Further studies of the reaction kernel structure and stabilization of jet diffusion flames

The structure of axisymmetric laminar jet diffusion flames of ethane, ethylene, acetylene, and propane in quasi-quiescent air has been studied numerically in normal earth gravity (1g) and zero gravity (0g). The time-dependent full Navier–Stokes equations with buoyancy were solved using an implicit, third-order accurate numerical scheme, including a C₃-chemistry model and an optically thin-media radiation model for heat losses. Observations of the flames were also made at the NASA Glenn 2.2-Second Drop Tower. For all cases of the fuels and gravity levels investigated, a peak reactivity spot, i.e., reaction kernel, was formed in the flame base, thereby holding a trailing diffusion flame. The location of the reaction kernel with respect to the burner rim depended inversely on the reaction-kernel reactivity or velocity. In the C₂ and C₃ hydrocarbon flames, the H₂–O₂ chain reactions were important at the reaction kernel, yet the CH₃ + O → CH₂O + H reaction, a dominant contributor to the heat-release rate in methane flames studied previously, did not outweigh other exothermic reactions. Instead of the C₁-route oxidation pathway in methane flames, the C₂ and C₃ hydrocarbon fuels dehydrogenated on the fuel side and acetylene was a major hydrocarbon fragment burning at the reaction kernel. The reaction-kernel correlations between the reactivity (the heat-release or oxygen-consumption rate) and the velocity, obtained previously for methane, were developed further for various fuels in more universal forms using variables related to local Damköhler numbers and Peclet numbers.     

Effect of stoichiometric mixture fraction on soot fraction and emission spectra with application to oxy-combustion

Many proposed oxy-combustion concepts for carbon capture incorporate the recycling of flue gas which is used as a dilution gas to aid in the control of temperature and heat flux. Improvements in efficiency may be realized by significantly reducing the recycle flue gas (RFG), however, in application, care must be taken to avoid excessive radiant heat flux and gas temperature. One of the features oxy-combustion, unlike air-fired combustion, is that the oxygen and dilution gases are initially separated. RFG can, for example, be strategically blended with either the fuel stream, or oxidizer stream, or both, which affects the stoichiometric mixture fraction, Z_st. In this work, the effects of the amount of dilution, or RFG, and Z_st on soot fraction are experimentally investigated in a laminar coflow flame. Carbon dioxide is employed as the dilution gas to simulate the recycling of dry flue gas. Soot fraction and temperature are quantitatively determined by a flame image processing technique. In addition, the visible and near-IR emission spectra are given. When dilution, or RFG, is reduced, while holding Z_st constant, soot formation and thermal radiation increase due to higher temperature. However, high temperature flames with reduced or zero soot are achieved by increasing Z_st via the combination of fuel dilution and oxygen enrichment. This study highlights the inherent flexibility of oxy-fuel combustion, which offers the opportunity to control flame temperature and gas volume while independently controlling soot formation and radiant heat transfer.     

Structure of propagating edge diffusion flames in hydrocarbon fuel jets

Direct numerical simulations with a C₃-chemistry model have been performed to investigate the transient behavior and internal structure of flames propagating in an axisymmetric fuel jet of methane, ethane, ethylene, acetylene, or propane in normal earth gravity (1g) and zero gravity (0g). The fuel issued from a 3-mm-i.d. tube into quasi-quiescent air for a fixed mixing time of 0.3 s before it was ignited along the centerline where the fuel–air mixture was at stoichiometry. The edge of the flame formed a vigorously burning peak reactivity spot, i.e., reaction kernel, and propagated through a flammable mixture layer, leaving behind a trailing diffusion flame. The reaction kernel broadened laterally across the flammable mixture layer and possessed characteristics of premixed flames in the direction of propagation and unique flame structure in the transverse direction. The reaction kernel grew wings on both fuel and air sides to form a triple-flame-like structure, particularly for ethylene and acetylene, whereas for alkanes, the fuel-rich wing tended to merge with the main diffusion flame zone, particularly methane. The topology of edge diffusion flames depend on the properties of fuels, particularly the rich flammability limit, and the mechanistic oxidation pathways. The transit velocity of edge diffusion flames, determined from a time series of calculated temperature field, equaled to the measured laminar flame speed of the stoichiometric fuel–air mixtures, available in the literature, independent of the gravity level.     

Characteristics and structure of inverse flames of natural gas

Characteristics and structure of nominally non-premixed flames of natural gas are investigated using a burner that employs simultaneously two distinct features: fuel and oxidiser direct injection, and inverse fuel and oxidiser delivery. At low exit velocities, the result is an inverse diffusion flame that has been noted in the past for its low NO_x emissions, soot luminosity, and narrow stability limits. The present study aimed at extending the burner operating range, and it demonstrated that the inverse flame exhibits a varying degree of partial premixing dependent on the discharge nozzle conditions and the ratio of inner air jet and outer fuel jet velocities. These two variables affect the flame length, temperature distributions, and stability limits. Temperature measurements and Schlieren visualisation show areas of enhanced turbulent mixing in the shear region and the presence of a well-mixed reaction zone on the flame centreline. This reaction zone is enveloped by an outer diffusion flame, yielding a unique double-flame structure. As the fuel–air equivalence ratio is decreasing with an increase in the inner jet velocity, the well-mixed reaction zone extends considerably. These findings suggest a method for establishing a flame of uniform high temperature by optimising the coaxial nozzle geometry and flow conditions. The normalised flame length is decreasing exponentially with the air/fuel velocity ratio. Measurements demonstrate that the inverse flame stability limits change qualitatively with varying degree of partial premixing. At the low premixing level, the flame blow-out is a function of the inner and outer jet velocities and the nozzle conditions. The flame blow-out at high degree of partial premixing occurs abruptly at a single value of the inner air jet velocity, regardless of the fuel jet velocity and almost independent of the discharge nozzle conditions.     

Synthesis of carbon nanotubes on Ni-alloy and Si-substrates using counterflow methane–air diffusion flames

We have conducted an experimental study to investigate the synthesis of multi-walled carbon nanotubes (CNTs) in counterflow methane–air diffusion flames, with emphasis on effects of catalyst, temperature, and the air-side strain rate of the flow on CNTs growth. The counterflow flame was formed by fuel (CH₄ or CH₄ + N₂) and air streams impinging on each other. Two types of substrates were used to deposit CNTs. Ni-alloy (60% Ni + 26% Cr + 14% Fe) wire substrates synthesized curved and entangled CNTs, which have both straight and bamboo-like structures; Si-substrates with porous anodic aluminum oxide (AAO) nanotemplates synthesized well-aligned, self-assembled CNTs. These CNTs grown inside nanopores had a uniform geometry with controllable length and diameter. The axial temperature profiles of the flow were measured by a 125 μm diameter Pt/10% Rh–Pt thermocouple with a 0.3 mm bead junction. It was found that temperature could affect not only the success of CNTs synthesis, but also the morphology of synthesized CNTs. It was also found, against previous general belief, that there was a common temperature region (1023–1073 K) in chemical vapor deposition (CVD) and counterflow diffusion flames where CNTs could be produced. CNTs synthesized in counterflow flames were significantly affected by air-side strain rate not through the residence time, but through carbon sources available for CNTs growth. Off-symmetric counterflow flames could synthesize high-quality CNTs because with this configuration carbon sources at the fuel side could easily diffuse across the stagnation surface to support CNTs growth. These results show the feasibility of using counterflow flames to synthesize CNTs for particular applications such as fabricating nanoscale electronic devices.     

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.     

Experimental and numerical study of premixed, lean ethylene flames

Ethylene is a key intermediate in the combustion mechanisms of most practical fuels. It plays also an important role in the formation of aromatic hydrocarbons and soot particules. The latter has motivated many experimental and numerical studies carried out on rich ethylene-air mixtures. Less studies have been devoted to lean mixtures, and the development of strategies based on lean, premixed flames to reduce soot and NO_x production requires additional experimental data in lean conditions. In this work, the chemical structure of lean premixed ethylene-oxygen-nitrogen flames stabilized on a flat-flame burner at atmospheric pressure was determined experimentally. The species mole fraction profiles were also computed by the Premix code (Chemkin II version) and four detailed reaction mechanisms. A very good agreement was observed for the main flame properties: reactants consumption, final products (CO₂, H₂O) and the main intermediates: CO and H₂. Marked differences occurred in the prediction of active intermediate species present in small concentrations. Pathways analyses were performed to identify the origins of these discrepancies. It was shown that the same reactions were involved in the four mechanisms to describe the consumption of ethylene, but with marked differences in their relative importance. C₂H₃ and CH₂HCO are the main radicals formed in this first step and their consumption increases the differences between the mechanisms either by the use of different kinetic data for common reactions or by differences in the nature of the consumption reactions.     

Compressible turbulent flame speeds of highly turbulent standing flames

In this paper we present the first measurement of turbulent burning velocities of a highly turbulent compressible standing flame induced by shock-driven turbulence in a Turbulent Shock Tube. High-speed schlieren, chemiluminescence, PIV, and dynamic pressure measurements are made to quantify flame–turbulence interaction for high levels of turbulence at elevated temperatures and pressure. Distributions of turbulent velocities, vorticity and turbulent strain are provided for regions ahead and behind the standing flame. The turbulent flame speed is directly measured for the high-Mach standing turbulent flame. From measurements of the flame turbulent speed and turbulent Mach number, transition into a non-linear compressibility regime at turbulent Mach numbers above 0.4 is confirmed, and a possible mechanism for flame generated turbulence and deflagration-to-detonation transition is established.     

Burning velocity of triple flames with gentle concentration gradient

We investigated the local flame speed of a two-dimensional, methane-air triple flame in a rectangular burner. The velocity fields and the concentration profiles were measured with particle image velocimetry and the Rayleigh scattering method, respectively. There was a requisite combination of initial velocity and initial concentration gradient for consistency of the local concentration gradient at the leading edge of the flame. In these cases, the flame curvatures were also consistent. Accordingly, the burning velocity, defined as local flow velocity at the triple point, was determined by the flame curvature. The burning velocity increased with increasing flame curvature, when the curvature was near zero. After that, the burning velocity decreased with increasing curvature. The peak value thus exceeded the adiabatic one-dimensional laminar burning velocity. Comparing the effects of the measured flame stretch rate on the flow strain κ_s and flame curvature κ_c, κ_s is larger and increases more rapidly than κ_c for flame curvatures satisfying 1/R_f < 250 m⁻¹ and then becomes constant while κ_c still increases for 250 m⁻¹ < 1/R_f, so that κ_c becomes much larger than κ_s. There is also a peak in burning velocity at roughly the transition in flame curvature specified above. Therefore, the burning velocity for a low concentration gradient correlates with the flame stretch rate.     

Experimental measurements of geometric properties of turbulent stratified flames

Combustion under stratified conditions is common in many systems. However, relatively little is known about the structure and dynamics of turbulent stratified flames. Two-dimensional imaging diagnostics are applied to premixed and stratified V-flames at a mean equivalence ratio of 0.77, and low turbulent intensity, within the corrugated flame range. The present results show that stratification affects the mean turbulent flame speed, structure and geometric properties. Stratification increases the flame surface density above the premixed flame levels in all cases, with a maximum reached at intermediate levels of stratification. The flame surface density (FSD) of stratified flames is higher than that of premixed flames at the same mean equivalence ratio. Under the present conditions, the FSD peaks at a stratification ratio around 3.0. The FSD curves for stratified flames are further skewed towards the product side. The distribution of flame curvature in stratified flames is broader and more symmetric relative to premixed flames, indicating an additional mechanism of curvature generation, which is not necessarily due to cusping. These experiments indicate that flame stratification affects the intrinsic behaviour of turbulent flames and suggest that models may need to be revised in the light of the current evidence.     

Three-dimensional direct simulations and structure of expanding turbulent methane flames

Direct numerical simulations (DNS) are ideally suited to investigate in detail turbulent reacting flows in simple geometries. For an increasing number of applications, detailed models must be employed to describe the chemical processes with sufficient accuracy. Despite the huge cost of such simulations, recent progress has allowed the direct numerical simulation of turbulent premixed flames while employing complete reaction schemes. We briefly describe our own developments in this field and use the resulting DNS code to investigate more extensively the structure of premixed methane flames expanding in a three-dimensional turbulent velocity field, initially homogeneous and isotropic. This situation typifies, for example, the initial flame development after spark ignition in a gas turbine or an internal combustion engine. First investigation steps have been carried out at low turbulence levels on this same configuration in the past Symposium, and we build on top of these former results. Here, a considerably higher Reynolds number is considered, the simulation has been repeated twice in to limit the possibility of spurious, very specific results, and several complementary post-processing steps are carried out. Characteristic features concerning the observed combustion regime are presented. We then investigate in a quantitative manner the evolution of flame surface area, global stretch-rate, flame front curvature, flame thickness, and correlation between thickness and curvature. The possibility of obtaining reliable information on flame front curvature from two-dimensional slices is checked by comparison with the exact procedure.     

Modelling of lifted turbulent diffusion flames in a channel mixing layer by the flame hole dynamics

The partial quenching structure of turbulent diffusion flames in a turbulent mixing layer is investigated by the method of flame hole dynamics as an effort to develop a prediction model for the turbulent flame lift off. The essence of the flame hole dynamics is derivation of the random walk mapping, from the flame-edge theory, which governs expansion or contraction of the quenching holes initially created by the local quenching events. The numerical simulation for the flame hole dynamics is carried out in two stages. First, a direct numerical simulation is performed for a constant-density fuel–air channel mixing layer to obtain the background turbulent flow and mixing fields, from which a time series of two-dimensional scalar-dissipation-rate array is extracted. Subsequently, a Lagrangian simulation of the flame hole random walk mapping, projected to the scalar dissipation rate array, yields a temporally evolving turbulent extinction process and its statistics on partial quenching characteristics. In particular, the probability of encountering the reacting state, while conditioned with the instantaneous scalar dissipation rate, is examined to reveal that the conditional probability has a sharp transition across the crossover scalar dissipation rate, at which the flame edge changes its direction of propagation. This statistical characteristic implies that the flame edge propagation instead of the local quenching event is the main mechanism controlling the partial quenching events in turbulent flames. In addition, the conditional probability can be approximated by a heavyside function across the crossover scalar dissipation rate.     

Hyperacceleration effects on turbulent combustion in premixed step-stabilized flames

Experimental results are presented from an investigation of the effects of large transverse accelerations on flame propagation and blowout limits in premixed step-stabilized flames. The accelerations, which exceed ±10,000 g in the present study, induce large body forces on the high-density reactants and low-density products. These body forces can substantially alter the flame propagation mechanisms and dramatically increase the flame blowout limits. Sustained centripetal accelerations a_c ≡ U²/R are created by flowing a premixed propane–air reactant stream with equivalence ratios 0.7  Φ  1.9 at various speeds U through a semicircular channel with radius R. A backward-facing step of height h on the radially outer (a_c > 0) or inner (a_c < 0) wall stabilizes the flame. For a_c > 0 the acceleration acts to force high-density reactants into the recirculation zone and low-density products into the reactant stream, while a_c < 0 forces hot products into the recirculation zone and impedes cold reactants from entering this zone. An otherwise identical straight channel provides corresponding baseline (a_c = 0) results for comparison. The flow speed U, equivalence ratio Φ, and step height h are systematically varied for a_c = 0, a_c > 0, and a_c < 0. Shadowgraph and chemiluminescence imaging show that as a_c→ +∞ the propagation of the flame across the channel becomes independent of the flame burning velocity and instead is primarily due to large-scale “centrifugal pumping” driven by the induced body forces. For a_c → −∞ the body forces effectively segregate reactants and products to produce a nearly flat flame. In both cases, for large |a_c| values the resulting blowout limits can be substantially higher than those at a_c = 0.     

Burning characteristics of candle flames in sub-atmospheric pressures: An experimental study and scaling analysis

Burning characteristics (mass burning rate, natural convection boundary layer thickness, flame height and dark zone height) of laminar diffusion flames produced by a candle at sub-atmospheric pressures in the range of P_∞?=?50–100?kPa were experimentally studied in a reduced-pressure chamber; such data are not reported to date. Scaling analysis was performed to interpret the pressure dependence. The new experimental findings for candle flames in the sub-atmospheric pressures were well interpreted by the proposed scaling laws: (1) the mass burning rate was higher for a candle with larger wick length, and it increased with increasing ambient pressure, a stagnant layer B-number model based on natural convection boundary (flame boundary layer thickness) was developed to scale the mass burning rate of candle flames at various pressures; (2) the flame boundary layer thickness was wider in lower pressure and can be well represented by a natural convection boundary layer solution; (3) flame height was higher for a candle with larger wick length, meanwhile the ratio of flame height to burning rate was independent of pressure; (4) the flame dark zone height representing a soot formation length scale changes little with pressure, meanwhile its ratio to the total flame height is scaled with pressure by $P_{\infty }^{?1/2}/L_{w,e}^{3/4}$ (L_w,e is effective wick length inside flame). This work provided new experimental data and scaling laws of candle flame behaviors in sub-atmospheric pressures, which provided information for future characterization and soot modeling for diffusion flames associated with melting and evaporation processes of solid fuels.     

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.     

Influence of advective heat flux on extinction scalar dissipation rate and velocity in negative edge flames

A counterflow flame geometry, which has previously been experimentally shown to produce stable negative edge flames, was studied using numerical simulations. In this geometry, the flame edge is formed off the counterflow centreline owing to a local increase in scalar dissipation rate. Hot products from the stable nonpremixed flame on the centreline flow through the edge at velocities of ～ 1–5 m/s. The size of the counterflow burner and the gas flowrates are varied in the simulations to alter the flame strength and velocity at the flame edge. The advection of products through the edge is shown to extend the flame extinction to higher scalar dissipation rates than required for centreline extinction. For high velocities, the scalar dissipation rate required for flame extinction can be related to the centreline extinction value by considering only the effect of energy addition to the flame edge via advection. However, for lower edge flame velocities, the effects of increased thermal and species diffusion through the edge must also be included. Since the advection at the edge is a product of both the local velocity and temperature gradient, a single correlation between the scalar dissipation rate and the negative edge flame velocity does not exist.     

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.     

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.