Chemical effects of added CO2 on the extinction characteristics of H2/CO/CO2 syngas diffusion flames

Chemical effects of added CO₂ on flame extinction characteristics are numerically studied in H₂/CO syngas diffusion flames diluted with CO₂. The two representative syngas flames of 80% H₂ + 20% CO and 20% H₂ + 80% CO are inspected according to the composition of fuel mixture diluted with CO₂ and global strain rate. Particular concerns are focused on impact of chemical effects of added CO₂ on flame extinction characteristics through the comparison of the flame characteristics between well-burning flames far from extinction limit and flames at extinction. It is seen that chemical effects of added CO₂ reduce critical CO₂ mole fraction at flame extinction and thus extinguish the flame at higher flame temperature irrespective of global strain rate. This is attributed by the suppression of the reaction rate of the principal chain branching reaction through the augmented consumption of H-atom from the reaction CO₂ + H→CO + OH. As a result the overall reaction rate decreases. These chemical effects of added CO₂ are similar in both well-burning flames far from extinction limit and flames at extinction. There is a mismatching in the behaviors between critical CO₂ mole fraction and maximum flame temperature at extinction. This anomalous phenomenon is also discussed in detail.     

Combustion stability limits and NOx emissions of nonpremixed ammonia-substituted hydrogen–air flames

The combustion stability (extinction) limits and nitrogen oxide (NO_x) emissions of nonpremixed ammonia (NH₃)–hydrogen (H₂)–air flames at normal temperature and pressure are studied to evaluate the potential of partial NH₃ substitution for improving the safety of H₂ use and to provide a database for the nonpremixed NH₃-substituted H₂–air flames. Considering coflow nonpremixed NH₃–H₂–air flames for a wide range of fuel and coflow air injection velocities (V_fuel and V_coflow) and the extent of NH₃ substitution, the effects of NH₃ substitution on the stability limits and NO_x emissions of the NH₃–H₂–air flames are experimentally determined, while the nonpremixed NH₃–H₂–air flame structure is computationally predicted using a detailed reaction mechanism. Results show significant reduction in the stability limits and unremarkable increase in the NO_x emission index for enhanced NH₃ substitution, supporting the potential of NH₃ as an effective, carbon-free additive in nonpremixed H₂–air flames. With increasing V_coflow the NO_x emission index decreases, while with increasing V_fuel it decreases and then increases due to the recirculation of burned gas and the reduced radiant heat losses, respectively. Given V_coflow/V_fuel the flame length increases with enhanced NH₃ substitution since more air is needed for reaction stoichiometry. The predicted flame structure shows that NH₃ is consumed more upstream than H₂ due to the difference between their diffusivities in air.     

Measurements in turbulent premixed bluff body flames close to blow-off

The structure of unconfined lean premixed methane–air flames stabilized on an axisymmetric bluff body has been examined for conditions increasingly closer to blow-off and during the blow-off event. Fast imaging (5 kHz) of OH¹ chemiluminescence and OH-PLIF and PIV (at 1 kHz) were used to obtain instantaneous and time-averaged images, temporal sequences, spectra of OH, 2-D estimates of flame surface density, curvature, turbulence statistics, and measurements of the duration of the blow-off transient. Blow-off was approached by slowly reducing the fuel flow rate, and the flame shape was seen to change from a cylindrical shape at stable burning conditions, with the flame brush closing across the flow at conditions close to the blow-off condition. This was followed by entrainment of fresh reactants from the downstream end of the recirculation zone (RZ), and fragmentation of the downstream flame parts. Just before the blow-off event, reaction fronts were observed inside the RZ, with progressive fragmentation occurring, leading to a shorter flame brush. Complete extinction occurred once the flame at the attachment point had been destroyed, and stabilization at the shear layers was no longer possible. Measurements showed a gradual reduction in FSD during the approach to blow-off and during the extinction event itself, and higher values of flame front curvature at conditions approaching extinction. The local Karlovitz number was estimated based on the local turbulence velocity and lengthscale characteristics and it reached a maximum value of about 10 at the location where the flame bends towards the axis. Quantification of the duration of the blow-off event showed that it was an order of magnitude longer than the characteristic timescale of the burner d/U_b. The measurements reported here are useful for model validation and for exploring the changes in turbulent premixed flame structure as extinction is approached.     

Effects of ammonia substitution on extinction limits and structure of counterflow nonpremixed hydrogen/air flames

The potential of partial ammonia substitution to improve the safety of hydrogen use was evaluated computationally, using counterflow nonpremixed ammonia/hydrogen/air flames at normal temperature and pressure. The ammonia-substituted hydrogen/air flames were considered using a recent kinetic mechanism and a statistical narrow-band radiation model for a wide range of flame strain rates and the extent of ammonia substitution. The effects of ammonia substitution on the extinction limits and structure, including nitrogen oxide (NO_x) and nitrous oxide (N₂O) emissions, of nonpremixed hydrogen/air flames were investigated. Results show reduction of the high-stretch extinction (i.e., blow-off) limits, the maximum flame temperature and the concentration of light radicals (e.g., H and OH) with ammonia substitution in hydrogen/air flames, supporting the potential of ammonia as a carbon-free, clean additive for improving the safety of hydrogen use in nonpremixed hydrogen/air flames. For high-stretched flames, however, NO_x and N₂O emissions substantially increase with ammonia substitution even though ammonia substitution reduces flame temperature, implying that chemical effects (rather than thermal effects) of ammonia substitution on flame structure are dominant. Radiation effects on the extinction limits and flame structure are not remarkable particularly for high-stretched flames.     

Onset of cellular instability in adiabatic H2/O2/N2 premixed flames anchored to a flat-flame heat-flux burner

The onset of cellular instability in adiabatic H₂/O₂/N₂ premixed flames anchored to a heat-flux burner is investigated numerically. Both hydrodynamic instability and diffusional-thermal instability are shown to play an important role in the onset of cellular flames. The burner can effectively suppress cellular instability when the flames are close to the burner, otherwise the burner can suppress the instabilities only at large wavenumbers. Because of differential diffusion, local extinction can occur in lean H₂/O₂/N₂ flames. When the flames develop to take on cellular shapes, the surface length, the overall heat release rate and the mean burning velocity are all increased. For near stoichiometric fuel-rich flames the mean burning velocity can increase by as much as 20%–30%. For lean flames with an equivalence ratio of 0.56, the mean burning velocity can be 2–3 times of the burning velocity of the corresponding planar flame.     

Lean premixed opposed jet flames in fractal grid generated multiscale turbulence

The opposed jet configuration presents an attractive canonical geometry for the evaluation of burning properties of turbulent flames with past studies typically limited to low Reynolds numbers. Fractal grid generated turbulence was used to remove the low turbulence level limitations associated with conventional perforated plate generators with the turbulent Reynolds number range moved from 50–120 to 130–318. Optimal grid configurations were determined with particular emphasis on reducing the impact of the flow upstream of the turbulence generators in order to facilitate simpler boundary conditions for computational studies. The resulting flow structures were analysed using proper orthogonal decomposition and conditional proper orthogonal decomposition. Velocity and reaction progress variable statistics, including conditional velocities and scalar fluxes, are reported for fuel lean methane, ethylene and propane flames approaching extinction. The instrumentation comprised particle image velocimetry with the flows to both nozzles seeded with 1 μm silicon oil droplets or 3 μm Al₂O₃ particles. Probability density functions were determined for the instantaneous location of the stagnation point to eliminate the possibility of low frequency bulk motion distorting velocity statistics. Probability density functions of flame curvature were determined using a multi-step flame front detection algorithm with estimates of the turbulent burning velocity provided along with a discussion of alternative determination methods. The data sets show that fractal grids generate multi-scale broadband turbulence and present an opportunity for a systematic evaluation of calculation methods for premixed turbulent flames that undergo a transition from non-gradient to gradient turbulent transport while approaching extinction.     

A computational study of oscillatory extinction of spherical diffusion flames

The transient behavior of burner-supported spherical diffusion flames was studied in the transport-induced limit of low mass flow rate and the radiation-induced limit of high mass flow rate which characterize the isola response of flame extinction. Oscillatory instability was observed near both steady-state extinction limits. The oscillation typically grows in amplitude until it becomes large enough to extinguish the flame. The oscillatory behavior was numerically observed using detailed chemistry and transport for methane (50%CH₄/50%He into 21%O₂/79%He) and hydrogen (100% H₂ into 21%O₂/79%He) diffusion flames where the fuel was issued from a point source, and helium was selected as an inert to increase the Lewis number, facilitating the onset of oscillation. In both methane and hydrogen flames, the oscillation always leads to extinction, and no limit cycle behavior was found. The growth rate of the oscillation was found to be slow enough under certain conditions to allow the flame to oscillate for over 450 s, suggesting that such oscillations can possibly be observed experimentally. For the hydrogen flames, however, the frequency of oscillation near the transport-induced limit is much larger, approximately 60 Hz as compared to 0.35 Hz for the methane flame, and the maximum amplitude of temperature oscillations was about 5 K. The distinctively different structures of the hydrogen and methane flames suggest that while both instabilities are thermal-diffusive in origin, oscillations in the hydrogen flames resemble those of premixed flames, while oscillations in the methane flames are non-premixed in character.     

A numerical study on the ability to predict the heat release rate using CH chemiluminescence in non-sooting counterflow diffusion flames

Numerical studies on 1-D, non-sooting counterflow diffusion flames were performed to determine the precision with which the total heat release rate can be calculated using light emission, namely, chemiluminescence, from the reaction zone. A detailed reaction mechanism, incorporating sub-reaction models for excited state radicals (CH^* and OH^*, where ^* denotes the excited state), was employed in this study. A set of 1-D, steady state conservation equations was solved under standard atmospheric conditions over a counterflow configuration utilizing the CHEMKIN-PRO package. A variety of fuels (CH₄ and C₃H₈), velocities (0.1 m/s − near the extinction condition), diluents (N₂, H₂O, CO₂, and Ar), detailed reaction mechanisms for C₁–C₃ hydrocarbons (GRI-Mech 3.0, Hai Wang’s-, and San Diego-mechanism), different sub-reaction models for excited radicals, and excited radical transport properties were examined for the current purpose. It was found that a one-to-one correlation between total chemiluminescence from CH^* and the total heat release rate cannot be sustained when the flame experiences a relatively high stretch and dilution, even though the condition is still far away from extinction. This trend is consistent with the different types of fuels, and it is understood that the reduction of the ethynyl radical (C₂H), a potential precursor of CH^*, is the main cause of the one-to-one correlation not being sustained. To this end, it was concluded that the observable light emission can only be used to predict the total heat release rate when non-sooting diffusion flames exist under velocity conditions from 0.1 m/s to 1.5 m/s. In other words, the chemiluminescence intensity does not always correlate with the total heat release rate of highly stretched flames found in practical combustors.     

Large Eddy Simulation of the Sandia Flame Series (D-F) using the Eulerian stochastic field method

Three turbulent piloted methane jet flames with increasing levels of local extinction (Sandia Flames D-F) have been computed using Large Eddy Simulation. The smallest unresolved scales of the flow, in which combustion occurs, are represented using the filtered probability density function method where the corresponding evolution equation is solved directly. A dynamic model for the sub-grid stresses together with a simple gradient diffusion approximation for the scalar fluxes is applied in conjunction with the linear mean square estimation closure for sub-filter scale mixing. An augmented reduced mechanism (ARM) derived from the full GRI 3.0 mechanism is incorporated to describe the chemical reaction. The results demonstrate the ability of the method in capturing quantitatively finite rate effects such as extinction and re-ignition in turbulent flames.     

A study on flame interaction between methane/air and nitrogen-diluted hydrogen–air premixed flames

Numerical and experimental studies are conducted to grasp downstream interactions between premixed flames stratified with two different kinds of fuel mixture. The selected fuel mixtures are methane and a nitrogen-diluted hydrogen with composition of 30% H₂ + 70% N₂. Extinction limits are determined for methane/air and (30% H₂ + 70% N₂)/air over the entire range of mixture concentrations. These extinction limits are shown to be significantly modified due to the interaction such that a mixture much beyond the flammability limit can burn with the help of a stronger flame. The lean extinction limit shows both the slanted segments of lower and upper branches due to the strong interaction with Lewis numbers of deficient reactant less than unity, while the rich extinction limit has a square shape due to the weak interaction with Lewis numbers of deficient reactant larger than unity. The regimes of negative flame speed show an asymmetric aspect with a single wing shape. The negative flame always appears only when methane is weak. The extent of interaction depends on the separation distance between the flames, which are the functions of the mixtures’ concentrations, the strain rate, the Lewis numbers, and the preferential diffusions of the penetrated hydrogen from the nitrogen-diluted hydrogen flame. The important role of preferential diffusion effects of hydrogen in the flame interaction is also discussed.     

A numerical study on the combustion limits of mesoscale methane jet flames with hydrogen addition

A meso-scale jet flame model was established for the flame ports of domestic gas stoves. The influences of hydrogen addition ratio (β = 0%–25%) on the combustion limits were explored. The results show that with the increase of hydrogen addition ratio, the blow-off limit increases obviously, while the extinction limit decreases slightly, namely, the combustible range expands significantly. Quantitative analysis was carried out in terms of chemical effect and thermal effect. It was found that hydrogen addition will reduce O₂ fraction in the pre-mixture for a constant equivalence ratio. Under near-extinction limit condition, since the flame is located at the nozzle exit, the external O₂ cannot be entrained into or diffuse into the upstream of the flame, which leads to the decrease of reaction rate. However, for the near-blow-off cases, the external O₂ can be entrained and diffuse into the flame, which compensates the difference of O₂ content in the pre-mixture. Therefore, the combustion reaction is enhanced by hydrogen addition because more H radicals can be produced. In addition, as the flame is located closer to the tube with the increase of hydrogen addition ratio, heat transfer between flame and tube wall is augmented and the preheating of fresh mixture is strengthened by the inner tube wall. This heat recirculation effect becomes especially notable in low velocity cases. In conclusion, the extension of extinction limit by hydrogen addition is attributed to the thermal effect, while the increase of blow-off limit is mainly due to the intensification of chemical effect.     

Stakeholder perceptions of CO2 capture and storage in Europe: Results from a survey

During 2006, a survey was conducted of European energy stakeholders (industry, government, environmental non-governmental organizations (NGOs), researchers and academicians and parliamentarians). A total of 512 responses was received from 28 countries as follows: industry (28%), research (34%), government (13%), NGOs (5%) and parliamentarians (4%). Three-quarters of the sample thought that widespread use of CO₂ capture and storage (CCS) was ‘definitely’ or ‘probably necessary’ to achieve deep reductions in CO₂ emissions between now and 2050 in their own country. Only one in eight considered that CCS was ‘probably’ or ‘definitely not necessary’. For a range of 12 identified risks, 20–40% thought that they would be ‘moderate’ or ‘very serious’, whilst 60–80% thought that there would be no risks or that the risks would be ‘minimal’. A particular risk identified by nearly half the sample is the additional use of fossil fuels due to the ‘energy penalty’ incurred by CCS. Further concerns are that development of CCS would detract from investment in renewable energy technologies. Half of the respondents thought that incentives for CCS should be set either at the same level as those for renewables or at a higher level. Environmental NGOs were consistently less enthusiastic about CCS than the energy industry.     

Study on stretch extinction limits of CH4/CO2 versus high temperature O2/CO2 counterflow non-premixed flames

Extinction limits of counterflow non-premixed flames with normal and high temperature oxidizers were studied experimentally and numerically for development of new-type oxygen-enriched mild combustion furnace. Extinction stretch rates of CH₄/CO₂ (at 300 K) versus O₂/CO₂ flames at oxygen mole fractions of 0.35 and 0.40 and oxidizer temperatures of 300 K, 500 K, 700 K and 1000 K were obtained. Investigation was also conducted for CH₄/N₂ (at 300 K) versus air (O₂/N₂) flames at the same oxidizer temperatures. An effect of radiative heat loss on stretch extinction limits of oxygen-enriched flames and air flames was investigated by computations with optical thin model (OTM) and adiabatic flame model (ADI). The results show influence of radiative heat loss on stretch extinction limits was not significant in relative high fuel mole fraction regions. The extinction curve of the oxygen-enriched flames with oxygen mole fraction of 0.35 was close to that of the air flames at the oxidizer temperature of 300 K. However, the extinction curve of air flames with high temperature oxidizer was comparable with that of oxygen-enriched flames with oxygen mole fraction of 0.40. Scaling analysis based on asymptotic solution of stretch extinction was applied and it was found that stretch extinction limits can be expressed by two terms. The first term is total enthalpy flux of fuel stream based on thermo-physical parameters. The second term is a kinetic term which reflects an effect of the chemical reaction rate on stretch extinction limits. OH radicals which play important roles in chain propagating and main endothermic reactions were used to represent the kinetic term of both oxygen-enriched and air flames. The global rates of OH formation in these two cases were compared to understand the contribution of kinetic term to stretch extinction limits. Variation of extinction curves of oxygen-enriched flames and air flames was well explained by the present scaling analysis. This offers an effective approach to estimate stretch extinction limits of oxygen-enriched flames based on those of air flames at the same oxidizer temperature.     

Extinction limit extension of unsteady counterflow diffusion flames affected by velocity change

The unsteady extinction limit of (CH₄ + N₂)/air diffusion flames was investigated in terms of the time history of the strain rate and initial strain rates. A spatially locked flame in an opposed-jet counterflow burner was perturbed using linear velocity variation, and time-dependent flame luminosity and unsteady extinction limits were measured with a high-speed intensified CCD (ICCD) camera. In addition, the transient maximum flame temperature and hydroxyl (OH) radical were measured as a function of time using Rayleigh scattering and OH laser-induced fluorescence, respectively. In this experiment, unsteady flames survive at strain rates that are much higher than the extinction limit of steady flames and unsteady extinction limits increase as the slope of the strain rate increases or as the initial strain rate decreases. We found that the equivalent strain rate represents well the unsteady behavior in the outer convective-diffusive layer of the flame. By using the equivalent strain rate, we were able to accurately estimate the contribution of the unsteady effect in the outer convective-diffusive layer to the extinction limit extension, and we also identified the unsteady effect in the inner diffusive-reactive layer of the flame. Consequently, the extension of unsteady extinction limits results from the unsteady effects of both the convective-diffusive layer and the diffusive-reactive layer. The former effect is dominant at the beginning of the velocity change, and the latter effect is dominant near the extinction limit.     

Adiabatic burning velocity and cellular flame characteristics of H2–CO–CO2–air mixtures

The objective of this work was to study the effect of dilution with carbon dioxide on the adiabatic burning velocity of syngas fuel (with various H₂/CO ratios)-air(21% O₂–79% N₂ by volume) mixtures along with detailed understanding of cellular flame structures. Heat flux method with a setup similar to that of de Goey and co-workers [1] was used for measurement of burning velocities. Validation experiments were done for H₂ (5%)–CO (95%)–air and H₂ (5%)–CO (45%)–CO₂ (50%)–air mixtures at various equivalence ratios and the results were in good agreement with published data in the literature. The mixtures considered in this work had 1:4, 1:1 and 4:1 H₂/CO ratio in the fuel and 40%, 50% and 60% CO₂ dilution. The burning velocity increased significantly with the increase in H₂ content in mixture of H₂–CO with fixed CO₂ dilution. The burning velocity reduced remarkably with carbon dioxide dilution in H₂–CO mixture due to reduction in heat release, flame temperature and thermal diffusivity of the mixture. The location of peak adiabatic burning velocity shifted from ? = 1.6 for 40% CO₂ to ? = 1.2 for 60% CO₂, whereas it remained unchanged with variation of H₂:CO ratio (4:1, 1:1 and 1:4) at a given CO₂ dilution. A comparison of experiments and simulations indicated that the Davis et al. [2] mechanism predicted burning velocities well for the most of experimental operating conditions except for rich conditions. For some lean mixtures, flames exhibited cellular structures. In order to explain the structures and generate profiles of various field variables of interest, computations of three dimensional porous burner stabilized cellular flames were performed using commercial CFD software FLUENT. Simulations for lean H₂ (25%)–CO (25%)–CO₂ (50%)–air mixtures (? = 0.6 and 0.8) produced cellular flame structures very similar to those observed in the experiments. It was found that the in the core region of a typical cell, stretch rate was positive, the volumetric heat release rate was high and the net reaction rate for the reaction O + H₂ ? H + OH and the net consumption rate of H₂ were both high.     

Effects of preferential diffusion on downstream interaction in premixed H2/CO syngas–air flames

Effects of strain rate and preferential diffusion of H₂ on flame extinction are numerically explored in interacting premixed syngas–air flames with the fuel compositions of 50% H₂ + 50% CO and 30% H₂ + 70% CO. Flame stability diagrams mapping lower and upper limit fuel concentrations at flame extinction as a function of strain rate are examined. Increasing strain rate reduces the boundaries of both flammable lean and rich fuel concentrations and produces a flammable island and subsequently even a point, implying that there exists a limit strain rate over which interacting flame cannot be sustained anymore. Even if effective Lewis numbers are slightly larger than unity on the lean extinction boundaries, the shape of the lean extinction boundary is slanted even at low strain rate, i.e. a_g = 30 s⁻¹ and is more slanted in further increase of strain rate, implying that flame interaction on lean extinction boundary is strong and thus hydrogen (as a deficient reactant) Lewis number much less than unity plays an important role of flame interaction. It is also shown that effects of preferential diffusion of H₂ cause flame interaction to be stronger on lean extinction boundaries and weaker on rich extinction boundaries. Detailed analyses are made through the comparison between flame structures with and without the restriction of the diffusivities of H₂ and H in symmetric and asymmetric fuel compositions. The reduction of flammable fuel compositions in increase of strain rate suggests that the mechanism of flame extinction is significant conductive heat loss from the stronger flame to ambience.     

Sensitivity analysis of transfer functions of laminar flames

The sensitivity of laminar premixed methane/air flames responses to acoustic forcing is investigated using direct numerical simulation to determine which parameters control their flame transfer function. Five parameters are varied: (1) the flame speed s_L, (2) the expansion angle of the burnt gases α, (3) the inlet air temperature T_a, (4) the inlet duct temperature T_d and (5) the combustor wall temperature T_w. The delay of the flame transfer function is computed for the axisymetric flames of Boudy et al. [1] and the slot flames of Kornilov et al. [2]. Stationary flames are first computed and compared to experimental data in terms of flame shape and velocity fields. The flames are then forced at different frequencies. Direct numerical simulations reproduce the flame transfer functions correctly. The sensitivity analysis of the flame transfer function is done by changing parameters one by one and measuring their effect on the delay. This analysis reveals that the flame speed s_L and the inlet duct temperature T_d are the two parameters controlling the flame delay and that any precise computation of the flame transfer function delay must first have proper models for these two quantities.     

Flame structures and behaviors of opposed flow non-premixed flames in mesoscale channels

An opposed flow non-premixed flame (OFNPF) in a narrow channel was chosen as a model of a non-premixed flame in a mesoscale combustion space or micro-combustor. The stabilization limits and behaviors of methane-air flames and propane-air flames were compared for various experimental parameters such as flow velocity, nozzle distance, nozzle width, channel gap, and fuel dilution. Flames could be stabilized in a wide range of strain rates (0.9–150 s⁻¹) and dilution ratios (∼80% nitrogen at the fuel side). The flame extinction limits were classified into three types and their mechanisms were investigated: higher-strain-rate (HSR) extinction limit determined by the flame stretch, lower-strain-rate (LSR) extinction limit determined by the conductive or convective heat loss from the flame, and fuel-dilution-ratio (FDR) extinction limit determined by the decrease in the heat release rate from the flames. The HSR extinction limits in mesoscale channels could be explained with a modified strain rate, and the LSR extinction limits could be explained by employing a premixed quenching theory in which the heat loss through the dead space near the wall was considered as a major extinction mechanism. Finally, the variation of the extinction limits with the FDR in both the HSR and the LSR conditions could be explained with a modified global reaction rate in which the variations in flame temperature and species concentrations were reflected. This study provides an essential model for the stabilization and extinction of non-premixed flames in mesoscale combustion spaces.     

Important role of chemical interaction on flame extinction in downstream interaction between stretched premixed H2-air and CO-air flames

Important role of chemical interaction in flame extinction is numerically investigated in downstream interaction among lean (rich) and lean (rich) premixed as well as partially premixed H₂- and CO-air flames. The strain rate varies from 30 to 5917 s⁻¹ until interacting flames cannot be sustained anymore. Flame stability diagrams mapping lower and upper limit fuel concentrations for flame extinction as a function of strain rate are presented. Highly stretched interacting flames are survived only within two islands in the flame stability map where partially premixed mixture consists of rich H₂-air flame, extremely lean CO-air flame, and a diffusion flame. Further increase in strain rate finally converges to two points. It is found that hydrogen penetrated from H₂-air flame (even at lean flame condition) participates in CO oxidation vigorously due to the high diffusivity such that it modifies the slow main reaction route CO + O₂ → CO₂ + O into the fast cyclic reaction route involving CO + OH → CO₂ + H. These chemical interactions force even rich extinction boundaries with deficient reactant Lewis numbers larger than unity to be slanted at high strain rate. Appreciable amount of hydrogen in the side of lean H₂-air flame also oxidizes the CO penetrated from CO-air flame, and this reduces flame speed of the H₂-air flame, leading to flame extinction. At extremely high strain rates, interacting flames are survived only by a partially premixed flame such that it consists of a very rich H₂-air flame, an extremely lean CO-air flame, and a diffusion flame. In such a situation, both the weaker H₂- and CO-air flames are parasite on the stronger diffusion flame such that it can lead to flame extinction in the situation of weakening the stronger diffusion flame. Important role of chemical interaction in flame extinction is discussed in detail.     

Numerical Study on the Morphology of a Re-Ignited Laminar Partially Premixed Flame with a Co-Axial Pilot Flame

Re-ignited partially premixed flame(PPF)is a quite extensive flame type in real applications,which is directly relevant to the local and global extinction and re-ignition phenomenon.The authors designed a model burner to establish laminar re-ignited PPFs.Numerical simulations were carried out to reveal the morphology of laminar re-ignited PPF.Based on the distributions of temperature,heat release and radicals,the morphologies of re-ignited flames were explored.W-shaped flames were formed under pilot-lean conditions.Line-shaped and y-shaped flames were formed under pilot-rich conditions.Both w-shaped and y-shaped flames had a triple-flame structure.The re-ignited flames can stand beyond the rich flammability limit.Additionally,OH distributions indicated both pilot flame and re-ignited flame well as it rapidly increased near the flame front.OH concentration did not increase visibly while CH2O concentration mildly increased during the mild re-ignition process in the pre-zone of the re-ignited PPF.According to the results of 0-D simulations using closed homogeneous reactor,both OH and CH2O reduced ignition time significantly.The results of this work are helpful for understanding re-ignited PPF more closely.