Sooting Diffusion Flames Research Articles

A numerical study on soot formation in methane-ethanol diffusion flames

• Soot volume fraction and particle number density increase with ethanol blending ratio. • The degree of soot enhancement decreases with further increase in ethanol blending ratio. • Effect of ethanol on soot concentration follows the order: chemical > dilution > thermal. • Thermal effect has a dominant influence on the flame temperature, but not on soot. • Ethanol addition promotes the HACA pathway and surface growth mechanism. Numerical simulation was carried out to investigate the effect of ethanol addition on the soot formation characteristics of methane diffusion flame. The ethanol blending ratio in methane ranged from 0 to 40 % in terms of the carbon contribution, ensuring a constant carbon flow rate for all cases. The sooting diffusion flames were modelled in an open-source environment, OpenFOAM, whereby-two different soot modelling approaches were attempted, including one detailed soot model and one semi-empirical soot model. Numerical results revealed that ethanol addition increased the soot volume fraction; however, further increase in the ethanol blending ratio (>20 %) reduced the relative enhancement of the soot concentration. Decoupling analysis was conducted to study the dilution, thermal, and chemical effects of ethanol separately. It was found that the dilution effect increased with the ethanol blending ratio. Also, the flame temperature was primarily affected by the thermal effect of ethanol. Despite these factors, the chemical effect of ethanol still played a dominant role in increasing soot formation. Analysis of the soot formation pathway revealed that ethanol addition promoted soot formation mainly through the acetylene addition reactions and polycyclic aromatic hydrocarbons (PAHs) addition reactions, which contributed to the hydrogen-abstraction-acetylene-addition (HACA) and surface growth mechanisms, respectively. Soot oxidation was enhanced with ethanol addition, which resulted in greater soot reduction; however, the overall effect of ethanol on soot formation remained positive.

Large Eddy Simulation of Lean Mixed-Mode Combustion Assisted by Partial Fuel Stratification in a Spark-Ignition Engine

Abstract Partial fuel stratification (PFS) is a promising fuel injection strategy to improve the stability of lean combustion by applying a small amount of pilot injection right before spark timing. Mixed-mode combustion, which makes use of end-gas autoignition following conventional deflagration-based combustion, can be further utilized to speed up the overall combustion. In this study, PFS-assisted mixed-mode combustion in a lean-burn direct injection spark-ignition (DISI) engine is numerically investigated using multi-cycle large eddy simulation (LES). A previously developed hybrid G-equation/well-stirred reactor combustion model for the well-mixed operation is extended to the PFS-assisted operation. The experimental spray morphology is employed to derive spray model parameters for the pilot injection. The LES-based model is validated against experimental data and is further compared with the Reynolds-averaged Navier–Stokes (RANS)-based model. Overall, both RANS and LES predict the mean pressure and heat release rate traces well, while LES outperforms RANS in capturing the cycle-to-cycle variation (CCV) and the combustion phasing in the mass burned space. Liquid and vapor penetrations obtained from the simulations agree reasonably well with the experiment. Detailed flame structures predicted from the simulations reveal the transition from a sooting diffusion flame to a lean premixed flame, which is consistent with experimental findings. LES captures more wrinkled and stretched flames than RANS. Finally, the LES model is employed to investigate the impacts of fuel properties, including heat of vaporization (HoV) and laminar burning speed (SL). Combustion phasing is found more sensitive to SL than to HoV, with a larger fuel property sensitivity of the heat release rate from autoignition than that from deflagration. Moreover, the combustion phasing in the PFS-assisted operation is shown to be less sensitive to SL compared with the well-mixed operation.

Femtosecond, two-photon, planar laser-induced fluorescence of carbon monoxide in flames.

Two-photon, planar laser-induced fluorescence (TP-PLIF) of carbon monoxide was performed in steady and driven flames using femtosecond (fs) laser pulses at 1kHz. Excitation radiation at 230.1nm (full-width at half-maximum bandwidth of 270 cm-1) was used to pump many rovibrational two-photon transitions in the B1∑+←X1∑+ system. Visible fluorescence in the range 362-516nm was captured using an image intensifier and high-speed camera. The signal dependence on excitation energy and wavelength is presented. Photolytic interferences from the ultraviolet laser were explored in a sooting diffusion flame. Using an excitation laser intensity of 1010 W/cm2, negligible photolytic interferences were observed, and PLIF imaging of dynamic flame events was performed at 1 kHz.

A paradigm shift in the interaction of experiments and computations in combustion research

A different approach to comparing experimental data and numerical simulation data is presented. Traditionally, when making comparisons with simulations, experimentalists have sought to measure the same fundamental quantities (e.g., mole fractions) that are output by numerical simulations. This approach often requires measurement of many variables to arrive at the desired quantity, and uncertainty may accumulate with each additional measurement. Because recent advances in computational resources have led to more detailed numerical models, more complete information is available within simulations. This allows for the possibility of using simulation results to derive predictions of measured signals (i.e., “computed signals”) rather than measuring many quantities to derive a single fundamental quantity. Three examples of comparing measured and computed signals are presented: NO laser-induced fluorescence (LIF) images in both non-sooting and sooting diffusion flames, and luminosity images of sooting diffusion flames. For illustration, the non-sooting LIF data is treated both by the traditional method of comparing fundamental quantities and by comparing measured and computed signals. In each example, the comparison of measured and computed signals yields quantitative information similar to that obtainable through comparison of traditional quantities, along with reduced noise in the experimental data.

Soot precursor measurements in benzene and hexane diffusion flames

To clarify the mechanism of soot formation in diffusion flames of liquid fuels, measurements of soot and its precursors were carried out. Sooting diffusion flames formed by a small pool combustion equipment system were used for this purpose. Benzene and hexane were used as typical aromatic and paraffin fuels. A laser-induced fluorescence (LIF) method was used to obtain spatial distributions of polycyclic aromatic hydrocarbons (PAHs), which are considered as soot particles. Spatial distributions of soot in test flames were measured by a laser-induced incandescence (LII) method. Soot diameter was estimated from the temporal change of LII intensity. A region of transition from PAHs to soot was defined from the results of LIF and LII. Flame temperatures, PAH species, and soot diameters in this transition region were investigated for both benzene and hexane flames. The results show that though the flame structures of benzene and hexane were different, the temperature in the PAHs–soot transition region of the benzene flame was similar to that of the hexane flame. Furthermore, the relationship between the PAH concentrations measured by gas chromatography in both flames and the PAH distributions obtained from LIF are discussed. It was found that PAHs with smaller molecular mass, such as benzene and toluene, remained in both the PAHs–soot transition and sooting regions, and it is thought that molecules heavier than pyrene are the leading candidates for soot precursor formation.

EM1-1: Behavior of PAHs and PM in a Diffusion Flame of Diesel Fuel(EM: Exhaust Emission Control and Measurement,General Session Papers)

To make clear the behavior of particulate matter (PM) and its precursor in a diffusion flame of diesel fuel, measurements of polycyclic aromatic hydrocarbons (PAHs) and PM were carried out. A sooting diffusion flame formed by a small pool combustion equipment were used for this purpose. Spatial distributions of PAHs in the diffusion flame were visualized by a laser induced fluorescence (LIF) method. PAHs concentrations were measured quantitatively using a gas chromatography (GC). Spatial distributions of PM in the test flame were also measured by a laser induced incandescence (LII) method. Average size of dry soot particles in PM was estimated from temporal changes of LII intensity. Moreover, PM size of the test flame was measured by a scanning mobility particle sizer (SMPS). Dry soot, soluble organic fraction (SOF) and sulfate components in PM were also measured by a PM analyzer. From these results, lighter PAHs such as benzene and toluene were changed gradually to heavier PAHs such as pyrene at the upstream part in flame. Lighter PAH species which did not change to heavier PAH species were remained in the sooting region and was released from the flame tip into surroundings.

Experimental Analysis of Soot Formation in Sooting Diffusion Flame by Using Laser-Induced Emissions

Two-dimensional images of OH fluorescence, polycyclic aromatic hydrocarbons (PAHs) fluorescence, and laser-induced incandescence (LII) from soot were measured in a sooting diffusion flame. To obtain an accurate OH fluorescence image, two images were taken with the laser wavelength tuned to (“on”) and away from (“off”) the OH absorption line. An accurate OH fluorescence image was obtained by subtracting the off-resonance image from the on-resonance image. For the PAH fluorescence and LII measurements, temporally resolved measurements were used to obtain the individual images; the LII image was obtained by detecting the LII signal after the PAH fluorescence radiation had stopped and the PAH fluorescence image was obtained by subtracting the LII image from the simultaneous image of PAH fluorescence and LII. Based on the obtained images, the relative location of OH, PAH, and soot in the flame was discussed in detail. To investigate the PAH size distribution in a sooting flame using LIF, an estimation strategy for PAH size is proposed. Emission spectra were measured at several heights in the flame using a spectrograph. Since the emission wavelength of PAH fluorescence shifts toward longer wavelengths with increasing PAH size, the main PAH components in the emission spectra could be estimated. The results suggest that PAH grows and the type of PAH changes as the soot inception region was approached. Near the soot inception region, we estimated that the PAHs, which have over 16 carbon atoms, mainly constituted the emission spectrum.

Measurements of absolute concentration profiles of C 2 in non-sooting and sooting diffusion flames by coupling cavity ring-down spectroscopy and laser induced fluorescence

Coupling between cavity ring down spectroscopy (CRDS) and laser induced fluorescence (LIF) methods has been implemented to measure nascent C 2 (a 3Π u) in both non-sooting and sooting flames stabilised on a Wolfhard-Parker burner at atmospheric pressure. In the non-sooting flame, relative concentration profiles have been recorded by LIF measurements and calibrated by CRDS in absolute concentration (maximum of 3.4 × 10 −9 in mole fraction). In the sooting flame, it is shown that LIF is unusable to measure nascent C 2 in the sooting regions due to the creation of additional C 2 from laser soot vaporisation, even by using extremely low laser energies. It is besides demonstrated that, although CRDS measurements were also affected by PAH/soot interference, relative profile determination and absolute quantification could be satisfactorily obtained. Complete C 2 profiles in the sooting flame are thus obtained by combining LIF and CRDS. Hence, LIF can be advantageously used to measure relative profiles of C 2 in the vicinity of the burner (non-sooting zone) and CRDS can then provide determination of C 2 profiles in the presence of soot. Absolute concentration has been determined by CRDS, providing a peak mole fraction of 6.8 × 10 −9. Based on a comparison with PAH and soot profiles issued from a previous work, the role of C 2 in soot formation mechanism is discussed. A comparison with earlier profiles of CH is also proposed, showing a good correlation between C 2 and CH radical profiles.

Two-color laser-induced incandescence and cavity ring-down spectroscopy for sensitive and quantitative imaging of soot and PAHs in flames

Laser-induced incandescence is a technique which enables the measurement of soot volume fractions. However, the laser-induced soot emission might be affected by a fluorescence background generally ascribed to the polycyclic aromatic hydrocarbon compounds (PAHs) present at the soot location. In this paper, spatially resolved distributions of PAH absorbance and soot are obtained in sooting diffusion flames. The original method developed here consists in comparing the emission distributions induced by two different laser wavelengths: (1) at 1064 nm emission signals are exempt from PAH fluorescence and (2) at 532 nm both soot incandescence and PAH emission contribute to the total signal. In addition, the absolute absorption coefficient of the PAH mixture is determined by comparing absorption measurements obtained by cavity ring-down spectroscopy (CRDS) at 1064 nm and 532 nm. The proposed method can provide highly sensitive 2D imaging of PAHs and soot using the fundamental and the second-harmonic frequencies of a single YAG laser. Finally, 2D distributions of PAH absorbance and soot volume fraction calibrated by CRDS are obtained in two diffusion flames, particularly in a very low-sooting flame exhibiting a maximum PAH absorbance of 6×10-4 cm-1 and a maximum soot volume fraction of 3 ppb only. The respective spatial distributions of PAHs and soot are shown to vary with the initial C/O ratio.

Rotational Temperature Measurement of NO in a Sooting Diffusion Flame through Laser-Induced Fluorescence Spectrum

LIF thermometry in a sooting diffusion flame was newly proposed based on a spectral matching method. A tunable ArF excimer laser was used to excite the several absorption lines in the D2Σ+←X2II (0, 1) band system of NO. In the case of sooting diffusion flame, emission spectrum consisted of NO and O2 fluorescence, and off-resonance emissions related to the soot. The O2 fluorescence and the off-resonance emissions overlapped on the NO fluorescence. However, the off-resonance emissions near the vacuum ultraviolet region (VUV) were weaker than the NO fluorescence, and thee (0, 3) band of the NO fluorescence, which appeared near the VUV region, was free from the O2 fluorescence. Therefore, NO rotational temperature in a sooting diffusion flame could be measured by the spectral matching method using the e (0, 3) band. By using this technique, NO rotational temperature in a propane diffusion flame could be measured.

Quantitative measurements of the CH radical in sooting diffusion flames at atmospheric pressure

The potential of Laser Induced Fluorescence detection of the CH radical using C–X (0–0) excitation is investigated in a sooting methane/air diffusion flame at atmospheric pressure. Fluorescence is detected using the very narrow (<0.4 nm) Q-branch of the C–X (0–0) band, which enables the measurement of CH in sooting flames without interference from PAH fluorescence and soot emissions. Absolute concentrations are obtained using Cavity Ring Down Spectroscopy. 1D CH profiles in the sooting zone are recorded using a CCD camera with an excellent signal-to-noise ratio. The C–X (0–0) excitation associated with Q-branch detection is shown to be three times more efficient than the B–X scheme.

Laser-induced incandescence and Raman measurements in sooting methane and ethylene flames

Laser-induced incandescence (LII) of soot particles is modeled using two non-linear coupled differential equations deduced from the energy- and mass-balance of the process. Experimental data of laminar sooting diffusion flames are taken and the model function is fitted to the individual data curves of every pixel in the two-dimensional flame image. The results of the least squares fit are maps of the soot particle radii and number densities in the flame. Methane and ethylene flames are examined and the results are compared with former measurements. The quality of this fitting algorithm is compared to the reproduction of the data curves by a conventional data evaluation method. The fitting routine for the methane flame is improved by providing local temperature data obtained by Raman measurements. Results for this flame are compared with the local gas concentrations.

Characterization of Soot Particles Generated in Non-sooting and Sooting Normal Diffusion Flames

Characteristics of carbon soot particles generated in diffusion flames were studied. Non-sooting and sooting normal diffusion flames using propane or ethylene as fuel were selected. In the flames, soot volume fraction was measured by a thermocouple, and primary particle diameter and cluster size were analyzed by TEM photographs. The characteristics of soot particles depended on flame(non-sooting or sooting) and fuel(propane or ethylene) type. Unlike the sooting diffusion flames, particle growth and oxidation processes were clearly observed in the non-sooting diffusion flames. In the sooting diffusion flames, soot particle size was slightly changed at the flame tip.

Behavior of Sooting Diffusion Flames under Electric Flelds with Ionic Winds.

This paper reports the effects of applying methods of electric field on a sooting diffusion flame. Essentially, two types of electric fields were tested in this experiment. One is a uniform electric field formed between parallel plate electrodes. The other is a non-uniform field formed between needle and plate electrodes. In the uniform field, the flame shape was changed by positive ions moving in the fields. Soot emission was suppressed with increasing the applied voltage, because an increase of the flame surface temperature promoted the soot oxidation. In the non-uniform field, on the contrary, ionic wind was formed by corona discharge. The ionic wind blew from a needle electrode toward a plate electrode, irrespectively of the polarity of the needle. The flame was inclined toward the plate electrode by this ionic wind. As the result, the surrounding air was entrained in the flame, and the suface temperature of flame also increased with increasing applied voltage. The soot suppression efficiency of the non-uniform electric field was higher than that of the uniform one.

Mapping of Soot Particles in a Weakly Sooting Diffusion Flame by Aerosol Techniques

The evolution of detailed particle size distributions has been measured along the centerline of an axisymmetric diffusion flame of CH 4 + Ar burning in air at 1 atm. Soot particles with mean diameters of 3–18 nm were observed. Changes in the size distribution exhibited zones where either nucleation, coagulation, or destruction of soot particles dominated. These highly sensitive measurements were made by microprobe sampling with an immediate dilution of 1:400, to quench the aerosol, and by subsequent application of aerosol measurement techniques. In parallel, the yield of photoemitted electrons from size-selected particles was determined. The yield shows a characteristic dependence on location in the flame, indicating changes of the particle’s surface. Multiphoton, time-of-flight mass spectrometry was used to investigate the correlation between polycyclic aromatic hydrocarbons in the flame and enhanced photoemission yield from the soot particles.

Soot volume fraction maps for normal and reduced gravity laminar acetylene jet diffusion flames

The study of soot particulate distribution inside gas jet diffusion flames is important to the understanding of fundamental soot particle and thermal radiative transport processes, as well as providing findings relevant to spacecraft fire safety, soot emissions, and radiant heat loads for combustors used in air-breathing propulsion systems. Compared to those under normal gravity (1-g) conditions, the elimination of buoyancy-induced flows is expected to significantly change the flow field in microgravity (O g) flames, resulting in taller and wider flames with longer particle residence times. Work by Bahadori and Edelman demonstrate many previously unreported qualitative and semi-quantitative results, including flame shape and radiation, for sooting laminar zas jet diffusion flames. Work by Ku et al. report soot aggregate size and morphology analyses and data and model predictions of soot volume fraction maps for various gas jet diffusion flames. In this study, we present the first 1-g and 0-g comparisons of soot volume fraction maps for laminar acetylene and nitrogen-diluted acetylene jet diffusion flames. Volume fraction is one of the most useful properties in the study of sooting diffusion flames. The amount of radiation heat transfer depends directly on the volume fraction and this parameter can be measured from line-of-sight extinction measurements. Although most Soot aggregates are submicron in size, the primary particles (20 to 50 nm in diameter) are in the Rayleigh limit, so the extinction absorption) cross section of aggregates can be accurately approximated by the Rayleigh solution as a function of incident wavelength, particles' complex refractive index, and particles' volume fraction.

Scattering Tomography and Its Application to Sooting Diffusion Flames

A new analytical tomographic reconstruction technique was developed for the determination of the extinction and scattering coefficient distributions in axisymmetric media. This method, called “scattering tomography,” was tested for several particle concentration profiles corresponding to those for diffusion flames. After that, a series of experiments were performed on sooting acetylene flames using an argon-ion laser nephelometer. The experimental results were reduced using both the transmission and scattering tomography techniques to obtain the extinction coefficient profiles. It was shown that in the center of the flame, the results from these two approaches were in good agreement. Scattering tomography can be used to determine both the absorption and scattering coefficient distributions in the medium. In addition to that, it is preferable over the transmission tomography if the medium is optically very thin and particles are predominantly scattering.

The influences of electric fields on soot formation and flame structure of diffusion flames

The influences of DC and AC electric fields, at frequencies up to 1.48 MHz and the maximum strength of about 6 kV/cm, on soot formation and flame structure were investigated using a counterflow type acetylene diffusion flame. The distributions of flame luminosity, soot volume fraction, flame temperature and OH concentration in flame were measured by non-invasive detection methods. Under the influence of electric fields, the changes in distribution of the soot volume fraction were confirmed. Electric fields of high frequency and high intensity reduced the soot volume fraction, whereas other electric fields increased it. The maximum values of flame temperature and OH concentration decreased. In the relationship between the maximum value of the soot volume fraction and the maximum temperature, the maximum soot volume fraction showed both increase and decrease with maximum temperatures depending on the frequencies and intensities of the electric fields, and both of them occurred at temperatures lower than 1900 K. The production of the incipient particles seemed to be the dominant process controlling the soot volume fraction due to the electric fields. The luminosity of a sooting diffusion flame was found to depend on the volume fraction and temperature of the soot particles in the flame. As for the behavior of the flame in the electric fields, the ionic wind effect was not found to be dominant in the present work, and the result of the previous simulation based on the ionic wind theory was not consistent with the present experimental results.

Computation of the radiative power loss in a sooting diffusion flame

Computation of the radiative power loss in a sooting diffusion flame

Coherent anti-stokes raman spectroscopic measurements of temperature and acetylene spectra in a sooting diffusion flame: Colloquium on combustion diagnostics

We report coherent anti-Stokes Raman spectroscopy (CARS) measurements of nitrogen and acetylene Q-branch spectra in a sooting laminar ethylene/air diffusion flame, previously characterized by thermocouple and laser scattering/extinction measurements. Good agreement between CARS and thermocouple temperature measurements was obtained throughout the flame. Effects of the presence of heavy soot-loading on the CARS measurements were minimized by reducing the acceptance angle and spectral bandwidth of the CARS detection system. We also present the first known CARS measurements of flame-generated acetylene. The acetylene spectra are in good agreement with preliminary calculated spectra.