primary particle diameter and capture quantitatively the effects of oxygen depletion on soot production. In all the flames, both the H-abstraction and acetylene addition (HACA) and PAH condensation contribute significantly to the soot mass growth, with the HACA slightly dominating the overall process. Reducing the oxygen concentration affects similarly all the formation and oxidation processes with a reduction in the range 30–40% from 21.0% to 18.9% and in the range from 60 to 70% as it is further reduced to 16.8%. Among the formation processes, the PAH condensation and surface growth by HACA have a similar sensitivity to oxygen depletion, followed by soot inception.

Effect of ammonia on the soot surface characteristics in ammonia/ethylene co-flow diffusion flames

As a carbon free and hydrogen rich fuel, ammonia is attracting more and more attention. The co-combustion of ammonia and hydrocarbon fuel can improve the combustion characteristics of ammonia and reduce CO2 emissions, and the influence of co-combustion of ammonia and hydrocarbon fuel on reducing soot emission characteristics has also aroused people's interest. In this study, the laminar co-flow ethylene diffusion flame with different ammonia addition ratios (0 %, 10 %, 20 %, and 30 %) was studied and the soot surface functional groups and graphitization structure characteristics were explored by Raman spectroscopy and X-ray photoelectron spectroscopy (XPS) respectively. The results showed that ammonia accelerated the soot graphitization process, improved the degree of graphitization, and increased the content of oxygen containing functional groups on the surface of soot particles. The XPS N1s analysis showed that, a certain amount of nitrogen-containing functional groups can be detected in ammonia/ethylene co-flow diffusion flames. The N-Ox content increased with the increase of ammonia ratio when the NHAB was>0.7. The N-5 content decreased linearly along the flame height for different the NH3 ratios, and the transforms between N-5, N-6 and N-Q were observed with the soot formation, growth and oxidation.

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.

Effects of NH3/H2/N2 addition on soot morphology and nanostructure in laminar co-flow ethylene diffusion flame

The present study investigated the effects of NH3/H2/N2 addition on soot morphology and nanostructure in laminar co-flow diffusion flame of ethylene, by adopting the method of thermophoretic sampling combined with transmission electron microscope (TEM). The volume fraction of NH3 varies from 10% to 30%, and the volume fraction of H2 and N2 are all 30%. The soot morphology evolution images were analyzed with the parameters representing size and fractal characteristics determined. Among the three diluents, the addition of NH3 is found to have the best suppressing effect with the smallest primary particle size and least accumulated aggregates, especially at the lower flame heights. Meanwhile, H2 addition is observed to advance the soot formation process due to the higher flame temperature. The soot particle images with a magnification time of 600,000 obtained from high-resolution transmission electron microscope (HRTEM) were processed by the processing code. The parameters describing the soot nanostructure were measured and analyzed with the fitted curves. It is found that the fringes of particles sampled from NH3 enriched flames tend to be shorter and more curved, and the inter fringe spacing tends to be larger, indicating the nanostructure is more disordered and the particles are easier to be oxidized. Compared with the flames with NH3 addition, particles from H2 enriched flames show a more ordered and compact nanostructure. Finally, simulations were performed to further interpret the effects. Calculated results show that the NH3 and H2 addition obviously suppress the production of larger PAHs, and NH3 is more effective than H2, but this difference gradually narrows for greater PAHs. Simulations also indicate that the peak value of precursors in NH3 enriched flame is delayed.

Effects of CO/H2/N2 addition on the soot morphology and nanostructure in laminar co-flow ethylene diffusion flame

This study investigated the effect of CO/H2/N2 addition on the morphological evolutions and nanostructures of soot generated from a laminar co-flow ethylene diffusion flame through utilizing the method of thermophoretic sampling and transmission electron microscopy (TEM). The additions were introduced into the inner tube along with the fuel stream, and the volume fractions of the additions are 10%, 20% and 30%. Specifically, the parameters of soot production, primary particle diameter (Dp), fractal dimension (Df), fractal pre-factor (kg) and the nanostructures were determined and analyzed. A reduction in soot production was observed with increasing proportion of the additives, demonstrating the addition of these diluent gases could inhibit the soot formation. Among the three additives, N2 is the most effective in reducing soot yield. The size analysis of soot particles and aggregates indicated the addition of CO or N2 could diminish the primary particle diameter and fractal parameters. The addition of H2 could reduce the primary particle diameter at the bottom flame while yields greater primary particles at 40 mm HAB. This result demonstrated that H2 addition may weaken the oxidation occurring on soot particles. High resolution transmission electron microscopy (HRTEM) analysis was also developed and the results showed that the addition of CO or H2 results in greater fringe length, smaller fringe tortuosity and inter-fringe spacing for soot particles, demonstrating the more orderly and compact lattice structure. In particular, the nanostructure parameters of soot particles in H2 enriched flame vary more significantly than those in CO enriched flame, demonstrating the soot nanostructure is more graphitized in H2 enriched flame. Different from the case of CO or H2 addition, the lattice characteristics for soot in the flames with N2 addition present inverse variation tendency.

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.

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%.

Soot production modeling in a laminar coflow ethylene diffusion flame at different Oxygen Indices using a PAH-based sectional model

A numerical study is carried out to, first of all, investigate the capability of a Polycyclic Aromatic Hydrocarbon (PAH)-based sectional particle dynamics soot model in the prediction of soot production in laminar coflow ethylene diffusion flames. The effects of different oxygen mole fractions in the oxidizer stream, called Oxygen Index (OI), ranging from 17% to oxygen-enriched conditions up to 33% are investigated. Secondly, the relative importance of species responsible for the increase in both soot formation and oxidation rates with increasing the OI is analyzed. The soot production model considers a detailed description of nucleation via collisions among heavy PAHs, particle aggregation, PAH condensation, surface growth and oxidation through the hydrogen abstraction acetylene addition (HACA) mechanism, and fragmentation of soot aggregates. Model predictions are compared with previously-published experimental data and numerical predictions obtained with a semi-empirical acetylene-based soot production model [Comb. Flame 160: 786–795 (2013)]. Results indicate that the flame structure, soot volume fraction and flame cross-section integrated soot volume fraction predicted by the PAH-based sectional soot model are in good agreement with the experimental data over the entire range of OI considered. The temperature and the concentrations of soot precursors increase with the OI, leading to higher soot nucleation, condensation, surface growth, and oxidation rates. Results show that the PAH-based sectional soot model represents a significant improvement over the semi-empirical acetylene based two-equation soot model studied earlier, especially in conditions far from the normal air conditions (21% of O2). With increasing the OI, the non-dimensional zone of influence does not change for the flame cross-section integrated soot formation rates but increases for the integrated soot oxidation rate. Nucleation occurs just above the burner rim and displays the largest increase in its rate with increasing the OI. Nucleation and condensation are mainly dependent on the concentrations of BGHIF and BAPYR. Condensation is also affected by the increase in the number of aggregates available for collision. Soot formation is mainly dominated by surface growth, through HACA, with a significant influence of soot condensation closer to the burner surface, regardless of the level of OI. The surface growth through HACA is mainly controlled not only by the acetylene concentration and the associated kinetic rate, but also by the specific soot surface area. Oxidation by O2 is found to dominate at the top of the flame, while oxidation by OH is dominant at the middle height of the flame. The oxidation rate by O2 increases more rapidly with increasing the OI than that by OH.

Experimental and Numerical Investigations of Soot Formation in Laminar Coflow Ethylene Flames Burning in O2/N2 and O2/CO2 Atmospheres at Different O2 Mole Fractions

This paper presents an experimental and numerical study of the distributions of the temperature and soot volume fraction in laminar coflow ethylene diffusion flames burning in O2/N2 and O2/CO2 atmospheres with the O2 mole fraction varying from 21 to 50% in both atmospheres. The fuel flow rate was maintained constant in all of the experiments and simulations. The two-color flame emission method based on the response spectrum of R and G bands of a three-color charge-coupled device camera was applied to measure the temperature and soot volume fraction. Numerical calculations were conducted using the C2 chemistry model [Appel–Bockhorn–Frenklach (ABF), Appel, J.; Bockhorn, H.; Frenklach, M. Kinetic modeling of soot formation with detailed chemistry and physics: Laminar premixed flames of C2 hydrocarbons. Combust. Flame 2000, 121 (1–2), 122–136, DOI: 10.1016/S0010-2180(99)00135-2] with formation of polycyclic aromatic hydrocarbons (PAHs) up to pyrene and a soot model incorporating the dimerization of two pyrene...

Numerical simulation and parametric sensitivity study of optical band gap in a laminar co-flow ethylene diffusion flame

A detailed population balance model is used to perform a parametric sensitivity study on the computed optical band gap (OBG) of polycyclic aromatic hydrocarbons (PAHs) in a laminar co-flow ethylene diffusion flame. Assuming that the OBG of PAHs in soot can be estimated by the energy difference between the highest occupied and lowest unoccupied molecular orbitals in single PAHs, OBG may be correlated with the number of aromatic rings in PAHs. This allows insights into which are the key species involved in the formation of soot. PAH size distributions are computed along the centreline and in the wings of the flame. We compare our simulations with experimentally determined soot volume fraction and OBG (derived from extinction measurements) from the literature. It is shown that the model predicts reasonably well the soot volume fraction and OBG throughout the flame. We find that the computed OBG is most sensitive to the size of the smallest PAH which is assumed to contribute to the OBG. The best results are obtained accounting for PAH contribution in both gas and particle phases assuming a minimum size of ovalene (10 rings). This suggests that the extinction measurements show a significant absorption by PAHs in the gas phase at the visible wavelength that is used, which has been demonstrated by experiments in the literature. It is further shown that PAH size distributions along the centreline and in the wings are unimodal at larger heights above burner. Despite the different soot particle histories and residence times in the flame, the PAH size associated with both modes are similar which is consistent with the near-constant OBG that is observed experimentally. The simulations indicate that the transition from the gas phase to soot particles begins with PAHs with as few as 16 aromatic rings, which is consistent with experimental observations reported in the literature.

Experimental and numerical study of the effects of the oxygen index on the radiation characteristics of laminar coflow diffusion flames

The oxygen concentration in the oxidizer stream (O2+N2) of laminar coflow ethylene diffusion flames is varied from 17% to 35% in order to study its influence on the flame height, the soot formation/oxidation processes, the smoke point characteristics, and the vertical distributions of radiative flux at a distance of 6.85cm from the flame axis. Measured values for all the investigated parameters are compared with the predictions provided by a numerical model based on a two-equation semiempirical acetylene-based soot model and on the statistical narrow-band correlated-k model to compute thermal radiation. Predictions are in overall reasonable agreement with the experiments for oxygen indices in the range 19–35%, whereas all the investigated parameters are significantly underestimated for an oxygen index of 17%. Measured flame heights based on CH∗ emission below, at, or slightly above the smoke point, as well as predicted stoichiometric flame heights, are found to be proportional to ζ=V˙f[D∞ln(1+1/S)]-1(T∞/Tad)0.67, where V˙f, D∞, S, and Tad are the volumetric flow rate of ethylene, the diffusion coefficient at ambient temperature, the stoichiometric molecular air-to-fuel ratio, and the adiabatic flame temperature. Soot formation processes are found to increase with the oxygen index, leading to higher values of the maximum soot volume fraction and the peak integrated soot volume fraction. In addition, the latter is reached at a nondimensional height (normalized by ζ of approximately 0.05, regardless of the oxygen index within the investigated range. Two regimes are evidenced: The first regime, occurring for oxygen indices lower than 25%, is dominated by soot oxidation and is characterized by an enhancement in both the maximum soot volume fraction and the fuel flow rate at the smoke point with the oxygen index. The second regime, occurring for oxygen indices higher than 25%, is dominated by soot formation; the rate of increase in the maximum soot volume fraction with the oxygen concentration is lower than in the first regime, whereas the fuel flow rate at the smoke point decreases. Finally, the peak of radiative flux increases with the oxygen index, but its rate of increase is also found to be considerably reduced for oxygen indices greater than 25%.

Influence of radiative property models on soot production in laminar coflow ethylene diffusion flames

Two axisymmetric laminar coflow non-smoking and smoking ethylene diffusion flames are studied numerically in order to assess the influence of different radiative property models on the soot formation and oxidation processes. Simulations are carried out by considering the Steady Laminar Flamelet (SLF) concept and a modified two-equation acetylene-based model to describe the soot nucleation, surface growth and oxidation processes. Several radiative property models are considered: the simple Optically Thin Approximation (OTA), the Weighted-Sum of Grey-Gases (WSGG), the Grey-Wide-Band model (GWB), the Statistical Narrow Band Correlated-k model (SNBCK) and the Full Spectrum Correlated-k model (FSCK). Comparisons between calculations carried out with the SNBCK model and experimental data show a reasonable agreement. Model results show that the choice of the radiative property models influence largely the soot prediction, especially in the upper part of the flame where oxidation occurs. Simulations show that the reabsorption of spectral gas and soot is an important feature and thus the commonly used OTA or grey models introduce large discrepancies. The GWB model leads to improved solutions, but it should be avoided if accurate predictions are desired. The FSCK provides equivalent results as compared to the SNBCK model with a substantial gain in CPU time.

An experimental and numerical study of the effects of dimethyl ether addition to fuel on polycyclic aromatic hydrocarbon and soot formation in laminar coflow ethylene/air diffusion flames

The effects of dimethyl ether addition to fuel on the formation of polycyclic aromatic hydrocarbons and soot were investigated experimentally and numerically in a laminar coflow ethylene diffusion flame at atmospheric pressure. The relative concentrations of polycyclic aromatic hydrocarbon species and the relative soot volume fractions were measured using planar laser-induced fluorescence and two-dimensional laser-induced incandescence techniques, respectively. Experiments were conducted over the entire range of dimethyl ether addition from pure ethylene to pure dimethyl ether in the fuel stream. The total carbon mass flow rate was maintained constant when the fraction of DME in the fuel stream was varied. Numerical calculations of nine diffusion flames of different dimethyl ether fractions in the fuel stream were performed using a detailed reaction mechanism consisting of 151 species and 785 reactions and a sectional soot model including soot radiation, inception of nascent soot particle due to collision of two pyrene molecules, heterogeneous surface growth and oxidation following the hydrogen abstraction acetylene addition mechanism, soot particle coagulation, and PAH surface condensation. The addition of a relatively small amount of dimethyl ether to ethylene was found experimentally to increase the concentrations of both polycyclic aromatic hydrocarbons and soot. The synergistic effect on polycyclic aromatic hydrocarbons persists over a wider range of dimethyl ether addition. The numerical results reproduce the synergistic effects of dimethyl ether addition to ethylene on both polycyclic aromatic hydrocarbons and soot, though the magnitude of soot volume fraction overshoot and the range of dimethyl ether addition associated with the synergistic effect of soot are less than those observed in the experiment. The synergistic effects of dimethyl ether addition to ethylene on many hydrocarbon species, including polycyclic aromatic ones, and soot can be fundamentally traced to the enhanced methyl concentration with the addition of dimethyl ether to ethylene. Contrary to previous findings, the pathways responsible for the synergistic effects of benzene, polycyclic aromatic hydrocarbons, and soot in the ethylene/dimethyl ether system are found to be primarily due to the cyclization of l-C 6H 6 and n-C 6H 7 and to a much lesser degree due to the interaction between C2 and C4 species for benzene formation, rather than the propargyl self-combination reaction route, though it is indeed the most important reaction for the formation of benzene.

Determination of the morphology of soot aggregates using the relative optical density method for the analysis of TEM images

The morphology of soot aggregates formed in a laminar coflow ethylene diffusion flame was studied by using the techniques of extractive thermophoretic sampling and subsequent image analysis of transmission electron microscopy (TEM) micrographs. A new image-analysis method, the relative optical density (ROD) method, is described in this paper and was used to obtain the morphological properties of soot aggregates. With this new method, the number of primary particles in each individual aggregate is determined by not only the projected area but also the optical density distribution within the projected area of the aggregate. As such, it attempts to address the three-dimensional nature of the aggregates. This method also calculates an overlap coefficient for each object in the TEM image that can be used to identify non-soot-aggregate structures. Unlike the conventional statistical method for fractal-like aggregate structure determination, ROD is independent of empirical constants based on correlations or numerical simulations. Using this new method, the parameters describing the morphology of soot aggregates generated in a laminar coflow ethylene diffusion flame, such as the geometric mean of the number of primary particles per aggregate, the fractal dimension, and the probability distribution function of aggregate size, were determined. ROD was found to be an accurate and reliable TEM image analysis method for studying the morphology of soot aggregates.

Determination of the soot absorption function and thermal accommodation coefficient using low-fluence LII in a laminar coflow ethylene diffusion flame

Effective temperatures of pulsed-laser-heated soot particles were derived from their thermal emission intensities using optical pyrometry in a laminar ethylene coflow diffusion flame. The present study concerns conditions of relatively low laser fluences under which soot particles are heated to temperatures below 3500 K to avoid complications of soot particle vaporization in both the experiment and the numerical calculations. The current nanoscale heat transfer model for laser-induced incandescence (LII) of soot was improved to account for the effect of the fractal structure of soot aggregates on the rate of heat loss to the surrounding gas. Mean primary soot particle diameter and mean aggregate size at the location of measurement were determined using the technique of thermophoretic sampling/transmission electron microscopy analysis. Numerical calculations based on the improved LII model were conducted to predict the soot particle temperature with known gas temperature, the heat conduction coefficient, the primary particle diameter, and the mean aggregate size, as well as values of assumed soot absorption function E(m) and the thermal accommodation coefficient of soot α. The experimentally observed soot temperature history, characterized by the peak value and the temporal decay rate, cannot be reproduced numerically using the values of E(m) and α found in the literature. By utilizing the experimental peak temperature and temporal decay rate new values of E(m) at 1064 nm and α were determined. Uncertainties in the derived values of E(m) and α caused by the uncertainty in the primary soot particle diameter and the mean aggregate size were analyzed. A novel method to determine the values of the soot absorption function E(m) and the thermal accommodation coefficient α was developed in the present study.