Ethylene Flames Research Articles

The role of DME addition on the evolution of soot and soot precursors in laminar ethylene jet flames

Dimethyl ether (DME) has received considerable attention as a fuel additive to reduce the emission of particulate matter (PM) due to its low-temperature chemistry, molecularly bound oxygen atom and the absence of CC bonds. However, the effect of DME addition on the evolution of soot and particularly soot precursors is not entirely understood. This study aims to shed light on this issue by blending different proportions of DME with diffusion, E60, and partially premixed, PP12, base cases of laminar ethylene flames using the Yale benchmark burner. Laser-induced fluorescence (LIF) intensity and decay time are used to characterize the structure and evolution of soot precursors, while laser-induced incandescence (LII) is utilized to determine the soot volume fraction (SVF) and the effective primary particle diameter (Dp). For the diffusion flames, the addition of 10% DME increases the concentrations of both soot and soot precursors. With the further addition of DME to 20%, the SVF decreases to levels similar to those of E60 and then decreases further with 30% DME addition. All diffusion flames with DME addition exhibit higher concentrations of soot precursors than those of the reference E60 case. For PP12, the addition of 10% DME shows similar concentrations of soot precursors and a slight reduction in the SVF which continues to decrease with further increases in DME additions to the PP12 flame. The addition of DME seems to have little effect on the soot particle diameters for all the studied flames. Overall, the PP flames result in smaller mean particle diameters than the diffusion flame counterparts.

Charge characteristics of sub-10 nm soot particles in premixed ethylene flames

In this work, the charge characteristics of sub-10 nm soot particles were studied in burner-stabilized-stagnation premixed ethylene flames, at an equivalent ratio of 2.0 and over the calculated maximum flame temperature (Tmax) range of 1665–1933 K, using micro-orifice probe sampling in tandem with neutralizer, half-mini differential mobility analyzer (Half-mini DMA) and electrometer. To obtain the charge fraction, the charged and total particle size distributions (PSDs) were measured with the neutralizer off and on, separately. Our results showed that both negatively charged particles (NCP) and positively charged particles (PCP) grew to bigger particles with the increase of height above burner (Hp). However, compared to PCP, NCP were relatively smaller in size, but higher in concentration, possibly because negative ions such as electrons diffused faster to coagulate with soot. Moreover, charge fraction decreased along Hp and newly nucleated soot particles were all neutral at higher Hp, which could be attributed to the higher temperature near the flame front, where the concentrations of ions were higher. Another interesting finding was the increase in charge fraction by two orders of magnitude (from 0.1% to 10%) as Tmax increased from 1665 K to 1933 K. All of these suggested the existence of particle-ions interaction close to the flame front, especially at high temperatures.

Study of soot functional groups and morphological characteristics in laminar coflow methane and ethylene diffusion flames with hydrogen addition

The soot in laminar coflow methane and ethylene diffusion flames with different hydrogen addition ratios is sampled by a capillary soot sampling system and analyzed by Fourier transform infrared spectroscopy (FTIR) and field emission scanning electron microscopy (FESEM). Soot functional groups and morphological characteristics are studied. The results show that with hydrogen addition, the relative contents of aromatic and aliphatic functional groups in the soot of methane flames increase, while the relative contents of the same functional groups in the soot of ethylene flames mostly show different trends, indicating that H2 has a chemical promotion and inhibition on soot in methane and ethylene flames, respectively. The relative content of oxygen-containing functional groups in the soot of both methane and ethylene flames with hydrogen addition increases. By adding H2, the coagulation and condensation of soot low in the methane flames are greatly enhanced, indicating that H2 promotes the formation and growth of PAH molecules. This is an important reason for the chemical promotion effect of H2 in methane flames. For the soot of C2H4 hydrogen-added flames, oxidation is relatively thorough such that a large number of soot branches are oxidized. This is an important reason for the chemical inhibition of H2.

Molecular Composition and the Optical Properties of Brown Carbon Generated by the Ethane Flame

Atmospheric “Brown Carbon” (BrC) is a complex mixture of organic compounds with diverse composition and variability of its light-absorbing properties. BrC formed by incomplete combustion of fossil ...

On the effect of pressure on soot nanostructure: A Raman spectroscopy investigation

Although the majority of existing combustion devices operate at high pressure conditions, most of our understanding of the soot formation process and soot physicochemical properties rely on studies performed at atmospheric pressure. Pressure is known to have nonlinear effects on combustion processes and a significant influence on soot formation; soot loading increases with increasing combustion pressure. Soot characteristics directly affect soot oxidation and optical/radiative properties, and it is desirable to have a better insight into them under high-pressure conditions. Due to scarcity of information on soot primary particles, aggregate morphology, and soot nanostructure relevant to high-pressure combustion, there are challenges in predicting soot oxidation and radiation, particularly at engine-relevant conditions.In this study, we perform Raman spectroscopy measurements on soot sampled from a set of laminar diffusion flames of ethylene at various pressures, ranging from atmospheric pressure to 12 bar. Our results show an increase in soot maturity as the pressure increases within the range of investigated pressures. In the examined co-flow flames, pressure seems to have an indirect influence on soot nanostructure through an earlier inception of soot, resulting in longer residence times of the carbon soot particles in the hot and reactive flame environment. It is found that soot maturity, tracked through the size of graphitic domains, La, increases linearly with residence times. The longer residence time of soot in high-pressure flames could be the main cause of the higher degree of graphitization observed, which suggests a greater resistance to oxidation with increasing pressure.

Photodegradation of 1,3,5-Tris-(2,3-dibromopropyl)-1,3,5-triazine-2,4,6-trione and decabromodiphenyl ethane flame retardants: Kinetics, Main products, and environmental implications

Photodegradation has been demonstrated as one of the important environmental factors affecting the fate of contaminants such as brominated flame retardants (BFRs). However, a number of emerging BFRs, particularly those with high bromine substitution, have rarely been investigated for their photodegradation kinetics. Our study evaluated photodegradation of two highly brominated FRs, 1,3,5-tris-(2,3-dibromopropyl)-1,3,5-triazine-2,4,6-trione (TDBP-TAZTO) and decabromodiphenyl ethane (DBDPE), under various conditions. The results indicated that the degradation kinetics was affected by UV irradiation wavelength, intensity, solvent type, as well as the structural characteristics. TDBP-TAZTO exhibited degradation half-lives (t1/2) of 23.5–6931 min under various UV irradiation conditions and 91.2 days under natural sunlight. Its degradation was much slower than that of DBDPE which exhibited t1/2 of 0.8–101.9 min under UV and 41.3 min under natural sunlight. A variety of degradation products were detected as a result of different breakdown pathways. This indicated that photodegradation could substantially influence the fate of these highly brominated FRs, resulting in a cocktail of degradation products as environmentally occurring contaminants. This could also complicate the evaluation of the ecological risks of these target flame retardants, given that degradation products generally possess physicochemical properties and biological effects different from their parent chemicals.

Impact of water-vapor addition to oxidizer on the thermal radiation characteristics of non-premixed laminar coflow ethylene flames under oxygen-deficient conditions

The effect of water-vapor addition to oxidizer on flame radiation is assessed experimentally and numerically through the study of laminar coflow ethylene-fueled non-premixed flames. Oxygen-deficient conditions were studied to closely represent real confined fire situations. Experimental soot volume fraction distributions, presented in a previous study, are complemented with temperature and radiation measurements. Experimental data are compared and complemented with numerical simulations. The relative importance of the thermal, chemical and dilution effects of water-vapor is also investigated through numerical modeling. Addition of water-vapor to oxidizer leads to a decrease in soot concentration, flame temperature, emitted radiation and radiant fraction, regardless of the Oxygen Index (OI). The measured temperatures in the sooting region by two-color pyrometry are in reasonably good overall agreement with the numerical predictions. Soot is the main radiation emitter in the ethylene flame under all the conditions studied, but its relative importance decreases with the water-vapor addition. CO2 is the second most important contributor. Depending on the OI, the relative importance of the water-vapor addition to flame radiation varies. At lower OI the dominant effect is thermal, while at higher OI (normal air condition) the effect of the water-vapor is shared by the three effects: dilution, chemical, and thermal.

Numerical Investigation of Local Heat-Release Rates and Thermo-Chemical States in Side-Wall Quenching of Laminar Methane and Dimethyl Ether Flames

The local heat-release rate and the thermo-chemical state of laminar methane and dimethyl ether flames in a side-wall quenching configuration are analyzed. Both, detailed chemistry simulations and reduced chemistry manifolds, namely Flamelet-Generated Manifolds (FGM), Quenching Flamelet-generated Manifolds (QFM) and Reaction-Diffusion Manifolds (REDIM), are compared to experimental data of local heat-release rate imaging of the lab-scale side-wall quenching burner at Technical University of Darmstadt. To enable a direct comparison between the measurements and the numerical simulations, the measurement signals are computed in all numerical approaches. Considering experimental uncertainties, the detailed chemistry simulations show a reasonable agreement with the experimental heat-release rate. The comparison of the FGM, QFM and REDIM with the detailed simulations shows the high prediction quality of the chemistry manifolds. For the first time, the thermo-chemical state during quenching of a dimethyl ether-air flame is examined numerically. Therefore, the carbon monoxide and temperature predictions are analyzed in the vicinity of the wall. The obtained results are consistent with previous studies for methane-air flames and extend these findings to more complex oxygenated fuels. Furthermore, this work presents the first comparison of the QFM and the REDIM in a side-wall quenching burner.

Soot Formation Model Performance in Turbulent Non-Premix Ethylene Flame: A Comparison Study

This paper presents results obtained from the application of a computational fluid dynamics (CFD) approach to modelling of non-premixed turbulent ethylene sooting flame. The study focuses on comparing the two soot models available in the Fluent in predicting the soot level in the turbulent non-premixed ethylene flame. A standard k-ε model and Eddy Dissipation model are utilized for the representation of flow field and combustion of the flame being investigated. For performance comparison study, a single step soot model of Khan and Greeves and two-step soot model proposed by Tesner are tested. The results of calculations are compared with experimental data for a turbulent sooting flame taken from literature. The results of the study show that a combination of the standard k-ε turbulence model and eddy dissipation model is capable of producing reasonable predictions of temperature both in axial and radial profiles; although further downstream of the flame over-predicted temperatures are evidence. With regard to soot model performance study, it shows that the two-step model clearly performed far better than the single-step model in predicting the soot level in ethylene flame at both axial and radial profiles.

Study on ether spray combustion and its products under sub- and super-critical environments

In order to explore the combustion process of fuel and its products under sub- and super-critical environments, the constant volume combustion bomb system was used. The high-pressure common rail system and single injection instrument were used to supply fuel. Images of spray and combustion processes were recorded by high speed video camera using shadow photographic method, and combustion products were measured with a gas analyzer. The experimental results show that the ether flame presents a bright spherical shape. At the critical pressure point, the spray combustion penetration is the shortest, and the overall flame area is the smallest. The combustion duration of ether increases with increasing ambient pressure, reaches the maximum at the critical pressure, and then decreases with further increasing pressure. The amount of CO emitted from the combustion of the ether continuously decreases with the increase of ambient pressure. The amount of HC changes abruptly at the critical pressure. The differences between combustion characteristics and products formation are mainly due to the differences in fuel physical parameters and its break-up and evaporation mechanisms under sub- and super-critical conditions.

Effects of brominated and organophosphate ester flame retardants on male reproduction

Environmental chemicals that interfere with the production and/or action of hormones may have adverse effects on male reproduction. This review focuses on the possible impact of exposure to flame retardant chemicals on male reproduction. Flame retardants are added to a wide variety of combustible materials to prevent fires from starting, slow their spread, and provide time to escape. However, these chemicals are often additive so they leach out into the environment. Governments have restricted the use of polybrominated diphenyl ether flame retardants based on evidence that they are persistent and bioaccumulate and have adverse effects on health. The phasing out of these "legacy" flame retardants has resulted in their replacement with alternatives, such as tetrabromobisphenol A and the organophosphate esters. To review the literature on the effects of brominated and organophosphate ester flame retardant chemicals on male reproduction. PubMed database was searched for studies reporting the effects of brominated and organophosphate ester flame retardants on male reproduction. Cell-based, animal model, and human studies provide evidence that the polybrominated diphenyl ethers act as endocrine-disrupting chemicals; further, exposure during critical windows of development may be associated with a permanent impact on male reproduction. In vitro and animal model data are accumulating with respect to the effects of tetrabromobisphenol A and organophosphate esters, but few studies have evaluated their impact on human health. More research on human exposure to replacement flame retardants and the possibility that they may be associated with adverse reproductive health outcomes is a high priority.

Temporal trends and determinants of PFR exposure in the Hokkaido Study.

The phase-out of polybrominated diphenyl ethers (PBDE) flame retardants has led to the rapid increase of alternatives such as phosphate flame retardants and plasticizers (PFRs) in many consumer products. Exposure to these additive chemicals is widespread and potentially harmful to humans and the environment. In the present study, we assessed the exposure to PFRs through the analysis of metabolites in urine collected from 7-year old children from Hokkaido, Japan between 2012 and 2017. This allowed us to investigate temporal and seasonal trends for PFR metabolite concentrations and to study determinants of exposure. Thirteen metabolites of seven PFRs were measured in morning spot urine samples (n=400). Multiple regression models were used to quantify the yearly increase in metabolite concentrations per sampling year. Information on the demographics, indoor environment and dietary habits of the participants were derived from self-administered questionnaires. PFR metabolite concentrations were comparable to our previous study of school children (7-12 years old). Eight PFR metabolites were detected in >50% of the samples. During the study time period, concentrations of three metabolites increased significantly: bis(1,3-dichloro-2-propyl) phosphate (BDCIPP; 13.3% per year), 1-hydroxy-2-propyl bis(1-chloro-2-propyl) phosphate (BCIPHIPP; 12.9% per year), and 2-ethylhexyl phenyl phosphate (EHPHP; 6.7% per year). We also found seasonality as a determinant for several PFR metabolites, with 2-fold higher levels in summer for BCIPHIPP and BDCIPP. Concentrations were also significantly impacted by ventilation habits. More frequent window opening or use of mechanical ventilation was consistently associated with higher levels of PFR metabolites in children's urine. This is the first study to show that human exposure to PFRs has increased in recent years in Japan, which indicates that further research into this class of chemicals is warranted.

Role of dimethyl ether in incipient soot formation in premixed ethylene flames

Emissions of soot particles exert crucial impact on human health and the environment. Previous researches show that a partial replacement of fossil fuels with oxygenated fuels is supposed to reduce soot emission. However, the crucial information of soot size distributions is unclear, although small particles are more dangerous than large particles. In this paper, dimethyl ether (DME) is selected among the oxygenated fuels for the investigation of the doping effect on nascent soot formation. DME was doped in the burner-stabilized rich premixed ethylene/oxygen/argon flames at various mixing ratios. Flame temperature profiles were measured using R-type thermocouples with radiation correction. A scanning mobility particle sizer (SMPS) with a sampling system was used to determine particle size distributions (PSDs) of soot-containing combustion products with immediate dilution. Thermo-gravimetric analysis (TGA) complemented with elemental analysis (EA) was conducted to detect the chemical properties of formed soot particles. Additionally, species profiles of the experimental flame conditions were provided from simulations. The experimental PSDs showed that DME addition could slow down soot evolution process, while lead to a slight increase in incipient soot with size below 4 nm. When DME added, the production of benzene was suppressed due to the reduced concentrations of acetylene and propargyl, and thus soot nucleation. Meanwhile, soot oxidiation was enhanced because the OH radical and the oxidizability of soot particles are both increased. Considering the slight increase of sub-4 nm soot, the effect of oxygenated-PAHs (OPAH) was emphasized. The production of OPAH suppressed soot surface growth, which leading to the relatively lower consumption of sub-4 nm soot in DME added flames.As a result, DME cannot be simply regarded as a clean fuel due to the enhanced formation of sub-4 nm soot particles. When applying oxygenated fuels into practice, it is necessary to pay more attention to the size distributions of the emitted particles, and conduct appropriate post-processing techniques to reduce the emissions of small particles.

Effects of carbon monoxide addition on the sooting characteristics of ethylene and propane counterflow diffusion flames

In this work, the effects of the addition of carbon monoxide (CO) on soot formation were investigated for ethylene and propane counterflow diffusion flames (CDFs). The work was partly motivated by experimental observations that the addition of CO monotonically reduced the sooting tendencies of ethylene diffusion flames; while it had an interesting non-monotonic influence on soot formation in propane flames. Quantitative laser-based soot measurements were performed in combination with numerical simulations using detailed soot models to provide kinetic insights into the role of CO in the soot evolution processes. To isolate the chemical effect of CO from dilution effects, additional studies on the effects of N2 addition were performed and the results were compared with those with CO addition. For ethylene flames, the experimental data and numerical results agreed to show that the addition of CO monotonically reduced soot formation, primarily due to its dilution effect. For propane flames, interestingly, the peak soot volume fraction was found to be slightly increased with 10% CO addition; higher level of CO addition finally led to a reduction of soot formation, but the quantitative extent of the reduction was much smaller as compared to that in the ethylene flame. In flames of both fuels, CO addition was seen to lead to a higher soot volume fraction than the same amount of N2 addition, indicating that CO chemically promoted soot formation. Further analysis showed that the chemical promoting effects of CO in propane flames was stronger than that of ethylene flames, which was due primarily to the different role CO played in the precursor formation /soot inception processes.

Investigations of autoignition and propagation of supersonic ethylene flames stabilized by a cavity

Two analysis methods for time scale and energy balance relevant to flame ignition and stabilization in cavity-stabilized flames are developed. The interaction time of hot product in the recirculation zone of the cavity with the surrounding unburned mixture and the reaction induction time of the mixture are estimated in the time scale method. The energy release from chemical reactions and the energy loss due to species exchange in the recirculation zone are included in the energy balance method. The autoignition and propagation of supersonic ethylene flames in a model supersonic combustor with a cavity is investigated first using highly resolved large eddy simulation. The evolutions of the two time scales are then calculated in the ignition process of the supersonic ethylene flames. It is found that the time scale theory is well valid in the flame propagation and stabilization stages. The rates of energy generation and loss are then analyzed in the cavity. It is found that initially the local energy generation rate is relatively small, resulting in slow net energy accumulation in the cavity. Then the energy generation increases due to the intermittent flame propagation in the cavity, whereas the energy loss oscillates consistently since the burned gas leaves the cavity. Also, energy generation and loss are generally balanced in the cavity and all tend to zero after the flame is globally stabilized. The two methods present the characteristic time scales and energy balancing during the transient ignition process for the first time.

On the ambiguity of premixed flame thickness definition of highly pre-heated mixtures and its implication on turbulent combustion regimes

Freely propagating, one-dimensional, laminar, premixed flames of ethylene–air are analysed at standard temperature and pressure (STP) and at elevated temperature and pressure conditions to reveal the differences in flame structure. For high-temperature and low-pressure conditions, there is a significant contribution to the total heat release in the post-flame region leading to an ambiguity in the definition of the characteristic flame structure thickness. The potential implications on the identification of the appropriate turbulent flame regime in Borghi or Re-Da regime diagrams are highlighted. The contributions of individual reactions to the total heat release across the entire flame structure are analysed to understand the impact of turbulence–chemistry interactions.

Nanosecond laser irradiation of soot particles: Insights on structure and optical properties

In spite of the advances in laser diagnostics in combustion, the effect of rapid laser irradiation on the physical/chemical properties of soot particles is far from being comprehensively understood. Optical properties, particle nanostructure and aggregate size of laser-irradiated soot particles are analyzed in this paper. Carbonaceous particles sampled from nitrogen-quenched diffusion flames of ethylene and methane are irradiated on-line by a 1064-nm short laser pulse. Wavelength-resolved extinction measurements in the visible range are used to follow their transformation by varying the laser energy density. A variation of the extinction coefficient of the irradiated particles compared to the extinction coefficient of the pristine ones is observed, especially for ethylene soot. The particle nanostructures are analyzed by Raman spectroscopy and the effect of laser irradiation on aggregate structure is evaluated by measuring particle size distributions. The results indicate that both soot nanostructure and optical properties are strongly dependent on the laser energy density when irradiated by a laser source. This is significant for ethylene soot, while for methane soot the degree of variation of such properties is less pronounced, at least in the investigated heating conditions.

The role of CO2 dilution on soot formation and combustion characteristics in counter-flow diffusion flames of ethylene

The effect on soot formation of carbon dioxide addition as diluent in ethylene/O2/Ar laminar counter flow diffusion flames has been examined both experimentally and with the help of a multi-sectional kinetic model. Different concentrations of CO2 have been used in the oxidizer stream to replace argon, whereas the amount of oxygen has been kept constant. Optical techniques have been adopted to measure particulate matter: laser-induced fluorescence (LIF) to detect carbon nanoparticles and laser-induced incandescence (LII) to detect soot. Significant reductions of both soot and carbon nanoparticles have been observed with the addition of CO2: by increasing the amount of CO2, the reduction of particulate matter increases. In the case of 100% CO2 as diluent, soot particles are completely depleted, whereas carbon nanoparticles are still formed but at a lower rate. Therefore, the net result of CO2 addition is the reduced emission of total particulate matter enriched in carbon nanoparticles. On the basis of the results of the kinetic model, particle reduction has been mostly attributed to a thermal effect due to the reduced adiabatic flame temperatures at increasing amounts of CO2 added and only partially to the chemical effect of CO2 in enhancing OH radical formation.

Experimental investigation of flame propagation in a meso-combustor

The present work aims to study the quenching of propagating flames in meso-combustors for which dimensions are of the order of quenching distances of hydrocarbon fuels. Combustion of gaseous fuels and subsequent flame propagation in a meso-scale combustor duct of square cross-section is studied experimentally. Premixed mixtures of methane, propane, and ethylene with air are considered. Two different variants of flame propagation states are found to occur in the meso-combustor, viz., one undergoing flame propagation till the combustor entry and quenching at the step and the other undergoing wall quenching. Regime transitions across these flame states are mapped comprehensively over a wide range of operating conditions. The radius of curvature of the flame and the dead space between the flame and the wall are determined for those conditions with the aid of curve fitting and image processing techniques using Matlab software. The spatial and temporal variation of both these parameters show a drastic increase during quenching in the wall-quenched case, while it remains nearly constant in the step quenched case. With increasing duct Reynolds number, the flame propagates slower, and the heat conduction to the wall leads to a decrease in the dead space and flattening of the flame, particularly at equivalence ratios corresponding to lower flame speeds. This flame-wall interaction is found to be low for methane, resulting in more heat loss and thereby wall-quenched flames compared to propane and ethylene. None of the ethylene flames were found to suffer wall quenching thereby making it a suitable fuel for meso-/micro-combustors among the three fuels used in the present work.

Insights on the effect of ethanol on the formation of aromatics

For decades, ethanol has played an important role as a biofuel, as its addition to hydrocarbon fuels has been associated with a reduction in soot formation. The chemical mechanisms behind this phenomenon, however, are still not clear. In this paper, we contribute to this research area by elucidating the mechanisms that participate in the formation of polycyclic aromatic compounds (PACs) in an ethanol-doped ethylene flame, using a combination of deterministic and stochastic computational techniques. We specifically focus on the formation of oxygenated PACs and the chemical interactions of pure hydrocarbons and oxygen in six ethylene/air premixed flames with similar temperature profiles but different equivalence ratios and ethanol doping percentages. Our simulations confirm that an increase in the ethanol content results in a reduction of the formation of acetylene, small aromatics, and large PAHs. At the same time, the number of oxygenated PACs reach a maximum at a height above burner around 2–3 mm where they constitute 45% of all PACs: most of the oxygenated structures are phenols, mixed with approximately 15% of furans, and a small amount of ethers. Overall, the results indicate that the formation pathways of oxygenated PACs can compete with pure hydrocarbon growth mechanisms in the rapid growth region up to a height above burner of 2–3 mm. The rate of PACs’ growth then gradually slows down, with pure hydrocarbon growth mechanisms being the main contributors in this region of the flame.