Syngas-air Mixtures Research Articles

Flame propagation dynamic characteristics and mechanisms of methane–syngas–air mixtures in a semi-enclosed duct

The composition of syngas is complex, often leading to the generation of methane, which impacts the explosive characteristics of syngas. The analysis focused on the kinetic characteristics and mechanisms of flame propagation in methane–syngas–air mixtures in a semi-enclosed duct under various equivalence ratios (φ = 0.8–1.4) and methane volume ratios (R = 0 %–70 %) under ambient temperature and pressure. The results indicated that the flame speed and pressure propagation of methane–syngas–air mixtures could be divided into three stages: a stable stage, an oscillatory increase stage, and an oscillatory decrease stage. In the stable stage, as the methane proportion in the premixed gas increased, the oscillations in the flame and pressure decreased. Turbulent flow of the mixed gas was observed due to restrictions and obstructions within the duct. This led to an increase in the peak pressure and flame speed of the mixed gas, thereby resulting in an oscillatory increase stage. Distorted tulip-shaped flames were also formed in the semi-enclosed duct. The flame of the pure syngas/air mixture exhibited a hemispherical shape and a finger-shaped and slope-shaped propagation structure. When 30 %, 50 %, or 70 % of the methane in the methane–syngas–air mixtures was added, the flame exhibited a propagation structure characterized by a hemispherical shape, finger shape, slope shape, flat shape, tulip shape, or distorted tulip shape. The flame front evolved with time from a linear growth stage to a nonlinear growth stage. The flame front speed of the methane–syngas–air mixtures was closely related to the pressure and flame structure. With the increase in the methane proportion in the methane–syngas–air mixtures, the concentration of key radicals and the rate of OH* generation and consumption gradually decreased. Sensitivity analysis revealed that with increasing methane proportion in the premixed gas, the most important reaction dominating OH* consumption changed from R15 (H + O2(+M) <=> HO2(+M)) to R138 (CH4 + OH <=> CH3 + H2O) and then to R112 (2CH3(+M) <=> C2H6(+M)); moreover, the most important reaction dominating OH* generation remained R1 (H + O2 <=> O + OH). The addition of methane interrupted the hydrogen–oxygen chain reaction of the methane–syngas–air explosion process.

Mathematical Modeling of the Hydrodynamic Instability and Chemical Inhibition of Detonation Waves in a Syngas–Air Mixture

This paper presents the results of the two-dimensional modeling of the hydrodynamic instability of a detonation wave, which results in the formation of an oscillating cellular structure on the wave front. This cellular structure of the wave, unstable due to its origin, demonstrates the constant statistically averaged characteristics of the cell size. The suppression of detonation propagation in synthesis gas mixtures with air using a combustible inhibitor is studied numerically. Contrary to the majority of inhibitors being either inert substances, which do not take part in the chemical reaction, or take part in chemical reaction but do not contribute to energy release, the suggested inhibitor is also a fuel, which enters into an exothermic reaction with oxygen. The unsaturated hydrocarbon propylene additive is used as an inhibitor. The dependence of the effect of the inhibitor content on the mitigation of detonation for various conditions of detonation initiation is researched. The results make it possible to determine a critical percentage of inhibitor which prevents the occurrence of detonation and the critical percentage of inhibitor which destroys a developed detonation wave.

Experimental study on the deformation and oscillation of premixed syngas/air flames in closed ducts

Experiments were conducted in two closed ducts, 100 × 100 × 1000 mm3 (AR = 10) and 60 × 60 × 2000 mm3 (AR = 33.33), to comparatively study the explosion behavior of premixed syngas-air mixtures. The stoichiometric syngas-air mixtures with the increase in hydrogen volume fraction were tested. The flame structure, flame tip, and overpressure dynamics in two ducts were analyzed, and the propagation behavior was compared according to the two flame types through the flame structure and flame tip dynamics. Results show that the development of the flame’s structure is predictable in AR = 10, but the development of the flame’s structure is complicated in AR = 33.33. Increasing both the hydrogen volume fraction and the aspect ratio of the duct increases the amplitude of the flame oscillation, and the flame in AR = 33.33 shows a stronger deformation and oscillation. There is a strong correlation between flame tip speed and overpressure profiles. The growth rates of Darrieus-Landau (DL) and Rayleigh-Taylor (RT) instabilities were presented and discussed, and the results show that both DL and RT instabilities significantly affect flame propagation behavior. Finally, the flame exhibits different structural changes, but it is confirmed that the response sources of the flame oscillation in the two ducts are the same, i.e., owing to the pressure wave’s excitation.

Interactions between high hydrogen content syngas–air premixed flames and homogeneous isotropic turbulence: Flame thickening

This paper reports the findings from a three-dimensional direct numerical simulation conducted to investigate the turbulent flame behaviors of premixed high-hydrogen content syngas (with 50% hydrogen on a per mole basis) and air mixtures. To accomplish this, a laminar flame front is placed in a homogeneous isotropic decaying turbulence field composed of a syngas–air mixture at an equivalence ratio of 0.7 and allowed to evolve for 1.4 eddy turnover times. Homogeneous isotropic turbulence is generated using a helical forcing function in a cubic domain with a grid size of 256 × 256 × 256. The Reynolds number based on the Taylor microscale, Reλ, is 57 for the generated turbulence field. The laminar flame front is placed at the center of the domain. The premixture enters the domain at a velocity of 8 m/s and at an initial temperature of 800 K. The pressure remains constant at 1 atm. In addition to quantifying the spatial and temporal evolution of turbulent characteristics and flame structure, the study also focuses on identifying turbulence/flame interactions, specifically, the impact of these interactions on flame thickness. Energy transfer from small to large scales, i.e., a reverse cascade is observed as a result of energy release due to chemical reactions at the small scales that is transferred to larger scales. The increase in turbulent intensities due to chemical reactions correlates with flame thickening.

Experimental and simulation study of premixed syngas-air deflagration dynamics with elevated temperature and CO2 addition

Experimental and dynamic analyses of the deflagration characteristics of laminar premixed syngas-air at different preheating temperatures and with different CO2 volume fractions were carried out in a rectangular half-open pipe. The effects of CO2 concentration and different initial temperatures on the flame structure evolution, flame structure profile and reaction rate of critical radicals, flame propagation speed, overpressure dynamics and hydrodynamic instability of syngas-air mixture were studied. The FFCM-1 mechanism was used to predict the laminar burning velocity of syngas-air under relevant conditions. The results revealed that the addition of CO2 inhibited the flame propagation and reduced the concentration of H, OH and O, thus reduced the laminar burning velocity. The increase in temperature promotes the chemical effect of CO2, and the interaction between the flame front and the pressure wave is more pronounced, prolonging the duration of the " tulip " flame. Adding CO2 reduces the flame front speed and overpressure, decreases the oscillation amplitude in late flame propagation, and inhibits the explosion intensity. Meanwhile, the temperature increase accelerates the flame propagation in the spherical and finger stages, and the maximum flame propagation speed and peak pressure appear earlier. In addition, as CO2 content and temperature rise, flame hydrodynamic instability is difficult to ignore. However, there is a lack of data from studies of syngas deflagration dynamics at higher temperatures and with higher CO2 additions. This suggests a focus on studies at higher temperatures as well as with higher CO2 additions to enable the development of accurate kinetic models for wide range of syngas combustion. Also, the higher the initial temperature, the longer the time required for heating.

State-to-state vibrational kinetics of diatomic molecules in laser-induced ignition of a syngas-air mixture: Modeling study

A thermally nonequilibrium model was created to describe the processes occurring in a syngas-air mixture when exposed to intense CO laser radiation. The model consistently describes the reactions of syngas oxidation in air and the state-to-state vibrational kinetics of CO, N2, O2, H2, and OH molecules and also takes into account the nonequilibrium vibrational excitation of polyatomic molecules in the mode approximation. Using the constructed model, the possibility of laser-induced ignition of syngas in air was analyzed. Special attention was paid to the influence of the efficiency of vibrational energy of CO molecules in overcoming the energy barriers of reactions on the simulation results. For the key reactions involving CO molecules, the values of the coefficients of vibrational energy usage were estimated employing the method of classical single-trajectory calculations. It was shown that these coefficients are the critical parameters determining the possibility of using the CO laser radiation to accelerate the ignition of syngas. In addition, we have shown that in the problem considered, because of the strong nonequilibrium excitation of CO vibrations, it is extremely important to take into account the deviation of the vibrational distribution functions of molecules from the Boltzmann one, which requires the use of a state-to-state model of vibrational nonequilibrium. The inaccuracy in calculating the laser-induced ignition time of syngas in air by means of a mode model can reach an order of magnitude.

Experimental and numerical study of the influence of initial temperature on explosion limits and explosion process of syngas-air mixtures

To study the effect of initial temperature of 30, 60, 90, and 120 °C on the explosion limits and the explosion process of the syngas-air mixtures, the explosion limits were tested by the explosive limit instrument, and the flame propagation process in the spherical pressure vessel was recorded by the high-speed camera. The ANSYS Fluent 3D software was used to simulate the explosion behavior of syngas-air mixtures. The results showed that with the increase of the initial temperature, the lower explosion limit of syngas decreased and the upper explosion limit increased, and the effect of initial temperature on the upper explosion limit of syngas was greater than that on the lower explosion limit. The flame development process in the simulation was consistent with that in the experiment, propagating outward spherically until it filled the entire container. Both experimental and numerical results presented the same trend of accelerating the flame propagation speed with the increase of initial temperature. In addition, the simulation also obtained multi-dimensional transient explosion parameters that were difficult to obtain in the experiment. The explosion process of syngas was analyzed by the explosion parameters such as temperature and pressure field in the explosion area. An increase in temperature decreased the maximum explosion pressure and shortened the time to reach the maximum explosion pressure.

Flame propagation mode transition of premixed syngas-air mixtures in a closed duct

Knowledge regarding the transformation of the flame propagation dynamics is of importance to predict the safety issues of explosion disasters. The current work experimentally investigated the premixed syngas-air flame propagating in a 2 m duct with a set of obstacles, focusing on the flame propagation mode transition with the change of hydrogen volume fraction (φ). Three modes were identified as the φ increases: steady flame propagation, oscillating flame propagation, and end-gas autoignition. The steady flame propagation mode occurred when φ = 0, where the slow oxidation of pure CO determines its propagation feature. The flame became unsteady and oscillated violently with a slight addition of hydrogen. Flame oscillation resulted from its interaction with pressure wave, and the strength of the pressure wave directly impacted the severity of oscillation. The autoignition appeared when φ ≥ 0.4, resulting from the secondary reflected pressure wave interacted with the region that the thermodynamic state was highly disturbed. What is more, the autoignition kernel migrated from the region ahead of the flame front to the end-wall region when φ ≥ 0.6. The reason is that syngas with higher φ has a lower temperature threshold for explosion and a shorter ignition delay.

Effects of Initial Turbulence on the Explosion Limit and Flame Propagation Behaviors of Premixed Syngas-Air Mixtures.

Syngas with important industrial applications has explosive hazards because of its flammability. It is necessary and valuable to study the combustion and explosion characteristics of syngas under actual working conditions. To explore the effects of initial turbulence on the explosion limits and the flame propagation behavior of the syngas–air mixtures, the explosion limits were tested by the explosive limit instrument, and the flame propagation process in the spherical pressure vessel was recorded by a high-speed camera. By adjusting the rotating speed of the stirrer to obtain turbulence of different intensities, the explosion limit and flame propagation behavior of syngas under different turbulent conditions were analyzed. The explosion limit of syngas in the macro-static state was 9.5–76.1%, and its flame front was relatively smooth. However, with the increase in turbulence intensity, both the upper and lower explosion limits of syngas decreased. The disturbance of turbulence made the flame shape change. The flame front was wrinkled, and the flame boundary was blurred, which became more and more obvious with the increase in turbulence intensity. The maximum velocity and duration of flame propagation increased with the increase in turbulence intensity. Under the same turbulence intensity, the flame propagation velocity generally augmented first and then lessened.

Experimental study on the explosion behaviors of premixed syngas-air mixtures in ducts

Explosion characteristics of premixed syngas-air mixtures at room temperature and atmospheric pressure were experimentally reported when the explosion flame propagates in ducts with various heights (H) and lengths (L). The discussion was based on flame morphology and pressure dynamics. The ratio of L/H and the ratio of H2/CO had a significant effect on the explosion flame behaviors as the explosion occurred in ducts. The structure of the explosion flame changes more drastically, as both the L/H ratio is large. The ratio of L/H affected the flame tip dynamics after the flame reached the duct wall, and the time of flame reaching the duct walls is divinable. For a given duct height, the shorter the duct length is, the faster flame propagates, and the maximum flame tip speed was higher as the duct length was small. For a given duct length, flame tip dynamics showed a nearly same development tendency, but the shorter the duct height, the faster the flame propagated. The venting pressure affected the overpressure dynamics, and the venting pressure increased with the increase of the L/H ratio and the H2/CO. For a given duct height, the overpressure reached the maximum value almost at the same time, and the longer duct length resulted in the greater maximum overpressure. Finally, for a given duct length, the higher duct height caused the higher maximum overpressure.

Experimental study on the premixed syngas-air explosion in duct with both ends open

Premixed flame of stoichiometric syngas-air mixture with various hydrogen volume fractions, 10% ≤ X (H2) ≤ 90%, propagating in a duct with both ends open is experimentally investigated in this study. Two representative ignition locations, i.e., Ig-1, locating at the center of the duct, and Ig-2, locating at the right open end, are considered. Results show that the tulip flame is first attained in the duct with both ends open at 10% ≤ X (H2) ≤ 50% as the flame is ignited at Ig-1. However, the flame maintains the convex shape with the cellular structure on the flame surface as the flame is ignited at Ig-2. The cellular structure results from Darrieus-Landau instability, but the Darrieus-Landau instability cannot invert the convex flame front. The flame tip and pressure dynamics have been examined. When the flame is ignited at Ig-1, the flame oscillates violently, and the overpressure profiles oscillate as a Helmholtz-type. When the flame is ignited at Ig-2, the left flame front propagates in an atmospheric pressure with a nearly constant speed. The prominent flame acceleration and oscillation are not observed at Ig-2 because of lacking flame acoustic interaction. What's more, the characteristic time of flame propagation has been compared. The time tw is shorter while the time tp is longer than the calculated value, and the time te has been delayed by both open ends. The flame propagation process is moderated as the flame propagates in the duct with both ends open.

Effect of the mixture initial temperature on the velocity field in the swirling flame

The paper presents the results of an experimental study of the flow structure of a turbulent swirling flame of a methane-air and syngas-air mixture at elevated temperature using modern optical methods PIV (Particle Image Velocimetry) and PLIF (Planar Laser-Induced Fluorescence). A comparison of the averaged flow characteristics of a high-swirl flame with an equivalence ratio Φ = 0.45 for syngas-air and Φ = 0.7 for methane-air mixture are performed with preheating the mixture to a temperature of 300, 400 and 500 K. The distribution of the fluorescence signal showed that the longitudinal size of the region of hot combustion products decreases significantly with increasing temperature of the mixture for both syngas-air and methane-air flames. For the flame of a mixture of methane-air and syngas-air heated to a temperature of 500 K, the size of the recirculation zone and velocity pulsation distribution are very similar, in contrast to a methane flame at lower preheating temperatures of the mixture.

Laminar burning velocities of LCV syngas-air mixtures at high temperature and pressure conditions

Laminar burning velocities of low-calorific value syngas-air mixtures are measured at elevated temperature and pressure conditions using externally heated diverging channel method. The technique facilitates direct measurement of laminar burning velocities simultaneously at elevated pressures and temperatures. The mixture burning velocities are observed to increase with mixture enthalpy at elevated temperatures and decrease with an increase in the mixture pressure. The measurements are carried out for a mixture temperature range of 300–650 K and up to 5 bar pressure. Based on these measurements a new relation for the variation of laminar burning velocity with pressure and temperature is proposed. The power-exponents of temperature and pressure were expressed as linear functions of pressure and temperature ratios, respectively. The comparison of the measured burning velocities with the best performing kinetic schemes (FFCM-1 and Aramco 3) revealed that a significant deviation in the predictions is observed, when a simultaneous increase in pressure and temperature are considered.

Experimental laminar burning velocity of syngas from fixed-bed downdraft biomass gasifiers

Laminar burning velocity is considered as being one of the fundamental properties of a premixed flame and reliable data are constantly required for practical applications. Although it is possible to find an extensive amount of experimental laminar burning velocity data for fuels containing one or two components, data for fuels with three or more components are scarce. The goal of this study is to help fill this gap by providing experimental laminar burning velocity data for a fuel mixture with five components (H2:CO:CO2:CH4:N2). The fuel composition utilized is proposed as surrogates of the fuel provided by a downdraft gasifier, the most common and the most efficient type of gasifier. Experimental measurements were carried out for different fuel-to-air equivalence ratios (0.88 < ϕ < 1.74) at atmospheric conditions, 954 mbar and 298 K. The method utilized was the conical-flame surface using OH PLIF images (Planar-Laser-Induced Fluorescence imaging of OH). The area method provides a good approximation of the unstretched laminar burning velocity. Experimental data were compared with the simulated results obtained from a CHEMKIN chemical kinetics software. The highest experimental laminar flame speed of a downdraft syngas air mixture was 0.3491 m s−1 and occurred when ϕ ∼1.3.

An experimental investigation on explosion behaviors of syngas-air mixtures in a vessel with a large blockage ratio perforated plate

Explosions may occur in the presence of obstacles, which was not given enough considerations. In this study, an experimental study was conducted to investigate the influence of a perforated plate on the explosion characteristics of syngas-air mixtures in a constant volume vessel. The syngas, containing 50% H2 and 50% CO, was selected as the test fuel. The maximum explosion pressure, heat loss to the vessel wall as well as the perforated plate, explosion time and deflagration index were examined. The turbulence enhancement factor, which was defined as the amplification of the burning velocity when a perforated plate was introduced, was also evaluated. The maximum explosion pressure was always decreased with a perforated plate due to the heat transferred to the obstacle. In addition, the maximum explosion pressure decreased as the distance between the obstacle and ignitor decreased. The heat loss to unit area of the obstacle was larger than that of the vessel walls. When a perforated plate existed, the explosion time was decreased. The maximum pressure rise rate could be affected by the obstacle in both positive and negative ways, depending on the blockage ratio and location of the perforated plate. Lastly, the turbulence enhancement factor was found to be up to 3 with a 0.99 blockage ratio.

Machine learning model to predict the laminar burning velocities of H2/CO/CH4/CO2/N2/air mixtures at high pressure and temperature conditions

An empirical model based on machine learning is developed for predicting the variation of the laminar burning velocities of H2/CO/CH4/CO2/N2/air mixtures with volumetric fractions as the independent variables at different elevated mixture temperatures and pressures. The proposed model is derived partly based on the measured burning velocities of syngas-air mixtures at elevated temperatures and pressures using diverging channel method, and partly established from the predictions using the FFCM-1 detailed kinetic model. The experiments at elevated pressures and temperature strongly agree with the predictions of the FFCM-1 kinetic model for PG1 (H2/CO/CO2/N2 = 15/15/15/55) syngas composition. Based on the detailed analysis of the experimental results, a power-law correlation considering the α, β variations is proposed: Su = Su,o * (Tu/Tu,o)α0+α1 (1−Pu/Pu,o) * (Pu/Pu,o)β0+β1 (1−Tu/Tu,o).Machine learning model (multiple linear-regression) was trained for the variables (Su,o, αo, α1, β0, β1) in the power-law correlation to enable the prediction of laminar burning velocity at various pressure and temperature conditions. The empirical model was developed with mole fractions of various components (H2/CO/CH4/CO2/N2) in the syngas composition and equivalence ratio as independent variables. The developed model was intended for low-calorific value syngas mixtures, and it performs exceedingly well without solving detailed governing equations, detailed chemistry, and transport equations. The proposed model is accurate for a wide range of syngas-air mixtures reported in the literature. A detailed comparison showed that the empirical model accurately predicts the laminar burning velocity with error <10%, for a wide range of H2/CO/CH4/CO2/N2/Air mixtures with 0.25 <XH2<0.70, 0.25 < XCO < 0.70, 0 <XCH4< 0.15, 0 <XCO2< 0.50, 0 <XN2< 0.70, for equivalence ratios of ϕ = 0.5–2.5, mixture temperatures from 300 to 650 K, and pressures from 1 to 5 atm.

Structure of a reacting flow of a turbulent swirling jet during combustion of a syngas–air mixture

The paper presents the results of an experimental study of the flow structure and features of combustion in a turbulent swirling lean pre-mixed syngas flame using modern planar optical laser-based methods. The measurement of the instantaneous velocity field was carried out using the particle image velocimetry (PIV), and location of the OH radicals in the jet were determined by the method of laser-induced fluorescence (PLIF). A comparison of the averaged flow characteristics of a high-swirl flame with an equivalence ratio Φ = 0.45 for syngas and Φ = 0.7 for methane are performed. Presence of a recirculation zone in the jet core is typical of both cases, however, the size of the central recirculation zone for the methane flame are approximately 1.5 times larger than that for the synthesis gas flame in both dimensions. The average distributions of the intensity of fluorescence signal of the OH radical shows a good coincidence of the position of the flame front in both cases, and lower fluorescence signal intensity for the synthesis gas flame.

A comparative investigation of premixed flame propagation behavior of syngas-air mixtures in closed and half-open ducts

A comparative investigation has been developed in conjunction with high-speed cameras and piezoelectric gauge to gain deep insight into the propagation behavior of premixed syngas-air flames. Experiments were carried out in two constant rectangular ducts, one closed duct (C-D) and one half-open duct (HO-D). One important finding is that the flame behavior in the C-D is different than that in the HO-D. The repeatable distorted tulip flame forms in the C-D while an unrepeatable distorted tulip shape forms in the HO-D. The repeated pressure-flame interactions result in a repetitive process of the tulip distortion. The downstream opening end of the duct has a significant influence on the flame propagation characteristics after the flame surface reaches the lateral walls. The opening end results in a higher maximum flame tip velocity, a lower overpressure and a longer plane formation time in the HO-D. Meanwhile, the position of the plane flame formation decreases as hydrogen fraction increases in the C-D while the opposite is true in the HO-D. Furthermore, the pressure build-up in both ducts are examined, and the maximum flame tip velocity is of importance for distorted tulip flame formation.

Demarcation of reaction effects on laminar burning velocities of diluted syngas–air mixtures at elevated temperatures

AbstractLaminar burning velocities of CO2/N2 (60%, 70%) diluted H2/CO/air mixtures were measured at elevated temperatures using the externally heated diverging channel method. The computed burning velocities using the two reaction mechanisms (Davis et al., Proc. Combust. Inst. 2005;30(1):1283–1292; FFCM‐1, http://nanoenergy.stanford.edu/ffcm1) are compared with the experimentally determined burning velocities. The accuracy of the chemical kinetic mechanisms at high dilution rates and elevated temperatures was investigated for various hydrogen fractions in the fuel composition. The burning velocity is observed to increase at high temperatures due to higher mixture enthalpy. The dilution effect on the variation of laminar burning velocity was stronger for the CO2 dilution case compared to N2 dilution. A comparison between the FFCM‐1 mechanism and experimental measurements shows an accurate depiction of the reaction chemistry regarding the prediction of laminar burning velocities. The role of third‐body reactions and direct inhibiting effect of N2 and CO2 molecules on burning velocity of diluted syngas–air mixtures is analyzed in detail. Detailed kinetic analysis revealed that the use of GRI 3.0 collision efficiency factors in the Davis mechanism helps in accurately predicting the burning velocities at elevated temperatures and high CO2 dilution rates. The thermal effect dominates the reduction in laminar burning velocity for N2 dilution case. The FFCM‐1 mechanism agrees well with the experiments for syngas flames diluted with N2 compared to the Davis mechanism. The addition of third‐body efficiency of N2 in the FFCM‐1 mechanism improved the predictions of laminar burning velocities for the N2 dilution case.

Hydrodynamic and chemical scaling for blow-off dynamics of lean premixed flames stabilized on a meso-scale bluff-body

Abstract Direct numerical simulations were conducted to investigate the effect of two parameters, density ratio and laminar flame speed, on the conditions of the onset of local extinction and blow-off of lean premixed flames, stabilized on a meso-scale bluff-body in hydrogen-air and syngas–air mixtures. A total of six simulation cases were considered as isolated comparison of the two parametric effects of the fluid dynamic instability and flame time scale. For all cases under study, the general flame development towards the blow-off limit showed a sequence of five distinct modes, with possible cyclic patterns among the different modes for a range of velocity conditions. The onset of local extinction was observed during the asymmetric vortex shedding and vortex street mode. As the density ratio is decreased, the flow inunder reviewstability is promoted through the increased sinuous mode, and such behavior was properly scaled by the Strouhal number. Although the blow-off velocity is altered by the fluid dynamic effects, the condition for the onset of local extinction and blow-off was mainly dictated by the competition between flow residence time associated with the lateral flame motion and ignition delay of the local mixtures. Time scale analysis supported the validity of the findings across all the cases investigated.