Pressure and temperature dependence of soot in highly controlled counterflow ethylene diffusion flames

Soot volume fraction and dispersion index were measured by pyrometry in a series of highly controlled counterflow diffusion flames, with peak temperatures, T_max, spanning a few hundred degrees and pressure covering the 0.1–0.8 MPa range. An unprecedented level of control was implemented by selecting flames with a self-similar structure to ensure that the normalized temperature-time history experienced by the reactants was the same, regardless of pressure. The self-similarity was verified by suitably rescaling the transverse coordinate with respect to a characteristic diffusion length. At constant T_max, the soot volume fraction increases approximately by two orders of magnitude as the pressure is raised from 1 atm to 4 atm, and by one to two additional orders of magnitude with an additional doubling of the pressure to 8 atm. At constant pressure, the soot load spans two to three orders of magnitude and soot formation exhibits increased sensitivity to temperature as the pressure is raised. Soot inception occurs near the flame, with an increase in soot concentration that becomes steeper at higher T_max. The increase is accompanied by a decrease in the dispersion exponent that is suggestive of dehydrogenation and aging of the particles and is sharper at higher T_max. Soot experiences continuous growth in a monotonically decreasing temperature field until it is convected away radially at the stagnation plane, with essentially no opportunity for oxidation. Evidence of two distinct mechanisms for soot formation was found: the classic high temperature, high activation energy process affecting soot formed in the vicinity of the flame and followed by dehydrogenation; and a relatively low-temperature, zero activation energy process, associated with the increase in volume fraction at low-temperatures in proximity of the stagnation plane. The latter is tentatively attributed to dimerization of aromatics, as revealed by the concurrent increase in the dispersion index corresponding to an increase in the particle hydrogen content.     

Polydisperse effects in jet spray flames

A laminar jet polydisperse spray diffusion flame is analysed mathematically for the first time using an extension of classical similarity solutions for gaseous jet flames. The analysis enables a comparison to be drawn between conditions for flame stability or flame blow-out for purely gaseous flames and for spray flames. It is found that, in contrast to the Schmidt number criteria relevant to gas flames, droplet size and initial spray polydispersity play a critical role in determining potential flame scenarios. Some qualitative agreement for lift-off height is found when comparing predictions of the theory and sparse independent experimental evidence from the literature.     

Polydisperse spray diffusion flames in oscillating flow

The phenomenon of droplet clustering or grouping found when a spray of droplets is moving in an oscillating host flow field is investigated for the case of a polydisperse spray that fuels a laminar co-flow diffusion flame. A mathematical solution is developed for the liquid phase based on use of small Stokes numbers for size sections into which the polydisperse spray size distribution is divided. Droplet clustering in the oscillatory flow field is accounted for by constructing a special model for the sectional vaporization Damkohler numbers in accordance with droplet size. Combining this with a formal solution for a gas phase Schvab-Zel'dovich variable yields the means whereby flame dynamics can be described. Results calculated from this solution demonstrate that preferential droplet size behaviour (with smaller droplets tending to cluster to a greater extent and reduce the vaporization Damkohler number more than larger ones) can have a major impact on the flame dynamics through local droplet enrichment with attendant consequences on the production of fuel vapour. The dynamics of the sort of flame (over- or under-ventilated) and the occurrence of flame pinching leading to multiple flame sheets are altered under these circumstances. However, potential control of the actual initial spray polydispersity may reduce the intensity of such effects.     

Axis switching in turbulent buoyant diffusion flames

Dynamics of buoyant diffusion flames from rectangular, square, and round fuel sources were investigated using direct numerical simulation (DNS). Fully three-dimensional simulations were performed employing high-order numerical methods and boundary conditions to solve governing equations for variable-density flow and finite-rate Arrhenius chemistry. Significant differences among the different cases were revealed in the vortex dynamics, entrainment rate, small-scale mixing, and consequently flame structures. Mixing and entrainment enhancement in non-circular flames in comparison with circular ones was explained using the Biot–Savart instability theory, which relates vortex dynamics to the local azimuthal curvature. An extension of the theory elucidated why rectangular flames entrain more efficiently and spread wider than square ones, although both configurations have corners. It also provided an explanation for the aspect ratio effects in the near field. In the far field, nonlinear effects were dominant and the general transport equations for vorticity were analyzed in detail. The corner effects and aspect ratio effects were shown to be augmented by the intricate interactions among vortex dynamics, combustion, and buoyancy through the various terms in the equations. The presence of corners in non-circular flames led to concentrated regions of fine-scale mixing and intense reactions centered around the corners. Moreover, the rectangular flames exhibited a different dynamic behavior from even the square one, by creating discrepancies in entrainment, mixing, and combustion between the minor and major axis directions. Increasing the aspect ratio exacerbated such directional discrepancies, and ultimately led to axis switching. It was the first time that axis switching was observed by DNS in a rectangular flame of aspect ratio 3, which raised further questions in combustion prediction and control. Finally, a unified explanation for corner and aspect ratio effects was given on the basis of the Biot–Savart instability theory and the vorticity transport equations.     

Oscillations and island evolution in radiating diffusion flames

We show how an island (isola) evolves out of the usual S-curve of steady states of diffusion flames when radiation losses are accounted for and how it eventually disappears when radiation increases further. At small activation temperatures there are never any islands. We show that stable oscillations evolve first out of perturbations of steady states on the S-curve at large Damköhler numbers. Only if the activation temperature is large enough do they also appear on the islands. The region of the stable oscillations grows larger as activation temperature decreases.     

Period doubling cascade in diffusion flames

Here it is shown that chaotic oscillations can appear after a series of period doublings in radiating diffusion flames when the activation temperature is high enough. It is also shown that period doubling cascades appear typically in very small regions and that they may not be observable if one starts with small perturbations of a steady flame.     

Diffusive-thermal instabilities of diffusion flames: onset of cells and oscillations

A comprehensive stability analysis of planar diffusion flames is presented within the context of a constant-density model. The analysis provides a complete characterization of the possible patterns that are likely to be observed as a result of differential and preferential diffusion when a planar flame becomes unstable. A whole range of physical parameters is considered, including the Lewis numbers associated with the fuel and the oxidizer, the initial mixture fraction, and the flow conditions. The two main forms of instability are cellular flames, obtained primarily in fuel-lean systems when the Lewis numbers are generally less than one, and planar pulsations, obtained in fuel-rich systems when the Lewis numbers are generally larger than one. The cellular instability is predominantly characterized by stationary cells of characteristic dimension comparable to the diffusion length, but smaller cells that scale on the reaction zone thickness are also possible near extinction conditions. The pulsating instability is characterized by planar oscillations normal to the flame sheet with a well-defined frequency comparable to the reciprocal of the diffusion time; high-frequency modes are also possible just prior to extinction. The analysis also alludes to other possible patterns, such as oscillating cellular structures, which result from competing modes of instability of comparable and/or disparate scales. The expected pattern depends of course on the underlying physical parameters. Consequently, stability boundaries have been identified for the onset of one or another form of the instability. The conditions for the onset of cellular and pulsating flames, as well as the predicted cell size and the frequency of oscillations, compare well with the experimental record.     

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.     

Dynamics and burning limits of near-limit hot,warm, and cool diffusion flames of dimethyl ether at elevated pressures

The near-limit diffusion flame regimes and extinction limits of dimethyl ether at elevated pressures and temperatures are examined numerically in the counterflow geometry with and without radiation at different oxygen concentrations. It is found that there are three different flame regimes—hot flame, warm flame, and cool flame—which exist, respectively, at high, intermediate, and low temperatures. Furthermore, they are governed by three distinct chain-branching reaction pathways. The results demonstrate that the warm flame has a double reaction zone structure and plays a critical role in the transition between cool and hot flames. It is also shown that the cool flame can be formed in several different ways: by either radiative extinction or stretch extinction of a hot flame or by stretch extinction of a warm flame. A warm flame can also be formed by radiative extinction of a hot flame or ignition of a cool flame. A general €-shaped flammability diagram showing the burning limits of all three flame regimes at different oxygen mole fractions is obtained. The results show that thermal radiation, reactant concentration, temperature, and pressure all have significant impacts on the flammable regions of the three flame regimes. Increases in oxidizer temperature, oxygen concentration, and pressure shift the cool flame regime to higher stretch rates and cause the warm flame to have two extinction limits. At elevated temperatures, it is found that there is a direct transition between the hot flame and warm flame at low stretch rates. The results also show that, unlike the hot flame, the cool flame structure cannot be scaled by using pressure-weighted stretch rates due to the its significant reactant leakage and strong dependence of reactivity on pressure. The present results advance the understanding of near-limit flame dynamics and provide guidance for experimental observation of different flame regimes.     

Carbon nanotube synthesis on catalytic metal alloys in methane/air counterflow diffusion flames

Various morphologies of multi-walled carbon nanotubes (MWNTs) are grown catalytically on metal-alloy probes in counterflow diffusion flames using methane as fuel. Carbon nanotube (CNT) properties and morphologies are investigated as functions of local gas-phase temperatures, C-related species concentrations (e.g. C₂H₂, CO), sampling positions, C₂H₂ adding to the fuel, and metal-alloy compositions (i.e., Fe, Fe/Cr, Ni/Cu, Ni/Ti, Ni/Cr, Ni/Cr/Fe). MWNTs grow optimally in non-sooty regions of the flames. C₂H₂ addition is found to promote direct synthesis of vertically well-aligned MWNTs with uniform diameters from Ni/Cr/Fe and Ni/Ti alloys.     

Computational and experimental study of JP-8, a surrogate, and its components in counterflow diffusion flames

Non-sooting counterflow diffusion flames have been studied both computationally and experimentally, using either JP-8, or a six-component JP-8 surrogate mixture, or its individual components. The computational study employs a counterflow diffusion flame model, the solution of which is coupled with arc length continuation to examine a wide variety of inlet conditions and to calculate extinction limits. The surrogate model includes a semi-detailed kinetic mechanism composed of 221 gaseous species participating in 5032 reactions. Experimentally, counterflow diffusion flames are established, in which multicomponent fuel vaporization is achieved through the use of an ultrasonic nebulizer that introduces small fuel droplets into a heated nitrogen stream, fostering complete vaporization without fractional distillation. Temperature profiles and extinction limits are measured in all flames and compared with predictions using the semi-detailed mechanism. These measurements show good agreement with predictions in single-component n-dodecane, methylcyclohexane, and iso-octane flames. Good agreement also exists between predicted and measured variables in flames of the surrogate, and the agreement is even better between the experimental JP-8 flames and the surrogate predictions.     

Dynamics of ball-like flames in extremely low-speed counterflow field in near-lean limit low-Lewis number mixture

For the understanding of ball-like flame behavior in counterflow field, transient three-dimensional computations with thermal-diffusion model were conducted for a low-Lewis number mixture near lean limit. Three types of flame behaviors were confirmed: stable spherical ball-like flame (spherical BLF) in A ≤ 0.010, stable non-spherical ball-like flame (non-spherical BLF) in 0.01 < A < 0.089 and splitting ball-like flame (splitting BLF) in A ≥ 0.089, where A is ordinary stretch rate normalized with laminar burning velocity S_L and thermal diffusivity α. Analysis of flame structure for non-spherical BLF located its center at the stagnation point showed that the maximum temperature on the stagnation plane was higher than that on the counterflow axis because of the small difference between the flame curvatures on the stagnation plane and that on the counterflow axis. With the increase of stretch rate, the maximum temperature of the non-spherical BLF on the stagnation plane increased and the position of maximum temperature got away from the stagnation point. The maximum temperature on the counterflow axis decreased and the position of maximum temperature got closer to the stagnation point. Existence of unburned fuel was also confirmed near the stagnation point at A = 0.085. Thus, net fuel velocity was newly introduced to evaluate the effect of the unburned fuel diffusion. The profile of the net fuel velocity revealed two peaks in the case of A < 0.050 and four peaks in the case of A > 0.050. In the case A > 0.050, the inner two peaks were found to be due to the diffusion of unburned fuel to the outward direction. The analyses on the peak positions showed that the flame splitting occurs when the positions of the inner two peaks of the net fuel velocity are located outside of the reference flame ball radius.     

Effect of Lewis number on premixed laminar lean-limit flames stabilized on a bluff body

In this paper, we present a study on the effect of Lewis number, Le, on the stabilization and blow-off of laminar lean limit premixed flames stabilized on a cylindrical bluff body. Numerical simulations and experiments are conducted for propane, methane and two blends of hydrogen with methane as fuel gases, containing 20% and 40% of hydrogen by volume, respectively. It is found that the Le?>?1 flame blows-off via convection from the base of the flame (without formation of a neck) when the conditions for flame anchoring are not fulfilled. Le?≤?1 flames exhibit a necking phenomenon just before lean blow-off. This necking of the flame front is a result of the local reduction in mass burning rates causing flame merging and quenching of the thin flame tube formed. The structure of these flames at the necking location is found to be similar to tubular flames. It is found that extinction stretch rates for tubular flames closely match values at the neck location of bluff-body flames of corresponding mixtures, suggesting that excessive flame stretch is directly responsible for blow-off of the studied Le?≤?1 flames. After quenching of the neck, the upstream part forms a steady and stable residual flame in the wake of the bluff body while the downstream part is convected away.     

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.     

A numerical study on extinction behaviour of laminar micro-diffusion flames

We conducted a numerical study on the fluid dynamic, thermal and chemical structures of laminar methane–air micro flames established under quiescent atmospheric conditions. The micro flame is defined as a flame on the order of one millimetre or less established at the exit of a vertically-aligned straight tube. The numerical model consists of convective–diffusive heat and mass transport with a one-step, irreversible, exothermic reaction with selected kinetics constants validated for near-extinction analyses. Calculations conducted under the burner rim temperature 300 K and the adiabatic burner wall showed that there is the minimum burner diameter for the micro flame to exist. The Damköhler number (the ratio of the diffusive transport time to the chemical time) was used to explain why a flame with a height of less than a few hundred microns is not able to exist under the adiabatic burner wall condition. We also conducted scaling analysis to explain the difference in extinction characteristics caused by different burner wall conditions. This study also discussed the difference in governing mechanisms between micro flames and microgravity flames, both of which exhibit similar spherical flame shape.     

The two-dimensional structure of low strain rate counterflow nonpremixed-methane flames in normal and microgravity

The structure and extinction of low strain rate nonpremixed methane–air flames was studied numerically and experimentally. A time-dependent axisymmetric two-dimensional (2D) model considering buoyancy effects and radiative heat transfer was developed to capture the structure and extinction limits of normal gravity (1-g) and zero gravity (0-g) flames. For comparison with the 2D modelling results, a one-dimensional (1D) flamelet computation using a previously developed numerical code was exercised to provide information on the 0-g flames. A 3-step global reaction mechanism was used in both the 1D and 2D computations to predict the measured extinction limit and flame temperature. Photographic images of flames undergoing the process of extinction were compared with model calculations. The axisymmetric numerical model was validated by comparing flame shapes, temperature profiles, and extinction limits with experiments and with the 1D computational results. The 2D computations yielded insight into the extinction mode and flame structure. A specific maximum heat release rate was introduced to quantify the local flame strength and to elucidate the extinction mechanism. The contribution by each term in the energy equation to the heat release rate was evaluated to investigate the multi-dimensional structure and radiative extinction of the 1-g flames. Two combustion regimes depending on the extinction mode were identified. Lateral heat loss effects and multi-dimensional flame and flow structure were also found. At low strain rates in 1-g flames (‘regime A’), the flame is extinguished from the weak outer edge of the flame, which is attributed to a multi-dimensional flame structure and flow field. At high strain rates, (‘regime B’), the flame extinction initiates near the flame centreline owing to an increased diluent concentration in the reaction zone, similar to the extinction mode of 1D flames. These two extinction modes can be clearly explained by consideration of the specific maximum heat release rate.     

Accurate treatment of size distribution effects in polydisperse spray diffusion flames: multi-fluid modelling,computations and experiments

An Eulerian multi-fluid model is validated by comparison with experimental measurements in the test case of laminar spray counterflow diffusion flames. Special attention is devoted, both from the modelling and experimental point of view, to the treatment of the droplet distribution tail, characterized by the rare occurrence of relatively large droplets carrying a non-negligible amount of mass. The Eulerian multi-fluid approach is shown to capture the dynamics, evaporation and heating of the droplets with a limited number of sections and, thus, at a modest cost. This simplification will be essential for the use of multi-fluid methods in multi-dimensional problems.     

On the two-reactant one-step activation-energy asymptotics for steady,adiabatic, planar flames with Lewis numbers of unity

Aspects of predictions of activation-energy asymptotics concerning the dependence of the burning velocity on the equivalence ratio are examined here through both asymptotic analyses and numerical computation. In typical hydrocarbon–air flames, the burning velocity achieves its maximum value for fuel-rich mixture, the cause being generally attributed to the effects of detailed chemical kinetics and unequal diffusivities of the reactants. The present results demonstrate the possibility of this attribute of the burning velocity occurring even when these two effects are absent. This is accomplished by parametrically studying the burning-velocity formula valid for all equivalence ratios under the conditions specified in the title of this article, with special attention paid to implications for hydrocarbon–air flames.     

Large eddy simulations of partially premixed ethanol dilute spray flames using the flamelet generated manifold model

The paper presents Large Eddy Simulations (LESs) for the Sydney ethanol piloted turbulent dilute spray flames ETF2, ETF6, and ETF7. The Flamelet Generated Manifold (FGM) approach is employed to predict mixing and burning of the evaporating fuel droplets. A methodology to match the experimental inflow spray profiles is presented. The spray statistical time-averaged results show reasonable agreement with mean and RMS data. The Particle Size Distribution (PSD) shows a good match downstream of the nozzle exit and up to x/D = 10. At x/D = 20 and 30 the PSD is under-predicted for droplets with mean diameter D10 > 20μm and over-predicted for the smaller size droplets. The simulations reasonably predict the reported mean flame structure and length. The effect of increasing the carrier velocity (ETF2–ETF7) or decreasing the liquid fuel injection mass flow rate (ETF2–ETF6) is found to result in a leaner, shorter flame and stronger spray–flow interactions. Higher tendency to local extinction is observed for ETF7 which is closer to blow-off compared to ETF2 and has higher scalar dissipation rates, higher range of Stokes number, and faster droplet response. The possible sources of LES-FGM deviations from the measurements are discussed and highlighted. In particular, the spray time-averaged statistical error contribution is quantified and the impact of the inflow uncertainty is studied. Sensitivity analysis to the pre-vaporized nozzle fuel mass fraction show that such small inflow perturbations (by ±?2% for the ETF2 flame) have a strong impact on the flame structure, and the droplets’ dynamics. Conditional scatter plots show that the flame exhibits wide range of mixing conditions and bimodal mixing lines particularly at upstream locations (x/D?相似文献