Laminar Diffusion Flames Research Articles

Hydrogen enrichment enhances soot formation in swirl-stabilized non-premixed turbulent combustion of ethylene in a model gas turbine combustor

A switch from fossil fuels to hydrogen is currently not feasible mostly due to supply and infrastructure issues. One of the possible approaches, and this is now practiced to a limited extent in industrial gas turbines, is to blend relatively small amounts of hydrogen with fossil fuels curbing the carbon dioxide emissions. However, studies assessing the influence of modest amounts of hydrogen blending with hydrocarbon fuels on soot processes yielded contradictory results. Most of these experimental and numerical studies were performed on laminar diffusion flames and studies on turbulent flames are scarce. One of the confounding factors in assessing the influence of hydrogen is selection of a control experiment in which the fossil fuel is blended with the same amount of an inert diluent. Using helium in the control experiment is preferable because of its similar transport properties and heat capacity to those of hydrogen. Hence, we studied the soot processes in a model gas turbine combustor in which the flame is stabilized by an air swirl. Swirl-stabilized platform ensures that with and without hydrogen/helium dilution, the hydrodynamics of the combustor stays fixed. Base fuel ethylene is supplemented with hydrogen or helium by the same amount to separate the dilution affects and assess the direct chemical interaction of hydrogen related to soot formation. Soot volume fraction and primary soot particle diameters were measured by auto-compensating laser induced-incandescence for all cases. Flow field data obtained using stereoscopic particle image velocimetry is utilized to ascertain the hydrodynamic effects on soot distribution due to addition of lighter species. Soot formation was found to be enhanced by the addition of hydrogen when allowance was made for the dilution effect using the helium doped flame experiments. Possible causes of this observation including the molecular diffusivities of hydrogen and helium, and chemical interaction are discussed.

A comparative study on soot and PAH formation of C10 naphthenic ring-containing species in laminar coflow diffusion flames

C10 naphthenic ring-containing species are important components in transportation fuels and have been widely used for surrogate formulation of diesel and jet fuels to represent cycloalkanes or aromatic fractions. In this study, experimental and modeling studies were carried out to compare the formation of typical polycyclic aromatic hydrocarbons (PAHs) and soot in coflow methane/air diffusion flames doped with n-butylcyclohexane, decalin and tetralin. The laser-induced incandescence (LII) and laser-induced fluorescence (LIF) techniques were applied to measure 2D maps of soot volume fractions and relative PAH concentrations, respectively. Yield sooting index (YSI) was obtained experimentally to indicate the fuel sooting tendency. It can be observed that the soot volume fractions, sooting tendencies and pyrene concentrations of C10 naphthenic ring-containing species follow the order of tetralin > decalin > n-butylcyclohexane. The chemical kinetic simulation reveals that decalin decomposes through two main pathways, including the H-atom abstraction route and ring opening route. Especially, the pathway of ring opening reactions coupled with dehydrogenation results in the strongest benzene formation process among the test components. In the reacting system of tetralin flames, ring opening reactions are suppressed, while the dominant decomposition way of tetralin is via H-atom abstraction reactions, which offer a more direct pathway to form naphthalene and indene without benzene formation, leading to the highest naphthalene concentration of tetralin flame and the strongest soot formation process among all the test components. This fundamental investigation aims to provide valuable data on sooting behaviors and PAH formation characteristics of typical C10 naphthenic ring-containing species and get insight into the kinetic pathways from fuels to soot precursors. Useful information has been obtained for soot reduction from the perspective of the fuel design and surrogate formulation in terms of the sooting tendency.

Critical conditions at liftoff limit of a laminar n-butane-air jet diffusion flame

The stability mechanism of laminar coflow jet diffusion flames in normal gravity has been studied computationally and experimentally. N-butane, the heaviest alkane in a gaseous state at ambient temperature and pressure, is used as the fuel since the reaction mechanism is similar to that of higher (liquid) hydrocarbons. The critical mean n-butane jet and coflowing air velocities at flame stability limits are measured using a small fuel tube burner (0.8 mm inner diameter). The time-dependent, axisymmetric numerical code with a detailed reaction mechanism (58 species and 540 reactions), molecular diffusive transport, and a radiation model, reveals a flame structure. A fuel-lean peak reactivity spot (i.e., reaction kernel), possessing the hybrid nature of diffusion-premixed flame structure at a constant temperature of ≈1560 K, is formed at the flame base and controls the flame stability. In a near-quiescent environment, the flame base resides below the fuel tube exit plane and thereby premixing is limited. As the coflowing air velocity is increased incrementally under a fixed fuel jet velocity, the flame base moves slightly above (≈1 mm) the burner exit and vigorous premixed combustion becomes prevailing. The local heat-release rate at the reaction kernel nearly doubles due to the increased convective oxygen flux (i.e., a blowing effect). The local Damköhler number, newly defined as a ratio of the square root of the local heat-release rate and the local velocity, decreases gradually first and drops abruptly at a critical threshold value and the flame base lifts off from the burner rim. The calculated coflow air velocity at liftoff is ≈0.38 m/s at the fuel jet velocity of 2 m/s, which is consistent with an extrapolated measured value of 0.41 m/s. This work has determined the critical Damköhler number at the stability limit quantitatively, for the first time, for laminar jet diffusion flames.

Effects of ammonia addition on soot formation in ethylene laminar diffusion flames. Part 2. Further insights into soot inception, growth and oxidation

Ammonia, as an alternative fuel, is attracting unprecedented attention due to the absence of CO2 emission during its combustion. Effects of ammonia co-firing on soot formation in ethylene laminar diffusion flames have been studied in the first part of this series of papers, addressing the soot volume fraction change and reaction kinetics of gaseous soot precursors. This paper further investigates the effect of ammonia on evolution behaviors of soot particle, namely soot inception, growth and oxidation in the ethylene co-flow diffusion flame. The soot volume fraction (SVF) and spatial distribution in flames without/with ammonia addition (5 and 20 vol%) were measured using laser-induced incandescence (LII) method. A newly constructed C2H4/NH3/PAHs kinetic model consisting of 163 components and 1018 reactions and the CoFlame codes dedicated to simulating laminar co-flow diffusion flame coupled with fixed sectional soot particle model were used to model the flames and soot inception, surface growth, and oxidation processes in them. Numerical results well captured the experimental observation on both peak SVF and flame height, and confirmed that ammonia inhibited the soot formation in the flame, showing a peak SVF decrease of 2–4 % per 1 % ammonia addition. The numerical results revealed that the ammonia addition inhibited all soot inception, surface growth via HACA (hydrogen-abstraction-acetylene-addition) and PAH condensation, and oxidation via OH/O2 processes. Most importantly, inhibition of HACA reactions induced by ammonia was identified as the main reason for reducing soot formation. Kinetic analysis revealed that adding ammonia created a new pathway to consume H radical while decreasing the rate of H radical-generating reactions, leading to a decrease in H radical and ultimately a reduction in the HACA reaction rate.

Impact of n-butanol substitution in ethylene on soot yields in laminar diffusion flames at pressures 3 to 10 bar

Bioalcohols are perceived to be cleaner burning fuels and generally are assumed to be soot suppressants when blended with hydrocarbon fuels. Butanol isomers display some superior physical and chemical properties as compared to methanol and ethanol, and could be blended with hydrocarbon based transportation fuels without substantial modifications in engine hardware. However, the controversy in the literature about the influence of lower alcohols on soot when blended with hydrocarbons at various proportions has not been resolved yet. Further, there is a lack of information on the dependence of soot production on pressure obtained from tractable flames of butanol-hydrocarbon blends. Such benchmark information on the behaviour of soot characteristics at elevated pressures would be helpful to facilitate a better interpretation of the soot emissions from engines fuelled with butanol-hydrocarbon blends as well as providing data that could be used for numerical simulation efforts. Soot volume fractions and temperature fields of laminar coflow diffusion flames of a nitrogen-diluted ethylene base fuel and of the base fuel doped with 10% n-butanol were measured at operating pressures of 3, 5, 8 and 10 bar in a high-pressure chamber with optical access. Radially resolved temperatures and soot volume fractions were inferred from spectrally resolved radiation emitted by soot from the flames, stabilized on a 3 mm diameter burner, in which carbon and nitrogen mass flow rates in the fuel stream were kept constant. Maximum soot yield decreased by about 25% to 45% when 10% of the carbon in the fuel stream is provided by n-butanol throughout the pressure range; although local enhancements in soot production were observed at 3 bar. However, increases in local soot production disappeared when the pressure is increased to 8 and 10 bar. Current results were compared to ethanol substitution in the same base fuel under identical conditions.

Experimental and numerical study of soot volume fraction and number density in laminar co-flow diffusion flames of n-decane/n-butanol blends

Normal decane was selected as a surrogate fuel for RP-3 aviation fuel, and soot formation characteristics and effects of n-decane (D) /n-butanol (B) cofiring were investigated experimentally and numerically. Combustion experiments of six n-decane/n-butanol blends (i.e., D100, D80, D60, D40, D20, D00, indicating the volumetric fraction of n-butanol in blends) were performed on co-flow diffusion flames. The volume fraction and morphological characteristics of soot at different heights of the flame axis were obtained using thermophoretic sampling and transmission electron microscopy analysis. A newly developed n-decane/n-butanol/PAH kinetic model consisted of 73 species and 275 reactions were further applied to model the soot formation process and ascertain the effects of n-butanol blending. Experimental and simulation results showed that soot loading (soot volume fraction and number density) decreased with increasing n-butanol blending ratios. Chemical kinetic analysis suggested that n-butanol addition decreased the rate of the HACA reaction as a result of a reduction of C2H2 and C2H3 (+M) ⇄ C2H2 + H (+M), ultimately leading to a reduction of soot loading.

Effects of ethanol and methyl formate additions on characteristics of pre-vaporised palm methyl ester laminar diffusion flame

Motivated by the growing interest in the application of renewable fuels and combustion efficiency, gaining a detailed understanding of the effect of oxygenated additives on the structure and shape of the biodiesel laminar coflow diffusion flame is of fundamental and practical importance. In the experiment, the amount of ethanol and methyl formate in the nitrogen-diluted biodiesel mixtures was 10% vol. while maintaining a constant fuel mole fraction. The results showed that adding ethanol and methyl formate decreased the flame height regardless of the variation of the fuel jet velocities, with methyl formate demonstrating a greater reduction effect on the flame height. Besides that, the addition of these oxygenated additives has a significant influence on the flame stability, which was characterised by the parameters such as liftoff height, liftoff velocity, and blowout velocity. The oxygenated additives addition reduced the flame stability limit, with methyl formate showing a greater decrease in flame stability limit based on the observation of the blowout velocity. The dilution effect of the oxygenated additives was identified as the dominant effect on the flame structure and stabilisation through the decoupling analysis when compared to the thermal and chemical effects. Further analysis revealed that the difference in the influence of the oxygenated additives on the combustion characteristics was attributed to the physicochemical properties of the fuel mixtures, such as the molecular diffusivity, energy density, and fuel decomposition. These parameters can be fundamentally related to the difference in the molecular structures between ethanol and methyl formate. On the other hand, the addition of these oxygenated additives decreased both carbon monoxide and nitrogen oxide concentrations; however, the differences in the pollutant concentrations between ethanol and methyl formate were insignificant.

The effects of 1-methylnaphthalene addition to n-dodecane on the formation of soot and polycyclic aromatic hydrocarbons in laminar coflow diffusion flames

1-Methylnaphthalene (1-MN) is a typical diaromatic component in petroleum-based fuels and has been widely chosen to represent aromatic fractions in surrogate fuels of diesel and jet fuels. In this study, experimental and modeling studies were performed to investigate the formation of typical polycyclic aromatic hydrocarbons (PAHs) and soot in coflow methane/air diffusion flames doped with 1-methylnaphthalene/n-dodecane mixtures, and explore chemical effects of 1-methylnaphthalen on key soot formation steps. The laser-induced incandescence (LII) and laser-induced fluorescence (LIF) techniques were used to obtained the soot volume fraction and relative concentration of typical PAHs along the flame centerline, respectively. A novel apparatus-independent indicator, yield aromatic index (YAI), was first proposed to quantify the formation tendency of a specific aromatic product in a flame. It can be found that the addition of 1-methylnaphthalene in the fuel mixtures has a significantly promoting effect on soot formation processes. The correlation between the mass fraction of 1-methylnaphthalene and sooting tendency shows a strong linear relationship. As to the PAH formation tendency, YAIA2R5 have the smaller deviations from corresponding YSIs than YAIA2&A3 and YAIA4, which indicating that A2R5 can be a type of important species leading to soot inception and have strong contribution to soot formation. Through the chemical kinetic analysis, A2R5 is proved to be a key coupling point of two A4 formation pathways, which start from n-dodecane and 1-methylnaphthalene, respectively. As a small amount of 1-methylnaphthalene is introduced into the fuel, the dominant approach to A2R5 rapidly shifts to a more efficient pathway, and is continually enhanced with increasing 1-methylnaphthalene at a moderate trend. The results reveal that 1-methylnaphthalene in the fuel can stride over the rate limiting step of the soot formation process of non-aromatic fuels, eventually leading to an obviously increasing sooting tendency. This study aims to provide useful information for the development of soot nucleation mechanisms and the control of soot formation processes.

Numerical Simulation of Biogas Combustion by Using a Finite Volume Based-Multispecies Transport Model

Abstract In this paper, we advocate the use of multispecies transport model coupled to global mechanisms instead of using detailed mechanisms, which are still not yet computational affordable for the majority of the research groups in Africa. The open-source sofware openFOAM® is used as the calculation platform. The obtained algorithm is validated by comparing its simulation results to the full Gas Research Institut (GRI)-3.0 mechanism-based simulations of Charest et al. (2014, “Numerical and Experimental Study of Soot Formation in Laminar Diffusion Flames Burning Simulated Biogas Fuels at Elevated Pressures,” Combust. Flame, 161, pp. 2678–2691) with quite satisfactory agreement for methane flames diluted with CO2. The new code is then used to investigate the axial flame temperature of locally produced biogases from pig slurry and cow dung respectively named BG L1, BG L2, BG B1, and BG B2. These biogases differ from each other in their composition. Methane proportion in the mixture ranges from 46% to 52%.

Investigation of the stability, radiation, and structure of laminar coflow diffusion flames of CH4/NH3 mixtures

The stability, radiation, and structure of laminar axisymmetric CH4/NH3/air diffusion flames have been studied using photographic images, spectrally resolved measurements of flame radiation, and the spatial distribution of temperature and major species mole fractions obtained by spontaneous Raman scattering. The fit procedure for the Raman spectra of NH3 includes a hitherto unquantified overtone feature, whose inclusion in the fit significantly improves the NH3 fraction obtained. Nitrogen is used to replace NH3 to separate chemical effects of NH3 addition from those due to dilution. The results show that NH3 addition drastically reduces radiation from carbon-containing species, with progressively increasing strong chemiluminescence from excited NO2 and NH2, indicating a substantial change in flame chemistry. While the Rayleigh/Mie scattering from soot particles is still observed in the Raman spectra at 28% NH3 addition, 46% NH3 in the fuel is seen to suppresses soot formation effectively. The measured axial and radial profiles of temperature and major species indicate a substantial contribution from radial transport from the reaction zone, seriously complicating the relation between composition, mixture fraction, and the corresponding equilibrium temperature and mole fractions.

Investigation of soot inception limits and chemiluminescence characteristics of laminar coflow diffusion flames in C/O ratio space

This study investigates soot inception limits of laminar coflow diffusion flames in the carbon-to-oxygen atom (C/O) ratio space, as well as the mechanism of flame structure affecting soot inception. And chemiluminescence characteristics in flames are evaluated. Oxy-ethylene flames with the same adiabatic flame temperature (2500 K) and different stoichiometric mixture fractions (Zst, 0.3–0.6) are investigated by relocating CO2 from oxidant to fuel. The position of soot inception and the chemiluminescence of CH*, C2*, and CO2* are measured by a hyperspectral imager. A numerical model with a detailed kinetic mechanism is used to obtain the temperature and gas-phase species concentration. The results prove that the critical C/O ratio favorable for soot inception is 0.53, and the critical temperatures on the flame centerline and sheet are 1322 K and 1632 K, respectively. The comparatively low temperature on the flame centerline can cause soot inception, as the polycyclic aromatic hydrocarbons (PAHs) concentration and residence time can guarantee the collision and cluster formation of PAHs. Although fuel pyrolysis is promoted on the flame sheet, oxygen-containing species dilutes the concentrations of fuel pyrolysis products and PAHs to limit soot inception, necessitating high temperatures for chemical bond formation. Furthermore, the mixture fraction, local C/O ratio, and net reaction and production rate are calculated, and the key reaction paths are also identified. The results demonstrate that the increase of Zst achieves the suppression of soot inception by lowering the formation of benzene and PAHs in the high C/O ratio (>0.5) zone, improving the oxygenation in the low C/O ratio (<0.5) zone, and compressing the C/O ratio space favorable for soot formation. Meanwhile, the (CH* and C2*) chemiluminescence zone with a C/O ratio of roughly 0.5 exists in the flame. With Zst increases, the CH* and C2* chemiluminescence diminishes at the flame root and enhances downstream, whereas CO2* chemiluminescence remains unaltered.

Probing ultrafast dynamics of soot in situ in a laminar diffusion flame using a femtosecond near-infrared laser pump and multi-color Rayleigh scattering probe spectroscopy.

Soot nanoparticles result from incomplete combustion of fossil fuels, and have been exhibited, when released into the atmosphere, to be detrimental to air quality and human health. However, because of the inert and non-luminescent properties, probing the dynamics of soot in situ is still a challenge. Here we report a strong near-infrared laser pump and multi-color Rayleigh scattering probe approach to reveal soot dynamics in situ in a n-pentanol/air laminar diffusion flame at femtosecond time resolution. A size-dependent dynamical process of the pump-laser-induced soot swelling at femtosecond time scale and subsequent shrinking back to its original size at picosecond time scale is observed, in which both the swelling rise time and the shrinking decay time increase monotonically as the initial sizes of soot nanoparticles become larger. By characterizing the evolution time and intensity of the multi-color scattered probe light, the spatial distributions of different sizes of soot particles from the inception to the burnout regions of the flame are mapped, which provide useful information on exploring the formation and growth mechanisms of soot particles in flames.

Machine learning techniques to predict the flame state, temperature and species concentrations in counter-flow diffusion flames operated with CH4/CO/H2-air mixtures

The usage of artificial intelligence (AI) is increasing in many fields of research, since complex physical problems can be ‘learned’ and reproduced by AI methods. Thus, instead of numerically solving partial differential equations, describing the physical processes in detail, appropriate AI methods can be used to decrease the calculation time significantly. In the present study, artificial neural networks (ANNs) were used to predict temperature and species concentrations in a laminar counter-flow diffusion flame. To improve the accuracy of the ANNs, a support vector machine (SVM) was used to subdivide the wide range of operating conditions (air–fuel ratio, strain rate, fuel mixture) into ‘flame’ and ‘no flame’ cases. Due to classification with the SVM the prediction performance of the ANNs was optimized and an average error to the reference values (GRI3.0) below 10 K for all cases was detected, whereas the calculation time was decreased by a factor of about 4,800 (solving the transport equations with GRI3.0).

Investigation of soot particles morphology and size distribution produced in a n-heptane/anisole laminar diffusion flame based on TEM images

The morphology and structure of soot particles in laminar diffusion flames of anisole/n-heptane have been investigated in this work. Soot morphology has been analyzed by thermophoretic sampling and transmission electron microscopy (TS-TEM). Further, a detailed TEM image process was performed and a set of fractal-based distribution analysis algorithms were proposed and applied in this study. The addition of anisole in n-heptane flames greatly influences the soot formation process, which leads to form the soot at the lower part of the flame and subsequently larger primary particle diameters. Moreover, the characteristics of the primary particle diameter distributions, the number of primary particles per aggregate, the aggregate sizes and the fractal dimensions of the anisole/n-heptane flames have been discussed in details.

Numerical Study of PAHs and Soot Emissions from Gasoline–Methanol, Gasoline–Ethanol, and Gasoline–n-Butanol Blend Surrogates

Soot formation is an intricate phenomenon, and soot propensity of a fuel is interwoven with the fuel composition, physical and chemical properties, and combustion environment. The present study examines the hypothesis that in addition to the chemical composition of the fuel, the sooting nature of the fuel is closely coupled with its chemical property known as octane sensitivity (S). With this motivation, the present study numerically investigates the effects of gasoline surrogate composition and its property, octane sensitivity (S), on polycyclic aromatic hydrocarbons (PAHs) and soot emissions. Four-component toluene primary reference fuel (TPRF)–alcohol blends, comprising iso-octane, n-heptane, toluene, and one of the three different alcohols- methanol, ethanol, and n-butanol, are used as gasoline surrogates. A total of 320 TPRF–alcohol mixtures, with S in the range of 1–10, are examined under laminar counterflow diffusion flame conditions. A detailed chemical mechanism coupled with a comprehensive soot model, which includes reactions for soot inception, surface growth, PAH condensation, and oxidation, is adopted. The analysis indicates that the toluene content in the fuel mixture has a prominent effect, while the alcohol content and octane sensitivity of the fuel have a weak correlation with the PAHs and soot. Thus, it is not clear if any of these three variables, namely, toluene content in the fuel, alcohol content in the fuel, and S, are individually sufficient to characterize the PAHs and soot across various blends. For this reason, a new variable (XCHO) based on the elemental composition of the fuel mixture is identified and it is shown that XCHO along with S of the fuel characterize soot emissions satisfactorily. Further, a reaction path analysis indicates that the efficacy of alcohols in reducing soot emissions follows the order: methanol > ethanol > n-butanol.

Soot formation in n-heptane/air laminar diffusion flames: Effect of toluene addition

The mixture of n-heptane and toluene is regarded as surrogate fuel for diesel. Under the premise of a certain carbon flow rate, the particle size, nanostructure, microcrystalline size and soot deposition morphology of three kinds of fuel laminar diffusion flames were studied by Transmission Electron Microscopy (TEM), High Resolution Transmission Electron Microscopy (HRTEM), X-ray diffractometer (XRD) and Field Emission Scanning Electron Microscopy (FESEM). The three fuels were pure n-heptane (T0), toluene / n-heptane blends with 5 vol% (T5) and 10 vol% (T10) toluene, respectively. Thermophoretic probe sampling and SiC fiber deposition were used to obtain soot samples. The results showed that with the increase of toluene content, the flame height increased and the temperature decreased at the same height. The addition of toluene increased the content of PAHs, which promoted the formation of soot, and increased the number and particle size of soot. At HAB = 30 mm, with the increase of toluene content, the graphitization degree of soot increased, which showed a larger soot fringe length and a smaller fringe tortuosity, and microcrystalline width (La) of soot particles increased gradually. It is worth noting that the size of a new structure in the soot deposition is counted for the first time, and it is found that its size is negatively correlated with the fringe length and microcrystalline width of soot.

Experimental investigation of the effect of hydrogen addition on the sooting limit and structure of methane/air laminar counterflow diffusion flames

The influence of hydrogen (H2) addition on the sooting limit and structure of methane/air laminar counterflow diffusion flames has been studied. Soot limits have been obtained using the laser light scattering technique for CH4/H2 mixtures with H2 concentration varying from 0–20% by volume in the fuel mixture for different strain rates. The resulting soot limit map shows a significant increase in the fuel concentration at the soot limit with increasing amounts of H2. To understand this trend, temperature and species concentration profiles have been measured and compared under the influence of H2 addition in (a) an incipiently sooting and (b) in a sooting CH4/air flame. Flame temperatures have been measured using a thin-wire S-type thermocouple and species profiles have been measured using the gas chromatography technique (GC). The results indicate a significant decrease in the amounts of higher hydrocarbons such as benzene and naphthalene (which are considered important soot precursors) with an increase in H2 concentration for the sooting flame. To explain this decrease in the concentration of higher hydrocarbons, pathway analysis has been performed to elucidate the influence of H2 on the formation and consumption of soot forming species.

A Comprehensive Study on Flame Length under Oxygen Enhanced Laminar Non-premixed Combustion

ABSTRACT The study of stoichiometric flame lengths of laminar jet or co-flow diffusion flames has been a major topic of research in last few decades, mostly focusing on flames of either undiluted or slightly diluted fuel reacting with air. However, the flame characteristics under extremely diluted fuel conditions are not well studied. Hence, a set of experiments and numerical simulations have been performed in the present work to study the flame lengths for a wide range of fuel and oxidizer dilutions using two different jet diameter burners (4 and 9 mm with identical volume flow rates). Both experimental and numerical results confirm that the 4 mm burner flame length is consistently higher than the 9 mm burner counterpart. This difference is attributed to the higher temperature of the 9 mm burner flame, which in turn is due to its lower conductive heat loss to the ambient. Several analytical models, available in the literature to predict the flame lengths, such as Roper, Li-Gordon-Williams (LGW), and Wang-Sunderland-Axelbaum (WSA), have also been analyzed and compared with the experimental data. A new analytical model is also proposed in this study by modifying the species diffusivity and flame temperature in WSA model. This revised model predicts the flame lengths better than the other analytical models, especially when the fuel stream is diluted.

U-Net applied to retrieve two-dimensional temperature and CO[formula omitted] concentration fields of laminar diffusion flames

Direct absorption spectroscopy measurement of the 4.3μm CO2 using interband cascade laser with a high spectral resolution, provides theoretical feasibility to retrieve the spatial distribution of CO2 temperature and concentration in flames. However, retrieving temperature and concentrations from the line-of-sight spectral measurements involves solving non-linear and ill-posed problems. Traditional reconstruction algorithms for axisymmetric flames require multiple line-of-sight measurements in both axial and radial directions. The complicated experimental system limits the reconstruction efficiency. Here, a novel reconstruction model is proposed and demonstrated based on the U-Net model, a fully convolutional network model composed by a encoder–decoder path forming a symmetric U-shaped architecture, which only requires measurements of the central axis of the flame to simultaneously and efficiently reconstruct the two-dimensional temperature and CO2 concentration fields for axisymmetric flames. The U-Net model is trained using datasets from simulated temperature, CO2 concentration and their corresponding spectral optical thickness. The proposed U-Net model has been applied to reconstruct the two-dimensional temperature and CO2 concentration fields of two previously measured laminar diffusion flames. Comparing with results from the traditional Abel inversion and two-line atomic fluorescence thermometry methods, the U-Net reconstruction model can better preserve the spatial continuity and correlation of flame temperature and CO2 concentration from information hidden in the flame spectral images, which considerably simplifies the measurement system and improves the reconstruction performance.

An investigation of thermal radiation from laminar diffusion flames in a tri-coflow burner with central oxygen

In natural gas combustion processes, oxygen-enriched combustion is often used to improve heat transfer and thermal efficiency. In this investigation, a triple-port burner is employed to investigate the effect of oxygen enrichment on thermal radiation when introducing oxygen into the center of a normal coflow diffusion flame, while maintaining an outer coflow of air. Two fuels are investigated: methane, being the primary constituent of natural gas and having a low propensity for soot formation, and ethylene, which acts as a reference fuel with a much higher propensity for soot formation. The flow rates of air in the outer annulus and fuel in the inner annulus are held constant, while the flow rate of oxygen in the central tube is systematically increased. Total radiation, which is converted to radiation loss fraction, is measured using a wide-angle radiometer. Soot volume fraction and soot temperature are measured using two-color ratio pyrometry with a digital camera.For low flow rates of central oxygen when an inner oxygen-fuel flame is apparent, the thermal radiation is increased due to an increase in soot concentration. At these low flow rates, thermal radiation is increased by 20% for methane and 26% for ethylene, as compared to a normal coflow flame without central oxygen. The increase in radiation from these flames results in a decreasing, or relatively constant soot temperature in the soot laden region as the increased soot concentration radiates heat from the flame, even though gas temperatures upstream near the central oxygen flame are quite high. For relatively high flow rates of central oxygen, radiation from the tri-coflow methane flame increases by up to 31% compared to a normal coflow flame, while radiation from the ethylene flame monotonically decreases. For both fuels, when oxygen flow rates are high, soot concentration decreases (due to a change in flame shape and enhanced oxidation rates) and soot temperature increases. Product gases are the dominant emitters of radiation in the methane flames studied. Soot is the dominant radiant emitter in the ethylene flames until high flow rates of central oxygen reduce the soot concentration, such that high temperature product gases take over as the dominant radiant emitters. Overall, low flow rates of central oxygen can increase thermal radiation from the coflow methane flame by 20% due to increased soot concentration, and high flow rates of central oxygen can increase thermal radiation by 31% due to increased gas temperatures. However, the methane flames with central oxygen still emit at least 41% less radiation than the coflow ethylene flame. These results indicate that when considering the effect of oxygen enrichment on thermal radiation, the change in overall flame (gas and soot) emissivity can be just as important as the change in temperature.