Formation of soot and nitrogen oxides in unsteady counterflow diffusion flames

The formation of pollutant species in turbulent diffusion flames is strongly affected by turbulence/chemistry interactions. Unsteady counterflow diffusion flames can be conveniently used to address the unsteady effects of hydrodynamics on the pollutant chemistry, because they exhibit a larger range of combustion conditions than those observed in steady flames.In this paper, unsteady effects on the formation of soot (and its main precursors) and nitrogen oxides (NO_x) are investigated by imposing harmonic oscillations on the strain rate of several counterflow diffusion flames fed with propane. Numerical results confirm that the dynamic response of each species is strongly affected by the strain rate oscillations and the characteristic time governing its chemistry. At low frequencies of imposed oscillations the soot and NO_x profiles show strong deviations from the steady-state profile. At large frequencies a decoupling between the concentration and the velocity field is evident. In particular, the formation of soot and NO_x is found less sensitive to velocity fluctuations for flames with large initial strain rate. The significant increase of soot and NO_x concentrations in unsteady conditions appears to be a function of both forcing frequency and flame global strain rate. Moreover, the cut-off frequency, defined as the minimum frequency above which the strain rate oscillations have negligible effects on the formation of each species, was found to be strongly dependent on the chemical characteristic time and the flame global strain rate, but only marginally affected by the amplitude of imposed oscillations.     

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.     

Soot properties of laminar jet diffusion flames in microgravity

The soot properties of round, non-buoyant, laminar jet diffusion flames are described, based on experiments carried out in microgravity conditions during three flights of the Space Shuttle Columbia (Flights STS-83, 94 and 107). Experimental conditions included ethylene- and propane-fueled flames burning in still air at an ambient temperature of 298 K and ambient pressures of 35-100 kPa. Measurements included soot volume fraction distributions using deconvolved laser extinction imaging and soot temperature distributions using deconvolved multiline emission imaging. Mixture fractions were estimated from the temperature measurements. Flow field modeling based on the work of Spalding is presented. It is shown that most of the volume of these flames is inside the dividing streamline and thus should follow residence time state relationships. Most streamlines from the fuel supply to the surroundings exhibit nearly the same maximum soot volume fraction and maximum temperature. The present work studies whether soot properties of these flames are universal functions of mixture fraction, i.e., whether they satisfy soot state relationships. Soot state relationships were observed, i.e., soot volume fraction was found to correlate reasonably well with estimated mixture fraction for each fuel/pressure selection. These results support the existence of soot property state relationships in steady non-buoyant laminar diffusion flames, and thus in a large class of practical turbulent diffusion flames through the application of the laminar flamelet concept.     

Measurement of the dimensionless extinction coefficient of soot within laminar diffusion flames

The dimensionless extinction coefficient (K_e) of soot must be known to quantify laser extinction measurements of soot concentration and to predict optical attenuation through smoke clouds. Previous investigations have measured K_e for post-flame soot emitted from laminar and turbulent diffusion flames and smoking laminar premixed flames. This paper presents the first measurements of soot K_e from within laminar diffusion flames, using a small extractive probe to withdraw the soot from the flame. To measure K_e, two laser sources (635 nm and 1310 nm) were coupled to a transmission cell, followed by gravimetric sampling. Coannular diffusion flames of methane, ethylene and nitrogen-diluted kerosene burning in air were studied, together with slot flames of methane and ethylene. K_e was measured at the radial location of maximum soot volume fraction at several heights for each flame. Results for K_e at both 635 nm and 1310 nm for ethylene and kerosene coannular flames were in the range of 9–10, consistent with the results from previous studies of post-flame soot. The ethylene slot flame and the methane flames have lower K_e values, in some cases as low as 2.0. These lower values of K_e are found to result from the contributions of (a) the condensation of PAH species during the sampling of soot, (b) the wavelength-dependent absorptivity of soot precursor particles, and, in the case of methane, (c) the negligible contribution of soot scattering to the extinction coefficient. RDG calculations of soot scattering, in combination with the measured K_e values, imply that the soot refractive index is in the vicinity of 1.75–1.03i at 635 nm.     

Flow-field effects on soot formation in normal and inverse methane–air diffusion flames

We investigate the effects of the flow-field configuration on the sooting characteristics of normal and inverse coflowing diffusion flames. The numerical model solves the time-dependent, compressible, reactive-flow, Navier-Stokes equations, coupled with submodels for soot formation and thermal radiation transfer. A benchmark calculation is conducted and compared with experimental data, and shows that computed peak temperatures and species concentrations differ from the experimental values by less than 10%, while the computed peak soot volume fraction differs from the experimental values by 10–40%, depending on height. Simulations are conducted for three normal diffusion flames in which the fuel/air velocities (cm/s) are 5/10, 10/10, and 10/5, and for an inverse diffusion flame (where the fuel and air ports have been reversed) with a fuel/air velocity of 10/10. The results show significant differences in the sooting characteristics of normal and inverse diffusion flames. This work supports previous conclusions from the experimental work of others. However, in addition, we use the ability of the simulations to numerically track soot parcels along pathlines to further explain the experimentally observed phenomena. In normal diffusion flames, both the peak soot volume fraction and the total mass of soot generated is several orders of magnitude greater than for inverse diffusion flames with the same fuel and air velocities. In normal diffusion flames, soot forms in the annular region on the fuel-rich side of the flame sheet, while in inverse flames, the soot forms in a fuel-rich region on top of the flame sheet. Surface growth is the dominant soot formation mechanism (compared to nucleation) for both types of flames; however, surface growth rates are much faster for normal diffusion flames compared to inverse flames. Soot oxidation rates are also much faster in normal flames, where the dominant soot-oxidizing species is OH, compared to inverse flames, where the dominant soot-oxidizing species is O₂. In the inverse flames, surface growth continues after oxidation has ceased, causing the peak soot volume fraction to be sustained for a long period of time, and causing the emission of soot, even though the quantity of soot is small. Comparison of soot formation among the three normal diffusion flames shows that the peak soot volume fraction and total mass of soot generated increases as the fuel-to-air velocity ratio increases. A larger fuel–air velocity ratio results in a longer residence time from the nucleation to the oxidation stage, allowing for more soot particle growth. When the fuel-to-oxidizer ratio decreases, there is less time for surface growth, and the particles cross the flame sheet (where they are oxidized) earlier, resulting in decreased soot volume fraction.     

Soot formation in high pressure laminar diffusion flames

The details of the chemical and physical mechanisms of the soot formation process in combustion remain uncertain due to the highly complex nature of hydrocarbon flames, and only a few principles are firmly established mostly for atmospheric conditions. In spite of the fact that most combustion devices used for transportation operate at very high pressures (e.g., aircraft gas turbines up to 40 atm, diesel engines exceeding 100 atm), our understanding of soot formation at these pressures is not at a desirable level, and there is a fundamental lack of experimental data and complementary predictive models. The focus of this review is to assess the experimental results available from laminar co-flow diffusion flames burning at elevated pressures. First, a brief review of soot formation mechanisms in diffusion flames is presented. This is followed by an assessment of soot diagnostics techniques, both intrusive and non-intrusive, most commonly used in soot experiments including the laser induced incandescence. Then the experimental results of soot measurements done at elevated pressures in diffusion flames are reviewed and critically assessed. Soot studies in shock tubes and in premixed flames are not covered. Smoke point fuel mass flow rate is revisited, and shortcomings in recent measurements are pointed. The basic requirements for tractable and comparable measurements as a function of pressure are summarized. Most recent studies at high pressures with aliphatic gaseous fuels show that the soot yield displays a unified behaviour with reduced pressure. The maximum soot yield seems to reach a plateau asymptotically as the pressure exceeds the critical pressure of the fuel. Lack of experimental data on the sensitivity of soot morphology to pressure is emphasized. A short summary of efforts in the literature on the numerical simulation of soot formation in diffusion flames at high pressures is the last section of the paper.     

Soot sheet dimensions in turbulent nonpremixed flames

A method is presented to automate the generation of the statistics of the characteristic length (L) and width (W) of the 2-D slices through 3-D soot sheets in turbulent nonpremixed flames. The method employs the simplified approximation of fitting an equivalent ellipse. The effectiveness of the method is first evaluated by comparison with two types of manual assessment. It is then applied to assess the evolution of the soot sheet dimensions in two turbulent nonpremixed propane flames. These two flames, had the same fuel flow rates but different global mixing rates, were produced by a precessing jet (PJ) and a simple jet (SJ) burner. The global mixing rate of the flames was observed to affect the shape of probability density functions (PDFs) of soot sheets L and W. The shapes of the PDFs of the flames reveal that burnout of the soot sheets in the PJ and SJ flame proceeded predominantly through the reduction of the soot sheet dimensions, and the number of soot sheets, respectively. Finally, a convenient way to estimate the actual maximum width of the single-branched soot sheets is proposed, and this value is estimated to be ∼20 mm for both flames.     

Parametric effects on sooting in turbulent acetylene diffusion flames

Optical extinction and temperature measurements have been made in free turbulent diffusion acetylene flames over a wide range of velocities and nozzle sizes. The measurements show that the soot volume profiles scale as the flame time constant. Soot formation and oxidation rates are controlled by mixing rates at low flow rates and by soot kinetics at high flow rates. The temperature in the downstream oxidizing region is the most important determinant of the flame's smoking propensity. Inert diluents greatly reduce soot formation and burnout rates and O₂ addition increases formation rates.     

Evidence of soot superaggregates in a turbulent pool fire

We report experimental observations of extremely large, 10–100 μm, soot aggregates in a blended methanol/toluene fueled turbulent pool fire, which are believed to be the first observation of “superaggregates” in a turbulent flame. Laser-induced incandescence images of soot volume concentration, at the center of the fire plume and at a height within the active flaming region, reveal the appearance of large-scale particle-like features across a broad range of apparent volume fraction, which emit at an intensity that is comparable with that of the laser-heated soot particles. We argue that the features in the incandescence images result from very large soot aggregates. This observation is supported by scanning electron microscope imaging of extracted soot that reveals large soot structures composed of much smaller chains of individual primary particles. Analysis of the soot aggregate structure from the electron-microscope images reveals a 1.8 fractal dimension at micron scales, comparable with commonly reported soot aggregate sizes from hydrocarbon flames. At larger scales of 10s of microns, comparable with the total aggregate size, a larger volume-filling fractal dimension of 2.5–2.6 is observed. This type of fractal structure is consistent with reported, but apparently rare, observations of soot superaggregates in heavily sooting laboratory laminar diffusion flames, but is encountered in the much larger meter-scale pool fire at much lower soot volume concentrations.     

Statistical modeling of thermal radiation transfer in buoyant turbulent diffusion flames

A statistical (Monte Carlo) method for radiative heat transfer has been incorporated in CFD modeling of buoyant turbulent diffusion flames in stagnant air and in a cross-wind. The model and the computational tool have been developed and applied to simulate both burner flames with controlled fuel supply rate and in self-sustained pool fires with burning rates coupled with flame radiation. The gas–soot mixture was treated either as gray (using the effective absorption coefficient derived from total emissivity data or the Planck mean absorption coefficient) or as non-gray (using the weighed sum of gray gases model). The comparison of predicted radiative heat fluxes indicates applicability of the gray media assumption in modeling of thermal radiation in case of high soot content. The effect of turbulence-radiation interaction is approximately taken into account in calculation of radiation emission, which is corrected to allow for temperature self-correlation and absorption-temperature correlation. In modeling buoyant propane flames in still air above 0.3 m diameter burner, extensive comparison is presented of the predictions with the measurements of gas species concentrations, temperature, velocity and their turbulent fluctuations, and radiative heat fluxes obtained in flames with different heat release rates. Similar to previously published experimental data, the predicted burning rate of flames above the acetone pools exposed to flame radiation increases with the pool diameter and approaches a constant level for large pool sizes. The magnitude of predicted burning rates is shown to be in agreement with the reported measurements. Augmentation of burning rate of the pool fire in a cross-wind because of increased net radiative heat flux received by the fuel surface and non-monotonic dependence of burning rate on cross-wind velocity, subject to the pool diameter, is predicted. The statistical treatment of thermal radiation transfer has been found to be robust and computationally efficient.     

Comparison of structures of laminar methane–oxygen and methane–air diffusion flames from atmospheric to 60 atm

A combined experimental and numerical study was conducted to examine the structure of laminar methane–oxygen diffusion flames in comparison with methane–air flames. Soot measurements made in these flames indicated that the maximum soot yields of methane–air flames are consistently higher than methane–oxygen flames at all pressures. The maximum soot yield of the methane–oxygen flames reaches a peak near 40 atm and then starts decreasing as the pressure further increased. The maximum soot yield of the methane–air flames plateaus at about 40 atm and does not change much with further increases in pressure. Methane–oxygen flames display a distinct two-zone structure which is visible from atmospheric pressure up to 60 atm. The inner zone, similar to hydrocarbon-air diffusion flames, has a yellow/orange colour and is surrounded by an outer blue zone. This outer zone was shown to have a stratified structure with a very steep equivalence ratio gradient. The main reactions in this zone were shown to be the oxidation of hydrogen and carbon monoxide produced within the inner zone. The methane–air diffusion flames had a thin layer of blue outer zone at atmospheric pressure; however, this zone completely disappeared when the pressure was increased above atmospheric. The presence of the two-zone structure in the methane–oxygen flames was attributed to the intensified penetration of oxygen into the core flow. The higher diffusivities, steeper oxygen concentration gradients, and enhanced entrainment increase the transport of oxygen to the flame. As such, there is sufficient oxygen present near the base of the flame to support the diffusion flame in the inner zone of the methane–oxygen flames. The abundance of oxygen near the centerline, even in the lower portion of the flame, also promotes the oxidation of soot.     

Formation,growth, and transport of soot in a three-dimensional turbulent non-premixed jet flame

The formation, growth, and transport of soot is investigated via large scale numerical simulation in a three-dimensional turbulent non-premixed n-heptane/air jet flame at a jet Reynolds number of 15,000. For the first time, a detailed chemical mechanism, which includes the soot precursor naphthalene and a high-order method of moments are employed in a three-dimensional simulation of a turbulent sooting flame. The results are used to discuss the interaction of turbulence, chemistry, and the formation of soot. Compared to temperature and other species controlled by oxidation chemistry, naphthalene is found to be affected more significantly by the scalar dissipation rate. While the mixture fraction and temperature fields show fairly smooth spatial and temporal variations, the sensitivity of naphthalene to turbulent mixing causes large inhomogeneities in the precursor fields, which in turn generate even stronger intermittency in the soot fields. A strong correlation is apparent between soot number density and the concentration of naphthalene. On the contrary, while soot mass fraction is usually large where naphthalene is present, pockets of fluid with large soot mass are also frequent in regions with very low naphthalene mass fraction values. From the analysis of Lagrangian statistics, it is shown that soot nucleates and grows mainly in a layer close to the flame and spreads on the rich side of the flame due to the fluctuating mixing field, resulting in more than half of the total soot mass being located at mixture fractions larger than 0.6. Only a small fraction of soot is transported towards the flame and is completely oxidized in the vicinity of the stoichiometric surface. These results show the leading order effects of turbulent mixing in controlling the dynamics of soot in turbulent flames. Finally, given the difficulties in obtaining quantitative data in experiments of turbulent sooting flames, this simulation provides valuable data to guide the development of models for Large Eddy Simulation and Reynolds Average Navier Stokes approaches.     

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.     

部分预混对扩散火焰中碳烟成核的影响

通过测量扩散火焰中开始形成碳核时的临界气动变形率Ｋｐ,对丙烷中加入空气,氮气和氧气时的碳烟生成进行了实验研究,介绍了利用碳烟颗粒和ＰＡＨ对偏振激光的散射性能差异测量Ｋｐ的方法;通过比较不同条件下的Ｋｐ,得出结论：部分预燃具有遏制扩散燃烧中碳烟成核的作用,有利于减少碳烟的生成。     

Measurement and analysis of soot inception limits of oxygen-enriched coflow flames

This study explores the criteria for soot inception in oxygen-enriched laminar coflow flames. In these experiments we select an axial height in the coflow flame at which to identify the sooting limit. The sooting limit is obtained by varying the amount of inert until luminous soot first appears at this predefined height. The sooting limit flame temperature is found to increase linearly with stoichiometric mixture fraction, regardless of fuel type. To understand these results, the relationships between flame structure, temperature, and local C/O ratio is explored through the use of conserved scalar relationships. Comparison of these relationships with the experimental data indicates that the local C/O ratio is a controlling parameter for soot inception in diffusion flames (analogous to the global C/O ratio in premixed flames). Analysis of experimental results suggests that soot inception occurs when the local C/O ratio is above a critical value. The values for critical C/O ratios obtained from the analysis of experiments using several fuels are similar in magnitude to the corresponding C/O ratios for premixed flames. In addition, temperatures and PAH fluorescence were measured to identify regions in these flames most conducive to particle inception. Results indicate that the peak PAH concentration lies along a critical iso-C/O contour, which supports a theory that soot particles first appear along this critical contour, given sufficient temperature.     

A model for sooting in diffusion flames

For laminar diffusion flames the onset of sooting can be predicted by a simple extension of the Bauke-Schumann theory of such flames. Deviations from these predictions may be caused by effectively finite soot-oxidation rates and by enhanced mixing due to turbulence. Both effects have been enhanced. It is shown that the fraction of carbon in the fuel liberated as soot is determined largely by the physical characteristics of the flame rather than by the detailed chemical nature of the fuel. The effect on soot production of the addition of oxygen to the fuel is studied under conditions for which the physical parameters are carefully controlled.     

LES model for sooting turbulent nonpremixed flames

In this work, an integrated Large Eddy Simulation (LES) model is developed for sooting turbulent nonpremixed flames and validated in a laboratory scale flame. The integrated approach leverages state-of-the-art developments in both soot modeling and turbulent combustion modeling and gives special consideration to the small-scale interactions between turbulence, soot, and chemistry. The oxidation of the fuel and the formation of gas-phase soot precursors is described by the Flamelet/Progress Variable model, which has been previously extended to account for radiation losses. However, previous DNS studies have shown that Polycyclic Aromatic Hydrocarbons (PAH), the immediate precursors of soot particles, exhibit significant unsteady effects due to relatively slow chemistry. To model these unsteady effects, a transport equation is solved for a lumped PAH species. In addition, due to the removal of PAH from the gas-phase, alternative definitions of the mixture fraction, progress variable, and enthalpy are proposed. The evolution of the soot population is modeled with the Hybrid Method of Moments (HMOM), an efficient statistical model requiring the solution of only a few transport equations describing statistics of the soot population. The filtered source terms in these equations that describe the various formation, growth, and destruction processes are closed with a recently developed presumed subfilter PDF approach that accounts for the high spatial intermittency of soot. The integrated LES model is validated in a piloted natural gas turbulent jet diffusion flame and is shown to predict the magnitude of the maximum soot volume fraction in the flame relatively accurately, although the maximum soot volume fraction is shown to be rather sensitive to the subfilter scalar dissipation rate model.     

Morphological characterization of the early process of soot formation by atomic force microscopy

Atomic Force Microscopy (AFM) has been used for the characterization of nanometric particles produced in rich flames. Very small particles (about 2 nm) have been found in pre-inception region of soot forming premixed flames, whereas both small nanoparticles as well as large soot particles have been found in the soot region of the flames. The smaller particles are very flat in shape if compared with the bigger ones, and this probably depends upon the different nature of the collected particles.Particle size distribution functions are reported for different sampling conditions. The results of AFM measurements are in good agreement with previous measurements performed with ultraviolet (UV) light scattering/extinction technique on the same flames.