Ethylene Flames Research Articles

Experimental and numerical study of a hybrid solar-combustor system for energy efficiency increasing

Coupling of traditional combustion technologies with solar thermal energy is fundamental to enlarge the field of applicability and to lower the costs of renewable energy sources. A possible solution is represented by direct irradiation of a flame with concentrated solar energy, so as to increase its temperature thanks to the high absorption coefficient of combustion by-products such as soot particles. In this study, we use a detailed model of soot formation and oxidation to simulate the behaviour of a coflowing diffusion ethylene flame with and without the exposure to a concentrated solar radiation. Modelling data are compared with experimental data reported in the literature. Results confirm that our model is able to reproduce the effect of radiation both matching the temperature and soot volume fraction increase. Successively, a modified version of the solar-combustion hybrid system has been setup to test the effect of solar light orientation on the combustion process. A simulator of concentrated solar energy has been used to directly irradiate the flame from the top. Thermocouple-based temperature measurements highlighted that the flame effectively absorbed solar radiation.

Optical band gap analysis of soot and organic carbon in premixed ethylene flames: Comparison of in-situ and ex-situ absorption measurements

The similarity of in-situ and ex-situ absorption/extinction properties was found by comparing data sets obtained in premixed flat ethylene flames (C/O = 0.77); both ex-situ absorption measurements at CNR of Naples and in-situ laser extinction/absorption measurements at Lund University. The optical band gap analysis was performed as a method to separate/evaluate the contributions to the UV–Visible absorption from organic carbon and soot by selecting two spectral regions; above 685 nm where only soot is assumed to absorb radiation and below 685 nm, where both soot and organic carbons absorb. Optical band gap for soot was analyzed separately from organic carbon, obtaining their individual contribution to the spectral absorption from inception, throughout the growth region up to soot aggregation. While the optical band gap of soot strongly decreased along the flame from 1 to 0.1 eV, the band gap for organic carbons remained constant at ∼1.7–1.8 eV. By using the relationship of the optical band gap with nanostructural parameters (layer length and aromatic rings number), the average molecular weight of organic carbon around 400 u resulted rather far away from the molecular weight of 1000–2000 mass units evaluated for incipient soot consistently with the view of PAH species sticking together for particle inception.

Study on soot evolution under different hydrogen addition conditions at high temperature by ReaxFF molecular dynamics

The physical and chemical properties evolution of soot particles was studied by ReaxFF MD simulation. The dimerization of PAHs, nucleation and coagulation, and graphitization from hydrocarbons to the final soot nanoparticle were investigated, and the chemical effects of the hydrogen addition on the formation mechanism of primary soot nanoparticle was explored by considering important polycyclic aromatic hydrocarbons (PAHs) in ethylene flames under high temperatures. The results obtained reveal that, in the first stage, the activated aliphatic hydrocarbons mainly grow into a large ring through the chain growth reactions, and then gradually form PAH-like molecules with inner ring bridging instead of directly generating the five or six-carbon ring structures. Hydrogenation effectively inhibited the soot nanoparticle nucleation at all stages. Due to a large number of vinyl and acryl groups attacked by the hydrogen to decarbonize and produce methane, a rapid decrease of the formation and growth rate of the PAHs was observed. With the increase of hydrogen, the nucleation and surface growth rate of soot particles are slowed down due to lose activity.

Chemical tracers guide identification of the location and source of persistent organic pollutants in juvenile Chinook salmon (Oncorhynchus tshawytscha), migrating seaward through an estuary with multiple contaminant inputs

Understanding the spatial extent, magnitude, and source of contaminant exposure in biota is necessary to formulate appropriate conservation measures to reduce or remediate contaminant exposure. However, obtaining such information for migratory animals is challenging. Juvenile Chinook salmon (Oncorhynchus tshawytscha), a threatened species throughout the US Pacific Northwest, are exposed to persistent organic pollutants (POPs), including polybrominated diphenyl ether (PBDE) flame retardants and polychlorinated biphenyls (PCBs), in many developed rivers and estuaries. This study used three types of complementary chemical tracer data (contaminant concentrations, POP fingerprints, and stable isotopes), to determine the location and source of contaminant exposure for natural- and hatchery-origin Chinook salmon migrating seaward through a developed watershed with multiple contaminant sources. Concentration data revealed that salmon were exposed to and accumulated predominantly PBDEs and PCBs in the lower mainstem region of the river, with higher PBDEs in natural- than hatchery-origin fish but similar PCBs in both groups, associated with differences in contaminant inputs and/or habitat use. The POP fingerprints of the natural-origin-fish captured from this region were also distinct from other region and origin sample groups, with much higher proportions of PBDEs in the total POP concentration, indicating a different contaminant source or habitat use than the hatchery-origin fish. Stable isotopes, independent tracers of food sources and habitat use, revealed that natural-origin fish from this region also had depleted δ15N signatures compared to other sample groups, associated with exposure to nutrient-rich wastewater. The PBDE-enhanced POP fingerprints in these salmon were correlated with the degree of depletion in nitrogen stable isotopes of the fish, suggesting a common wastewater source for both the PBDEs and the nitrogen. Identification of the location and source of contaminant exposure allows environmental managers to establish conservation measures to control contaminant inputs, necessary steps to improve the health of Chinook salmon and enhance their marine survival.

On the coagulation efficiency of carbonaceous nanoparticles

In this paper we derived the interaction energy between two spherical nanoparticles from the pair-wise Lennard-Jones attractive and repulsive potentials of the constituent atoms of the two particles, and proposed a coagulation efficiency model based on the average particle kinetic energy and the potential well depth (i.e. the minimum interaction energy) between two colliding particles. To test the performance of this new coagulation efficiency model, we applied it in detailed population balance modelling of soot particle size distributions (PSDs), and found better agreement with the measured PSDs in a benchmark premixed ethylene flame than that using the unit coagulation efficiency, especially in the range of small particles with mobility diameter less than 5 nm. Moreover, the agreement between the computed and the measured primary particle size distribution (PPSD) was also improved with the new coagulation efficiency model.

Confined spherically expanding flame method for measuring laminar flame speeds: Revisiting the assumptions and application to C1[sbnd]C4 hydrocarbon flames

The spherically expanding flame under constant volume method was introduced in 1934 by Lewis and von Elbe as a means to study laminar flame propagation at engine-relevant conditions. Despite its potential, this method has not been utilized extensively due to concerns regarding the underlying assumptions and data uncertainty. In the present study, the intricacies of the experimental approach as well as the models and assumptions involved during data interpretation were reassessed with the aid of direct numerical simulations. Results confirmed that stretch effects are negligible during the compression stage of the experiment for a wide range of Lewis numbers. Additionally, it was shown that the laminar flame speed is sensitive to the flame radius stressing thus the requirement that the modeling of flame radius needs to be done with the highest possible accuracy by accounting properly for product dissociation and thermal radiation from the burned gases. It was also shown, that the equilibrium assumption is valid for modeling the flame radius as a function of pressure and, as expected, the kinetic and transport effects are negligible. Subsequently, laminar flame speeds were measured for methane, ethane, ethylene, propane, propylene, n-butane, 1-butene, and isobutene flames for 8–30 atm pressures and 400–520 K unburned mixture temperatures. A hybrid thermodynamic/radiation model was utilized to interpret the experimental observables and derive the laminar flame speed by accounting for spectrally dependent emission from and absorption by the burned gases as well as product dissociation during compression. The data were found to be consistent with measurements obtained in spherically expanding flame experiments under constant pressure conditions and predictions using a number of current kinetic models.

Probing soot structure and electronic properties by optical spectroscopy

In this work, the physicochemical transformation of soot particles during the initial stages of formation and growth was investigated in a laminar premixed flame of ethylene and air. The selected flame condition allowed producing two classes of carbon nanoparticles, namely incipient and primary soot particles, on the basis of their size distribution. The two classes of particles have been selectively collected in well distinct flame zones for further chemico-physical characterization. The optical band gap decreases as particle size grows up. Particle medium range order and structure at molecular level have been investigated by Raman and fluorescence spectroscopy. Slight changes in particle composition were observed for incipient and primary soot. Both kinds of particles are made of a similar ensemble of fluorescence centers, which produce excitation dependent emission irrespective of particle sizes. In agreement with the Raman spectra, the molecular constituents of the particles are polyaromatic compounds with an average size of the order of ovalene molecules that only slightly increase from incipient to primary soot particles. These results suggest that the observed decrease of the optical band gap, as particle size increases, typical of a quantum-dot behavior, is not due to changes in particle composition at molecular level. Further investigations are needed to investigate possible evolution in supramolecular organization of the particles as they grow in flame and its effect on the optical band gap.

A reduced and robust reaction mechanism for toluene and decalin oxidation with polycyclic aromatic hydrocarbon predictions

In this work, a reduced toluene-decalin reaction mechanism containing 108 species and 566 reactions was proposed for computational fluid dynamics (CFD) simulations and polycyclic aromatic hydrocarbon (PAH) formation predictions in combustion engines. The present mechanism was validated with the available literature data including ignition delay time (IDT) determined in shock tubes and rapid compression machines (RCM), species mole fractions measured in premixed flames and jet stirred reactors (JSR), and laminar flame speeds. Moreover, a sensitivity analysis was performed on toluene and decalin flames to further investigate the main formation pathways of four representative PAHs: benzene (A1), naphthalene (A2), phenanthrene (A3) and pyrene (A4). In general, the simulation results exhibited a reasonable agreement with the experimental data. The negative temperature coefficient (NTC) behaviors of decalin IDTs were accurately reproduced by the proposed mechanism. The PAH species concentrations were also well captured for ethylene and benzene flames. The sensitivity analysis results indicated that the main formation reactions for PAHs have a strong link with ring structures. The decompositions of toluene and decalin primarily contributed to the formation of A1. For toluene, A2 was formed by the reactions A1- + C4H3 = A2 and IC4H5 + A1 = A2 + H2 + H, while for decalin, the self-combination reaction of C5H5 became the main pathway. In addition, the reactions of the aromatic molecules and radicals significantly promoted the formation of A3 and A4 for both toluene and decalin.

Experimental and numerical study of soot formation in counterflow diffusion flames of gasoline surrogate components

Soot formation is experimentally and numerically investigated in laminar counterflow diffusion flames burning ethylene and three typical gasoline surrogate components; n-heptane, iso-octane, and toluene. Laser-induced incandescence and a light extinction technique are employed to determine the soot volume fraction within the well-controlled region of the burner. The experiments are performed across a wide range of strain rates and stoichiometric mixture fractions. From the experimental data, sensitivities of soot formation on strain rate and stoichiometric mixture fraction are derived for each fuel. The fuels show significantly different sensitivities. For iso-octane and n-heptane, a higher sensitivity of soot production on the strain rate is observed as compared to ethylene and toluene. Moreover, the sensitivities of soot formation on the strain rate increase with increasing stoichiometric mixture fraction. One-dimensional simulations of the flames investigated experimentally were performed using two different detailed chemical kinetic mechanisms, detailed chemical soot models, and the hybrid method of moments as well as a discrete sectional method to describe soot dynamics. The models are capable of predicting the soot volume fraction of the ethylene flames with remarkable accuracy, whereas for the gasoline surrogate components, the overall soot volume fractions are overpredicted for all tested models. In iso-octane flames, soot nucleation and PAH condensation rates are particularly enhanced. A reaction pathway analysis shows that in ethylene flames, the formation of benzene mostly originates from acetylene, while for iso-octane, large amounts of iso-butenyl form propyne, propargyl, and then benzene.

Exergy losses in premixed flames of dimethyl ether and hydrogen blends

A second-law thermodynamic analysis was conducted for stoichiometric premixed dimethyl ether (DME)/hydrogen (H2)/air flames at atmospheric pressure. The exergy losses from the irreversibility sources, i.e., chemical reaction, heat conduction and species diffusion, and those from partial combustion products were analyzed in the flames with changed fuel blends. It is observed that, regardless of the fuel blends, chemical reaction contributes most to the exergy losses, followed by incomplete combustion, and heat conduction, while mass diffusion has the least contribution to exergy loss. The results also indicate that increased H2 substitution decreases the exergy losses from reactions, conduction, and diffusion, primarily because of the flame thickness reduction at elevated H2 substitution. The decreases in exergy losses by chemical reactions and heat conduction are higher, but the exergy loss reduction by diffusion is slight. However, the exergy losses from incomplete combustion increase with H2 substitution, because the fractions of the unburned fuels and combustion intermediates, e.g., H2 and OH radical, increase. The overall exergy losses in the DME/H2 flames decrease by about 5% with increased H2 substitution from 0% to 100%.

Estimating the internal and surface oxidation of soot agglomerates

Oxidation of soot takes place inside and on the surface of its constituent primary particles at a rate that depends on temperature, T, and O2 concentration. Even though accurate oxidation kinetics are essential in both industrial uses and environmental impact of soot, they are often derived neglecting internal particle oxidation and the structure of such soot agglomerates. Here, the detailed evolution of the fractal-like agglomerate soot mass, m, and mobility diameter, dm, during both internal and surface oxidation is determined by a moving sectional model. The model predictions are in excellent agreement with oxidation data of mature ethylene soot dm for T = 900–1200 K. The oxidation mode index, a, given by the ratio of the characteristic O2 reaction and diffusion times is used to quantify the contributions of internal and surface oxidation of soot. At low Τ (e.g., < 1100 K), O2 diffuses into the primary particles and reacts with bulk soot, hardly altering the dm and yielding a > 3. As T increases, surface oxidation becomes dominant, decreasing both dm and a. The common assumption that soot agglomerates are spheres underestimates their dm up to 50 % during oxidation. Coupling this detailed moving sectional model with soot mobility size distributions can yield realistic soot oxidation rates. Accounting for soot morphology and internal oxidation shows that the classic NSC rate increasingly underestimates (by 3–7 times) the oxidation rate of soot (from ethylene and toluene flames) with decreasing temperature (900–1800 K) and/or oxygen concentration (0.2–21 vol %).

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.

Liftoff Characteristics of Ethylene and Ethane Jet Flames in Coflow

AbstractThis work aimed to figure out the liftoff characteristics of ethylene and ethane, which are both promising fuels in the new energy era for flames in coflow. A free jet condition, air coflow...

Investigation on the chemical effects of dimethyl ether and ethanol additions on PAH formation in laminar premixed ethylene flames

This paper presents a comparative study focusing on the chemical effects of dimethyl ether (DME) and ethanol (EtOH) on PAH formation in fuel-rich laminar premixed ethylene flames using laser-induced fluorescence (LIF) diagnostic. To investigate the chemical effects, the equivalence ratios, dilution ratios, and temperature profiles were kept nearly constant while DME and EtOH were separately blended into the ethylene-base flame. The LIF testing data showed that the PAH formation was observably suppressed by the DME and EtOH additions, and DME was much more effective at suppressing the PAH formation than EtOH. To reveal the mechanism of the inhibiting effects of oxygenated fuels on PAH formation, chemical kinetic analysis was conducted. The model results captured the variation tendency of PAHs observed in experiments and indicated that the decreased concentration of carbon which was available to form the first aromatic ring (benzene) is the main reason for PAH reduction with EtOH and DME additions. Moreover, during the growth process from benzene to PAHs, the C2H2-addition reactions, which played a key role in the formation of new aromatic rings, were suppressed due to less C2H2 formed in EtOH and DME addition flames. There was less C2H2 produced from DME than that from EtOH, therefore, DME had a stronger inhibiting effect than EtOH on PAH formation in laminar premixed ethylene flames.

A post processing technique to predict primary particle size of sooting flames based on a chemical discrete sectional model: Application to diluted coflow flames

A numerical post-processing strategy to reconstruct the size of soot primary particles is presented in this work in the context of the chemical sectional models. The proposed technique is based on solving the transport equation of the primary particle number density for each considered aggregate size. The chemical source terms are derived from the corresponding chemical reactions.The validity of the proposed approach is tested on target flames of the International Sooting Flame (ISF) Workshop. In particular, first, a laminar premixed ethylene flame is considered. Profiles of soot volume fraction and mean primary particle size are compared between simulation and measurements and a satisfactory agreement is observed, validating the proposed post-processing strategy. Second, a laminar coflow ethylene flame is put under the scope. Numerical results are compared to experimental data once again in terms of soot volume fraction and primary particle size. The sensitivity to model parameters, such as accounting for surface rounding and the choice of the smallest aggregating particle size, is explored.Once validated, the effect of dilution on the mean primary particle diameter in laminar diffusion flames is examined. The general trends observed experimentally are recovered. The correlation between temperature, precursor concentrations, soot volume fraction and primary particle diameter is explored. Finally, formation rates and residence time along the particle trajectories are investigated to explain the effect of dilution on the spatial localization of the biggest particles along the flame. The relation between the soot volume fraction and the mean primary particle diameter is examined.

Radiant fraction from sooting jet fires

The objective of this article is to investigate numerically the radiative structure of methane, ethylene and acetylene lab-scale jet flames ranging from the transitional to the momentum-driven regimes. The numerical model involves a hybrid flamelet/transported PDF method coupled to an acetylene-benzene soot production model and a Wide-Band Correleted-K gas radiation model. Model predictions in terms of mean and rms soot volume fraction and temperature, integrated soot volume fraction and radiant fraction are in reasonable agreement with the available experimental data. In particular, the model reproduces quantitatively the decrease in radiant fraction observed as the flow becomes momentum driven. This behavior results mainly from two mechanisms: (i) an increase in flame self-absorption due to an enhancement in flame volume and (ii) for the ethylene and acetylene flames a reduction in the soot emission per unit flame volume owing to a strong decrease in soot production. In addition, for a given fuel, gas emission per unit flame volume remains approximatively constant as the exit strain rate increases whereas the soot emission per unit flame volume and the characteristic soot volume fraction scale with the Kolmogorov time scale. It was also found that competitive mechanisms govern the effects of Turbulence-Radiation Interaction (TRI) on radiant fraction. Enhancement mechanisms are due to gas emission TRI and temperature self-correlation effects on soot emission whereas inhibiting mechanisms results from the negative correlation between soot volume fraction and temperature. Enhancement mechanisms dominate in weakly sooting methane flames and taking TRI into account increases the radiant fraction. On the other hand, inhibiting mechanisms become significant in moderately and highly sooting fuels, with their importance increasing with both the fuel sooting propensity and the Reynolds number. For flames dominated by soot radiation, the inhibiting mechanisms prevail and taking TRI into account reduces the radiant fraction.

An experimental apparatus to measure soot morphology at high pressures using multi-angle light scattering

In this work, a novel experimental setup is described which is designed and built specifically to study soot morphology using light scattering and extinction techniques at elevated pressures. The experimental setup consists of a counterflow burner housed inside a pressure vessel. A unique feature of this pressure vessel is the four curved optical windows which can provide the required optical access for light scattering measurements in order to infer the morphological parameters of soot. Using this setup, N2-diluted ethylene and air counterflow flames are stabilized from 3 to 5 atm. Global strain rate (a) of 30 s−1 is maintained at all conditions and all the flames studied are soot formation (SF) flames. Light scattering by soot is measured between 15° to 165° at different locations along the axis of the burner. Ratio of total scattering to absorption (ρsa), path averaged soot volume fraction (f v), mean primary particle size (dp), mean radius of gyration of aggregates (Rgm) and fractal dimension (Df) are calculated from multi-angle light scattering and extinction data using Rayleigh–Debye–Gans theory for fractal aggregates (RDG-FA). ρsa, f v, dp, and Rgm increase as the pressure is raised. The scattering contribution in these measurements vary from 1.3% to 16% of absorption which suggests that wide angle optical access is essential for accurate measurements of f v. Df equal to 1.27 is measured near the flame at 3 atm which increases as the particles are convected away from the flame and Df increases to 1.98 at 5 atm.

Low temperature combustion of methane/alkenes mixtures

The use of methane is commonly adopted as thermal diluent and chemical inhibitor in the partial oxidation of alkenes, in order to avoid the occurrence of unwanted reactions and full oxidation. Moreover, methane-alkene mixtures are commonly present in several industrial applications such as petrochemical processes.In this work, experimental data for pure ethylene and propylene flames in presence of air were collected, as first. Then the effect of alkenes addition on methane premixed flames was investigated. These fuel compositions were investigated by means of a heat flux burner at different gaseous compositions. The measurements were compared with experimental data retrieved from current literature as well as with the results of a detailed kinetic mechanism.The data collected in this work suggest the existence of two chemical regimes, resulting in a methane-dominated kinetic, in case of alkene addition lower than 20 %v/v, and alkene-dominated kinetic, in case of higher alkenes content. The production of soot precursors for methane combustion was found to be significantly increased by the addition of propylene. Finally, the maximum reactivity, expressed in terms of laminar burning velocity, undergoes a non-negligible shift toward a richer composition of methane in the presence of alkene.The agreement between the experimental data and numerical predictions allowed for further evaluation of the chemical interactions, product and heat profiles as a function of the initial composition. A criterion based on the flame structure was posed and successfully implemented to identify the temperature values giving unsafe or undesired conditions in the epoxidation processes.

Flame-formed carbon nanoparticles exhibit quantum dot behaviors

We examine the quantum confinement in the photoemission ionization energy in air and optical band gap of carbon nanoparticles (CNPs). Premixed, stretched-stabilized ethylene flames are used to generate the CNPs reproducibly over the range of 4-23 nm in volume median diameter. The results reveal that flame-formed CNPs behave like an indirect band gap material, and that the existence of the optical band gap is attributed to the highest occupied molecular orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) gap in the polycyclic aromatic hydrocarbons comprising the CNPs. Both the ionization energy and optical band gap are found to follow closely the quantum confinement effect. The optical band gaps, measured both in situ and ex situ on the CNPs prepared in several additional flames, are consistent with the theory and the baseline data of CNPs from stretched-stabilized ethylene flames, thus indicating the observed effect to be general and that the particle size is the single most important factor governing the variation of the band gap of the CNPs studied. Cyclic voltammetry measurements and density functional theory calculations provide additional support for the quantum dot behavior observed.

Effect of nickel acetylacetonate addition on soot inception and growth in an ethylene flame studied by using in situ small-angle X-ray scattering

Small-angle X-ray scattering (SAXS) technique has been used to study soot formation and growth in an ethylene flame doped with Ni(acac)2 (nickel acetylacetonate) at atmospheric pressure. An undoped flame under the same conditions was also studied to compare with the doped flame. The results show that the Ni(acac)2 addition affects particle size, shape and morphology significantly at certain flame heights. It has reducing effect on the primary soot particle size at the flame heights between 17 and 34 mm. It delays the growth of primary soot particles at soot growth region. The soot volume fraction in the doped flame is smaller than that in the undoped flame by factors of 1.5–4 at different heights above the burner. In the most heights of the soot surface growth region (HAB=13–30 mm), the Porod exponents for the doped flame are smaller than those in the undoped flame, suggesting that the addition of Ni(acac)2 in the ethylene flame induces a rougher surface and more irregular shape of the soot primary particles.