Numerical study of the effects of gravity on soot formation in laminar coflow methane/air diffusion flames under different air stream velocities

Numerical simulations of laminar coflow methane/air diffusion flames at atmospheric pressure and different gravity levels were conducted to gain a better understanding of the effects of gravity on soot formation by using relatively detailed gas-phase chemistry and complex thermal and transport properties coupled with a semi-empirical two-equation soot model. Thermal radiation was calculated using the discrete-ordinates method coupled with a non-grey model for the radiative properties of CO, CO₂, H₂O, and soot. Calculations were conducted for three coflow air velocities of 77.6, 30, and 5 cm/s to investigate how the coflowing air velocity affects the flame structure and soot formation at different levels of gravity. The coflow air velocity has a rather significant effect on the streamwise velocity and the fluid parcel residence time, especially at reduced gravity levels. The flame height and the visible flame height in general increase with decreasing the gravity level. The peak flame temperature decreases with decreasing either the coflow air stream velocity or the gravity level. The peak soot volume fraction of the flame at microgravity can either be greater or less than that of its normal gravity counterpart, depending on the coflow air velocity. At sufficiently high coflow air velocity, the peak soot volume fraction increases with decreasing the gravity level. When the coflow air velocity is low enough, soot formation is greatly suppressed at microgravity and extinguishment occurs in the upper portion of the flame with soot emission from the tip of the flame owing to incomplete oxidation. The numerical results provide further insights into the intimate coupling between flame size, residence time, thermal radiation, and soot formation at reduced gravity level. The importance of thermal radiation heat transfer and coflow air velocity to the flame structure and soot formation at microgravity is demonstrated for the first time.     

A computational study of soot formation and flame structure of coflow laminar methane/air diffusion flames under microgravity and normal gravity

A numerical study is conducted of methane–air coflow diffusion flames at microgravity (μg) and normal gravity (1g), and comparisons are made with experimental data in the literature. The model employed uses a detailed gas phase chemical kinetic mechanism that includes PAH formation and growth, and is coupled to a sectional soot particle dynamics model. The model is able to accurately predict the trends observed experimentally with reduction of gravity without any tuning of the model for different flames. The microgravity sooting flames were found to have lower temperatures and higher volume fraction than their normal gravity counterparts. In the absence of gravity, the flame radii increase due to elimination of buoyance forces and reduction of flow velocity, which is consistent with experimental observations. Soot formation along the wings is seen to be surface growth dominated, while PAH condensation plays a more major role on centreline soot formation. Surface growth and PAH growth increase in microgravity primarily due to increases in the residence time inside the flame. The rate of increase of surface growth is more significant compared to PAH growth, which causes soot distribution to shift from the centreline of the flame to the wings in microgravity.     

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.     

Influence of Strouhal number on pulsating methane–air coflow jet diffusion flames

Four periodically time-varying methane–air laminar coflow jet diffusion flames, each forced by pulsating the fuel jet's exit velocity U _j sinusoidally with a different modulation frequency w _j and with a 50% amplitude variation, have been computed. Combustion of methane has been modeled by using a chemical mechanism with 15 species and 42 reactions, and the solution of the unsteady Navier–Stokes equations has been obtained numerically by using a modified vorticity-velocity formulation in the limit of low Mach number. The effect of w _j on temperature and chemistry has been studied in detail. Three different regimes are found depending on the flame's Strouhal number S = aw _j /U _j , with a denoting the fuel jet radius. For small Strouhal number (S = 0.1), the modulation introduces a perturbation that travels very far downstream, and certain variables oscillate at the frequency imposed by the fuel jet modulation. As the Strouhal number grows, the nondimensional frequency approaches the natural frequency of oscillation of the flickering flame (S ? 0.2). A coupling with the pulsation frequency enhances the effect of the imposed modulation and a vigorous pinch-off is observed for S = 0.25 and S = 0.5. Larger values of S confine the oscillation to the jet's near-exit region, and the effects of the pulsation are reduced to small wiggles in the temperature and concentration values. Temperature and species mass fractions change appreciably near the jet centerline, where variations of over 2 % for the temperature and 15 % and 40 % for the CO and OH mass fractions, respectively, are found. Transverse to the jet movement, however, the variations almost disappear at radial distances on the order of the fuel jet radius, indicating a fast damping of the oscillation in the spanwise direction.     

Implementation of an advanced fixed sectional aerosol dynamics model with soot aggregate formation in a laminar methane/air coflow diffusion flame

An advanced fixed sectional aerosol dynamics model describing the evolution of soot particles under simultaneous nucleation, coagulation, surface growth and oxidation processes is successfully implemented to model soot formation in a two-dimensional laminar axisymmetric coflow methane/air diffusion flame. This fixed sectional model takes into account soot aggregate formation and is able to provide soot aggregate and primary particle size distributions. Soot nucleation, surface growth and oxidation steps are based on the model of Fairweather et al. Soot equations are solved simultaneously to ensure convergence. The numerically calculated flame temperature, species concentrations and soot volume fraction are in good agreement with the experimental data in the literature. The structures of soot aggregates are determined by the nucleation, coagulation, surface growth and oxidation processes. The result of the soot aggregate size distribution function shows that the aggregate number density is dominated by small aggregates while the aggregate mass density is generally dominated by aggregates of intermediate size. Parallel computation with the domain decomposition method is employed to speed up the calculation. Three different domain decomposition schemes are discussed and compared. Using 12 processors, a speed-up of almost 10 is achieved which makes it feasible to model soot formation in laminar coflow diffusion flames with detailed chemistry and detailed aerosol dynamics.     

An experimental and modeling study of tetramethyl ethylene pyrolysis with polycyclic aromatic hydrocarbon formation

Iso-olefins, in the C₅–C₈ range can potentially be blended with renewable gasoline fuels to increase their research octane number (RON) and octane sensitivity (S). RON and S increase with the degree of branching in iso-olefins and this is a desirable fuel anti-knock quality in modern spark-ignited direct-injection engines. However, these iso-olefins tend to form larger concentrations of aromatic species leading to the formation of polycyclic aromatic hydrocarbons (PAHs). Thus, it is important to understand the pyrolysis chemistry of these iso-olefins. In this study, a new detailed chemical kinetic mechanism is developed to describe the pyrolysis of tetramethyl ethylene (TME), a symmetric iso-olefin. The mechanism, which includes the formation of PAHs, is validated against species versus temperature (700–1160 K) measurements in a jet-stirred reactor at atmospheric pressure and in a single-pulse shock tube at a pressure of 5 bar in the temperature range 1150–1600 K. Synchrotron vacuum ultraviolet photoionization mass spectrometer (SVUV-PIMS) and gas chromatography (GC) systems were used to quantify the species in the jet-stirred reactor and in the single-pulse shock tube, respectively. The mechanism derives its base and PAH chemistry from the LLNL PAH sub-mechanism. The predictions are accurate for most of the species measured in both facilities. However, there is scope for mechanism improvement by understanding the consumption pathways for some of the intermediate species such as isoprene. The formation of 1, 2, and 3-ring aromatic species such as benzene, toluene, naphthalene and phenanthrene measured experimentally is analyzed using the chemical kinetic mechanism. It is found that the PAH formation chemistry for TME under pyrolysis conditions is driven by both propargyl addition reactions and the HACA mechanism.     

Characteristics of laminar lifted flames in coflow jets with initial temperature variation

Characteristics of laminar lifted flames of propane highly diluted with nitrogen have been investigated by varying the initial temperature in coflow jets. The result showed that the lifted flame maintained the tribrachial structure up to the initial temperature of 900 K and the liftoff height decreased with initial temperature and dilution ratio. The overall behavior of liftoff heights correlated well with the jet velocity scaled by the stoichiometric laminar burning velocity, emphasizing the importance of the stoichiometric laminar burning velocity on the propagation speed of tribrachial flame. The exponent of the liftoff height with jet velocity in the relation of increased with initial fuel mole fraction, which has been attributed to the differential diffusion between propane and diluent nitrogen. Consequently, nitrogen concentration varied along the stoichiometric contour, which affected the propagation speed. Also, the exponent increased with initial temperature due to the sensitiveness of the propagation speed variation with nitrogen dilution on initial temperature. The liftoff conditions have been observed for the jet velocity even smaller than the stoichiometric laminar burning velocity at relatively low initial temperatures. This can be attributed to the effect of the buoyancy. Liftoff velocities accounting for the relative buoyancy effect were found to have a satisfactory correlation with regardless of initial temperatures and nitrogen dilution.     

Effects of soot absorption and scattering on LII intensities in laminar coflow diffusion flames

Absorption and scattering of laser-induced incandescence (LII) intensities by soot particles present between the measurement volume and the detector were numerically investigated at detection wavelengths of 400 and 780 nm in a laminar coflow ethylene/air flame. The radiative properties of aggregated soot particles were calculated using the Rayleigh-Debye-Gans polydisperse fractal aggregate theory. The radiative transfer equation in emitting, absorbing, and scattering media was solved using the discrete-ordinates method. The radiation intensity along an arbitrary direction was obtained using the infinitely small weight technique. The effects of absorption and scattering on LII intensities are found to be significant under the conditions of this study, especially at the shorter detection wavelength and when the soot volume fraction is higher. Such a wavelength-dependent signal-trapping effect leads to a lower soot particle temperature estimated from the ratio of uncorrected LII intensities at the two detection wavelengths. The corresponding soot volume fraction derived from the absolute LII intensity technique is overestimated. The Beer-Lambert relationship can be used to describe radiation attenuation in absorbing and scattering media with good accuracy provided the effective extinction coefficient is adequately.     

Laser-induced fluorescence measurements and modeling of absolute CH concentrations in strained laminar methane/air diffusion flames

We have applied linear laser-induced fluorescence to obtain spatially resolved profiles of CH radicals in laminar methane/air and methane/nitric oxide/air counterflow diffusion flames at atmospheric pressure. Excitation and detection of transitions in the A–X band and calibrating the optical detection efficiency via Rayleigh scattering allowed the determination of absolute radical concentrations. Flames at strain rates from 59 to 269 s⁻¹ were studied to characterize the strain rate dependence of the CH concentration. The work shows that CH concentrations increase with increasing strain rate. Comparisons have been made with predicted CH levels obtained using two different chemical kinetic mechanisms (Lindstedt et al. and GRI-Mech. 3.0). Computed concentrations are shown to be in good agreement with experimental data. It was furthermore found that the addition of up to 600 ppm NO to the fuel did not have a measurable effect on the CH radical concentration. This is also in agreement with predictions from both mechanisms. The current work has shown that measurements of absolute CH radical concentrations are possible in non-premixed flames without the need for spatial temperature or quenching corrections.     

Numerical investigation of laminar cross-flow non-premixed flames in the presence of a bluff-body

A knowledge of flame stability regimes in the presence of cylindrical bluff-bodies of various dimensions is essential to design non-premixed burners. The reacting flow field in such cases is reported to be three-dimensional and unsteady. In the literature, only a few experimental investigations with limited measurements are available. Therefore, in this work, a detailed numerical study of laminar cross-flow non-premixed methane–air flames in the presence of a square cylinder is presented. The flow, temperature, species and reaction fields have been predicted using a comprehensive transient three-dimensional reacting flow model with detailed chemical kinetics and variable thermo-physical properties, in order to get a good insight into the flame stabilisation phenomena. Further, analyses of quantities such as local equivalence ratio, cell Damköhler number, species velocity, net consumption rate of methane, which are not easily obtained through experiments even with detailed diagnostics, have been carried out. The influence of the flow field due to varying inlet velocity of the oxidiser, in the presence of the bluff-body, on flame anchoring location has been analysed in detail. Local equivalence ratio contours obtained from non-reacting flow calculations are seen to be quite useful in analysing the mixing process and in the prediction of flame anchoring locations when the flames are not separated. Cell Damköhler number has been calculated using cell size, species velocity of the fuel, which is a derived quantity, and the net reaction rate of the fuel. The flame zone, which is customarily inferred from the contours of temperature, CO and OH, is also shown to be predicted well by the contour line corresponding to a Damköhler number equal to unity. The net reaction rate of CH₄ and the net rates of two dominant reactions, which consume methane, show clearly the variation in the flame anchoring locations in these three cases. Further, the three-dimensionality of these flames are analysed by plotting the mean temperature contours in y–z planes. Finally, the unsteadiness in the separated flame case is analysed.     

Effect of velocity gradient on propagation speed of tribrachial flames in laminar coflow jets

The effect of velocity gradient on the propagation speed of tribrachial flame edge has been investigated experimentally in laminar coflow jets for propane fuel. It was observed that the propagation speed of tribrachial flame showed appreciable deviations at various jet velocities in high mixture fraction gradient regime. From the similarity solutions, it was demonstrated that the velocity gradient varied significantly during the flame propagation. To examine the effect of velocity gradient, detail structures of tribrachial flames were investigated from OH LIF images and Abel transformed images of flame luminosity. It was revealed that the tribrachial point was located on the slanted surface of the premixed wing, and this slanted angle was correlated with the velocity gradient along the stoichiometric contour. The temperature field was visualized qualitatively by the Rayleigh scattering image. The propagation speed of tribrachial flame was corrected by considering the direction of flame propagation with the slanted angle and effective heat conduction to upstream. The corrected propagation speed of tribrachial flame was correlated well. Thus, the mixture fraction gradient together with the velocity gradient affected the propagation speed.     

Combined experimental and computational study of laminar, axisymmetric hydrogen–air diffusion flames

We investigate the structure of two-dimensional, axisymmetric, laminar hydrogen–air flames in which a cylindrical fuel stream is surrounded by coflowing air, using laser-diagnostic and computational methods. Spontaneous Raman scattering and coherent anti-Stokes Raman scattering (CARS) are used to measure the distributions of major species and temperature. Computationally, we solve the governing conservation equations for mass, momentum, energy, and species, using detailed chemistry and transport. The fuel is diluted with nitrogen (1:1) to reduce heat transfer to the burner, to match the zero temperature gradient at the fuel exit. Three average fuel exit velocities are studied: 18, 27, and 50 cm/s. Comparisons of the measured and computed results are performed for radial profiles at a number of axial positions, and along the axial centerline. Peak major species mole fractions and temperatures are quantitatively predicted by the computations, and the axial species profiles are predicted to within the experimental uncertainty. In the radial profiles studied, base-case computations excluding thermal diffusion of light species were in excellent agreement with the measurements. While the addition of thermal diffusion led to some discrepancy with the measured results, the magnitude of the differences was no more than 25%. The computations predicted the axial centerline profiles from the burner exit to the maximum temperature well, though the experimental temperatures in the downstream mixing region decreased somewhat faster than the computed profiles. Radiative losses are seen to be negligible in these flames, and changes in transport properties and variations in initial flow velocities generally led to only modest changes in the axial profiles. The results also show that the detailed axial profiles of major species and temperature at different fuel jet velocities scale quantitatively with the jet velocity.     

Blending effect of methanol on the formation of polycyclic aromatic hydrocarbons in the oxidation of toluene

Methanol has been considered as a potential renewable liquid fuel and blending it with gasoline and diesel is an effective way to reduce greenhouse gas emissions from the transport sector. To understand the mixing effect of methanol on the formation of polycyclic aromatic hydrocarbons (PAHs) and oxygenated PAHs (OPAHs), the fuel-rich oxidation of toluene with and without methanol was studied using a flow reactor at atmospheric pressure, temperatures from 1050 to 1350 K, equivalence ratio of 9.0, and residence time of 1.2 s. The blending ratio of methanol was varied as 0% and 50% on a molar basis. Gas chromatograph mass spectrometer was employed to identify and quantify PAHs and OPAHs in gaseous products. A kinetic model on PAH growth up to five ring structures was used to investigate the blending effect on PAH and OPAH formation. Both experiment and modeling showed that PAH and OPAH production at lower temperatures was unexpectedly promoted in toluene/methanol oxidation compared with toluene oxidation, while their production in toluene oxidation was identical with or larger than that in toluene/methanol oxidation at elevated temperatures. In methanol oxidation, no PAHs were produced under the current experimental conditions. Kinetic analysis indicated that high methanol reactivity produced several radicals, such as OH, H, and HO₂, which promoted toluene reactivity at lower temperatures, resulting in the enlargement of PAH and OPAH formation in toluene/methanol oxidation compared to neat toluene oxidation. When the temperature was increased, the effect of methanol blending was diminished based on the kinetic analysis. These results suggest that oxygenated fuels do not necessarily reduce PAH production, but promote it under some conditions.     

Effects of radiation heat loss on laminar premixed ammonia/air flames

Ammonia (NH₃) direct combustion is attracting attention for energy utilization without CO₂ emissions, but fundamental knowledge related to ammonia combustion is still insufficient. This study was designed to examine effects of radiation heat loss on laminar ammonia/air premixed flames because of their very low flame speeds. After numerical simulations for 1-D planar flames with and without radiation heat loss modeled by the optically thin model were conducted, effects of radiation heat loss on flame speeds, flame structure and emissions were investigated. Simulations were also conducted for methane/air mixtures as a reference. Effects of radiation heat loss on flame speeds were strong only near the flammability limits for methane, but were strong over widely diverse equivalence ratios for ammonia. The lower radiative flame temperature suppressed the thermal decomposition of unburned ammonia to hydrogen (H₂) at rich conditions. The equivalence ratio for a low emission window of ammonia and nitric oxide (NO) in the radiative condition shifted to a lower value than that in the adiabatic condition.     

The formation of polycyclic aromatic hydrocarbons from the supercritical pyrolysis of 1-methylnaphthalene

In order to better understand the pyrolytic reactions leading to the formation of polycyclic aromatic hydrocarbons (PAH) and carbonaceous solids under supercritical conditions, we have pyrolyzed the model fuel 1-methylnaphthalene (critical temperature, 499 °C; critical pressure, 36 atm) in an isothermal silica-lined stainless-steel reactor coil at 585 °C, 110 atm, and 140 s. Analysis of the reaction products by high-pressure liquid chromatography with diode-array ultraviolet-visible absorbance detection and mass spectrometric detection has led to the identification of 37 individual 2- to 7-ring PAH—fifteen of which have never before been reported as products of 1-methylnaphthalene pyrolysis. The absence, among the reaction products, of single-ring aromatics and acetylene indicates that there is no aromatic ring rupture in this reaction environment, and the structures of each of the 5- to 7-ring PAH products reveal the intactness of the two 2-ring naphthalene units required in their construction. Proposed reaction pathways involving species plentiful in the reaction environment—1-naphthylmethyl radical, methyl radical, 1-methylnaphthalene, naphthalene, and 2-methylnaphthalene—account for the formation of the observed 5- to 7-ring PAH products. These reaction pathways, along with consideration of bond dissociation energies and relative abundances of reactant species, account for the extremely high product selectivity exhibited by the observed product PAH. The detection of seven 8- and 9-ring PAH, each requiring construction from three naphthalene or methylnaphthalene units, provides evidence that the types of reaction mechanisms outlined here—for the combination of two naphthalene entities to form 5- to 7-ring PAH—are also likely to apply to the combination of three and more such entities in the formation of larger-ring-number PAH and eventually carbonaceous solids.     

Effect of electric fields on reattachment and propagation speed of tribrachial flames in laminar coflow jets

The effects of electric fields on the reattachment of lifted flames have been investigated experimentally in laminar coflow jets with propane fuel by applying high voltages to the fuel nozzle. In case of AC, the frequency has also been varied. Results showed that reattachment occurred at higher jet velocity when applying the AC voltages, thus the stabilization limit of attached flames was extended by the AC electric field. Higher voltage and lower frequency of the AC were found to be more effective. On the contrary, the effect of DC was found to be minimal. To understand the early onset of the reattachment with the AC, occurring at higher jet velocity, the influence of AC electric fields on the propagation speed of tribrachial flame edge was investigated during the transient reattachment processes. The propagation speed increased reasonably linearly with the applied AC voltage and decreased inversely to the distance between the flame edge and the nozzle electrode. Consequently, the enhancement in the propagation speed of tribrachial flame edge was correlated well with the electric field intensity, defined as the applied AC voltage divided by the distance.