Measurements of soot, OH, and PAH concentrations in turbulent ethylene/air jet flames

This paper presents results from an investigation of soot formation in turbulent, non-premixed, C₂H₄/air jet flames. Tests were conducted using a H₂-piloted burner with fuel issuing from a 2.18 mm i.d. tube into quiescent ambient air. A range of test conditions was studied using the initial jet velocity (16.2-94.1 m/s) as a parameter. Fuel-jet Reynolds numbers ranged from 4000 to 23,200. Planar laser-induced incandescence (LII) was employed to determine soot volume fractions, and laser-induced fluorescence (LIF) was used to measure relative hydroxyl radical (OH) concentrations and polycyclic aromatic hydrocarbons (PAHs) concentrations. Extensive information on the structure of the soot and OH fields was obtained from two-dimensional imaging experiments. Quantitative measurements were obtained by employing the LII and LIF techniques independently. Imaging results for soot, OH, and PAH show the existence of three soot formation/oxidation regions: a rapid soot growth region, in which OH and soot particles lie in distinctly different radial locations; a mixing-dominated region controlled by large-scale motion; and a soot-oxidation region in which the OH and soot fields overlap spatially, resulting in the rapid oxidation of soot particles. Detailed quantitative analyzes of soot volume fractions and OH and soot zone thicknesses were performed along with the temperature measurement using the N₂-CARS system. Measurements of OH and soot zone thicknesses show that the soot zone thickness increases linearly with axial distance in the soot formation region, whereas the OH zone thickness is nearly constant in this region. The OH zone thickness then rapidly increases with downstream distance and approximately doubles in the soot-oxidation region. Probability density functions also were obtained for soot volume fractions and OH concentrations. These probability density functions clearly define the spatial relationships among the OH, PAH concentrations, the soot formation, and oxidation processes.     

Chaotic map models of soot fluctuations in turbulent diffusion flames

In this paper, we introduce a methodology to characterize time-dependent soot volume fraction fluctuations in turbulent diffusion flames via chaotic maps. The approach is based on the hypothesis that fluctuations of properties in turbulent flames are deterministic in nature, rather than statistical. Our objective is to develop models of these fluctuations to be used in comprehensive algorithms to study the nature of turbulent flames and the interaction of turbulence with radiation. To this end we measured the time series of soot scattering coefficient in an ethylene diffusion flame from light scattering experiments and fit these data to linear combinations of chaotic maps of the unit interval. Both time series and power spectra can be modeled with reasonable accuracy in this way. © 1998 Elsevier Science Ltd. All rights reserved.     

The blowout mechanism of turbulent jet diffusion flames

The complicated flame stabilization mechanisms and flame/flow interactions in the blowout of turbulent nonpremixed jet flames are experimentally studied using phenomenological observation, 2D Rayleigh scattering, 2D laser-induced predissociative fluorescence (LIPF) images of OH, and particle image velocimetry (PIV) techniques. The blowout process may be categorized into four characteristic regions: pulsating, onset of receding, receding, and extinction. Based on experimental findings, a blowout mechanism is proposed. The maximum “waistline” point of the stoichiometric contour, defined as the point where the radial distance between the elliptic stoichiometric contour and the jet axis reaches a maximum value, can be regarded as the dividing point separating the unstable and stable regions for the lifted flame in the blowout process. If the flame base is pushed beyond the maximum “waistline” point, the flame will step into the pulsating region and become unstable, triggering the blowout process. The triple flame structure is identified and found to play an important role in flame stabilization within the stable liftoff and pulsating regions. In the pulsating region, the stabilization point of the triple flame moves along the stoichiometric contour, stabilizing the flame where the flame base is bounded by the contours of lean and rich limits. If the flame is pushed beyond the tip of the stoichiometric contour, the stabilization point and triple flame structure vanish and the flame becomes lean. The flame then recedes downstream continuously and finally extinguishes.     

Effect of free stream turbulence on NOx and soot formation in turbulent diffusion CH4-air flames

A two-dimensional axisymmetric RANS numerical model was solved to investigate the effect of increasing the turbulence intensity of the air stream on the NOx and soot formation in turbulent methane diffusion flames. The turbulence–combustion interaction in the flame field was modelled in a k − ε/EDM framework, while the NO and soot concentrations were predicted through implementing the extended Zildovich mechanism and two transport equations model, respectively. The predicted spatial temperature gradients showed acceptable agreement with published experimental measurements. It was found that the increase of free stream turbulence intensity of the air supply results in a significant reduction in the NO formation of the flame. Such phenomenon is discussed by depicting the spatial distribution of the NO concentration in the flame. An observable reduction of the soot formation was also found to be associated with the increase of inlet turbulence intensity of air stream.     

Conditional moment closure prediction of soot formation in turbulent, nonpremixed ethylene flames

Results obtained from incorporating a semiempirical soot model into a first-order conditional moment closure (CMC) approach to modeling turbulent nonpremixed flames of ethylene and air are presented. Soot formation is determined via the solution of two transport equations for soot mass fraction and particle number density, with acetylene and benzene employed as the incipient species responsible for soot nucleation, and the concentrations of these species calculated using a detailed gas-phase kinetic scheme involving 463 reactions and 70 species. The study focuses on the influence of differential diffusion of soot particles on soot volume fraction predictions. The results of calculations are compared with experimental data for three sooting ethylene flames and, in general, predictions of mixing and temperature fields within the three flames show good agreement with data. Soot volume fraction predictions are found to be in significantly better accord with data when differential diffusion is accounted for in the CMC-based soot model, supporting the importance of such effects in sooting flames, as previously noted by Kronenburg et al. in relation to methane combustion. Overall, the study demonstrates that the CMC-based soot model, when used in conjunction with a model of differential diffusion effects, is capable of accurately predicting soot formation in turbulent nonpremixed ethylene-air flames.     

Experimental and kinetic studies of soot formation in methanol-gasoline coflow diffusion flames

Methanol has been considered to be a potential alternative fuel to reduce soot emissions from GDI engine. In order to fully understand the effect of methanol addition on soot formation, the 2-D distribution of soot volume fraction in methanol/gasoline laminar diffusion flames was measured quantitatively with two-color laser induced incandescence (TC-LII) technique. In addition, the Methanol-TRF-PAH mechanism is constructed and used to analyze the formation pathways of soot precursors based on the CHEMKIN PRO 0-D constant pressure reactor. In this experiment, the blending ratio of methanol/gasoline was set as M0/20/40/60/80. Considering the carbon content decreasing due to methanol addition, carbon mass flow rate was remained constant. The experimental results showed that methanol is able to decrease the soot significantly, while the effect of methanol on soot reduction is weakened with the increasing methanol ratio. Compared with pure gasoline, the average soot volume fraction in the M20, M40, M60, and M80 flames decrease by 48.2%, 70.4%, 83.8%, and 97.7%, and the peak soot volume fraction decrease by 41.5%, 64.1%, 75.8% and 91.8% respectively. There is little soot formation in the M80 flame, inferring the pure methanol hardly forms soot. The kinetic analysis showed that mole fraction of A1-A4 decrease with the increasing methanol ratio. For the toluene-containing fuel M0-M80, A1 is mainly formed by C₆H₅CH₃ + H = A1 + CH₃ and oxidized by A1 + OH = A1- + H₂O. A4 is mainly produced by C₆H₅CH₂ + C₉H₇ = A4 + 2H₂ and oxidized by H-abstraction reaction with H or OH radical. The major reaction pathways of A1 and A4 formation are consistent under different methanol blending ratios. The soot reduction as methanol added mainly attributes to aromatics dilution effect. In addition, the formation process of soot precursors is largely affected by chemical processes of OH, CH₃, HO₂ radicals.     

Large eddy simulation of soot formation in a turbulent non-premixed jet flame

A recently developed subgrid model for soot dynamics [H. El-Asrag, T. Lu, C.K. Law, S. Menon, Combust. Flame 150 (2007) 108-126] is used to study the soot formation in a non-premixed turbulent flame. The model allows coupling between reaction, diffusion and soot (including soot diffusion and thermophoretic forces) processes in the subgrid domain without requiring ad hoc filtering or model parameter adjustments. The combined model includes the entire process, from the initial phase, when the soot nucleus diameter is much smaller than the mean free path, to the final phase, after coagulation and aggregation, where it can be considered in the continuum regime. A relatively detailed but reduced kinetics for ethylene-air is used to simulate an experimentally studied non-premixed ethylene/air jet diffusion flame. Acetylene is used as a soot precursor species. The soot volume fraction order of magnitude, the location of its maxima, and the soot particle size distribution are all captured reasonably. Along the centerline, an initial region dominated by nucleation and surface growth is established followed by an oxidation region. The diffusion effect is found to be most important in the nucleation regime, while the thermophoretic forces become more influential downstream of the potential core in the oxidation zone. The particle size distribution shows a log-normal distribution in the nucleation region, and a more Gaussian like distribution further downstream. Limitations of the current approach and possible solution strategies are also discussed.     

The effects of hydrogen addition on soot formation in counterflow diffusion n-heptane flames

The effects of H₂ addition on soot formation are investigated in counterflow diffusion n-heptane flames. Three effects including chemical, thermal, and dilution are fully isolated and characterized by additions of H₂, He, and Ar. Soot volume fractions are measured using LE-calibrated LII technique, and flame temperatures are measured using OH-TLAF method along with a thermocouple. Numerical simulations are conducted with a detailed mechanism with soot model. The simulated soot volume fractions and flame temperatures are in good agreement with experimental data. The experimental results show that H₂ addition can greatly reduce the soot formation. It is also found that the chemical and dilution effects suppress soot formation, while the thermal effect with increasing flame temperature promotes soot formation. Kinetic analysis suggests that HACA growth rate could be the dominant factor that controls the final soot formation through the three effects due to H₂ addition.     

Dissipation length scales in turbulent nonpremixed jet flames

Line imaging of Raman/Rayleigh/CO-LIF is used to investigate the energy and dissipation spectra of turbulent fluctuations in temperature and mixture fraction in several flames, including CH₄/H₂/N₂ jet flames at Reynolds numbers of 15,200 and 22,800 (DLR-A and DLR-B) and piloted CH₄/air jet flames at Reynolds numbers of 13,400, 22,400, and 33,600 (Sandia flames C, D, and E). The high signal-to-noise ratio of the 1D Rayleigh scattering images enables determination of the turbulent cutoff wavenumber from 1D dissipation spectra. The local length scale inferred from this cutoff is analogous to the Batchelor scale in nonreacting flows. The measured thermal dissipation spectra in the turbulent flames are shown to be similar to the model spectrum of Pope for turbulent kinetic energy dissipation. Furthermore, for flames with Lewis number near unity, the 1D dissipation spectra for temperature and mixture fraction are shown to follow nearly the same rolloff in the high-wavenumber range, such that the cutoff length scale for thermal dissipation is equal to or slightly smaller than the cutoff length scale for mixture fraction dissipation. Measurements from the piloted CH₄/air flames are used to demonstrate that a surrogate cutoff scale may be obtained from the dissipation spectrum of the inverse of the Rayleigh signal itself, even when the Rayleigh scattering cross section varies through the flame. This suggests that the cutoff length scale determined from Rayleigh scattering measurements may be used to define the local resolution requirements and optimal data processing procedures for accurate determination of the mean mixture fraction dissipation, based upon Raman scattering measurements or other multiscalar imaging techniques.     

Stochastic modeling of finite-rate chemistry effects in hydrogen-air turbulent jet diffusion flames with helium dilution

Stochastic simulations of turbulent hydrogen-air jet diffusion flames at three different dilution rates with helium are implemented using the ‘one-dimensional turbulence’ (ODT) model. The approach is based on one-dimensional unsteady solution of boundary layer equations to represent molecular processes and a stochastic implementation of turbulent advection. The 1D scalar and streamwise momentum profiles represent radial profiles within the flames; while, the unsteady evolution of the solution is interpreted as a downstream evolution of the radial scalar and streamwise momentum profiles. Multiple realizations of jet simulations are used to compute conditional statistics of major species, NO, and temperature. The ODT computations are implemented with a five-step reduced mechanism for hydrogen combustion and an optically-thin radiation model. Computed conditional statistics of temperature, major and minor species are compared to the experimental data from a set of documented flames at Sandia National Labs. Reasonable qualitative and quantitative agreement between computed and measured statistics is found, including very good predictions of NO mean and RMS profiles. Both computation and experiment exhibit the role of dilution in enhancing finite-rate chemistry effects, which vary as a function of downstream distance and fuel dilution.     

Synergistic effect on soot formation in counterflow diffusion flames of ethylene-propane mixtures with benzene addition

The characteristics of the formation of polycyclic aromatic hydrocarbon (PAH) and soot in counterflow diffusion flames of ethylene/propane mixtures have been investigated experimentally to identify the effect of fuel structure. The synergistic effect, that is, the enhancement of PAH and soot formation by the fuel mixing of ethylene and propane has been further analyzed to examine the suggested mechanisms based on the competition between PAH and soot growths through the H-abstraction-C₂H₂-addition (HACA) mechanism and the incipient ring formation through the propargyl recombination reaction. To mitigate the effect of incipient ring formation on the synergistic effect, a small amount of benzene was added to the fuel stream. Planar laser-induced incandescence and laser-induced fluorescence techniques were employed to measure relative soot volume fractions and PAH concentrations, respectively. Results showed that the synergistic effect on soot formation remained, even though the synergistic effects for relatively small aromatics mitigated with the benzene addition. Larger size PAHs have shown enhanced synergistic effects compared to smaller size PAHs regardless of benzene addition. These results implied that the role of propane mixing on the synergistic effect cannot be explained solely by the incipient ring formation via a propargyl recombination reaction; thus, it is suggested that the C₃ pathways could also contribute to the growth of PAH species.     

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.     

Modeling extinction and reignition in turbulent flames

The conditional moment closure method (CMC) has been extended to improve reactive species predictions in flames with significant local extinction and reignition. Simple first-order closure of the conditionally averaged reaction rate term does not give satisfactory results due to large fluctuations around the conditional mean and an alternative closure is suggested here. The new closure is based on a precomputed parameterized reference field that maps reactive species mass fractions as functions of mixture fraction and sensible enthalpy. During the computations, the reference field is continuously adjusted to ensure consistency with the CMC solution and doubly conditioned chemical source terms that are functions of time, space, mixture fraction, and sensible enthalpy can thus be obtained. Integration over sensible enthalpy space yields the improved singly conditioned chemical source term that can be used for the solution of the CMC equations. Full closure can be achieved by assuming a β-PDF for the probability distribution in sensible enthalpy space and an additional conditional variance equation needs to be solved. The overall agreement between the measured and the computed variance is satisfactory and the extended CMC model is applied to Sandia Flames D, E, and F. Excellent predictions of temperature, major species, intermediates, and NO are obtained in Flames D and E while temperature predictions can be significantly improved in Sandia Flame F.     

Laser measurements of scalars in turbulent diffusion flames

Laser diagnostics based on incoherent interaction between fight and matter can provide quantitative scalar data in reacting flows as linear calibration procedures are achievable. However, in turbulent diffusion flames, where mixing and chemistry interact, a closure relation is still required as the signal intensity depends on both the instantaneous composition and the reaction progress in the probed volume. The properties of the main scalars in tubulent non-premixed flames are examined in order to provide coupling functions that can be used to complete the laser diagnostics, and also to underline the quantities and assumptions that are worth experimental investigation. Laser measurements by Mie, Rayleigh, Raman and induced fluorescence techniques are successively reviewed as well as recent combined measurements where two diagnostics are simultaneously performed to provide correlated data which are essential for interpretation.     

Multiple mapping conditioning of velocity in turbulent jet flames

Multiple mapping conditioning (MMC) has emerged as a new approach to model turbulent reacting flows. This study revises the standard MMC closure for velocity in turbulent jet flows from linearity in the reference space to linearity in the composition space. This modeling amendment ensures that the standard velocity model in conditional moment closure studies can now be used for MMC computation as well. A simplified model for the velocity-dependence of MMC drift coefficients is derived without loss of generality and is implemented for the revised velocity closure. Modeling results have been corroborated against the Direct Numerical Simulation database of a spatially evolving, planar turbulent jet flame. The revised model shows marked improvement over standard MMC closure in predicting velocity statistics close to the nozzle.     

Stability characteristics of non-premixed turbulent jet flames of hydrogen and syngas blends with coaxial air

The stability characteristics of attached hydrogen (H₂) and syngas (H₂/CO) turbulent jet flames with coaxial air were studied experimentally. The flame stability was investigated by varying the fuel and air stream velocities. Effects of the coaxial nozzle diameter, fuel nozzle lip thickness and syngas fuel composition are addressed in detail. The detachment stability limit of the syngas single jet flame was found to decrease with increasing amount of carbon monoxide in the fuel. For jet flames with coaxial air, the critical coaxial air velocity leading to flame detachment first increases with increasing fuel jet velocity and subsequently decreases. This non-monotonic trend appears for all syngas composition herein investigated (50/50 → 100/0% H₂/CO). OH^∗ chemiluminescence imaging was performed to qualitatively identify the mechanisms responsible for the flame detachment. For all fuel compositions, local extinction close to the burner rim is observed at lower fuel velocities (ascending stability limit), while local flame extinction downstream of the burner rim is observed at higher fuel velocities (descending stability limit). Extrema of the non-monotonic trends appear to be identical when the nozzle fuel velocity is normalized by the critical fuel velocity obtained for the single jet cases.     

Stability characteristics of lifted turbulent partially premixed jet flames

The stability characteristics of partially premixed turbulent lifted methane flames have been investigated and discussed in the present work. Mixture fraction and reaction zone behavior have been measured using a combined 2-D technique of simultaneous Rayleigh scattering, Laser Induced Predissociation Fluorescence (LIPF) of OH and Laser Induced Fluorescence (LIF) of C₂H_x. The stability characteristics and simultaneous mixture fraction-LIPF-LIF measurements in three lifted flames with originally partially premixed jets at different mean equivalence ratio and Reynolds number are presented and discussed in this paper. Higher stability of partially premixed flames as compared to non-premixed flames has been observed. Lifted, attached, blow-out and blow-off regimes have been addressed and discussed in this work. The data show that the mixture fraction field on approaching the stabilization region is uniquely characterized by a certain level of mean and rms fluctuations. This suggests that the stabilization mechanism is likely to be controlled by premixed flame propagation at the stabilization region. Triple flame structure has been detected in the present flames, which is likely to be the appropriate model at the stabilization point.     

Strategy for PLIF single-shot HCO imaging in turbulent methane/air flames

Formyl (HCO) has since long been recognized as a common intermediate species and a potential local indicator of the major heat release in hydrocarbon combustion. Consequently, the detection of HCO is desirable especially in turbulent flames of practical relevance. However, due to the low concentration and low fluorescence quantum yield, single-shot based detection of HCO with planar laser-induced fluorescence (PLIF) has been a real challenge for experimentalists. In the present paper, a series of systematic investigations have been performed in order to develop a strategy for single-shot HCO PLIF detection in methane/air premixed flames. Potential spectral interference and applicable combustion conditions were analyzed in stable laminar flames employing fluorescence detection with high spectral and spatial resolution for different laser wavelengths. The wavelength 259.004 nm was identified as optimum in giving the maximum signal and minimum spectral interference from other species (e.g., OH and hot O₂). Photolytically generated HCO from formaldehyde (CH₂O) was also observed, which restricts the applicable laser fluence to below 2.5 J/cm² in order to diminish the influence of CH₂O down to 5%. Besides, large hydrocarbon species generated in rich flames were found to contribute a considerable interference which can hardly be screened out. This limits the application of the HCO PLIF technique to lean premixed flames. Finally, by employing an optimized alexandrite laser system, single-shot HCO PLIF imaging in a turbulent methane/air flame is demonstrated, indicating the feasibility of further application of this technique to turbulent combustion systems.