Oxygen Mole Fraction Research Articles

Experimental and kinetic study on the extinction characteristics of ammonia-dimethyl ether diffusion flame

Combustion with dimethyl ether is regarded as one of the effective means to improve the combustion stability of ammonia gas. In-depth understanding of the combustion characteristics of ammonia-dimethyl ether mixture, especially the extinction limits, is the basis for the development of related combustion technology. In this work, the extinction limits of diffusion flames of NH3, DME and NH3-DME mixture were obtained through experiments, and a kinetic model was developed to accurately describe the combustion characteristics of NH3-DME, especially for the extinction limits. The extinction characteristics of NH3-DME mixture were analyzed based on this model. The experimental results show that blending DME in the fuel or increasing the oxygen concentration in the oxidant can effectively expand the combustion limits of the NH3 diffusion flame. When the oxygen mole fraction in the oxidant is increased to 27%, the strain rate at extinction of the NH3 diffusion flame is 3.3 times of the original value. When NH3 is mixed with 10% volume fraction of DME, the increase is 3.9 times. Numerical analysis shows that the chemical and thermal effects of DME promote the combustion of NH3-DME mixture, and when the DME mole fraction in fuel exceeds 33%, the effect of chemical effect is bound to exceed the thermal effect. With the increase of the strain rate, the reaction zone of the flame becomes thinner, the escape rate of active radicals increase, and the heat release rate of per unit mass of fuel decreases, which leads to the continuous decrease of the flame temperature and eventually leads to flame extinction. The addition of DME increases the thickness of flame, increases the amount of radical generation and improves the heat release rate of the reaction process, thus promoting the combustion and improving the stability of NH3 diffusion flame.

Demonstrating steady burning for small flat materials in microgravity in a quiescent ambient

The likelihood of steady burning in microgravity is examined with Burning Rate Emulation (BRE) using a gas burner with a flat 25 mm diameter porous surface with two embedded heat flux sensors. The data are greatly expanded from a previous work. The fuel mixture used in this analysis are ethylene and ethylene diluted with nitrogen resulting in a range of heats of combustion from 12 to 47.2 kJ/g. An analytical solution of these flames was utilized to compute their steadiness, temperatures at flame sheet, and sizes. 103 flames were ignited in an ambient with nominal oxygen mole fractions of 0.21, 0.265, 0.34 and 0.40. 49 of those flames burned for the entire duration of the test before the fuel supply was terminated, while the remainder self-extinguished. The flames attained at least 90% of their asymptotic heights for all 49 fuel terminated flames. An estimated critical steady flame temperature ranging from 1100 to 1200 K was observed segregating the sustained and self-extinguished flames. Flame temperatures for the 49 terminated flames were above the critical temperature range indicating sustained steady burning, whereas below they self-extinguished. The measured flame sizes, and radiative fractions are reported at the endpoint of the terminated tests. An empirical correlation of the radiative fraction based on radiation theory with Ω, a dimensionless parameter, was used in prediction for real fuels at steady burning in microgravity. A flammability diagram, as a plot of the emulated fuel heat of gasification with its burning mass flux, for the range of steady data, is presented comparing how real fuels might burn steadily in normal and microgravity conditions. The results suggest that fuel mass burning rates would be about 3 to 4-times higher in 1-g. The heats of gasification and combustion of some real fuels are presented and compared against the burner flames.

Combustion behavior of single iron particles-part I: An experimental study in a drop-tube furnace under high heating rates and high temperatures

Micrometric spherical particles of iron in two narrow size ranges of (38–45) µm and (45–53) µm were injected in a bench scale, transparent drop-tube furnace (DTF), electrically heated to 1400 K. Upon experiencing high heating rates (104–105 K/s) the iron particles ignited and burned. Their combustion behavior was monitored pyrometrically and cinematographically at three different oxygen mole fractions (21%, 50% and 100%) in nitrogen. The results revealed that iron particles ignited readily and exhibited a bright stage of combustion followed by a dimmer stage. There was evidence of formation of envelope micro-flames around iron particles (nanometric particle mantles) during the bright stage of combustion. As the burning iron particles fell by gravity in the DTF, contrails of these fine particles formed in their wakes. Peak temperatures of the envelope flames were in the range of 2500 K in air, climbing to 2800 K in either 50% or 100% O2. Total luminous combustion durations of particles, in the aforesaid size ranges, were in the range of 40–65 ms. Combustion products were bimodal in size distribution, consisting of micrometric black magnetite particles (Fe3O4), of sizes similar to the iron particle precursors, and reddish nanometric iron oxide particles consisting mostly of hematite (Fe2O3).

Soot formation from n-heptane counterflow diffusion flames: Two-dimensional and oxygen effects

Soot formation of n-heptane is experimentally and numerically studied in low-strain rate (K = 50 s−1) counterflow soot formation (SF) flames. Flame temperatures and soot volume fractions at different oxygen mole fractions on the oxidizer stream (xO2=0.3–0.45) are measured by using thermocouple-calibrated OH two colors laser-induced fluorescence (2C-PLIF) and laser-induced incandescence (LII) calibrated by light extinction methods, respectively. Mono (MAHs) and polycyclic aromatic hydrocarbons (PAHs) are also qualitatively measured by using the PAHs-LIF technique. Good agreement between measured and predicted maximum flame temperature (Tmax) and peak soot volume fraction (fv,peak) is obtained. However, discrepancies in the shape of the entire soot volume fraction profiles along the axial centerline are observed between 1D and 2D simulations. This result is found to be primarily related to the thermophoretic and radial effects in the low strain rate flames investigated, which hamper neglecting the underlying 2D effects. MAH/PAH peak mole fractions and fv,peak increase from xO2=0.3 to xO2=0.45 due to the related increase of flame temperature, which fosters n-heptane pyrolysis near the fuel nozzle leading to higher concentrations of C1C4 hydrocarbons. The latter are found to grow to MAHs/PAHs and finally to soot particles through analogous pathways independently of xO2. However, the higher flame temperature in the sooting region at xO2=0.45 leads to soot particles and aggregates characterized by larger sizes and more dehydrogenated than those produced at xO2=0.3.

Sensitivity of soot formation to strain rates in counterflow diffusion flames of various C3-C5 alkanes and alcohols

In this work, the sensitivities of soot formation to the variations of strain rates have been studied both experimentally and numerically in counterflow diffusion flames of fourteen C3-C5 alkanes and alcohols. The results showed that an increase in strain rate would lead to a decrease of soot volume fraction in all the tested flames. Except for tert-butanol and tert-pentanol, the sensitivity of soot volume fraction to strain rate was observed to be lower for fuels with a higher sooting tendency. When the soot loadings in flames of the different fuels were matched at a reference strain rate by adjusting oxygen mole fraction, their dependence on strain rate became comparable, except that there was a more substantial reduction in soot volume fraction for tert-butanol and tert-pentanol flames. Kinetic pathway analysis found that the main reason for the faster decline of soot loading in tert-butanol flame has to with the unique molecular structure of tert-alcohols which led to a unique pathway for the formation of molecular soot precursors. The present work served as a first test in alcoholic fuels of the general trend that was previously observed in experiments with aliphatic fuels regarding the sensitivities of soot formation to strain rate. The present data identified tert-alcohols as outliers; corresponding numerical simulation helped to explain the observed peculiarities in flames of tert-alcohols.

Cr Poisoning in SOC Stacks

Solid oxide cell stacks (SOCs) usually use a Cr-containing alloy as the interconnect, which requires the air electrode not only to exhibit satisfying electrochemical performance, but also a high resistance to Cr poisoning. This is because during operation volatile Cr species will diffuse from the oxide scales on the interconnect to the air electrode, leading to a fast degradation. Currently, Forschungszentrum Jülich (FZJ) applies an MCF (MnCo1.9 Fe0.1O3–δ) coating by means of atmospheric plasma-spraying (APS) on the interconnect. This coating reduces the degradation rate from 0.9 % / kh to ~ 0.3 % / kh. However, according to distribution of relaxation time (DRT) figures, Cr poisoning is still a major contribution to overall degradation in the beginning of the operation.In order to further alleviate issues of Cr poisoning, an LSC (La0.6Sr0.4CoO3 –δ) air electrode was investigated. Based on previous studies, the LSC electrode was expected to be more resistant to Cr poisoning compared to LSCF (La0.58Sr0.4Co0.2Fe0.8O3 –δ). Long-term tests demonstrated a lower degradation rate for LSC air electrodes compared to LSCF air electrodes and LSC adsorbed less Cr than LSCF even if the operation time of the SOC stack with LSC air electrode was twice longer [1]. For improving the understanding of the degradation mechanisms of Cr poisoning on LSC and LSCF, an accelerated test was carried out. The test was done with a rainbow stack that used LSC (RU A) and LSCF (RU B) air electrodes in different repetitive units (RU) of the stack. To quickly degrade the air electrodes, no protective coatings were added on the interconnects and 1.4509 stainless steel was used instead of Crofer 22 APU. Besides, the operation condition (775 ºC, 0.75 A/cm2) was harsher than that of previous tests (730 ºC, 0.5 A/cm2). Due to fast degradation, the stack was shut down after only 1,700 h operation. Latest analysis of this stack showed the peak attributed to air electrode polarization in the DRT deconvolution for EIS spectra of RU A cells increased evidently while that of RU B cells barely changed. This was a contradictory finding to previous understanding as it indicated it was LSCF instead of LSC that was more resistant to Cr poisoning. A possible reason might be a lower chemical stability of LSC to volatile Cr species at high temperature and current density. To clearly reveal the reason, Cr distribution and deposited Cr amount in the air electrode needs to be determined. Post-test characterization, such as SEM, EDX and ICP-OES, is undergoing.In addition, a steady-state multiphysical three-dimensional model of an SOC was developed with the open-source software OpenFOAM. Momentum transfer, mass transfer and electrochemical reactions were successfully included in the model. A coupled solver was applied to solve the ionic and electronic potential across different regions. The model was verified by the comparison of the simulation results of the OpenFOAM model with the simulation results of an equal model developed with the commercial software COMSOL, whereas the same mesh, geometry, and parameters were used. Also, the OpenFOAM model was validated by a comparison of the simulation results with experimental results under different operation modes (e.g., fuel cell and electrolysis modes) and temperatures. Assisted by the model, the spatial distribution of the local mole fraction of oxygen and local overpotential in the air electrode could be revealed, which was helpful in analyzing Cr poisoning. As expected, it was found that the overpotential mainly became non-zero at the interface between the air electrode and the electrolyte. Since Cr is mainly observed near the interconnect interface of the LSCF air electrode in post-test examinations, the Cr deposition is expected to primarily arise from chemical reactions. Plus, owing to higher current density on the rib, the mole fraction of oxygen was also lower under rib, where more Cr deposition can be triggered owing to lower oxygen partial pressure. Micro-kinetic modelling of the reduction of adsorption sites owing to Cr poisoning is under development.After the governing equations related to degradation will be included, the model will become a transient-state model which is capable of predicting the voltage with evolution of time.

0-D composition and performance analysis of an air-breathing radiofrequency ion thruster

AbstractThis paper presents a physics-based 0-D steady-state model of a radiofrequency ion thruster (RIT) that is valid for any propellant blend, and hence, any plasma mixture, and the model is used to predict the drag compensation capability and plasma composition of an air-breathing RIT operating in very-low Earth orbit (VLEO). This study fills a gap in the modeling of air-breathing electric propulsion with an air-breathing RIT model that is validated with experimental data of a RIT operating with an $${\mathrm{N}}_{2}$$ N 2 /$${\mathrm{O}}_{2}$$ O 2 mixture. The model expands upon current 0-D RIT models by accounting for the dissociation of neutral molecules and molecular ions and by applying particle conservation for each ion and neutral species. Atomic oxygen in the atmosphere is assumed to completely recombine into $${\mathrm{O}}_{2}$$ O 2 in the collector, and the model is used to calculate an air-breathing RIT’s thrust, discharge efficiency, mass utilization efficiency, and discharge plasma composition at altitudes between 80 and 150 km. At 1 kW of total input power and an optimal flow rate of 0.17 mg/s, the net thrust was limited to 3 mN at all altitudes considered. At these same operating conditions, the mass utilization efficiency and discharge efficiency were approximately 0.15 and 870 eV/ion, respectively. To increase the performance of the thruster, the magnetic field should be increased in order to increase the beam current. The mole fraction of atomic oxygen in the discharge chamber was found to be between 0.19 and 0.27 depending on the altitude, indicating that grid erosion should be a focus of future studies.

Optical diagnostics on coal ignition and gas-phase combustion in co-firing ammonia with pulverized coal on a two-stage flat flame burner

Gradual substitution of coal with green ammonia is a practical approach for the coal power phasedown at a minimal cost of modification, but the ignition and gas-phase reaction during co-firing NH3 with coal remain largely unclear. In this work, we investigate the co-combustion behaviors of NH3 and a high-volatile coal on a two-stage flat flame burner. Remarkably, the post-flame oxygen mole fraction Xi,O2 of the inner stage can be manipulated to reproduce a proper reducing-to-oxidizing environment that coal particles experience in the practical combustor. We first reveal that, under certain values of Xi,O2 and NH3 co-firing energy ratios ENH3, the reaction intensity (manifested by OH-PLIF signals) in the NH3-coal flame is stronger than burning either pure coal or NH3. This synergetic effect originates from an NH3-combustion-induced enhancement of volatile release. We then propose a characteristic time scale τOH from the OH signals for the initiation of overall reactions in the system. In the case of Xi,O2=0, τOH monotonically increases with ENH3, while for Xi,O2=0.2, the trend transitions to a decreasing one. It can be interpreted by comparing τOH with the characteristic O2 diffusion time, coal particle heating time, and the coal pyrolysis time under different Xi,O2. Furthermore, the coal particle ignition in coal-NH3 flames can no longer be determined by visual images. Instead, we apply CH* chemiluminescence to identify the stages of coal particle ignition and volatile combustion in the NH3-coal flame. While NH3 addition has both positive (elevating temperatures & diluting coal particles) and negative (consuming O2) effects on coal ignition, the combined influence of ENH3 is marginal on coal ignition delay time. On the other hand, the volatile combustion time decreases linearly with ENH3, suggesting a pure effect of reduced coal feed rate.

Experimental and kinetic modeling studies on oxidation of n-heptane under oxygen enrichment in a jet-stirred reactor

Oxygen-enriched combustion can improve thermal efficiency and reduce CO, unburned hydrocarbons and soot emissions in internal combustion engines. There are significant differences in the combustion process and pollutant emissions between oxygen-enriched and air condition. In order to develop an improved understanding of the effect of oxygen concentration on n-heptane oxidation, the jet-stirred reactor oxidation characteristics of n-heptane were experimentally tested at temperatures of 450–950 K and pressures of 1.03 atm with the residence time (τ) of 2 s, under the condition of oxygen mole fractions from 0.055 to 0.6. Meantime, the simulations were performed on CHEMKIN. Results show that, firstly, with the increase of oxygen concentration, the NTC (Negative Temperature Coefficient) effect is weakened, mainly due to the strengthening of low-temperature reaction path, which is also the reason for the improvement of ignition stability in cold-start condition of engines. Secondly, the activity of the reaction system improves with the increase of oxygen concentration. Within 500–750 K, it is due to enhancement of the low-temperature reaction path. When the temperature is higher than 750 K, the improvement is due to the enhancement of decomposition of H2O2 and addition reaction of H and O2. Thirdly, with the increase of oxygen concentration, both the generation and consumption of CO are strengthened, which leads to the tip of the diesel spray flame changing from soot area under air condition to the CO chemiluminescence.

Numerical simulation of double chemically reacting particles moving in hot O2/CO2 atmospheres

The entrained-flow gasifier operates with large numbers of discrete particles, and an individual particle has an impact on the reactions of the particles nearby. Therefore, taking the double particles as an example to explore the conversion process offers a reference for developing sub-models that available for reactor-scale gasification simulation. In this study, a CFD-based model was developed to describe the reaction of double coal char particles in O2/CO2 atmospheres. The influence of Reynolds number and oxygen mole fraction on the flame morphology evolution, the Stefan flow, and the carbon consumption rate were discussed to understand the conversion of the double particles. The results show that the severe reaction area around the double particles gradually migrates towards the front of particle 1 with the increase of oxygen mole fraction, and the area of high-temperature zone decreases multi-modally with the increase of both factors. Reynolds number is the main factor affecting mass transfer. The overall carbon consumption rates of the double particles grow linearly with the increase of the oxygen mole fraction, and the rate of particle 1 is greater than that of particle 2. The Reynolds number has a greater effect on intensifying the difference between the carbon consumption rates of the double particles.

CFD analysis of air and oxygen mixing during non-invasive ventilation therapy

The Non-Invasive Ventilator (NIV) is used widely for the treatment of patients suffering from respiratory disorders. The bi-level positive pressure in NIV therapy is provided using pressurized air. The supplementary oxygen is added to the circuit during NIV therapy. The mixture of oxygen and air is provided to the patient through the face mask. At present, the higher FiO2 (i.e., the mole fraction of oxygen in the mixture of air and oxygen) is difficult to achieve in NIV therapy. The higher FiO2 is required for critical patients and the use of NIV therapy is limited to selected minority patients. Thus, a strong need to increase FiO2 exists in NIV therapy. The Computation Fluid Dynamics (CFD) method is used to simulate the mixing of air and oxygen in NIV therapy. The CFD solver is validated by simulating the present solution which is an oxygen adapter and comparing simulation results with hand calculations. The new mixing chamber is suggested. The testing of the mixing chamber was carried out in the previously validated CFD solver and results were obtained. The work aimed at enhancing FiO2 at outlet of the mixing chamber. The effect of geometric parameters of the mixing chamber, viz. convergent-divergent angles, flow direction, and throat diameter is discussed in the results. The ANSYS Design Explorer is used to study the overall effect of all parameters and optimization of parameters. The optimal combination of the above parameters showed a 40 % improvement in FiO2 compared to the oxygen adapter. The optimal combination of the above parameters can provide support for research, design, and manufacturing.

Pore-Resolving Simulations of Biomass Char Particle Combustion

Biomass char morphology affects combustion behavior at the particle scale for zone II conditions, in which both heterogeneous reaction and intra-particle diffusion govern the overall rate. Furthermore, particle-scale processes affect reactor-scale outputs, and reactor-scale simulations are sensitive to particle-scale models. However, most char particle combustion models employ coarse-grained, effective-continuum approaches, which treat all porosity at the subgrid-scale. Effective-continuum approaches are not valid or accurate in the presence of large, irregular pores which can approach the size of the particle. A 3-D, pore-resolving CFD simulation approach using real biomass char particle geometries obtained from X-ray micro-computed tomography (micro-CT) is therefore used to examine the impact of morphology on zone II combustion for pulverized (∼100 µm) biomass char particles for the first time. In contrast to larger, millimeter to centimeter sized particles, the sub-millimeter, high aspect ratio biomass char particles exhibited localized reactant penetration into the innermost regions of the particles, facilitated by the presence of large pores connected to the external surface. The oxygen mole fraction distributions were governed by the large pore morphology, were non-monotonic with distance from the surface, and achieved minima in thick microporous char regions surrounding the large pores. A comparison between the pore-resolving simulation and an equivalent, spatially resolved, effective-continuum simulation revealed that even in the microporous char, the effective-continuum model underpredicted reactant penetration. A careful comparison was then performed between 30 pore-resolving particle simulations and several effectiveness factor models that employed particle-specific parameters. Commonly used uniform cylinder models significantly underpredicted effectiveness factors for these real pulverized pine char particles, while accessible hollow cylinder models achieved less than 10% relative error when averaged over all 30 particles.

Regression of solid polymer fuel strands in opposed-flow combustion with gaseous oxidizer

The combustion of solid fuels is a complex feedback loop, coupling the decomposition of the solid fuel into volatile gases with the gas-phase combustion which is responsible for the heat flux that drives decomposition. This study aims to explore the combustion of a solid fuel, hydroxyl-terminated polybutadiene (HTPB), with different mixtures of oxygen and nitrogen in an opposed-flow burner (OFB) configuration to better understand these coupled processes. An experimental OFB setup is described, which utilizes a nichrome wire and linear variable differential transformer (LVDT) to capture regression rate and shadowgraph imaging to measure flame thickness. Experimental measurements are compared with results from a complimentary one-dimensional opposed-flow combustion model with a pyrolyzing solid fuel boundary condition that conserves mass, species, and energy at the solid-gas interface. The oxidizer mass flux, ratio of oxygen to nitrogen, and separation distance of the fuel and oxidizer are varied to understand their influence on the combustion process and subsequently their effect on the regression rate. In numerical results, fuel regression rate increases when oxygen mole fraction or mass flux increase, or when separation distance decreases. Experimental regression rates and flame thicknesses are compared to simulated results. Though the actual values do not agree exactly, numerical and experimental results are reasonably close and present similar trends. These results demonstrate the utility of simple optical diagnostics in measuring OFB flames and provide a starting point for future opposed-flow combustion model improvements.

“Qualitative and time analysis of the gaseous constituents transport processes in a micro-tunnel on the basis of in-situ measurements”

This paper describes air pressure, relative humidity (RH), temperature and oxygen molar fraction (xO2) measurements that have been carried out over several years in a hundred meter long and 75 cm diameter micro-tunnel (MT) experiment at the Andra's Meuse/Haute-Marne underground research laboratory (MHM URL). Sensors were placed all along, every 20 m inside the MT, and in the access drift. RH and xO2 time evolutions exhibit very complicated profiles, showing many singularities. These features can be well explained by taking into account mainly the corrosion of the steel liners, the water evaporation from the surrounding saturated clay and a leak between the head of the MT excavation and the access drift. Airflow through the leak is driven by the drift pressure seasonal and short time fluctuations. A simple model involving coupled infinite and finite reservoirs made it possible to reproduce xO2 evolution and to calculate the air fluxes associated. Drift pressure variations induce longitudinal compression-dilatation processes in the system that are closely estimated using suitable analytical formulas. By taking into account diffusion driven leaks at the liners interfaces, it was possible to build a global model from which the results compare very well with experimental data.

Spherical gas-fueled cool diffusion flames

An improved understanding of cool diffusion flames could lead to improved engines. These flames are investigated here using a spherical porous burner with gaseous fuels in the microgravity environment of the International Space Station. Normal and inverse flames burning ethane, propane, and n-butane were explored with various fuel and oxygen concentrations, pressures, and flow rates. The diagnostics included an intensified video camera, radiometers, and thermocouples. Spherical cool diffusion flames burning gases were observed for the first time. However, these cool flames were not readily produced and were only obtained for normal n-butane flames at 2 bar with an ambient oxygen mole fraction of 0.39. The hot flames that spawned the cool flames were 2.6 times as large. An analytical model is presented that combines previous models for steady droplet burning and the partial-burning regime for cool diffusion flames. The results identify the importance of burner temperature on the behavior of these cool flames. They also indicate that the observed cool flames reside in rich regions near a mixture fraction of 0.53.

Molecular beam mass spectrometry study on plasma-assisted low-temperature oxidation of ethylene

The chemical kinetics of plasma-assisted low-temperature oxidation of ethylene (C2H4) is explored by molecular beam mass spectrometry and kinetic modeling. In the experiments, a dielectric barrier discharge (DBD) flow reactor coupled with photoionization and electron ionization molecular beam mass spectrometry (PI-MBMS/EI-MBMS) is employed to provide detailed speciation information, including ions, radicals, and molecules in C2H4/O2/Ar systems (∼340 K, 30 Torr). A series of hydrocarbons and oxygenated intermediates including ethenol, methyl hydroperoxide, ethyl hydroperoxide, diacetylene, vinylacetylene, and hydroxyacetaldehyde is detected for the first time in this system. A kinetic mechanism that contains both combustion and plasma reaction networks is developed. The path fluxes of fuel consumption and oxygenates formation are obtained based on the rate of production analysis. Based on our experimental observations and modeling analysis, new insights into the kinetics of C2H4/O2/Ar plasmas are provided. The fuel is mainly consumed through OH addition, dissociation after e/Ar* collision, reactions with O/O(1D), and dissociative ionization. H abstraction reactions that are favored in alkane plasmas contribute little to the fuel consumption of ethylene. Formation pathways of C1 oxygenates are mainly initiated by CH3 produced by O/O(1D) reactions with fuel, and the subsequent pathways of OH/H addition to the fuel are responsible for the formation of most C2 oxygenates. Oxygen addition-elimination reactions of C2H4OH isomers are the sources of ethenol formation. The concentration profiles for some of the species under different oxygen mole fractions are measured. The two distinct patterns of species mole fraction profiles with increasing oxygen concentrations indicate the different dominating formation pathways, namely oxidation pathways and non-oxidation pathways. And the competition between oxidation effects and plasma effects is displayed by the non-monotonic mole fractions profiles of oxidation products.

Incipient sooting tendency of oxygenated fuels doped in ethylene counterflow diffusion flames

The incipient sooting tendencies of oxygenated fuels in counterflow diffusion flames were systematically assessed by doping selected oxygenate fuels into the baseline fuel of ethylene with various mixing ratios. Laser light scattering and planar laser-induced fluorescence (PLIF) techniques were adopted. The critical oxygen mole fractions at different mixing ratios were identified with the use of laser scattering. It is found that the critical oxygen mole fraction varies with oxygenates, suggesting that the effect of mixing ethylene with oxygenates is highly sensitive to the types of oxygenate. While the doping of acetone moderately promoted the incipient sooting tendency, the doping of methanol, acetic acid, formic acid, and diethyl carbonate suppressed the formation of incipient soot. On the other hand, the doping of ethanol, dimethyl carbonate, diethyl ether, and dimethoxymethane had weak influences on the incipient sooting tendency. The production of polycyclic aromatic hydrocarbons (PAHs) of different sizes were measured by PLIF. The results show there was a strong correlation between the incipient sooting tendency and the production of large PAHs. One important finding is that, at their sooting limits, the total oxygen-to-carbon ratio of a stoichiometric fuel/oxygen mixture is linearly correlated with the mixing ratio of the ethylene-oxygenate mixture. This slope of these two quantities can be defined as an incipient sooting index (ISI) of oxygenated fuels for ranking their tendency of incipient sooting formation. Having the slopes correlated well with the oxygenates’ hydrogen/oxygen ratio, a reasonably accurate prediction of critical oxygen mole fractions from the molecule information of oxygenated fuels can be achieved.

Characteristics of oxy-methane flame in an axial/tangential swirl jet burner

This work designs an axial/tangential swirl jet burner to establish stable CH4/O2/CO2 combustion under a wide range of operating conditions. As the flow rate of the axial fuel stream increases, the flame is lifted and stabilized near the downstream wall. The lifted flame is robust and shows good combustion completeness. The critical fuel flow rate for flame lift-off is affected by the oxygen mole fraction of the oxidant flow. The diagram controlling flame attachment and lift-off is investigated experimentally to derive the critical Schmidt number. In addition, high-speed photography and planar laser-induced fluorescence (PLIF) techniques are employed to resolve instantaneous flame structures near lift-off. It is visualized that as the flame height increases at larger fuel flow rates, the flame layer encounters local extinction due to the stretching of the surrounding high-speed oxidant flow. The local stretch rate measured in cold flow agrees well with the theoretic extinction strain rate. Further, a fitting line is obtained to summarize the effect of flow conditions and burner geometry on flame lift-off height. Particularly, the normalized lift-off height shows a good linear relationship with the Reynolds number of the axial fuel jet.

Diffuse Back-Illumination Extinction Imaging Of Soot Formation From A Liquid Fuel Film

The transient combustion of a liquid iso-octane film, isolated from the often co-existing combustion of liquid sprays, was investigated within the nominally quiescent ambient of a constant volume chamber using a custom-made liquid fuel film generation system. Soot formation throughout the combustion process, from ignition to extinction, was visualized using high-speed diffuse back-illumination extinction imaging technique, providing temporally resolved spatial distribution of soot optical thickness (KL) in the chamber. The impact of ambient pressure and ambient oxygen content on soot formation was examined over a range of 2 – 5 bar (absolute) and 16 – 30% (in terms of molar fraction of oxygen), respectively. Regardless of the test conditions, the fuel film combustion entailed three stages, namely flame initiation, steady burning and flame extinction. While the ambient oxygen content was kept constant, the flame gradually became turbulent-like and the flame flickering less distinct as ambient pressure was increased. The total amount of soot generated within the chamber was found to first increase then decrease with the ambient pressure, due to the competing impacts of increasing pressure on promoting soot-formation reaction rate and enhancing mixing of fuel vapour with the entrained air. Increasing ambient oxygen content, on the other hand, consistently enhanced soot formation, which may be associated with its impact on boosting flame temperature and consequently liquid fuel evaporation rate. In addition, flame flickering remained distinct for ambient oxygen content above atmospheric level, while becoming substantially less observable for that below atmospheric level. Flickering frequency, for all test conditions with distinct flame flickering, had a value of approximately 10 Hz and gradually increased with time during the steady burning stage, suggesting the shrinkage of the fuel film diameter. Flickering of the flame resulted in fluctuations in the total amount of soot presented in the combustion chamber. The power of this fluctuation varied with time, and this temporal variation of fluctuation power also depended on experimental conditions, due to its dependency on the combined effects of soot concentration and volume of the flame.

Signal-enhanced Raman spectroscopy with a multi-pass cavity for quantitative measurements of formaldehyde, major species and temperature in a one-dimensional laminar DME-air flame

While Raman spectroscopy provides opportunities for simultaneous detection and quantitative analysis of multiple species, it also suffers from certain limitations, such as low signal strength, which often makes it unsuitable for the study of minor species. The present work employs a high-repetition-rate and high-power laser and re-introduces an amplification technique to improve the detection limit, providing an up to 45-time signal enhancement. The amplification is achieved by employing a multi-pass cavity, and different modes were tested and optimized for the measurements. This multi-pass setup has enabled the detection of formaldehyde (CH2O), which is present in stoichiometric dimethyl ether (DME)-air flames on a sub-percent level, and, therefore, known to be challenging to study with Raman spectroscopy. Furthermore, Raman cross-sections of CH2O and DME, which have not been reported in the literature to the authors’ knowledge, are obtained ab initio using Raman scattering activities in the literature. The cross-sections are 6.63 and 22.43, respectively, for CH2O and DME, normalized by the Raman cross-section of nitrogen. The CH2O detection limit in flame with was estimated to 40 ppm with the present experimental settings. Mole fractions of CH2O, nitrogen (N2), water (H2O), carbon dioxide (CO2), oxygen (O2), DME (CH3OCH3), carbon monoxide (CO), and hydrogen (H2) measured in the flame show good agreement when compared with modeling results obtained using two chemical kinetic mechanisms. Flame temperatures were evaluated from the N2 Raman signals, and the steep temperature gradient is successfully resolved experimentally with the multi-pass setup and well predicted by the models.