Soot studies of laminar diffusion flames with recirculation zones

This paper describes the unusual sooting structure of three flames established by the laminar recirculation zones of a centerbody burner. The vertically mounted burner consists of an annular air jet and a central fuel jet separated by a bluff-body. The three ethylene fueled flames are identified as: fully sooting, donut-shape, and ring-shape sooting flames. Different shapes of the soot structures are obtained by varying the N₂ dilution in the fuel and air jets while maintaining a constant air and fuel velocity of 1.2 m/s. All three flames have the unusual characteristic that the soot, entrained into the recirculation zone, follows discrete spiral trajectories that terminate at the center of the vortex. The questions are what cause: (1) the unusual sooting structures and (2) the spiral trajectories of the soot? Flame photographs, laser sheet visualizations, and calculations with a 2D CFD-based code (UNICORN) are used to answer these questions. The different sooting structures are related to the spiral transport of the soot, the spatial location of the stoichiometric flame surface with respect to the vortex center, and the burnout of the soot particles. Computations indicate that the spiral trajectories of the soot particles are due to thermophoresis.     

Measurement of nitric oxide concentrations in flames by using electronic-resonance-enhanced coherent anti-Stokes Raman scattering

We have measured nitric oxide (NO) concentrations in flames by using electronic-resonance-enhanced coherent anti-Stokes Raman spectroscopy (ERE-CARS). Visible pump and Stokes beams were tuned to a Q-branch vibrational Raman resonance of NO. A UV probe beam was tuned into resonance with specific rotational transitions in the (v"=1,v'=0) vibrational band in the A(2)Sigma(+)-X(2)Pi electronic transition, thus providing a substantial electronic-resonance enhancement of the resulting CARS signal. NO concentrations were measured at levels down to 50 parts in 10(6) in H(2)/air flames at atmospheric pressure. NO was also detected in heavily sooting C(2)H(2)/air flames at atmospheric pressure with minimal background interference.     

Addition of NO2 to a laminar premixed ethylene-air flame: Effect on soot formation

Experiments were conducted on a laminar premixed ethylene-air flame at equivalence ratios of 2.34 and 2.64. Comparisons were made between flames with 5% NO₂ added by volume. Soot volume fraction was measured using light extinction and light scattering and fluorescence measurements were also obtained to provide added insight into the soot formation process. The flame temperature profiles in these flames were measured using a spectral line reversal technique in the non-sooting region, while two-color pyrometry was used in the sooting region. Chemical kinetics modeling using the PREMIX 1-D laminar flame code was used to understand the chemical role of the NO₂ in the soot formation process. The modeling used kinetic mechanisms available in the literature. Experimental results indicated a reduction in the soot volume fraction in the flame with NO₂ added and a delay in the onset of soot as a function of height above the burner. In addition, fluorescence signals—often argued to be an indicator of PAH—were observed to be lower near the burner surface for the flames with NO₂ added as compared to the baseline flames. These trends were captured using a chemical kinetics model that was used to simulate the flame prior to soot inception. The reduction in soot is attributed to a decrease in the H-atom concentration induced by the reaction with NO₂ and a subsequent reduction in acetylene in the pre-soot inception region.     

Experimental low-stretch gaseous diffusion flames in buoyancy-induced flowfields

The present study experimentally investigates the structure and instabilities associated with extremely low-stretch (1 s⁻¹) gaseous diffusion flames. Ultra-low-stretch flames are established in normal gravity by bottom burning of a methane/nitrogen mixture discharged from a porous spherically symmetric burner of large radius of curvature. OH-PLIF and IR imaging techniques are used to characterize the reaction zone and the burner surface temperature, respectively. A flame stability diagram mapping the response of the ultra-low-stretch diffusion flame to varying fuel injection rate and nitrogen dilution is explored. In this diagram, two main boundaries are identified. These boundaries separate the stability diagram into three regions: sooting flame, non-sooting flame, and extinction. Two distinct extinction mechanisms are noted. For low fuel injection rates, flame extinction is caused by heat loss to the burner surface. For relatively high injection rates, at which the heat loss to burner surface is negligible, flame radiative heat loss is the dominant extinction mechanism. There also exists a critical inert dilution level beyond which the flame cannot be sustained. The existence of multi-dimensional flame phenomena near the extinction limits is also identified. Various multi-dimensional flame patterns are observed, and their evolutions are studied using direct chemiluminescence and OH-PLIF imaging. The results demonstrate the usefulness of the present burner configuration for the study of low-stretch gaseous diffusion flames.     

Nitrous oxide conversion in laminar premixed flames of CH4 + O2 + Ar

Experimental measurements of the adiabatic burning velocity in neat and NO formation in CH₄ + O₂ + Ar flames doped with small amounts of N₂O are presented. The oxygen content in the oxidizer was varied from 15 to 17%. Non-stretched flames were stabilized on a perforated plate burner at 1 atm. The Heat Flux method was used to determine burning velocities under conditions when the net heat loss of the flame is zero. Adiabatic burning velocities of methane + oxygen + argon mixtures were found in satisfactory agreement with the modeling. The NO concentrations in the flames doped with N₂O (100 ppm in the argon stream before mixing) were measured in the burnt gases at a fixed distance from the burner using probe sampling. Axial profiles of [NO] were found insensitive to the downstream heat losses. Experimental dependencies of [NO] versus equivalence ratio had a maximum between φ = 1.1 and 1.2. Calculated concentrations of NO were in good agreement with the measurements. In lean flames calculated concentrations of NO strongly depends on the rate constant of reaction N₂O + O=NO + NO if too high values proposed in the literature are employed. These new experimental data thus allowed for validation of the key reactions of the nitrous oxide mechanism of NO formation in flames.     

Laser diagnostics of NO reburning in fuel-rich propene flames

Absolute NO concentrations were measured by laser-induced fluorescence (LIF) in three different fuel-rich non-sooting propene flames (φ=1.5, 1.8 and 2.3). The experiments were performed in low-pressure premixed propene flames with 0.2%-1% NO added. Laser diagnostics was applied as a tool for investigating reburn chemistry. The Q₂(25.5) line in the A-X(0,0) band was excited because of the small temperature dependence of its ground state population. The NO fluorescence lifetimes were measured directly and compared to theoretical values. The initial NO levels are strongly reduced in all three flames. According to modeling results, the HCN mole fraction increases strongly with stoichiometry. As guidelines for laser diagnostics applications in such systems, the modeling results were analyzed with respect to the main reaction channels and reaction partners in fuel-rich flames. Received: 1 March 2000 / Revised version: 20 April 2000 / Published online: 20 September 2000     

A structural study of premixed hydrogen-air cellular tubular flames

Two dimensionally spatially resolved structural measurements are reported for cellular phenomena in lean laminar premixed hydrogen-air tubular flames. Laser-induced Raman scattering and chemiluminescence imaging are combined to investigate low Lewis number lean hydrogen-air flames. The strong effect of thermal-diffusive imbalance is observed in radial profiles interpolated through the centers of reaction and extinction zones. In the flame cell, the equivalence ratio is ～80% higher than the inlet mixture, resulting in a peak flame temperature of 1600 K that is 550 K above the adiabatic flame temperature of the inlet mixture (1055 K). In the adjacent extinction zone, the temperatures are ～900 K lower than the peak flame temperature and the equivalence ratio is similar to the inlet mixture. Despite doubling the global stretch rate from 200 s^?1 to 400 s^?1, the enhancement of local equivalence ratio and peak temperature in the flame cell remain similar. This enhancement seems dependent on the local cellular flame curvature, that is similar between both cases. With strong preferential diffusion effects, cellular flames offer unique validation data to improve the accuracy of current molecular transport modeling techniques.     

Tunable KrF laser-induced fluorescence of C2 in a sooting flame

2 in a flame, excited by a tunable KrF laser near 248 nm. The first comprises several P and R lines of the (1,0) band of the e ³Π_g-a ³Π_u Fox–Herzberg system, with fluorescence bands extending past 350 nm. The second is the band head region of the (7,1) band of the D ¹Σ_u ⁺←B^′ 1Σ_g ⁺ system, with fluorescence at 232 nm from D to the X ¹Σ_g ⁺ ground state. Neither band has been previously observed in any environment. The flame in these experiments is highly sooting, and the C₂ seen here is likely produced by laser vaporization of the soot with subsequent laser photolysis of a C₂ precursor. In a rich flame, this fluorescence could cause interferences in other studies such as KrF laser Raman scattering. Moreover, signal level calculations suggest native C₂ near 10 ppm could be readily observed using the Fox–Herzberg excitation. Raman measurements of major species (X≥0.01) in the same flame, using the KrF laser, are in good agreement with a model prediction. Received: 2 April 1998/Revised version: 8 June 1998     

Computational and experimental study of oxygen-enhanced axisymmetric laminar methane flames

Three axisymmetric laminar coflow diffusion flames, one of which is a nitrogen-diluted methane/air flame (the ‘base case’) and the other two of which consist of nitrogen-diluted methane vs. pure oxygen, are examined both computationally and experimentally. Computationally, the local rectangular refinement method is used to solve the fully coupled nonlinear conservation equations on solution-adaptive grids. The model includes C2 chemistry (GRI 2.11 and GRI 3.0 chemical mechanisms), detailed transport, and optically thin radiation. Because two of the flames are attached to the burner, thermal boundary conditions at the burner surface are constructed from smoothed functional fits to temperature measurements. Experimentally, Raman scattering is used to measure temperature and major species concentrations as functions of the radial coordinate at various axial positions. As compared to the base case flame, which is lifted, the two oxygen-enhanced flames are shorter, hotter, and attached to the burner. Computational and experimental flame lengths show excellent agreement, as do the maximum centreline temperatures. For each flame, radial profiles of temperature and major species also show excellent agreement between computations and experiments, when plotted at fixed values of a dimensionless axial coordinate. Computational results indicate peak NO levels in the oxygen-enhanced flames to be very high. The majority of the NO in these flames is shown to be produced via the thermal route, whereas prompt NO dominates for the base case flame.     

Critical temperature and reactant mass flux for radiative extinction of ethylene microgravity spherical diffusion flames at 1 bar

In microgravity combustion, where buoyancy is not present to accelerate the flow field and strain the flame, radiative extinction is of fundamental importance, and has implications for spacecraft fire safety. In this work, the critical point for radiative extinction is identified for normal and inverse ethylene spherical diffusion flames via atmospheric pressure experiments conducted aboard the International Space Station, as well as with a transient numerical model. The fuel is ethylene with nitrogen diluent, and the oxidizer is an oxygen/nitrogen mixture. The burner is a porous stainless-steel sphere. All experiments are conducted at constant reactant flow rate. For normal flames, the ambient oxygen mole fraction was varied from 0.2 to 0.38, burner supply fuel mole fraction from 0.13 to 1, total mass flow rate, ṁ_total, from 0.6 to 12.2 mg/s, and adiabatic flame temperature, T_ad, from 2000 to 2800 K. For inverse flames, the ambient fuel mole fraction was varied from 0.08 to 0.12, burner supply oxygen mole fraction from 0.4 to 0.85, ṁ_total from 2.3 to 11.3 mg/s, and T_ad from 2080 to 2590 K. Despite this broad range of conditions, all flames extinguish at a critical extinction temperature of 1130 K, and a fuel-based mass flux of 0.2 g/m²-s for normal flames, and an oxygen-based mass flux of 0.68 g/m²-s for inverse flames. With this information, a simple equation is developed to estimate the flame size (i.e., location of peak temperature) at extinction for any atmospheric-pressure ethylene spherical diffusion flame given only the reactant mass flow rate. Flame growth, which ultimately leads to radiative extinction if the critical extinction point is reached, is attributed to the natural development of the diffusion-limited system as it approaches steady state and the reduction in the transport properties as the flame temperature drops due to increasing flame radiation with time (radiation-induced growth.)     

CARS spectral fitting with multiple resonant species using sparse libraries

The coherent anti‐Stokes Raman spectroscopy (CARS) technique is often used in the study of turbulent flames. Fast and accurate algorithms are needed for fitting CARS spectra for temperature and multiple chemical species. This paper describes the development of such an algorithm. The algorithm employs sparse libraries whose size grows more slowly with number of species than a regular library. It was demonstrated by fitting synthetic ‘experimental’ dual‐pump CARS spectra containing four resonant species (N₂, O₂, H₂ and CO₂), both with added noise and without it, and by fitting experimental spectra from a H₂ air flat flame produced by a Hencken burner. In the four‐species example, the library was nearly an order of magnitude smaller than the equivalent regular library (fitting times are correspondingly faster), and the fitting errors in the absence of added noise were negligible compared to the random errors associated with fitting noisy spectra. When fitting noisy spectra, weighted least squares fitting to signal intensity, as opposed to least squares fitting or least squares fitting to square root of intensity, minimized random and bias errors in fit parameters. Copyright © 2011 John Wiley & Sons, Ltd.     

UV raman spectroscopy of H2-air flames excited with a narrowband KrF laser

Raman spectra of H₂ and H₂O in flames excited by a narrowband KrF excimer laser are reported. Observations are made over a porous-plug, flat-flame burner reacting H₂ in air, fuel-rich with nitrogen dilution to control the temperature, and with a H₂ diffusion flame. Measurements made from UV Raman spectra show good agreement with measurements made by other means, both for gas temperature and relative major species concentrations. Laser-induced fluorescence interferences arising from OH and O₂ are observed in emission near the Raman spectra. These interferences do not preclude Raman measurements, however.     

Simultaneous Raman/LIF measurements of major species and NO in turbulent H2/air diffusion flames

A single-pulse spontaneous Raman scattering apparatus, based on a flashlamp-pumped dye laser, was used to determine the concentrations of the major species and the temperature in turbulent H₂/N₂/air jet diffusion flames. The concentrations of nitric oxide were simultaneously measured by Laser-Induced Fluorescence (LIF) after excitation of theA ^{2 +}–X ² transition with a Nd: YAG-pumped dye laser. Some fundamentals of the employed methods, including the calibration procedure, quenching corrections, and accuracy are discussed. Besides a detailed study of the experimental technique, a main goal of the presented investigations was the generation of comprehensive data sets of high accuracy from well-defined turbulent flames which allow for a quantitative comparison with model calculations. Two flames with different fuel dilution and Reynolds numbers were investigated in a pattern of typically 100 measuring locations each comprising 300 single shots. In addition, four flames with different flow velocities but same fuel composition were compared with respect to their temperature and NO concentration profiles. The results show that differential diffusion plays an important role in these flames, especially near the flame base, where the temperature is increased above the adiabatic flame temperature and deviations from adiabatic equilibrium are large. The correlations between NO and mixture fraction and NO and temperature reveal characteristic features of the different flames.     

Simultaneously calibrated two-line atomic fluorescence for high-precision temperature imaging in sooting flames

Simultaneously calibrated, non-linear two-line atomic fluorescence (SC-nTLAF) thermometry for application in turbulent sooting flames has been developed to increase the precision of single-shot, planar measurements of gas temperature. The technique has been demonstrated in both steady and turbulent sooting flames, showing good agreements with previous optical measurements. The SC-nTLAF involves imaging simultaneously laser-induced fluorescence (LIF) of atomic indium in both the target flame and a non-sooting calibration flame for which the temperature distribution is known. The LIF intensities from the reference flame enable correction for fluctuations, not only in the laser power, but also in the laser mode. The resulting precision was found to be ±67 K and ±75 K (based on one standard deviation) in the rich and oxidizing regions of a steady sooting flame for which the measured temperature was 1610 K and 1854 K, respectively, with a spatial resolution of 550 × 550 µm². This corresponds to a relative precision of ∼ 4.1%. The resulting precision in the single-shot temperature images for a well-characterized, lifted ethylene jet diffusion flame (fuel jet Reynolds number = 10,000) compares favorably with previously reported data obtained with shifted-vibrational coherent anti-Stokes Raman spectroscopy (CARS), together with increased spatial resolution. The planar imaging also provides more details of the temperature distribution, particularly in the flame brush region, which offers potential for measurement of more parameters, such as gradients and spatial corrections. The new calibration method has also achieved a significant time-saving in both data collection and processing, which is an estimated total of ∼ 60%–70% compared with conventional nTLAF.     

Computational and experimental investigation of the interaction of soot and NO in coflow diffusion flames

A combined computational and experimental investigation that examines the relationship of soot formation and NO in coflow ethylene air diffusion flames is presented. While both NO and soot formation are often studied independently, there is a need to understand their coupled relationship as a function of system parameters such as fuel type, temperature and pressure. The temperature decrease due to radiative losses in systems in which significant soot is produced can affect flame length and other temperature-dependent processes such as the formation of NO. The results of a computational model that includes a sectional representation for soot formation with a radiation model are compared against laser-induced fluorescence measurements of NO. The sooting characteristics of these flames have been studied previously. Experimentally, a laser near 225.8 nm is used to excite the γ(0, 0) band in NO. Spectrally resolved fluorescence emission is imaged radially, for the (0, 0), (0, 1), (0, 2), (0, 3), and (0, 4) vibrational bands, at varying axial heights to create a two-dimensional image of NO fluorescence. A reverse quenching correction is applied to the computational results to determine an expected fluorescence signal for comparison with experimental results. Modeling results confirm that Fenimore NO is the dominant mechanism for NO production and suggest that for lightly sooting flames (peak soot volume fraction < 0.5 ppm), soot reduces only the Zeldovich NO formation (by a factor of two). For flames with increased soot levels (peak soot volume fraction ∼ 4 ppm), the model indicates not only that Zeldovich NO decreases by a factor of 2.5 through radiation loss, but that non-Zeldovich NO is reduced in the top center of the flame by about 30% through the oxidation of soot.     

The effects of burner stabilization on Fenimore NO formation in low-pressure, fuel-rich premixed CH4/O2/N2 flames

We investigate the effects of varying the degree of burner stabilization on Fenimore NO formation in fuel-rich low-pressure flat CH₄/O₂/N₂ flames. Towards this end, axial profiles of flame temperature and OH, NO and CH mole fractions are measured using laser-induced fluorescence (LIF). The experiments are performed at equivalence ratios between 1.3 and 1.5. The flame temperature is seen to decrease by 200-300 K, with a concomitant decrease in OH mole fraction, upon reducing the total flow rate from 5 to 3 L/min, thus increasing stabilization. At equivalence ratios between 1.3 and 1.5, this decrease in flow rate lowers the maximum CH mole fraction by a factor of 2, and the NO mole fraction by ∼40% in all flames studied. Integrating the reaction rate for CH + N₂ to estimate Fenimore NO formation, using the rate coefficient in GRI-Mech 3.0, and the measured temperatures and CH profiles show very good agreement with the measured NO mole fraction for ? = 1.3 and 1.4, supporting the current choice for this rate. This agreement also shows that the increase in residence time caused by increased stabilization is an important factor in the ultimate impact of the changes in CH mole fraction on NO formation. The results at ? = 1.5 suggest that substantial quantities of fixed nitrogen species, e.g., HCN, are only slowly oxidized in the post-flame zone under these conditions, leading to a significant discrepancy between the measured NO mole fraction and that obtained by integrating over the CH profile. Detailed calculations using GRI-Mech 3.0 predict the experimental results at ? = 1.3 nearly quantitatively, but show increasing differences with the measurements for both CH and NO profiles with increasing equivalence ratio.     

The OH collision broadening parameter in the 2900°K H2-NO flame

The collision broadening parameter of the Q₁6 line in the OH (0, 0) A → X u.v. transition has been determined by the curve of growth method in atmospheric pressure nitric oxide-hydrogen flames, in the temperature range 2800–2900°K. An average value of 0.05 was found for this parameter for five flames varying in feed composition between 40 and 65% NO. The exact flame gas compositions and temperatures for which these determinations were made are given in the paper. The data obtained are in agreement with those obtained by Carrington² for atmospheric hydrogen-oxygen flames at 2600°K.     

Effect of fuel/air ratio and aromaticity on the molecular weight distribution of soot in premixed n-heptane flames

Soot growth from inception to mass-loading is studied in a wide range of molecular weights (MW) from 10⁵ to 10¹⁰u by means of size exclusion chromatography (SEC) coupled with on-line UV-visible spectroscopy. The evolution of MW distributions of soot is also numerically predicted by using a detailed kinetic model coupled with a discrete-sectional approach for the modeling of the gas-to-particle process. Two premixed flames burning n-heptane in slightly sooting and heavily sooting conditions are studied. The effect of aromatic addition to the fuel is studied by adding n-propylbenzene (10% by volume) to n-heptane in the heavily sooting condition. A progressive reduction of the MW distribution from multimodal to unimodal is observed along the flames testifying the occurrence of particle growth and agglomeration. These processes occur earlier in the aromatic-doped n-heptane flame due to the overriding role of benzene on soot formation which results in bigger young soot particles. Modeled MW distributions are in reasonable agreement with experimental data although the model predicts a slower coagulation process particularly in the slightly sooting n-heptane flame. Given the good agreement between model predictions and experiments, the model is used to explore the role of fuel chemistry on MW distributions. Two flames of n-heptane and n-heptane/n-propylbenzene in heavily sooting conditions with the same temperature profile and inert dilution are modeled. The formation of larger soot particles is still evident in the n-heptane/n-propylbenzene flame with respect to the n-heptane flame in the same operating conditions of temperature and dilution. In addition the model predicts a larger formation of molecular particles in the flame containing n-propylbenzene and shows that soot inception occurs in correspondence of their maximum formation thus indicating the importance of molecular growth in soot inception.     

Chirped-probe-pulse femtosecond CARS thermometry in turbulent spray flames

This paper presents temperature measurements in turbulent dilute and dense spray flames using single-laser-shot chirped-probe-pulse femtosecond coherent anti-Stokes Raman spectroscopy (CPP-fs-CARS). This ultrafast technique, with a repetition rate of 5 kHz, is applied to the piloted Sydney Needle Spray Burner (SYNSBURN^TM). The burner system features air-blast atomization of liquid injected from a needle that can be translated within a co-flowing air stream. The pilot-stabilized spray flames can range between the two extremes of dense and dilute by physically translating the needle tip relative to the burner's exit plane. The CPP-fs-CARS set-up has achieved integration times of 3 picoseconds (ps) as well as spatial resolution of approximately 800 µm along beam propagation and 60 µm in the transverse dimension. Brief details of the technique, calibration, correction of interferences, and spectral fitting processes are presented along with estimates of the associated error. The measurements are compared against well-established, line Raman–Rayleigh data for temperature collected in a turbulent CH₄/air jet diffusion flame, which is largely non-sooting. At peak gaseous flame temperatures of up to 2512 K, the relative accuracy and precision were 2.8% and ±3.4%, respectively. Measurements in turbulent spray flames are shown after applying the relevant corrections based on non-resonant background (NRB) behavior and camera saturation effects on the shape of the CARS signal spectrum. Preliminary mapping of the temperature fields demonstrates the wealth of information available in this dataset which will provide insights into the spatio-temporal structure of spray flames once relevant statistical analysis is applied.     

Quantitative C2H2 measurements in sooty flames using mid-infrared polarization spectroscopy

Quantitative measurements of acetylene (C₂H₂) molecules as a combustion intermediate species in a series of rich premixed C₂H₄/air flames were non-intrusively performed, spatially resolved, using mid-infrared polarization spectroscopy (IRPS), by probing its fundamental ro-vibrational transitions. The flat sooty C₂H₄/air premixed flames with different equivalence ratios varying from 1.25 to 2.50 were produced on a 6 cm diameter porous-plug McKenna type burner at atmospheric pressure, and all measurements were performed at a height of 8.5 mm above the burner surface. IRPS excitation scans in different flame conditions were performed and rotational line-resolved spectra were recorded. Spectral features of acetylene molecules were readily recognized in the spectral ranges selected, with special attention to avoid the spectral interference from the large amount of coexisting hot water and other hydrocarbon molecules. On-line calibration of the optical system was performed in a laminar C₂H₂/N₂ gas flow at ambient conditions. Using the flame temperatures measured by coherent anti-Stokes Raman spectroscopy in a previous work, C₂H₂ mole fractions in different flames were evaluated with collision effects and spectral overlap between molecular line and laser source being analyzed and taken into account. C₂H₂ IRPS signals in two different buffering gases, N₂ and CO₂, had been investigated in a tube furnace in order to estimate the spectral overlap coefficients and collision effects at different temperatures. The soot-volume fractions (SVF) in the studied flames were measured using a He–Ne laser-extinction method, and no obvious degrading of the IRPS technique due to the sooty environment has been observed in the flame with SVF up to ～2×10^?7. With the increase of flame equivalence ratios not only the SVF but also the C₂H₂ mole fractions increased.