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.     

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.     

Stationary combustion regimes and extinction limits of one-dimensional stretched premixed flames in a gap between two heat conducting plates

Stationary combustion regimes, their linear stability and extinction limits of stretched premixed flames in a narrow gap between two heat conducting plates are studied by means of numerical simulations in the framework of one-dimensional thermal-diffusion model with overall one-step reaction. Various stationary combustion modes including normal flame (NF), near-stagnation plane flame (NSF), weak flame (WF) and distant flame (DF) are detected and found to be analogous to the same-named regimes of conventional counterflow flames. For the flames stabilized in the vicinity of stagnation plane at moderate and large stretch rates (which are NF, NSF and WF) the effect of channel walls is basically reduced to additional heat loss. For distant flame characterized by large flame separation distance and small stretch rates intensive interphase heat transfer and heat recirculation are typical. It is shown that in mixture content / stretch rate plane the extinction limit curve has ε-shape, while for conventional counterflow flames it is known to be C-shaped. This result is quite in line with recent experimental findings and is explained by extension of extinction limits at small stretch rates at the expense of heat recirculation. Analysis of the numerical results makes possible to reveal prime mechanisms of flame quenching on different branches of ε-shaped extinction limit curve. Namely, two upper limits are caused by stretch and heat loss. These limits are direct analogs of the upper and lower limits on conventional C-shaped curve. Two other limits are related with weakening of heat recirculation and heat dissipation to the burner. Thus, the present study provides a satisfactory explanation for the recent experimental observations of stretched flames in narrow channel.     

On the existence of steady-state gaseous microgravity spherical diffusion flames in the presence of radiation heat loss

Whether steady-state gaseous microgravity spherical diffusion exist in the presence of radiation heat loss is an important fundamental question and has important implications for spacecraft fire safety. In this work, experiments aboard the International Space Station and a transient numerical model are used to investigate the existence of steady-state microgravity spherical diffusion flames. Gaseous spherical diffusion flames stabilized on a porous spherical burner are employed in normal (i.e., fuel flowing into an ambient oxidizer) and inverse (i.e., oxidizer flowing into an ambient fuel) flame configurations. The fuel is ethylene and the oxidizer oxygen, both diluted with nitrogen. The flow rate of the reactant gas from the burner is held constant. It is found that steady-state gaseous microgravity spherical diffusion flames can exist in the presence of radiation heat loss, provided that the steady-state flame size is less than the flame size for radiative extinction, and the flame develops fast enough that radiation heat loss does not drop the flame temperature below the critical temperature for radiative extinction (1130 K). A simple model is provided that allows for the identification of initial conditions that can lead to steady-state spherical diffusion flames. In the spherical, infinite domain configuration, the characteristic time for the diffusion-controlled system to effectively reach steady-state is found to be on the order of 100,000 s. Despite a narrow range of attainable conditions, flames that exhibit steady-state behavior are observed aboard the ISS for up to 870 s, even with the constraint of a finite boundary. Steady-state flames are simulated using the numerical model for over 100,000 s.     

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.     

Extinction and flame bifurcations of stretched dimethyl ether premixed flames

Extinction limits and flame bifurcation of lean premixed dimethyl ether–air flames are numerically investigated using the counterflow flame with a reduced chemistry. Emphasis is paid to the combined effect of radiation and flame stretch on the extinction and flammability limits. A method based on the reaction front is presented to predict the Markstein length. The predicted positive Markstein length agrees well with the experimental data. The results show that flow stretch significantly reduces the flame speed and narrows the flammability limit of the stretched dimethyl ether–air flame. It is found that the combined effect of radiation and flow stretch results in a new flame bifurcation and multiple flame regimes. At an equivalence ratio slightly higher than the flammability limit of the planar flame, the distant flame regime appears at low stretch rates. With an increase in the equivalence ratio, in addition to the distant flame, a weak flame isola emerges at moderate stretch rates. With a further increase in the equivalence ratio, the distant flame and the weak flame branches merge together, resulting in the splitting of the weak flame branch into two weak flame branches, one at low stretch and the other at high stretch. Flame stability analysis demonstrates that the high stretch weak flame is also stable. Furthermore, a K-shaped flammability limit diagram showing various flame regimes and their extinction limits is obtained.     

On the global structure and asymptotic stability of low-stretch diffusion flame: Forced convection

The present work analyzes cylindrical diffusion flames (Tsuji burner) under low stretch condition, considering fuel injection also from the backward region of the burner. To highlight the fundamental aspects of this flame, some assumptions are imposed, like constant thermodynamic and transport coefficients, unitary Lewis number and no radiative heat loss. It is also considered potential flow model and incompressible Navier–Stokes model. Despite the simplicity of the former model, results from both models show good agreement. Also, an asymptotic analysis describing the problem far from the burner is able to capture the most important mechanisms controlling the flame, then the flame shape is determined and the dependence of the characteristic length scales on Peclet number (based on the burner properties), free stream velocity and stoichiometry is revealed. The results show that the flame width is proportional to the mass stoichiometric coefficient and reciprocal to the Peclet number the 1/4 power and free stream velocity the 3/4 power, and that the flame height is proportional to the square of the mass stoichiometric coefficient and to the square root of the ratio of Peclet number to free stream velocity. In addition, an asymptotic stability analysis reveals low-stretch flame extinction to be caused by reduction in fuel and oxidizer concentrations, which provides the range of the stoichiometric coefficient for stable regime, and at the same time the range of heat released.     

Three-dimensional numerical simulations of cellular jet diffusion flames

Recent experimental investigations have demonstrated that the appearance of particular cellular states in circular non-premixed jet flames significantly depends on a number of parameters, including the initial mixture strength, reactant Lewis numbers, and proximity to the extinction limit (Damköhler number). For CO₂-diluted H₂/O₂ jet diffusion flames, these studies have shown that a variety of different cellular patterns or states can form. For given fuel and oxidizer compositions, several preferred states were found to co-exist, and the particular state realized was determined by the initial conditions. To elucidate the dynamics of cellular instabilities, circular non-premixed jet flames are modeled with a combination of three-dimensional numerical simulation and linear stability analysis (LSA). In both formulations, chemistry is described by a single-step, finite-rate reaction, and different reactant Lewis numbers and molecular weights are specified. The three-dimensional numerical simulations show that different cellular flames can be obtained close to extinction and that different states co-exist for the same parameter values. Similar to the experiments, the behavior of the cell structures is sensitive to (numerical) noise. During the transient blow-off process, the flame undergoes transitions to structures with different number of cells, while the flame edge close to the nozzle oscillates in the streamwise direction. For conditions similar to the experiments discussed, the LSA results reveal various cellular instabilities, typically with azimuthal wavenumber m = 1–6. Consistent with previous theoretical work, the propensity for the cellular instabilities is shown to increase with decreasing reactant Lewis number and Damköhler number.     

Characteristics of microjet methane diffusion flames

Characteristics of microjet methane diffusion flames stabilized on top of the vertically oriented, stainless-steel tubes with an inner diameter ranging from 186 to 778 μ m are investigated experimentally, theoretically and numerically. Of particular interest are the flame shape, flame length and quenching limit, as they may be related to the minimum size and power of the devices in which such flames would be used for future micro-power generation. Experimental measurements of the flame shape, flame length and quenching velocity are compared with theoretical predictions as well as detailed numerical simulations. Comparisons of the theoretical predictions with measured results show that only Roper's model can satisfactorily predict the flame height and quenching velocity of microjet methane flames. Detailed numerical simulations, using skeletal chemical kinetic mechanism, of the flames stabilized at the tip of d = 186, 324 and 529 μ m tubes are performed to investigate the flame structures and the effects of burner materials on the standoff distance near extinction limit. The computed flame shape and flame length for the d = 186 μm flame are in excellent agreement with experimental results. Numerical predictions of the flame structures strongly suggest that the flame burns in a diffusion mode near the extinction limit. The calculated OH mass fraction isopleths indicate that different tube materials have a minor effect on the standoff distance, but influence the quenching gap between the flame and the tube.     

Measurement and simulation of partially-premixed cellular tubular flames

Experimental and numerical simulation results are reported of partially-premixed cellular tubular flames. Parametric measurements across stretch rate and equivalence ratio are taken by chemiluminescent imaging and are presented for the first time. Select hybrid cases with both cellular and non-cellular flame structures are examined with laser-induced spontaneous Raman scattering. Results are spatially resolved in two dimensions and radial interpolations of reaction and extinction zones are compared to numerical simulations using multicomponent transport and detailed chemical kinetics. Experimental cell structures and extinction zones are well predicted by numerical simulation, with discrepancies of temperature and H₂O and temperature primarily observed in locations with moderate and high mole fractions of CO₂. A novel cellular structure, denoted as a “split-cell” flame, is reported for the first time with both chemiluminescent imaging and Raman scattering. Results indicate that partially-premixed flames are valuable as experimental and numerical benchmarks to advance fundamental combustion research.     

Bifurcation and extinction limit of stretched premixed flames with chain-branching intermediate kinetics and radiative loss

Premixed counterflow flames with thermally sensitive intermediate kinetics and radiation heat loss are analysed within the framework of large activation energy. Unlike previous studies considering one-step global reaction, two-step chemistry consisting of a chain branching reaction and a recombination reaction is considered here. The correlation between the flame front location and stretch rate is derived. Based on this correlation, the extinction limit and bifurcation characteristics of the strained premixed flame are studied, and the effects of fuel and radical Lewis numbers as well as radiation heat loss are examined. Different flame regimes and their extinction characteristics can be predicted by the present theory. It is found that fuel Lewis number affects the flame bifurcation qualitatively and quantitatively, whereas radical Lewis number only has a quantitative influence. Stretch rates at the stretch and radiation extinction limits respectively decrease and increase with fuel Lewis number before the flammability limit is reached, while the radical Lewis number shows the opposite tendency. In addition, the relation between the standard flammability limit and the limit derived from the strained near stagnation flame is affected by the fuel Lewis number, but not by the radical Lewis number. Meanwhile, the flammability limit increases with decreased fuel Lewis number, but with increased radical Lewis number. Radical behaviours at flame front corresponding to flame bifurcation and extinction are also analysed in this work. It is shown that radical concentration at the flame front, under extinction stretch rate condition, increases with radical Lewis number but decreases with fuel Lewis number. It decreases with increased radiation loss.     

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.     

Unsteady effects on flame extinction limits during gaseous and two-phase flame/vortex interactions

In highly fluctuating flows, it happens that high values of the strain-rate do not induce extinction of the flame front. Unsteady effects minimize the flame response to rapidly varying strain fields. In the present study, the effects of time-dependent flows on non-premixed flames are investigated during flame/vortex interactions. Gaseous flames and spray flames in the external sheath combustion regime are considered. To analyse the flame/vortex interaction process, the velocity field and the flame geometry are simultaneously determined using particle imaging velocimetry and laser-induced fluorescence of the CH radical. The influence of vortex flows on the extinction limits for different vortex parameters and for different gaseous and two-phase flames is examined. If the external perturbation is applied over an extended period of time, the extinction strain-rate is that corresponding to the steady-state flame, and this critical value mainly depends on the fuel and oxidizer compositions and the injection temperature. If the external perturbation is applied during a short period of time, extinction occurs at strain-rates above the steady-state extinction strain-rate. This deviation appears for flow fluctuation timescales below steady flame diffusion timescales. This behaviour is induced by diffusive processes, limiting the ability of the flame to respond to highly fluctuating flows. With respect to unsteady effects, the spray flames investigated in this article behave essentially like gaseous flames, because evaporation takes place in a thin layer before the flame front. Extinction limits are only slightly modified by the spray, controlling process being the competition between aerodynamic and diffusive timescales.     

Theoretical and experimental studies on mesoscale flame propagation and extinction

Mesoscale flame propagation and extinction of premixed flames in channels are investigated theoretically and experimentally. Emphasis is placed on the effect of wall heat loss and the wall–flame interaction via heat recirculation. At first, an analytical solution of flame speed in mesoscale channels is obtained. The results showed that channel width, flow velocity, and wall thermal properties have dramatic effects on the flame propagation and lead to multiple flame regimes and extinction limits. With the decrease in channel width, there exist two distinct flame regimes, a fast burning regime and a slow burning regime. The existence of the new flame regime and its extended flammability limit render the classical quenching diameter inapplicable. Furthermore, the results showed that at optimum conditions of flow velocity and wall thermal properties, mesoscale flames can propagate faster than the adiabatic flame. Second, numerical simulation with detailed chemistry demonstrated the existence of multiple flame regimes. The results also showed that there is a non-linear dependence of the flame speed on equivalence ratio. Moreover, it is shown that the Nusselt number has a significant impact on this non-linear dependence. Finally, the non-linear dependence of flame speed on equivalence ratio for both flame regimes is measured using a C₃H₈–air mixture. The results are in good agreement with the theory and numerical simulation.     

A structural study of premixed hydrogen-air cellular tubular flames

Two dimensionally spatially resolved structural measurements are reported for cellular phenomena in lean laminar premixed hydrogen-air tubular flames. Laser-induced Raman scattering and chemiluminescence imaging are combined to investigate low Lewis number lean hydrogen-air flames. The strong effect of thermal-diffusive imbalance is observed in radial profiles interpolated through the centers of reaction and extinction zones. In the flame cell, the equivalence ratio is ～80% higher than the inlet mixture, resulting in a peak flame temperature of 1600 K that is 550 K above the adiabatic flame temperature of the inlet mixture (1055 K). In the adjacent extinction zone, the temperatures are ～900 K lower than the peak flame temperature and the equivalence ratio is similar to the inlet mixture. Despite doubling the global stretch rate from 200 s^?1 to 400 s^?1, the enhancement of local equivalence ratio and peak temperature in the flame cell remain similar. This enhancement seems dependent on the local cellular flame curvature, that is similar between both cases. With strong preferential diffusion effects, cellular flames offer unique validation data to improve the accuracy of current molecular transport modeling techniques.     

Unsteady propagation of premixed methane/propane flames in a mesoscale disk burner of variable-gaps

Unsteady flame propagation of air-premixed methane and propane flames was investigated in a new mesoscale disk burner, of which disk-gap could be precisely varied. To begin with, the quenching disk-gaps on the flammability limits were measured. In most cases, with the slight increase of the disk-gap, cellular flame structures could be generated. The initiation of such cellular structures could be explained by the thermally induced hydrodynamic instability, and it could be enhanced if the Lewis number was sufficiently small. When the disk-gap was sufficiently larger than a critical value (approximately 1.5 times the quenching distance), the cellular flame structure changed into a smooth one in the azimuthal direction. With a further increase of the disk-gap, the flame propagation velocities approached to constant values. These values were comparable to the laminar burning velocities except for the propane-rich conditions, in which much larger propagation velocities were observed. The flame stretch effects (coupled with Le-effects) within a narrow space were suspected as the reason. The structural transition of the premixed flame could be investigated successfully through various disk-gaps, from the smallest quenching-scale to the ordinary large scales via whole mesoscales including Hele–Shaw scales.     

Influences of heat flux on extinction characteristics of steady/unsteady premixed stagnation flames

This paper investigates the extinction characteristics of premixed stagnation flames (PSFs) with controlled heat losses and flow disturbances. The low-frequency air flow pulsations that imitate the operational transients in practical combustors were specially introduced. The tunable diode-laser absorption spectroscopy (TDLAS) measurement was applied to obtain the temperature profile and wall heat flux. It is found that, for steady flame with a fixed equivalence ratio, the extinction stretch rate dramatically increases as the wall heat flux decreases. The extinction criterion is summarized as a global Karlovitz number of 0.57 by establishing a relationship between the global and local stretch rates. Numerical simulations reveal that the local extinction Karlovitz number of steady PSFs is approximately 1.0 regardless of the conditions such as heat flux and equivalence ratio. Further experiments present that the air pulsations with a repetition of ～5 Hz significantly deteriorate the flame stability. Particularly, for unsteady perturbed flames, the extinction stretch rate exhibits a nonlinear trend, yielding two regimes with discrepant sensitivities to wall heat flux. The unsteady simulation then highlights a local stretch rate overshoot in the presence of pulsation. It is caused by the time delay between the inlet velocity and flame front movement that eventually leads to poor flame stability. Moreover, in the high heat-flux regime, a smaller local stretch rate overshoot results in the weak dependence of extinction limits on heat fluxes.     

Minimum ignition energy and propagation dynamics of laminar premixed cool flames

Laminar premixed cool flames, induced by the coupling of low-temperature chemistry and convective-diffusive transport process, have recently attracted extensive interest in combustion and engine research. In this work, numerical simulations have been conducted using a recently developed open-source reacting flow platform reactingFOAM-SCT, to investigate the minimum ignition energy (MIE) and propagation dynamics of premixed cool flames in a 1D spherical coordinate. Results have shown that when ignition energy is below the MIE of regular hot flames, a class of cool flames could be initiated, which allow much wider flammability limits, both lean and rich, compared to hot flames. Furthermore, the overall cool flame propagation dynamics exhibit intrinsic similarity to those of hot flames, in that, they begin with an ignition kernel propagation regime, followed by two transition regimes, and eventually reach a normal flame propagation regime. However, a spherical expanding cool flame responds completely differently to stretch. Specifically, a regular outwardly propagating hot spherical flame accelerates with increasing stretch rate when the mixture Le < 1 and decelerates when Le > 1. However, it is found that a cool flame always tends to decelerate with increasing stretch rate regardless of mixture composition, exhibiting unique flame aerodynamic characteristic. This research discovers novel features of premixed cool flame initiation and propagation dynamics and sheds light on flame transition, spark-ignition system design, and advanced engine combustion control.