Flames Of Fuels Research Articles

Combustion condition predictions for C2-C4 alkane and alkene fuels via machine learning methods

The accurate and rapid prediction of hydrocarbon type was a precondition for the utilization of fossil fuels with high efficiency and safety. In this study, machine learning based techniques were used to predict the type and equivalence ratio of flames of C2-C4 alkane and alkene fuels based on the differences in flame morphology between various combustion conditions. The test results of different machine learning algorithms, including ANN, SVM, SVR, KNN, MLR, and RF were compared in detail using statistical methods. Results indicated that ANN, SVM, KNN, and RF all exhibited an outstanding performance in predicting the types of C2-C4 alkane and alkene flames, achieving accuracies of 95.7 %, 96.3 %, 93.8 %, and 96.5 %, respectively. For the prediction of the equivalence ratio among these fuels, the mean absolute percentage errors of the ANN, SVR, MLR, and RF were only 5.6 %, 3.8 %, 8.2 %, and 3.8 %, respectively. The performance of SVM, SVR, and RF algorithms was significantly superior to that of ANN, MLR, and KNN algorithms for flame prediction. Moreover, the data of feature analysis revealed that the importance level of designed features exhibited a significant distinction between different prediction targets. For predicting the type of C2-C4 alkane and alkene fuels, the features associated with blue region showed a stronger importance level. However, the yellow region related features played a more significant role for the prediction of the equivalence ratio.

Investigation of the Coupling Schemes between the Discrete and the Continuous Phase in the Numerical Simulation of a 60 kWth Swirling Pulverised Solid Fuel Flame under Oxyfuel Conditions

Investigation of the Coupling Schemes between the Discrete and the Continuous Phase in the Numerical Simulation of a 60 kWth Swirling Pulverised Solid Fuel Flame under Oxyfuel Conditions

Structure of Low Stretched Non-Premixed Counterflow Flames Stabilized in Planar Channel: Mass Spectrometric Study and Numerical Simulation

ABSTRACT In this paper, a methodology for experimental investigation of chemical and thermal structure of weakly stretched counterflow flames under terrestrial conditions is proposed. The non-premixed counterflow flames are stabilized in a narrow channel between quartz plates, and the flame gases are sampled by a microprobe inserted via a thin slit in one of the quartz plates. The gas samples are then analyzed by mass spectrometry. The results of the measurements of mole fraction profiles of the main components of methane-air non-premixed flames, namely, CH4, O2, H2O, CO2, and CO, are presented. The temperature profiles are also reconstructed using the measurement data for the gas composition. The experimental profiles are compared to the results of the 3D numerical simulations undertaken within the global reaction mechanisms (one- and four-step) and a detailed reaction mechanism (San-Diego). The results of this work clearly show that the thermal and chemical structures of the counterflow flame in the planar channel are essentially non-one-dimensional, and 3D-calculations are necessary to correctly predict the flame behavior in this configuration. The methodology demonstrated in the current work can be successfully employed for the investigation of the low stretched counterflow flames of various hydrocarbon fuels and fuel blends. This will allow verifying and developing the reaction kinetic models capable of quantitative prediction of weakly stretched flame behavior in confined conditions.

Resonance enhanced multiphoton ionization detection of aromatics formation in fuel-rich flames

Polycyclic aromatic hydrocarbons (PAHs) are formed in fuel-rich and sooting hydrocarbon flames. These compounds and smaller aromatic flame constituents, such as benzene, toluene and naphthalene can be detected by flame-sampling molecular beam mass spectrometry (MBMS). They are readily ionized by photoionization with UV light, either in resonant or non-resonant ionization processes. The multiphoton ionization of benzene, toluene, and styrene with MBMS detection was investigated as a function of laser power and wavelength and the quantitative relation between ion signal and mole fractions in the flames was established. These measurements were prerequisites for the measurement of quantitative mole fraction profiles of these species in flames of various fuels. In addition, the measurements provide relative mole fraction profiles of various other aromatic flame species. The results are compared to measurements of aromatics in butadiene, propyne, and allene flames by single-photon ionization with vacuum ultraviolet radiation from a synchrotron source and other literature data.

Optical diagnosis study of fuel volatility on combustion characteristics of spray flame and wall-impinging flame

Finding suitable alternative fuels and reducing carbon emissions are important development directions for future engines. Many carbon-neutral fuels, such as alcohol fuels are widely used, and they are more volatile than traditional diesel fuel. Therefore, exploring the development characteristics of spray flames and near-wall flames of different volatile fuels has important theoretical significance and application value for the optimization of fuel characteristics and the development of efficient and clean combustion technology. In this paper, the flame structure, flame development characteristics and soot evolution of three fuels, including n-hexane, 74%v n-heptane mixed 26%v isooctane (PRF26) and 53%v n-dodecane mixed 47%v isooctane (D53) were tested, that the cetane number of three fuels is essentially same. High speed self-luminous imaging and RGB two-color method were used to investigated the effect of fuel volatility on combustion. Results can be seen as following. Firstly, during the early stages of n-hexane combustion, a blue premixed flame can be found, and there is a pale-yellow flame in the later stage. As the boiling point increases, the area of the blue flame in the first stage gradually decreases, the area of the yellow flame in the later stage gradually increases, and the spatially integrated natural luminosity (SINL) of flame gradually increases. In addition, as the boiling point increases, the initial spontaneous combustion ignition point is away from the nozzle, the ignition delay time reduces and flame area becomes larger. Furthermore, the flame of high boiling fuels result in higher soot emissions. Secondly, in the event of flame wall-impingement, the near-wall flame of the three fuels has higher flame SINL and more soot formation in comparison to the free spray flame. The flame development of the three fuels is similar. However, there are some differences in the flame brightness distribution. The bright flame areas in n-hexane and PRF26 are concentrated near the wall, while the bright flame area in the D53 flame is concentrated on the wall and the flame head area rolled up with a larger distribution area. The flame near the wall is impacted by the wall temperature that the lower wall temperature leads to prolonged fuel evaporation and mixing time, which increases the ignition delay time. In conclusion, fuel volatility has less influence on flame structure and development, but a significant impact on ignition delay time and emissions. Regardless of whether wall impingement occurs, higher volatile fuels can extend the ignition delay time, increase the flame area and decrease soot emissions from combustion.

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.

Sooting transition diagnostics in counter-flow flames of C4 isomer fuels

Macroscopic transition processes of soot formation for 1-butene, isobutene, n-butane and isobutane counterflow diffusion flames from sooting-free to heavy-sooting were studied by varying fuel concentration (XF) in the fuel stream. A novel optical diagnostic algorithm was proposed to classify different sooting transition stages in flames. Effects of saturated/unsaturated bonds and linear/branched structures of fuel molecules on the sooting transition were investigated in details. Flame species were detected and quantified by on-line gas chromatography and also kinetic analysis was performed. KL factor was obtained by optical diagnostics to qualitatively compare soot concentrations. Results showed that XF required for 1-butene flame to reach the critical state of macroscopic sooting transition was the lowest, followed by isobutene, isobutane and n-butane flames. Fuels with higher sooting tendencies could achieve entire sooting transition process with a narrower range of XF. Linear butane was more difficult to achieve the sooting transition than branched butane, while linear 1-butene was easier to form soot than branched butene, reflecting the difference between isomeric structures acting on C4 alkenes and alkanes. These might be largely due to the close association of allyl (C3H5-A) with 1-butene. Higher C3 species content in isobutane flame made it have higher sooting tendency than n-butane.

Role of acoustic wave on extinguishing flames coupling with water mist

Water mist plays an indispensable role in various fire extinguishing scenarios, but its efficiency is restricted by the interaction area of droplets and the flame. With the characteristics of simple operation and limited environmental pollution, acoustic waves can deliver water droplets to flame and have the potential to enhance the fire extinguishing efficiency of water mist. However, our understanding of the synergistic effect of acoustics and water mist on suppressing flame is still limited. In this study, the enhancement mechanisms of acoustic waves in extinguishing ethanol fueled flames are experimentally investigated. The effects of critical parameters related to sound pressure and acoustic-induced air speed on flame behaviors are comprehensively discussed. Initially, the flames cannot be extinguished by the pure water mist. Coupling with acoustic waves, the response of the flames significantly changes. For the acoustic wave of a sine signal, the frequency of 30 Hz induces a greater air speed, which is superior to the frequency of 40 Hz and 50 Hz in suppressing the flame. The acoustically enhanced mechanism is perhaps related to the acoustic-induced wind, which causes the flame oscillation. The flame and the wick expose the larger area to be cooled directly by the water mist when the flame is “pulled” and “pushed” by the acoustic-induced wind. A dimensionless number θ is finally estimated for determining the critical flame extinction condition for this collaborative system. These results could advance our understanding of the underlying enhancement mechanisms in coupling acoustics with water mist and provide scientific insights into applying this novel suppression strategy.

Characterization of Propane Fueled Flames: A Significant Source of Brown Carbon

In this study, we developed a framework for interpreting the in situ morphological properties of black carbon (BC, also referred to as “soot” due to combustion relevance) mixed with primary organic aerosol. Integration of the experiment considering primary organic aerosol (POA) evaporation from the soot particles was examined using a Differential mass–mobility analyzer (DMA) and showed the untold story of the mixing of BC and POA. We also hypothesize that morphological transformation of soots and determined such as (i) the evaporation of externally and internally mixed POA led to a decline in the particle number and size of monodisperse aerosol; (ii) presence of externally mixed BC was interpreted from the occurrence of two peaks of soot upon heating; (iii) heat-induced collapse of the BC core possibly resulted from the evaporation of material from the voids and effect of heat; (iv) volume equivalent to changes in the mobility diameter represented evaporation of POA from the surface and collapse upon heating. POA constituted a high fraction (20–40% by mass) of aerosol mass from these flames and was predominantly (i.e., 92–97% by mass) internally mixed with BC. POA was found to be highly light absorptive, i.e., an Ångström absorption exponent (AAE) value of (in general) >1.5 was estimated for BC + POA at 405/781 nm wavelengths. Interestingly, a much more highly absorptive POA [mass absorption cross-section (MAC)-5 m2 g−1] at 405 nm was discovered under a specific flame setting, which was comparable to MACs of BC particles (8–9 m2 g−1).

PAH formation characteristics in hydrogen-enriched non-premixed hydrocarbon flames

The utilisation of hydrogen with conventional hydrocarbons offers an excellent opportunity to decarbonise current energy systems without significant hardware upgrades. However, this presents fresh scientific challenges, one of which is the difficulty in effective control of pollutant soot emissions due to complex reaction kinetics of hydrogen enriched flames. This paper focuses on polycyclic aromatic hydrocarbons (PAHs), which are the building blocks of soot and responsible for its carcinogenicity. Detailed understanding of the effect of H2 on the underlying processes of PAH formation and growth is important for the development of effective strategies to curtail PAH formation and hence, reduce soot emissions from combustion systems.In this study, an experimental methodology was employed to analyse PAH formation and growth characteristics of laminar inverse diffusion flames of various hydrocarbon fuels (alkanes and alkenes) enriched with H2 using simultaneous planar laser induced fluorescence (PLIF) imaging of PAHs and hydroxyl radicals (OH). OH PLIF was used to indicate peak temperature locations in the flame (flame front), while PAH PLIF was used to determine PAH formation characteristics. Methane (CH4) was also separately added to the same hydrocarbon fuels to study effects of carbon-bound hydrogen addition, in comparison to H2 addition. It was observed that only the addition of H2 to CH4 showed significant variation in the magnitude of PAH reduction levels as the length along the flame front, Lf increased. The results also showed that while the addition of H2 was more effective in reducing the rate of PAH fluorescence signal increase (indicative of concentration growth) when compared to CH4 addition, both fuels showed two distinct regions in the PAH growth curve; a steep growth region followed by a slower growth region. This is potentially indicative of the self-limiting nature of PAH formation and growth. The study concluded that the growth rate of PAHs lies within a narrow band irrespective of the fuel bonding, molecular structure and the H:C ratio of the fuel mixtures tested.

Sensitivity of soot formation to strain rate in steady counterflow flames determines its response under unsteady conditions

This work reports an original experimental investigation on the sooting characteristics of counterflow diffusion flames (CDFs) subject to sinusoidal oscillations of strain rates. The study was motivated by the necessity to consider transient responses of soot formation when using the laminar flamelet model to simulate soot formation in turbulent flames. Acoustic forcing was applied through loudspeakers installed at the inlet end of both the oxidizer and fuel nozzles of the counterflow burner to generate strain rate oscillations at the desired frequency and amplitudes. Particle image velocity and planar laser induced incandescence were used respectively to measure flow field and the soot volume fraction in a time-resolved manner. Counterflow flames of C2H4 and C3H8 fuels were tested with oxidizer streams of various oxygen mole fractions (XO). The experimental data revealed notable differences in unsteady soot responses between CDFs of C2H4 and C3H8 at the same XO. However, after matching the soot loadings of the two CDFs to have the same sensitivity to strain rate at steady states, their responses to unsteady strain oscillation became almost identical. We experimentally demonstrated that the unsteady response of soot formation to oscillating strain rates, as characterized by phase lag and amplitude attenuations, are strongly correlated with the sensitivity of the soot loading to strain rate under steady conditions. The results may have important implications for the development of transient flamelet models that incorporate unsteady effects on soot formation.

Study on Extinguishing Limits of Liquid Fuel Flames in Confined Spaces

To sustain a flame following fuel ignition, it must receive oxygen from the surroundings at a concentration above a stoichiometrically-determined threshold; otherwise, the flame will decrease in intensity or extinguish. These thresholds vary depending on the type of fuel. Using the aforementioned factors, it is possible to predict and control the characteristics of flames in confined spaces. While the combustion characteristics of flames exposed to the atmosphere have been actively studied under controlled conditions, it is more challenging to predict and remediate the impact of variables, which manifest in real fire sites. In this study, following the ignition of a flame in a confined space with limited oxygen supply, oxygen and carbon dioxide levels and temperature at this space were measured according to changes in the fuel supply capacity. The extinguishing limit oxygen concentration as well as the extinguishing time decreased in relation to the size of the initial source. These results can guide the design and control of fire-fighting equipment for the early detection and extinguishing of fires.

H2 Impact on Combustion Kinetics, Soot Formation, and Nox Emission of Hydrocarbon Fuel Flames

H2 Impact on Combustion Kinetics, Soot Formation, and Nox Emission of Hydrocarbon Fuel Flames

A Comparative Study of Gaseous Fuel Flame Characteristics for Different Bluff Body Geometries

A Comparative Study of Gaseous Fuel Flame Characteristics for Different Bluff Body Geometries

Chemical and Sooting Structures of Counterflow Diffusion Flames of Butanol Isomers: An Experimental and Modeling Study

ABSTRACT This work reports an experimental and numerical analysis on the sooting characteristics of butanol isomers. Light extinction and gas chromatography were used to measure soot and gas-phase species, respectively. Kinetic analysis of the tested flames was performed with detailed gas-phase mechanism and a sectional soot model. The present work aims to provide an understanding on the effects of butanol isomeric structures on sooting tendencies. For a comprehensive analysis, flames of both neat butanol fuels and butanol/hydrocarbon mixtures were studied. The results showed that the relative ranking of sooting tendencies among the butanol isomers were similar in neat butanol flames and in butanol-doped ethylene flames. In addition, we show that a small amount of butanol addition in ethylene flame enhanced soot formation. It was found that different isomeric structures mainly affected the formation of C3H3, which led to the different concentrations of important aromatic soot precursors. In the butanol-doped ethylene flames, the blending of the four butanol isomers increased the number of C3H3 formation pathways, which in turn significantly increased the production of benzene. Structural effects explaining for the differences in the sooting tendencies of the four butanol isomers in neat butanol flames and ethylene/butanol flames were found to be the same.

Laser ignition of iso-octane and n-heptane jets under compression-ignition conditions

This work aims to investigate the effect of laser-induced plasma ignition (LI) on combustion behaviours of iso-octane (a gasoline surrogate) at compression-ignition (CI) conditions. A high-energy laser was used to force the fuel ignition at a quiescent-steady environment inside an optically accessible constant-volume combustion chamber with 900K ambient gas temperature, 22.8kg/m3 ambient gas density and 21 vol.% O2 concentration. A diesel surrogate (n-heptane) was tested at a lower charge temperature of 735K to offset its higher fuel reactivity than the iso-octane, such that the flames of both fuels can have a similar lift-off length. Forced laser ignition was introduced either before or after the natural autoignition timing of the fuels. The laser was focused at the jet axis 15mm and 30mm from the nozzle. High-speed schlieren imaging, heat release analysis and flame luminosity measurement were applied to the flames. The high-speed schlieren imaging was used to monitor the flame structure evolution of the natural ignition and LI cases. Due to laser ignition, the flame lift-off lengths decrease, with which the uncertainties in the lift-off distances reduce by more than 80%. The laser-affected flame bases return back to the natural flame base locations. The uncertainties in the lift-off lengths also increase, as the flame stabilisation locations approach the natural lift-off distances. Under the test conditions of this work, the rates at which the iso-octane flames shift downstream are slower than the n-heptane cases. The heat release rate profiles show high heat release from the flames following the LI events, before transitioning to lower steady values. The flame luminosity measurements indicate a strong correlation between the LI affected lift-off length and increased soot formation. The luminosity levels decrease as the flame base shifts downstream over time.

A Validation Study for RANS Based Modelling of Swirling Pulverized Fuel Flames

A swirling pulverized coal flame is computationally investigated. A Eulerian–Lagrangian formulation is used to describe the two-phase flow. Turbulence is modelled within a RANS (Reynolds averaged numerical simulation) framework. Four turbulence viscosity- (TV) based models, namely the standard k-ε model, realizable k-ε model, renormalization group theory k-ε model, and the shear stress transport k-ω model are used. In addition, a Reynolds stress transport model (RSM) is employed. The models are assessed by comparing the predicted velocity fields with the measurements of other authors. In terms of overall average values, the agreement of the predictions to the measurements is observed to be within the range 20–40%. A better performance of the RSM compared to the TV models is observed, with a nearly twice as better overall agreement to the experiments, particularly for the swirl velocity. In the second part of the investigation, the resolution of the discrete particle phase in modelling the turbulent particle dispersion (TPD) and particle size distribution (SD) is investigated. Using the discrete random walk model for the TPD, it is shown that even five random walks are sufficient for an accuracy that is quite high, with a less than 1% mean deviation from the solution obtained by thirty random walks. The approximation of the measured SD is determined by a continuous Rosin–Rammler distribution function, and inaccuracies that can occur in its subsequent discretization are demonstrated and discussed. An investigation on the resolution of the SD by discrete particle size classes (SC) indicates that 12 SC are required for an accuracy with a less than 1% mean deviation from the solution with 18 SC. Although these numbers may not necessarily be claimed to be sufficiently universal, they may serve as guidance, at least for SD with similar characteristics.

Structure and propagation speed of autoignition-assisted flames of jet fuels

The flame structure and propagation speed of autoignition-assisted fuel-lean and fuel-rich flames of jet fuels are numerically and analytically investigated at elevated pressures and temperatures. The results show that while the propagation speed increases dramatically for large residence time, for fuels with strong low temperature chemistry (LTC), the propagation speed may first increase slightly at relatively small residence time. The species evolution in time shows that with the increase of propagation speed, the trajectories for autoignition-assisted flames first rapidly and then gradually converge to the autoignition trajectory. Furthermore, transport budget analysis shows that as the propagation speed increases, the balance in the high temperature reaction zone evolves from reaction-diffusion balance to reaction-convection balance, demonstrating the transition from flame propagation to autoignition. An analytical model, which only needs to calculate the corresponding 0D autoignition process, is proposed to predict the autoignition-assisted flame speed. The good qualitative agreement with the 1D simulation result indicates that the autoignition assistance on the propagation speed mainly results from the temperature rise ahead of the preheat zone. Finally, different scaling correlations for the normalized flame propagation speed Sl/Sl0 are compared, showing a universal scaling for n-heptane, n-dodecane, and Jet-A over a wide range of pressures (5–35 atm), temperatures (600–1015 K), and equivalence ratios (0.5–1.2) with the normalized maximum mole fractions of CO.

On the Application of Electron Energy-Loss Spectroscopy for Investigating Nanostructure of Soot from Different Fuels

Soot is characterized by a multiscale structural organization; the only diagnostic tool that can give access to it is the transmission electron microscope (TEM). However, as it is a diffraction-based technique, TEM images only conjugate aromatic systems and, thus, it is particularly useful to combine it with electron energy-loss spectroscopy (EELS), which is able to provide quantitative information about the relative abundance of sp3 and sp2 hybridized carbon. In this paper, a method for the EELS spectrum analysis of carbonaceous materials, recently developed for electron-irradiated graphite and glassy carbon composition analysis, has been applied for the first time on soot samples, in order to test its performance in soot nanostructure study in combination with TEM and high-resolution TEM (HRTEM). Soot samples analyzed were collected in the soot inception region of premixed flames of different hydrocarbon fuels. EELS, in agreement with TEM and HRTEM, showed a quite disordered and heterogeneous structure for young soot, with a relatively low sp2 content and slight presence of fullerene-like structures, more evident in the case of methane soot hinting to the effect of more saturated aliphatic fuels on soot characteristics at soot inception.

Numerical Investigation of Negative Temperature Coefficient Effects on Sooting Characteristics in a Laminar Co-flow Diffusion Flame.

It is a common sense that diesel engines produce worse soot emission than gasoline engines, even though gasoline direct injection also brings about terrible sooting tendency. However, reports showed that diesel emits less soot than gasoline in laminar diffusion flames, which implies that soot emission is a combined effect of multiple factors, such as the combustion mode, physical properties of the fuel, and also fuel chemistry. This work, thus, conducted numerical calculations in laminar co-flow diffusion flames of fuels with different negative temperature coefficient (NTC) behaviors in an order of n-heptane > iso-octane > toluene to solely evaluate the chemical effect, especially the role of low-temperature combustion on soot formation. 2-Dimensional simulations were carried out to obtain the soot distributions, and 0-dimensional simulations were performed to analyze the chemical kinetics of polycyclic aromatic hydrocarbon (PAH) formation and low-temperature reaction sensitivities. The grids of the 2-D model converged at 80(r) × 196(z), and the boundary conditions of both models were set to eliminate the influence of physical factors as much as possible. The results showed that there were three main reactions associated to the formation of aromatic hydrocarbons A1 at the first-stage combustion in the n-heptane flame and the iso-octane flame, in which the reaction of C7H15 + O2 = C7H15O2 enhances the NTC behavior. The first two reaction pathways generated larger molecular hydrocarbons and were unfavorable by A1 formation and therefore inhabit the PAH formation, and 49.8% of C7H16 reacted through the large molecular pathways, while the percentage for C8H18, with weaker NTC behavior, was only 37%. Toluene with even weaker NTC behavior showed no low-temperature oxidation. Therefore, in a more general case, fuels with stronger NTC behavior smoke less, and this conclusion could be promising potential to reduce soot emission in future.