Stabilization, propagation and instability of tribrachial triple flames

A tribrachial (or triple) flame is one kind of edge flame that can be encountered in nonpremixed mixing layers, consisting of a lean and a rich premixed flame wing together with a trailing diffusion flame all extending from a single point. The flame could play an important role on the characteristics of various flame behaviors including lifted flames in jets, flame propagation in two-dimensional mixing layers, and autoignition fronts. The structure of tribrachial flame suggests that the edge is located along the stoichiometric contour in a mixing layer due to the coexistence of all three different types of flames. Since the edge has a premixed nature, it has unique propagation characteristics. In this review, the propagation speed of tribrachial flames will be discussed for flames propagating in mixing layers, including the effects of concentration gradient, velocity gradient, and burnt gas expansion. Based on the tribrachial edge structure observed experimentally in laminar lifted flames in jets, the flame stabilization characteristics including liftoff height, reattachment, and blowout behaviors and their buoyancy-induced instability will be explained. Various effects on liftoff heights in both free and coflow jets including jet velocity, the Schmidt number of fuel, nozzle diameter, partial premixing of air to fuel, and inert dilution to fuel are discussed. Implications of edge flames in the modeling of turbulent nonpremixed flames and the stabilization of turbulent lifted flames in jets are covered.     

Mechanism on oscillating lifted flames in nonpremixed laminar coflow jets

The oscillating lifted flame in a laminar nonpremixed nitrogen-diluted fuel jet is known to be a result of buoyancy, though the detailed physical mechanism of the initiation has not yet been properly addressed. We designed a systematic experiment to test the hypothesis that the oscillation is driven by competition between the positive buoyancy of flame and the negative buoyancy of a fuel stream heavier than the ambient air. The positive buoyancy was examined with various flame temperatures by changing fuel mole fraction, and the negative buoyancy was investigated with various fuel densities. The density of the coflow was also varied within a certain range by adding either helium or carbon dioxide to air, to study how it affected the positive and negative buoyancies at the same time. As a result, we found that the range of oscillation was well-correlated with the positive and the negative buoyancies; the former stabilized the oscillation while the latter triggered instability and became a source of the oscillation. Further measurements of the flow fields and OH radicals evidenced the important role of the negative buoyancy on the oscillation, detailing a periodic variation in the unburned flow velocity that affected the displacement of the flame.     

Stabilization mechanism of lifted flame edge in the near field of coflow jets for diluted methane

The stabilization mechanism of lifted flames in the near field of coflow jets has been investigated experimentally and numerically for methane fuel diluted with nitrogen. The lifted flames were observed only in the near field of coflow jets until blowout occurred in the normal gravity condition. To elucidate the stabilization mechanism for the stationary lifted flames of methane having the Schmidt number smaller than unity, the behavior of the flame in the buoyancy-free condition, and unsteady propagation characteristics after ignition were investigated numerically at various conditions of jet velocity. It has been found that buoyancy plays an important role for flame stabilization of lifted flames under normal gravity, such that the flame becomes attached to the nozzle in microgravity. The stabilization mechanism is found to be due to the variation of the propagation speed of the lifted flame edge with axial distance from the nozzle in the near field of the coflow as compared to the local flow velocity variation at the edge.     

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.     

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.     

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 Schmidt number on non-monotonic liftoff height behavior in laminar coflow-jet flames with diluted methane and ethylene

Stabilization characteristics of laminar lifted jet flames in a coflow were investigated experimentally to elucidate the effect of Schmidt number in methane and ethylene fuels diluted with N₂, He, and Ar. A non-monotonic (decreasing and then increasing) liftoff height (H_L) behavior with jet velocity (U₀) was observed previously for methane fuel diluted with N₂. To further elucidate the fuel Schmidt number (Sc_F) effect in exhibiting such a non-monotonic (U-shaped) behavior, various diluents (N₂, He, and Ar) were added to the fuel streams and methane and/or ethylene fuels were used. The result showed three flame types in terms of Sc_F and fuel density; nozzle-attached flame, stationary lifted flame, and oscillating flame. Among stationary lifted flames, two distinct H_L behaviors with U₀ were observed; monotonic and non-monotonic H_L behaviors. A critical Schmidt number (Sc_F,cr1) existed over which monotonically increasing behavior was observed. A second critical Schmidt number (Sc_F,cr2) also existed such that U-shaped behavior was observed for Sc_F,cr2<Sc_F<Sc_F,cr1. An oscillating lifted flame was observed for Sc_F<Sc_F,cr2. The oscillating and stationary lifted flames can be categorized in terms of the density differences among the fuel, air, and burnt gas. For the increasing H_L cases (including the increasing regime in U-shaped behavior), H_L behavior can be characterized in terms of Sc_F, the density difference between fuel and air, Sc_F,cr1, and U₀. While the decreasing H_L regime in the U-shaped behavior can be characterized with Sc_F and/or the Richardson number (defined based on the density difference between fuel and air). Oscillating flames were observed with the frequency range of 2.1–2.7 Hz by the repetitive action of positive (by burnt gas) and negative (when the fuel heavier than air) buoyancies.     

Lean or ultra-lean stretched planar methane/air flames

Lean premixed combustion has potential advantages of reducing pollutants and improving fuel economy. In some lean engine concepts, the fuel is directly injected into the combustion chamber resulting in a distribution of lean fuel/air mixtures. In this case, very lean mixtures can burn when supported by hot products from more strongly burning flames. This study examines the downstream interaction of opposed jets of a lean-limit CH₄/air mixture vs. a lean H₂/air flame. The CH₄ mixtures are near or below the lean flammability limit. The flame composition is measured by laser-induced Raman scattering and is compared to numerical simulations with detailed chemistry and molecular transport including the Soret effect. Several sub-limit lean CH₄/air flames supported by the products from the lean H₂/air flame are studied, and a small amount of CO₂ product (around 1% mole fraction) is formed in a “negative flame speed” flame where the weak CH₄/air mixture diffuses across the stagnation plane into the hot products from the H₂/air flame. Raman scattering measurements of temperature and species concentration are compared to detailed simulations using GRI-3.0, C₁, and C₂ chemical kinetic mechanisms, with good agreement obtained in the lean-limit or sub-limit flames. Stronger self-propagating CH₄/air mixtures result in a much higher concentration of product (around 6% CO₂ mole fraction), and the simulation results are sensitive to the specific chemical mechanism. These model-data comparisons for stronger CH₄/air flames improve when using either the C₂ or the Williams mechanisms.     

A comparison of computational and experimental lift-off heights of coflow laminar diffusion flames

As a sensitive marker of changes in flame structure, the number densities of excited-state CH (denoted CH^*), and excited-state OH (denoted OH^*) are imaged in coflow laminar diffusion flames. Measurements are made both in normal gravity and on the NASA KC-135 reduced-gravity aircraft. The spatial distribution of these radicals provides information about flame structure and lift-off heights that can be directly compared with computational predictions. Measurements and computations are compared over a range of buoyancy and fuel dilution levels. Results indicate that the lift-off heights and flame shapes predicted by the computations are in excellent agreement with measurement for both normal gravity (1g) and reduced gravity flames at low dilution levels. As the fuel mixture is increasingly diluted, however, the 1g lift-off heights become underpredicted. This trend continues until the computations predict stable flames at highly dilute fuel mixtures beyond the 1g experimental blow-off limit. To better understand this behavior, an analysis was performed, which indicates that the lift-off height is sensitive to the laminar flame speed of the corresponding premixed mixture at the flame edge. By varying the rates of two key “flame speed” controlling reactions, we were able to modify the predicted lift-off heights so as to be in closer agreement with the experiments. The results indicate that reaction sets that work well in low dilution systems may need to be modified to accommodate high dilution flames.     

Experimental measurements of two-dimensional planar propagating edge flames

The study of edge flames has received increased attention in recent years. This work reports the results of a recent study into two-dimensional, planar, propagating edge flames that are remote from solid surfaces (called here, “free-layer” flames, as opposed to layered flames along floors or ceilings). They represent an ideal case of a flame propagating down a flammable plume, or through a flammable layer in microgravity. The results were generated using a new apparatus in which a thin stream of gaseous fuel is injected into a low-speed laminar wind tunnel thereby forming a flammable layer along the centerline. An airfoil-shaped fuel dispenser downstream of the duct inlet issues ethane from a slot in the trailing edge. The air and ethane mix due to mass diffusion while flowing up towards the duct exit, forming a flammable layer with a steep lateral fuel concentration gradient and smaller axial fuel concentration gradient. We characterized the flow and fuel concentration fields in the duct using hot wire anemometer scans, flow visualization using smoke traces, and non-reacting, numerical modeling using COSMOSFloWorks. In the experiment, a hot wire near the exit ignites the ethane-air layer, with the flame propagating downwards towards the fuel source. Reported here are tests with the air inlet velocity of 25 cm/s and ethane flows of 967-1299 sccm, which gave conditions ranging from lean to rich along the centerline. In these conditions the flame spreads at a constant rate faster than the laminar burning rate for a premixed ethane-air mixture. The flame spread rate increases with increasing transverse fuel gradient (obtained by increasing the fuel flow rate), but appears to reach a maximum. The flow field shows little effect due to the flame approach near the igniter, but shows significant effect, including flow reversal, well ahead of the flame as it approaches the airfoil fuel source.     

Dynamics and stability of premixed hydrogen-air flames in square microchannels with wall temperature gradients

The dynamics and stability of premixed hydrogen-air flames in square microchannels with heated walls were investigated through three-dimensional direct numerical simulations. The inlet velocity and equivalence ratio were 1.5 m/s and 0.5. The effect of the wall temperature gradient characteristics on the flame dynamics and stability was examined varying the width and location of the wall temperature gradient for a channel height of 1.5 mm. Five distinct flame modes were observed at different wall temperature profiles: flame with repetitive extinction-ignition (FREI), pulsating flame, laterally oscillating flame, spinning flame, and steady flame modes. Furthermore, transitions between these flame modes were observed for specific inflow and boundary conditions. The effect of the channel height on the flame stability was investigated by varying the channel height from 1.0 mm to 1.677 mm for a fixed wall temperature gradient. As the channel height was increased, four of the flame modes, namely, FREI, laterally oscillating flame, spinning flame, and steady flame modes appeared sequentially. To determine whether this sequential appearance was associated with the variation of the wall heat loss, the maximum wall temperature was changed by small amounts. For a lower wall temperature, the laterally oscillating flame mode transitioned to the FREI mode, and for a higher wall temperature, unstable flame modes such as the FREI and laterally oscillating flame modes disappeared, resulting in stable flame.     

Decreasing liftoff height behavior in diluted laminar lifted methane jet flames

Stabilization of laminar lifted coflow jet flames of nitrogen-diluted methane was investigated experimentally and numerically. As the fuel jet velocity was increased, two distinct behaviors in liftoff height were observed depending on the initial fuel mole fraction; a monotonically increasing trend and a decreasing and then increasing trend (U-shaped behavior). The former was observed in the jet-developing region and the latter in the jet-developed region. Because the decreasing behavior of liftoff height with jet velocity has not been observed at ambient temperature, the present study focuses on decreasing liftoff height behavior. To elucidate the physical mechanism underlying the U-shaped behavior, numerical simulations of reacting jets were conducted by adopting a skeletal mechanism. The U-shaped behavior was related to the buoyancy. At small jet velocities, the relative importance of the buoyancy over convection was strong and the flow field was accelerated in the downstream region to stabilize the lifted flame. As the jet velocity increased, the relative importance of buoyancy decreased and the liftoff height decreased. As the jet velocity further increased, the flame stabilization was controlled by jet momentum and the liftoff height increased.     

Control of oscillating combustion and noise based on local flame structure

To control combustion oscillations, the characteristics of an oscillating swirl injection premixed flame have been investigated, and control of oscillating combustion and noise based on local flame structure has been conducted. The r.m.s. value of pressure fluctuations and noise level show significantly large values between = 0.8 and 1.1. The beating of pressure fluctuations is observed for the large oscillating flame conditions in this combustor. Relationship between beating of pressure fluctuations and local flame structure was observed by the simultaneous measurement of CH/OH planar laser induced fluorescence and pressure fluctuations. The local flame structure and beating of pressure fluctuations are related and the most complicated flame is formed in the middle pressure fluctuating region of beating. The beating of pressure fluctuations, which plays important roles in noise generation and nitric oxide emission in this combustor, could be controlled by injecting secondary fuel into the recirculating region of oscillating flames. Injecting secondary fuel prevented lean blowout, and low NO_x combustion was also achieved even for the case of pure methane injection as a secondary fuel. By injecting secondary fuel into the recirculating region near the swirl injector, the flame lifted from the swirl injector and its reaction region became uniform and widespread, hence resulting in low nitric oxide emission. Secondary mixture injection, fuel diluted with air, is not effective for control of combustion oscillations suppression and lean blowout prevention.     

Air/fuel mixing in jet flames

The paper examines eight diverse regimes in which fuels can mix and react with air. These comprise: (i) Lifted subsonic; and (ii) supersonic jet flames, with (iii) and without (iv) cross flows; (v) Rim-attached flames; (vi) Early Downwash flames; (vii) Downwash-attached jet flames; and (viii) Fire Whirls.Correlations of characteristics within these regimes are principally in terms of a dimensionless Flow Number, U*, Cross Flow Reynolds number, Re_c, and, for Fire Whirls, a dimensionless Critical Velocity, CV. Boundaries of seven of the eight regimes are identified, through plots of U*, against Re_c, and of the eighth through a plot of CV against U*. The circumstances of transitions between regimes are identified. The study involves a variety of CH₄ cross flow flame measurements, in a wind tunnel. Cross flows can initially create a small lee-side flame downwash, due to the depression in pressure. With increasing fuel flow this might extend 1.3 m downwards from the horizontal tip of the vertical burner. Jet flames can attach to the downwash, which can become significant above Re_c ≈ 2000. More extensive downwash might further delay blow-off. Regime boundaries are constructed on the U*/Re_c diagram covering lifted flames, early downwash, and downwash-attached flames. The most powerful flames tend to be lifted, choked, flames, with cross flow, and fire whirls. Combustion becomes less efficient at high Re_c and low U*, although CH₄ was efficiently reacted.Experimental values of the ratio of fuel to air velocity, u/u_c, of CH₄ flames ranged between about 10 and 30 for lifted flames, and between 0.3 and 3.6, at blow-off, for rim-attached flames. The latter comprise an important category, often intermediate between lifted flames and downwash-attached flames.     

浮力对矩形射流扩散火焰影响的大涡模拟

本文对浮力作用下的矩形射流扩散燃烧过程进行了三维大涡模拟。数值模拟结果展示了浮力作用下矩形射流扩散火焰的动态弯曲过程,比较分析了射流速度对火焰刚性的影响,发现射流速度越高火焰弯曲程度越小、燃料喷射距离越远。对浮力作用下的水平射流横截面流动分析结果表明,由于流向涡的卷吸作用在局部区域存在逆着浮力方向的流动。     

The effect of co-flow stream velocity on turbulent non-premixed jet flame stability

The stability behaviour of non-premixed jet flames in a co-flowing air stream was investigated experimentally. The experimental data obtained indicate that there exists a range of co-flow velocity where two distinctly different extinction limits can occur at the same co-flow velocity depending on whether the flame is lifted or attached at ignition. Results show that co-flow velocity has a much greater effect on the blowout limits of lifted flames than on the blowoff limits of attached flames. The blowout limit of lifted flames initially increase linearly with co-flow velocity independent of nozzle diameter until a peak value is reached, after which it decreases rapidly with increasing co-flow velocity. Such behaviour appears to be governed by two different mechanisms. A model for predicting lifted flame blowout limits has been developed. It is based on the ratio of the Kolmogorov time scale and the chemical time scale as a function of a jet similarity parameter. The model was used to predict the blowout limits for methane as well as the effect of diluents in either fuel or co-flow stream. Results show very good agreement with experimental data in the current investigation.     

Dynamic behavior of diffusion flame interacting with a large-scale vortex by laser imaging techniques

Instantaneous and simultaneous measurements of two-dimensional temperature and OH-LIF profiles by combining Rayleigh scattering with laser induced fluorescence (LIF) were demonstrated in a nitrogen-diluted hydrogen (H₂ 30% + N₂ 70%) laminar normal diffusion flame interacting with a large-scale vortex by oscillating central fuel flow or in an inverse diffusion flame by oscillating central airflow. The dynamic behavior of the diffusion flame extinction and reignition during the flame–vortex interaction processes was investigated. The results obtained are described as follows. (1) The width of the reaction zone decreases remarkably, and a decrease in flame temperature and OH-LIF is seen with increasing central airflow in an inverse diffusion flame. OH-LIF increases, and temperature does not change with increasing central fuel flow in a normal diffusion flame. The computations predict the experimental results well, and it is revealed that flame temperature characteristics result from the preferential diffusion of heat and species, which induces excess enthalpy or on enthalpy deficit, and an increase or decrease in H₂ mole fraction in the flame. (2) When a large velocity fluctuation is given to the central flow, the temperature and the OH-LIF at the reaction zone become thin at the convex and circumferential part of the vortex where a high temperature layer exists, and the temperature at the reaction zone is lowered in the inverse flame and the normal flame. (3) The width and temperature of the reaction zone interacting with the vortex recover quickly to that of the laminar steady flame after the vortex passing in the normal flame, but the recovery to that of the steady flame after the vortex passing is delayed in the inverse flame. (4) When a remarkably large velocity fluctuation is given to the central airflow in the inverse flame, thinning of temperature and reaction zone starts at the convex and circumferential part of the vortex, resulting in a and flame extinction completely occurs at the tail part of the vortex and makes the pair of edge flames. The outside edge flame reignites and connects with the upstream reaction zone. The inside edge flame finally extinguishes as the supply of fuel is interrupted by the outside edge flame.     

Dynamics of triple-flames in ignition of turbulent dual fuel mixture: A direct numerical simulation study

Pilot-ignited dual fuel combustion involves a complex transition between the pilot fuel autoignition and the premixed-like phase of combustion, which is challenging for experimental measurement and numerical modelling, and not sufficiently explored. To further understand the fundamentals of the dual fuel ignition processes, the transient ignition and subsequent flame development in a turbulent dimethyl ether (DME)/methane-air mixing layer under diesel engine-relevant conditions are studied by direct numerical simulations (DNS). Results indicate that combustion is initiated by a two-stage autoignition that involves both low-temperature and high-temperature chemistry. The first stage autoignition is initiated at the stoichiometric mixture, and then the ignition front propagates against the mixture fraction gradient into rich mixtures and eventually forms a diffusively-supported cool flame. The second stage ignition kernels are spatially distributed around the most reactive mixture fraction with a low scalar dissipation rate. Multiple triple flames are established and propagate along the stoichiometric mixture, which is proven to play an essential role in the flame developing process. The edge flames gradually get close to each other with their branches eventually connected. It is the leading lean premixed branch that initiates the steady propagating methane-air flame. The time required for the initiation of steady flame is substantially shorter than the autoignition delay time of the methane-air mixture under the same thermochemical condition. Temporal evolution of the displacement speed at the flame front is also investigated to clarify the propagation characteristics of the combustion waves. Cool flame and propagation of triple flames are also identified in this study, which are novel features of the pilot-ignited dual fuel combustion.     

Strain rate and fuel composition dependence of chemiluminescent species profiles in non-premixed counterflow flames: comparison with model results

A detailed comparison has been conducted between chemiluminescence (CL) species profiles of OH^?, CH^?, and C₂ ^?, obtained experimentally and from detailed flame kinetics modeling, respectively, of atmospheric pressure non-premixed flames formed in the forward stagnation region of a fuel flow ejected from a porous cylinder and an air counterflow. Both pure methane and mixtures of methane with hydrogen (between 10 and 30 % by volume) were used as fuels. By varying the air-flow velocities methane flames were operated at strain rates between 100 and 350 s^?1, while for methane/hydrogen flames the strain rate was fixed at 200 s^?1. Spatial profiles perpendicular to the flame front were extracted from spectrograms recorded with a spectrometer/CCD camera system and evaluating each spectral band individually. Flame kinetics modeling was accomplished with an in-house chemical mechanism including C₁–C₄ chemistry, as well as elementary steps for the formation, removal, and electronic quenching of all measured active species. In the CH₄/air flames, experiments and model results agree with respect to trends in profile peak intensity and position. For the CH₄/H₂/air flames, with increasing H₂ content in the fuel the experimental CL peak intensities decrease slightly and their peak positions shift towards the fuel side, while for the model the drop in mole fraction is much stronger and the peak positions move closer to the fuel side. For both fuel compositions the modeled profiles peak closer to the fuel side than in the experiments. The discrepancies can only partly be attributed to the limited attainable spatial resolution but may also necessitate revised reaction mechanisms for predicting CL species in this type of flame.