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Large hydrocarbon fuels are used for ground and air transportation and will be for the foreseeable future. A key parameter when burning these fuels is the turbulent consumption speed, which is the velocity at which fuel and air are consumed through a turbulent flame front. Such information can be useful as a model input parameter and for validation of modeled results. In this study, turbulent consumption speeds were measured for three jet-like fuels using a premixed turbulent Bunsen burner. The burner was used to independently control turbulence intensity, unburned temperature, and equivalence ratio. Each fuel had similar heat releases (within 2%), laminar flame speeds (within 5–15%), and adiabatic flame temperatures. Despite this similarity, for constant ReD and turbulence intensity, A2 fuel (i.e. jet-A) has the highest turbulent flame speeds and remains stable (i.e. without tip quenching) at lower ϕ than the other fuels. In contrast the C1 fuel, which consists of heavily-branched alkanes, contains no aromatics and has a relatively high average molecular weight, has the slowest turbulent flame speeds and is the most prone to tip quenching. The C1 fuel has the highest stretch sensitivity, in general, as indicated by calculated Markstein numbers. This work shows that turbulent flame speeds and tip stability of multi-component large hydrocarbon fuels can be sensitive to the chemical class of its components. [ABSTRACT FROM AUTHOR]

Due to the high costs associated to experimental testing of hydrogen (H 2) explosions, numerical simulations play an essential role in the design of safe industrial systems. In this work, we investigate the use of the Thickened Flame Model (TFM) to perform simulations of H 2 deflagrations. Specific issues arise when considering hydrogen: (i) due to differential diffusion, the flame is very sensitive to stretch; (ii) instabilities of the flame front are generated and lead to an increased flame propagation speed. Both of these phenomena are affected by flame thickening. First, the flame sensitivity to stretch is artificially increased by the thickening. The recently developed Ma-TFM model proposes a correction of diffusive fluxes to reach the correct Markstein length and is selected in this work. Secondly, thermo-diffusive and hydrodynamic flame instabilities are also altered by thickening. By computing the dispersion relationship for a hydrogen/air mixture, we show that the instability scales are proportional to the thickening factor F. A simple subgrid scale (SGS) model, based on the computations of laminar unstable 2-D and 3-D flames, is then considered to take into account the effect of instabilities. The Ma-TFM and SGS instability models are applied together for the first time on a semi-industrial vented lean hydrogen deflagration, leading to promising results. In particular, it is found that considering 3-D unstable flames is necessary to accurately predict the experimental pressure peaks. [Display omitted] • Stretch corrected Thickened Flame Model coupled to a model for flame instabilities. • Thermodiffusive instability scales are proportional to the thickening factor. • Successful simulation of a semi-industrial vented hydrogen deflagration. • 3-D nature of thermo-diffusive instabilities needs to be considered. [ABSTRACT FROM AUTHOR]

The introduction of the pilot flame can stabilize the lean premixed flame and promote its industrial application. However, the interaction mechanism between the pilot and main flames is complicated. To reveal the effect of the pilot flame on the main flame, a laminar lean premixed flame adjacent to a rich premixed pilot flame on one side and a similar lean premixed flame on the other side was considered. A two-dimensional numerical model was adopted with detailed chemistry and species transport, also with no artificial flame anchoring boundary conditions. The results show that the pilot flame could promote the main flame stabilized in different locations with various shapes, by adjusting the stretch, heat transfer, and preferential diffusion in a complicated manner. The pilot flame improves the local equivalence ratio and transfer more heat to the main flame. The growth of the pilot flame equivalence ratio and inlet velocity enhances the combustion on the rich side of the main flame and helps it anchor closer to the flame wall. Both the curvature and strain rate show a significant effect on the flame root, which contributes to the main flame bending towards the pilot flame. [ABSTRACT FROM AUTHOR]

In the current work, we investigated hydrogen/air flame propagation under supergravity conditions. Results show that when gravity is in the same/opposite direction as flame propagation, it leads to acceleration/deceleration of the flame, and that such an effect could substantially modify the flame propagation and structure at high gravity levels. Furthermore, for the absolute and relative flame propagation speeds, the gravity-affected flame speed shows opposite trends as the absolute flame speed is more affected by the local induced flow field, while the relative flame speeds are controlled by the super-adiabatic or sub-adiabatic flame temperature. The gravity-affected thermal and chemical flame structures are also examined through the influence of the mixture equivalence ratio, pressure, and flame stretch. [ABSTRACT FROM AUTHOR]

This paper investigates laminar premixed hydrogen/air flame-wall interactions (FWI) subject to flame stretch (curvature and strain rate). Direct numerical simulations of finite-rate chemical reactions and in-depth molecular diffusion are used to numerically study two-dimensional (2D) V-shaped flames, with one side functioning as an isothermal cold wall that interacts with the flame and the other as a symmetrical center for free-propagating flame. An isometric grid with a grid resolution of 10 μm is used to resolve the FWI zone. It is discovered that the wall at the flame tip experiences increased negative stretching and enhanced local preferential diffusion which reduces the intensity of local combustion. This effect is especially noticeable at low equivalent ratios. Additionally, there is a close relationship between quenching behavior and flame dynamics. The present results show that the Markstein numbers for both the sidewall quenching flame and the freely propagating flame (Ma SWQ and Ma FF) are negative in lean hydrogen flames and positive in rich hydrogen flames. Cold wall has less impact on Ma. Furthermore, an intersection in the equivalence ratio is observed in the lean-fuel hydrogen/air flame, located at Φ = 0.8–1.0, with a change in the sign of Ma. • Resolved simulation of steady flame-wall interaction of hydrogen flames over a wide range of equivalent ratios. • Analysis of the V-shaped flame with/without wall from a thermodynamics perspective. • The wall increased negative flame stretching and enhanced local preferential diffusion. • Comparison of Markstein numbers between sidewall quenching flames and free propagating flames. • A crossover of equivalence ratio was observed in fuel poor hydrogen/air flames. [ABSTRACT FROM AUTHOR]

One-dimensional numerical simulations of an unstretched propagating hydrogen/air premixed flame and two-dimensional numerical simulations of outwardly propagating hydrogen/air laminar premixed flames are performed using a flamelet-generated manifold (FGM) method. The effects of considering preferential diffusion and flame stretch in the FGM method are investigated in detail. The results of the one-dimensional numerical simulations show that the flame behavior can be qualitatively reproduced without considering both these two phenomena, but it can be predicted more precisely by considering preferential diffusion. On the other hand, the results of the two-dimensional numerical simulations show that considering both these two phenomena is essential to accurately predict the behavior of stretched hydrogen/air premixed flames at an equivalence ratio of less or more than 0.7. The results also show that the consideration of preferential diffusion and flame stretch is less important at an equivalence ratio of around 0.7. In addition, the validity of an assumption that the Lewis numbers of each species are constant, which is used in most previous studies, is investigated. The results show that assuming constant Lewis numbers worsens the prediction accuracy. The computational cost of the FGM method considering both preferential diffusion and flame stretch is about 1/15 that of the detailed calculation for the two-dimensional numerical simulations, which indicates the efficiency of the extended FGM method.

Importance of the considerations of preferential diffusion and flame stretch effects in the flamelet-generated manifold (FGM) method on the prediction accuracy is investigated by two-dimensional numerical simulations of cylindrical NH3/air premixed flames, under the conditions of an unburnt gas temperature of 673K, an ambient pressure of 2MPa, and equivalence ratios of 0.8 to 1.2. Results of the numerical simulations using the detailed chemistry, in which 32 species and 204 reactions are directly solved in the physical space without the FGM method, show that the mixture fraction in the burnt gas increases from the unburnt gas value when considering the preferential diffusion effect, whereas it remains flat when assuming the unity Lewis number. This means that assuming the unity Lewis number causes the underprediction and overprediction of the burnt gas temperature under fuel-lean and fuel-rich conditions, respectively. Results of the numerical simulations using the FGM methods show that considering the preferential diffusion and flame stretch effects in the FGM method is important for accurate prediction of the flame propagating speed, and the effectiveness is more evident for the flame stretch effect than for the preferential diffusion effect for the NH3/air premixed flames.

The introduction of the pilot flame can stabilize the lean premixed flame and promote its industrial application. However, the interaction mechanism between the pilot and main flames is complicated. To reveal the effect of the pilot flame on the main flame, a laminar lean premixed flame adjacent to a rich premixed pilot flame on one side and a similar lean premixed flame on the other side was considered. A two-dimensional numerical model was adopted with detailed chemistry and species transport, also with no artificial flame anchoring boundary conditions. The results show that the pilot flame could promote the main flame stabilized in different locations with various shapes, by adjusting the stretch, heat transfer, and preferential diffusion in a complicated manner. The pilot flame improves the local equivalence ratio and transfer more heat to the main flame. The growth of the pilot flame equivalence ratio and inlet velocity enhances the combustion on the rich side of the main flame and helps it anchor closer to the flame wall. Both the curvature and strain rate show a significant effect on the flame root, which contributes to the main flame bending towards the pilot flame.

A premixed and thermo-diffusively unstable turbulent hydrogen-air flame-in-a-box case is simulated in conjunction with the flame particle tracking (FPT) method. The flame is located in the flamelet regime. The focus lies on the assessment of memory effects in local flame dynamics. By tracking flame particles on an iso-surface of the flame during flame-turbulence interaction, the time history of flame speed and flame stretch can be recorded for each point on the flame iso-surface in a Lagrangian reference frame. The results reveal a time delay between the local flame speed and flame stretch signal, showing that previous values of flame stretch affect currently observed values of flame speed. Furthermore, by choosing flame particles whose trajectories are dominated by single frequencies, the time delay can be quantified. While plotting instantaneous values of flame speed and flame stretch results in a large scattering for turbulent flames, a quasi-linear correlation can be achieved by shifting the time signal of flame stretch according to the time delay. The time delay itself depends on the local flow time scale, which is expressed as a local Damköhler number. There is, however, an important difference between consumption and displacement speed. While most analyses in the literature are limited to the flame displacement speed, the flame consumption speed is evaluated for each flame particle in this work as well, which shows a strong correlation with the local equivalence ratio even at unsteady conditions. As the flame particles move toward regions with more negative flame stretch, the consumption speed decreases as the flame locally extinguishes. At the same time, the diffusive component of the displacement speed increases, as the tangential component of the diffusive flux increases in regions with strong negative flame curvature. [ABSTRACT FROM AUTHOR]

Development of a premixed flame theory that includes the effects of flame stretch and curvature has been at the forefront of combustion research. Diffusion dominated flames such as highly curved flame balls present a challenging flame structure that has not been included in current flame stretch theory so far. In such flames, the relationship between consumption speed and negative displacement speed usually marks the boundary of what flame stretch theory can predict. In this work, our objective is to derive a general formulation which naturally includes this relationship. We use flamelet equations derived using a mass based stretch rate and show that if the diffusion flux at the unburnt side is not ignored, as is normally done in flame stretch theory, a formulation that can describe the propagation of different types of premixed flames can be derived. Based on the thin reaction zone assumption, solutions from theory are verified against numerical results for 1D ideal flame balls. Further verification is done for multi-dimensional ball-like flames, where both convection and diffusion dominated regions are present for highly curved flames. It is shown that the extended theory is able to predict the flame kinematics in a better way by describing the diffusion dominated flame propagation as well. [ABSTRACT FROM AUTHOR]

Preferential diffusion plays an important role especially in hydrogen flames. Flame stretch significantly affects the flame structure and induces preferential diffusion. A problematic phenomenon occurring in real combustion devices is flashback, which is influenced by non-adiabatic effects, such as wall heat loss. In this paper, an extended flamelet-generated manifold (FGM) method that explicitly considers the preferential diffusion, flame stretch, and non-adiabatic effects is proposed. In this method, the diffusion terms in the transport equations of scalars, viz. the progress variable, mixture fraction, and enthalpy, are formulated employing non-unity Lewis numbers that are variable in space and different for each chemical species. The applicability of the extended FGM method to hydrogen flames is investigated using two- and three-dimensional numerical simulations of hydrogen-air flame flashback in channel flows. The results of the extended FGM method are compared with those of detailed calculations and other FGM methods. The two-dimensional numerical simulations show that considering both preferential diffusion and flame stretch improves the prediction accuracy of the mixture fraction distribution and flashback speed. The three-dimensional numerical simulations show that the prediction accuracy of the flashback speed, backflow region, and distributions of physical quantities near the flame front is improved by employing the extended FGM method, compared with the FGM method that considers only the heat loss effect. In particular, the extended FGM method successfully reproduced the relationship between the reaction rate and curvature. These results demonstrate the effectiveness of the extended FGM method. Novelty and Significance Statement The novelty of this research is the development of a flamelet-generated mani-fold (FGM) method that explicitly considers preferential diffusion, flame stretch, and non-adiabatic effects. To the best of the authors' knowledge, no studies have performed numerical simulations of pure hydrogen flames using such an FGM method. The developed FGM method was applied to numerical simula- tions of hydrogen-air premixed flame flashback at an equivalence ratio of 0.5 and reproduced the flashback speed of lean hydrogen-air premixed flame. The applicability of the FGM method to the numerical simulation of hydrogen-air flashback is reported first. This research is significant because the FGM method is one of the most widely used combustion models for premixed combustion, and the development of an accurate FGM method will contribute to the engineering field. The accurate prediction of the flame flashback attempted in this study is particularly important for the development of hydrogen-fueled combustion devices. [ABSTRACT FROM AUTHOR]

• The effect of jet-impinging structures on flame stability is investigated. • The jet-impinging structure contributes to the enhancement of the near-field turbulence intensity of the micro-mixing nozzle jet. • Reducing the flame stretch rate of the shear layer contributes to flame anchoring. Micromix combustion has the potential to achieve ultra-low NOx emissions. The jet-impinging structure is generally applied for micromix nozzles to obtain a well-mixing performance. However, its influence on flame stability is worth investigating, especially for natural gas flames, characterized by limited flame stability. In this study, the effect of the jet-impinging structure was investigated experimentally and numerically. Combustion characteristics, including flame shapes, OH* distributions, and lean blow-off limits, were obtained experimentally for two types of nozzles. Compared with the round nozzles without the jet-impinging structure, the flames generated by the micromix nozzles with the jet-impinging structure have lower lift-off height, shorter flame height, and lower lean blow-off equivalence ratio. Numerical simulations show that the jet-impinging structure contributes to increasing the turbulence intensity, as a result, the turbulent flame propagation is enhanced. Meanwhile, the structure produces thicker shear layers which decrease the local flame stretch rate, and local extinctions caused by flame stretching are partially avoided. In general, this work demonstrates the viability of applying the jet-impinging structure to extend the flame stability limit and generate compact flames. [ABSTRACT FROM AUTHOR]

In the present work, direct numerical simulation data of a laboratory-scale high Karlovitz number (Ka) jet flame with a strong mean shear and a high Ka freely propagating planar flame without a mean shear were analyzed to explore the influence of flame orientation and mean shear on flame stretch in highly turbulent flames. Three flame fronts, i.e. the leading, in-plane and trailing front, were identified for the jet flame. The leading and trailing front is defined as the flame normal being facing the downstream and upstream regions, respectively, while the flame normal of the in-plane front is near the cross-stream plane. It was found that the flame stretch is positive in the leading front, and is negative in the trailing front of the jet flame. As for the planar flame, the statistics of the flame stretch are similar in various flame fronts. The components of the flame stretch are analyzed. The normal vector of the leading front is aligned with the most compressive strain rate in the jet flame, resulting in a large tangential strain rate, while the tangential strain rate of the trailing front is significantly lower. In contrast, the alignment characteristics and the strain rates of the planar flame are not sensitive to the flame orientation. The value of the stretch due to curved propagating flame front is large and negative in the trailing front of the jet flame due to the negative correlation between the displacement velocity and curvature, which is weak in the leading and in-plane front. The correlation between the displacement velocity and curvature is, however, similar in various fronts of the planar flame. Overall, it is suggested that the effects of flame orientation and mean shear should be accounted for in understanding turbulence-flame interactions in practical combustion devices.

Stratified propane air flames stabilized in the near wake region of a premixer/disk arrangement were investigated for a variety of inlet mixture compositions and preheating temperatures. The employed burner is capable of anchoring flames at very lean mixtures, expanding the lean flammability limits and promoting flame stabilization, at global equivalence ratio values of Φ = 0.13 at 743 K. Particle Image Velocimetry and OH ∗ Chemiluminescence Image analysis were performed to investigate flame stabilization characteristics at four inlet preheat levels ranging from 300 to 743 K, for fuel-lean reactant concentrations. Inlet mixture reactivity, 2D aerodynamic stretch rates, Damköhler ( Da ) and Karlovitz ( Ka ) numbers and flame brush thicknesses were estimated and analyzed to elucidate the effects of turbulence, inlet preheat and mixture composition on the flame topology and anchoring characteristics. Results show that the simultaneous stratification and preheat of the inlet mixture has a notable effect on the turbulent flame stabilization, with Ka numbers up to 60, for the highly preheated, ultra-lean flames, suggesting that the flames are chemically controlled with turbulence playing a less significant role. Moreover, the 2D aerodynamic stretch rates, evaluated along the mean flame front path, revealed that flame quenching requires both a certain level of mean stretch and a sufficiently large probability of these values to be exerted on the flame. Considering the stretch rate distributions and the respective Da numbers along the flame sheet it could be conjectured that laminar extinction stretch rates can be used as reference, for turbulent flames, only when the local Da number exceeds unity. [ABSTRACT FROM AUTHOR]

Ammonia is a promising sustainable fuel, however, its low reactivity creates challenges in industrial applications. In this study, ammonia/methane mixtures were considered for premixed and non-premixed counterflow flames. The extinction stretch rate was measured over a wide range of ammonia/methane mixing ratios and compared to 1D numerical results from four different mechanisms. Additionally, for counterflow premixed twin flames, quantitative analysis based on the comparison of experimental and numerical FWHM of OH and NO profiles was performed. Results showed that in premixed flames, all the mechanisms investigated were inadequate for predicting the extinction stretch rate, specifically for lean flames. In non-premixed flames, Okafor's mechanism was accurately predicting the extinction stretch rate. For the FWHM analysis, the numerical mechanisms overpredicted both OH and NO apparition in the flame, except for Tian's mechanism which underpredicted OH apparition. GRI Mech 3.0 performed well for small quantities of ammonia but failed to reproduce the pure ammonia case. Okafor's and UCSD mechanisms gave better predictions of experimental trends, though overestimated both OH and NO apparition. In the FWHM analysis of both OH and NO, the lower R2 values in the lean region suggest that the lean region should be the focus for mechanism improvement. [ABSTRACT FROM AUTHOR]

An improved embedded-manifold/Navier–Stokes numerical methodology is developed to simulate the propagation of premixed flames within the context of the hydrodynamic theory. The method is computationally tractable, permitting calculations to not only be extended to larger physical domains but also to span a broader parametric space of physicochemical parameters. The focus of this paper is to examine the susceptibility of centrally ignited, freely propagating and outwardly expanding circular flames to small amplitude disturbances and observe the flame's development through the onset of the hydrodynamic instability. The numerical simulations, validated by a linear stability analysis, show that for mixtures with Lewis numbers above criticality, thermo-diffusive effects exert stabilising influences which dominate at small flame radii, initially suppressing the growth of all disturbances. Consistent with the linear theory, simulations show the flame initially remaining stable and demonstrate the existence of a particular mode which is the first to grow. This mode is said to dictate the cellular pattern observed experimentally at the onset of instability. The variation in critical flame radius with respect to the Markstein length and thermal expansion coefficients are in quantitative agreement with these analytical results. [ABSTRACT FROM AUTHOR]

The current research experimentally studied flame speed and stretch under engine in-cylinder conditions. A direct-injection, spark-ignition, and optically accessible engine was utilized to image the flame propagation, and E10 was selected as the fuel. Also, three fuel–air mixtures (stoichiometric, lean, and rich) were examined. The flame front was located by processing high-speed images. This study introduces a novel approach for calculation of equivalent spherical flame radius for analysis of flame speed and stretch. Flame front propagation analysis showed that during the flame propagation period, the stretch decreased until the flame front touched the piston surface. This was a common trend for stoichiometric, lean, and rich mixtures, which occurred because the flame radius was the dominant factor in the stretch calculation. In addition, the rich fuel–air mixture showed a lower flame stretch compared to stoichiometric or lean mixture. This was the result of a lower Markstein number for the rich fuel–air mixture. To study the sensitivity of different fuel–air mixtures to the flame stretch, the trajectory of the flame centroid was tracked until the flame front touched the piston surface. The results showed that the end centroid for the lean mixture deviated from the start point more than those of the rich and stoichiometric mixtures because the lean mixture had a higher flame stretch and lower flame speed. Furthermore, comparing the flame stretch at three different engine speeds revealed that increasing the engine speed increased the flame stretch, especially during the early flame development period. According to previous studies which discussed flame stretch as a flame extinguishment mechanism, the probability of flame extinction is higher when the engine speed is higher. Also, uncertainty analysis was conducted to determine the effect of camera setting on the flame stretch. Results showed that a maximum relative uncertainty of 4.5% occurred during the early flame development. [ABSTRACT FROM AUTHOR]

According to the US Energy Information Administration, fossil fuels will remain the main source of energy for transportation over the next decades and thus the combustion of these fuels remains an important concern. This research studied the flame propagation under engine in-cylinder conditions and developed a correlation for turbulent burning velocity based on the global flame stretch concept. To study the impact of engine operation on flame stretch, two speeds, two loads, and three fuel-air mixtures were investigated. The flame front was determined by processing images of the flame natural luminosity. A turbulent burning velocity model was developed using dimensional analysis. The model showed that the turbulent burning velocity decreased due to flame stretching. Higher engine speeds increased the turbulent burning velocity by increasing the turbulent intensity, yet a tradeoff between the flame stretch and the turbulent burning velocity due to higher engine speed was observed. In cases where the flame distortion was very high, the flame stretch may cancel out any benefits of a large enflamed area. Incorporating the flame stretch into the burning velocity model and coupling the developed model with GT-Power simulation software revealed that the stretch may result in a 35% reduction in turbulent burning velocity. • The developed model showed that the turbulent burning velocity decreased due to flame stretching. • Higher engine speeds increased the turbulent burning velocity by increasing the turbulent intensity. • A tradeoff between the flame stretch and the turbulent burning velocity due to higher engine speed was observed. • In cases where the flame distortion was very high, the flame stretch may cancel out any benefits of a large enflamed area. [ABSTRACT FROM AUTHOR]

From experimental spark ignition (SI) engine studies, it is known that the slow-down of early flame kernel development caused by the (Le > 1)-property of common transportation-fuel/air mixtures tends to increase cycle-to-cycle variations (CCV). To improve the fundamental understanding of the complex phenomena inside the flame structure of developing flame kernels, an engine-relevant DNS database is investigated in this work. Conclusive analyses are enabled by considering equivalent flame kernels and turbulent planar flames computed with Le > 1 and Le = 1. In Part I of the present study (Falkenstein et al., Combust. Flame, 2020), a reduced representation of the local mixture state was proposed for the purpose of this analysis. Fluctuations in heat release rate attributed to differential diffusion were found to be governed by the parameters local enthalpy, local equivalence ratio, and H-radical mass fraction. Here, a coupling relation for the diffusion-controlled mixture parameter local enthalpy with local flame geometry and structure is derived, characterized by the key parameters κ and |∇ c |/|∇ c | lam. The analysis shows that the large positive global mean curvature intrinsic to the flame kernel configuration may detrimentally affect the local mixture state inside the reaction zone, particularly during the initial flame kernel development phase. External energy supply by spark ignition may effectively bridge over this critical stage, which causes the impact of global mean flame kernel curvature to be small under the present conditions compared to the overall effect of Le ≠ 1 observed in a statistically planar flame. Once ignition effects have decayed, the mixture state inside the reaction zone locally exhibits an identical dependence on |∇ c | as in a strained laminar flame. This implies that differential diffusion effects at Karlovitz numbers representative for part-load conditions are not weakened by small-scale turbulent mixing, which is undesirable for the engine application, but can be favorable in terms of modeling. [ABSTRACT FROM AUTHOR]

Although experimental evidence for the correlation between early flame kernel development and cycle-to-cycle variations (CCV) in spark ignition (SI) engines was provided long ago, there is still a lack of fundamental understanding of early flame/turbulence interactions, and accurate models for full engine simulations do not exist. Since the flame kernel is initiated with small size, i.e. with large positive curvature, differential diffusion is expected to severely alter early flame growth in non-unity-Lewis-number (Le ≠ 1) mixtures as typically used in engines. In this work, a DNS database of developing iso-octane/air flame kernels and planar flames has been established with flame conditions representative for stoichiometric engine part-load operation. Differential diffusion effects on the global heat release rate are analyzed by relating the present findings to equivalent flames computed in the Le = 1 limit. It is shown that in the early kernel development phase, the normal propagation velocity is significantly reduced with detrimental consequences on the global burning rate of the flame kernel. Besides this impact on the overall mass burning rate, the initial production of flame surface area by the normal propagation term in the flame area balance equation is noticeably reduced. By using the optimal estimator concept, it is shown that strong fluctuations in local heat release rate inherent to Le ≠ 1 flames in the thin reaction zones regime are mainly contained in the parameters local equivalence ratio, enthalpy, and H-radical mass fraction. Differential diffusion couples the evolution of these parameters to the unsteady flame geometry and structure, which is analyzed in Part II of the present study (Falkenstein et al., Combust. Flame, 2020). [ABSTRACT FROM AUTHOR]

This paper examines the dynamics of flame annihilation in turbulent premixed flames using a DNS dataset of Haghiri et al. (J. Fluid Mech. 843, 29 2018). Two theoretical frameworks are considered. The relative contributions of the reaction rate, normal diffusion and flame curvature to the flame displacement speed are first considered. A revised form of Markstein's theory describing the displacement speed as a function of the local Karlovitz number is then used, including consideration of the relative importance of strain rate and curvature. Comparison of the statistics of flame annihilation to those of the entire flame surface suggests that this modified Markstein number might be sufficient to estimate the displacement speed for most annihilation events. This has important implications for level-set type combustion modelling where correct prediction of the displacement speed is a key part of describing the evolution of the flame surface. Given the significance of flame annihilation in sound generation by turbulent premixed flames (Haghiri et al. J. Fluid Mech. 843, 29 2018; Brouzet et al. Combust. Flame 204, 268 2019), these findings also have implications for the modelling of premixed combustion noise. [ABSTRACT FROM AUTHOR]

• Three propagation phenomena are observed in the premixed hydrogen/ammonia/air flame. • The addition of hydrogen increases the burning rate of ammonia, causing the effect of the volumetric force instability to fade away. • Flame thickness and critical instability radius decrease monotonically with increasing hydrogen content. • Critical instability radius and Markstein length both decrease rapidly with increasing initial pressure. To improve the safety and reliability of hydrogen energy, this paper studied the flame propagation and instability characteristics of hydrogen/ammonia/air mixture under different hydrogen contents, equivalence ratios and initial pressures. The results show that three typical phenomena are observed in the premixed hydrogen/ammonia/air flame propagation process under different experimental conditions. In a stable flame, the roughness of the flame surface does not increase, while during unstable flame expansion, the cracking of the flame surface splits and increases. In addition, under the condition of low equivalence ratio and hydrogen content, significant flame uplift is observed. With the increase in hydrogen content, the flame thickness and critical instability radius of the hydrogen/ammonia/air mixture decrease monotonously. The increase of hydrogen content mainly strengthens the hydrodynamic instability, leading to the weakening of flame stability. With the increase of the equivalence ratio, the Markstein length and the critical instability radius increase monotonically. The minimum value of flame thickness appears at the equivalence ratio of 1.0. The Lewis number is always less than 1 for poor combustion and greater than 1 for rich combustion. An increase in the hydrogen content only moves the Lewis number further away from 1. In rich burn, the thermal-diffusion is beneficial to flame stability, and the flame thickness increases with the increasing equivalence ratio. Under the combined effect of thermal-diffusion and hydrodynamic instability, the overall stability of the hydrogen/ammonia/air flame gradually increases with the increasing equivalence ratio. With the increase of initial pressure, the flame thickness decreases gradually. The critical instability radius and Markstein length both decrease rapidly with the increase of initial pressure. [ABSTRACT FROM AUTHOR]

This study investigated the flame propagation and turbulent burning velocity of hydrogen/n-heptane/air mixtures subjected to differential diffusion and flame stretch induced by turbulence at different hydrogen molar fractions (X H2 = 0–100%), equivalence ratios (ϕ = 0.5–1.0) and turbulence intensities (u' = 0–3.62 m/s). The experiments were carried out on a medium-scale, fan-stirred cylindrical combustion chamber generating near-isotropic turbulence. Results show that as hydrogen content increases, the turbulent flame propagation speed tends to be affected by stronger differential diffusion and weakened turbulence, characterized by decreasing Lewis number (Le) and Karlovitz number (Ka) within a similar range of turbulence intensity. Among them, the effect of differential diffusion on the normalized turbulent propagation speed (d < r > / d t σ S L ) appears to be well correlated with Re T,flame Le -2, where Re T,flame represents the turbulent flame Reynolds number. However, inconsistencies in the fitting parameters for this scale are observed at different X H2 , highlighting the impact of substantial variations in flame stretch induced by turbulence. Therefore, the normalized turbulent flame propagation velocity under varying hydrogen contents is proposed to be accurately predicted by d < r > / d t σ S L -1 = 0.07 K a 0.32 (R e T , flame L e − 2) 0.5 . The synergistic effects of Le and Ka can also account for the peak phenomenon in the normalized turbulent burning velocity (S T / S L) observed at 85% hydrogen content. Furthermore, the quantitative characterization of turbulence and differential diffusion under extensive experimental data (Ka = 0.03–90) is studied. As evidenced by the Damköhler theoretical hypothesis, the two-stage trend between S T / S L and (u' / S L) Le -1 is identified with a transition near unity Ka. The modified correlations for turbulent burning velocity in terms of (u' / S L) Le -1 and Ka 0.2 are obtained with consideration of the differential diffusion and flame stretch rate induced by turbulence, which show good predictive performance for data at different turbulence regime diagrams regardless of fuel type and equipment size. [ABSTRACT FROM AUTHOR]

[Display omitted] • Demonstrate the blowout of different diffusion flames remotely by a moving air vortex ring. • The air vortex ring with less than 1% of the flame power is sufficient to cause blow-off. • The instant release of vortex core kinetic energy is defined as the main factor causing flame extinction. • Propose a Damköhler number criterion to identify blow-off limits for different fuels and burners. The blow-off is one of the typical flame extinction mechanisms, and such a principle has been widely used in firefighting when the water-based extinguisher is limited. This work explores the blow-off extinctions of different diffusion flames by air vortex ring. The vortex ring harnesses its kinetic energy within the fast-rotating vortex core, enabling the transmission of power over several meters to blow off a remote flame. The power required for vortex ring blow-off is found to be two to three orders of magnitude smaller than the power of the flame itself, demonstrating exceptional energy efficiency. It is observed that the poloidal flow (circulation) surrounding the vortex core can stretch the flame base to the critical state and then cause instantaneous extinction. To explain the vortex-induced blow-off limit, a critical Damköhler number that accounts for the competition between fuel gas flow and flame stretch was formulated. This work provides a fundamental understanding of the extinction mechanism by vortex ring, and it offers technical guidelines for using air as a flame extinguisher for remote firefighting within minimum energy input. [ABSTRACT FROM AUTHOR]

Flames in practical combustors and engines are inevitably subject to the effects of upstream chemical reaction progress and stretch, induced by elevated thermodynamic conditions and flow non-uniformities, respectively. Recent shock tube experiments and simulation studies on flame propagation have shown that flame speed under engine-relevant conditions can be enhanced with non-negligible upstream chemical reaction progress, especially when low-temperature heat release is involved in the unburnt mixture. On the other hand, depending on the mixture equivalence ratio and diluents, nonequidiffusion (including the non-unity Lewis number effect and the preferential diffusion effect) can couple with the flame stretch to fundamentally affect the flame propagation, which can manifest as either facilitation or suppression. Depending on the transport property of a reacting mixture, there hence can be either inhibition or promotion from unburnt reaction progress and stretch effects on flame propagation. In the current work, through one-dimensional numerical simulations of transient planar and spherical flames of n -heptane/air under elevated thermodynamic conditions, the combined effects of upstream chemical reaction progress and stretch on flame propagation are investigated. Results show that for both lean and rich n -heptane/air mixtures, flame speed can be substantially promoted with reaction progress, while the rich mixtures can exhibit opposite stretch dependence after first-stage ignition. Different definitions of effective Lewis number are adopted to explain the change in Markstein length for spherical flame in lean and rich mixtures reformed by the low temperature chemistry. This work fills an important gap in laminar premixed flame research relevant to practical combustion systems and can provide useful insight into local turbulent flame behaviors and phi-sensitivity in engine combustion. [ABSTRACT FROM AUTHOR]

This paper investigates a predictive model that considers the impact of stretch on the dynamic response of laminar premixed conical and V- flames; the flame stretch consists of two components: the flame curvature and flow strain. The steady and perturbed flame fronts are determined via the linearized G -equation associated with the flame stretch model. Parameter analyses of the effects of Markstein length L , flame radius R and unstretched flame aspect ratio β are also conducted. Results show that the flame stretch reduces the steady flame height, with this effect being more significant for larger Markstein lengths and smaller flame sizes. The effects of flame stretch on perturbed flames are evaluated by comparing the flame transfer function (FTF) considering the flame stretch and not. For flames of different sizes, the impact of flame stretch on FTF gain can be divided into three regions. When both β and R are relatively small, due to the decrease in steady flame height and the impact of flow strain, the FTF gain increases. As β and R gradually increase, the FTF gain of the conical flame oscillates periodically while the FTF gain of the V-flame decreases, primarily due to the flame curvature enhancing the flame front disturbance and the wrinkle counteracting effect. When β and R are large, a disruption in wrinkle counteracting effect ensues, leading to a significant increase in FTF gain. Furthermore, as the actual flame height is reduced, the flame stretch also reduces the FTF phase lag which is related to the disturbance propagation time from the flame root to the tip. [ABSTRACT FROM AUTHOR]

The manner in which an ultra-lean hydrogen flame stabilizes and blows off is crucial for the understanding and design of safe and efficient combustion devices. In this study, we use experiments and numerical simulations for pure H2-air flames stabilized behind a cylindrical bluff body to reveal the underlying physics that make such flames stable and eventually blow-off. Results from CFD simulations are used to investigate the role of stretch and preferential diffusion after a qualitative validation with experiments. It is found that the flame displacement speed of flames stabilized beyond the lean flammability limit of a flat stretchless flame (ϕ=0.3) can be scaled with a relevant tubular flame displacement speed. This result is crucial as no scaling reference is available for such flames. We also confirm our previous hypothesis regarding lean limit blow-off for flames with a neck formation that such flames are quenched due to excessive local stretching. After extinction at the flame neck, flames with closed flame fronts are found to be stabilized inside a recirculation zone.

An experiment was set up to investigate the correlation between flame velocity and flame stretch in moderately stretched laminar jet flames of methane–air mixtures at different equivalence ratios. Gas flow velocities were obtained by Particle Image Velocimetry (PIV) from images of axial sections of seeded flows, and the location of the flame front was related to the gradient of luminosity. Flame stretch was determined and shown to be mainly due to flame curvature, with a small contribution from the gas strain rate. Inlet gas velocities not much larger than the planar flame velocity led to short rounded flames for which the classical Markstein relation was experimentally validated at any point of the flame fronts. However, the Markstein relation ceases to be uniformly valid for larger inlet gas velocities, in the order of ten times the planar flame velocity, for which the flame adopts a slender shape. These tall flames are nearly conical and obey the Markstein relation far from their tips, but a superlinear increase of flame velocity with stretch is observed when the tip is approached. Furthermore, the local velocity of these flames depends not only on the stretch but also on the inlet velocity of the gas. This behavior has been analyzed in terms of an empirical correction to the Markstein relation that is quadratic in the flame stretch with a coefficient that depends on the gas inlet velocity. The experimental results suggest that this coefficient is zero below a certain threshold value of this velocity and increases as the square root of the excess of gas inlet velocity above this threshold. This kind of sharp transition controlled by the velocity of the incoming gas is reminiscent of a typical critical behavior and is tentatively taken to reveal a stability exchange between rounded (weakly curved) and slender (strongly curved) flames. [ABSTRACT FROM AUTHOR]

Abstract Compared to current refrigerants, next-generation refrigerants are more environmentally benign but more flammable. The laminar burning velocity is being used by industry as a metric to screen refrigerants for fire risk and it is also used for kinetic model development and validation. This study reports measurements of difluoromethane/air flame burning velocities for equivalence ratios from 0.9 to 1.4 in a spherical, constant volume device. Experimental burning velocities produced with the aid of an optically thin radiation model are about 17% greater than those obtained with an adiabatic model. Characterization of flame stretch based on the product of Markstein and Karlovitz numbers indicates that while many experimental data are nearly stretch-free, those for slower burning velocities, smaller flame radii, and leaner conditions may not be. Limiting the data to regions estimated to be stretch-free requires extrapolation away from the experimental conditions to extract burning velocities near ambient conditions, e.g., at (298 K, 101 kPa). Lower uncertainty, desirable for kinetic model validation, is obtained by interpolating between experimental conditions, e.g., at (400 K, 304 kPa). Since thermal radiation and flame stretch were found to affect the inferred burning velocities of difluoromethane/air constant volume spherical flames, these effects should also be considered during data reduction of other mildly flammable refrigerants. [ABSTRACT FROM AUTHOR]

Abstract Direct Numerical Simulations (DNS) are conducted to study the statistics of flame surface stretch in turbulent jet premixed flames. Emphasis is placed on the rates of surface production and destruction and their scaling with the Reynolds number. Four lean methane/air turbulent slot jet flames are simulated at increasing Reynolds number and up to Re ≈ 22 × 103, based on the bulk velocity, slot width, and the reactants' properties. The Karlovitz number is held approximately constant and the flames fall in the thin reaction zone regime. The simulations feature finite rate chemistry and mixture-average transport. Our data indicate that the area of the flame surface increases up to the streamwise position corresponding to 80% of the average flame length and decreases afterwards as surface destruction overtakes production. It is observed that the tangential rate of strain is responsible for the production of flame surface in the mean and surface destruction is due to the curvature term. In addition, it is found that these two terms are both significantly larger than their difference, i.e., the net surface stretch.The statistics of the tangential strain rate are in good agreement with those for infinitesimal material surfaces in homogeneous isotropic turbulence. Once scaled by the Kolmogorov time scale, the means of both contributions to stretch are largely independent of location and equal across flames with different values of the Reynolds number. Surface destruction is due mostly to propagation into the reactants where the surface is folded into a cylindrical shape with the center of curvature on the side of the reactants. The joint statistics of the displacement speed and curvature of the reactive surface are nuanced, with the most probable occurrence being that of a negative displacement speed of a flat surface, while the surface averaged displacement speed is positive as expected. [ABSTRACT FROM AUTHOR]

Abstract Premixed flames in turbulent, shear flows are disturbed by a superposition of broadband, turbulent fluctuations, and coherent disturbances originating from acoustic disturbances and/or hydrodynamic instabilities. Prior studies have shown nonlinear interactions between such coherent and stochastic disturbances on the flame. This paper investigates the effect of these interactions on the correlation between tangential flame stretch and curvature, an important quantity in turbulent premixed flame physics. High speed particle image velocimetry is used to measure the flow field in an experimental facility where harmonic wrinkles are introduced into a premixed flame that is subjected to varying turbulence intensities, while Mie scattering images are used to extract flame edges. Results show a clear correlation between phase-averaged tangential flame stretch and flame curvature. The magnitude of the fluctuations in both of these quantities decreases with increase in turbulence intensity, even while their overall correlation remains strong. Results from a linearized analysis of the perturbation of the approach flow by a curved, laminar flame are shown to capture the key features of the experimental data. These results suggest that the phase-averaged tangential stretch-curvature correlation is due to gas expansion effects, causing divergence of the approach flow in regions of positive flame curvature, the same mechanism that is responsible for the Darrieus–Landau instability. This result shows that turbulent flames in shear flows with large scale coherent structures, are subjected to not only mean strain, but also coherent modulations in strain rate. [ABSTRACT FROM AUTHOR]

An experimental study investigated the frequency characteristics of a stretched cylindrical diffusion flame in response to sinusoidal air flow velocity oscillation. The flame has a convex curvature with respect to the air stream. The fuel was methane diluted with nitrogen, and oxidizer air. The flame was formed in the air stream side from the stagnation surface. The oscillation frequency f ranged from 10 Hz to 250 Hz. In order to clarify the influence of an oscillation amplitude A of the air flow velocity on the flame responses, three kinds of amplitude were chosen. The flame was photographed by the high speed video camera, and flame radius rf, flame thickness δ, and flame luminosity Lf were analyzed. The air flow velocity va at the nozzle outlet was measured by using particle image velocimetry. Though velocity gradient of the air flow ga responds quickly to the changing va regardless of f and A, the gradient of the fuel flow gf delays with increasing f and A. Here, ga correlates to the flame stretch effect, and gf impacts the fuel transportation. Response of rf to varying va also delays with increasing f and A. δ/rf, which relates to the flame curvature effect, responds complicated with the change in f and A. Oscillation amplitudes of Lf have maximum values with respect to f regardless of A. These maximum values are greater than those of a steady flame, over the same air velocity fluctuation ranges. This complex change in the Lf with respect to f is closely related to the phase difference in the respective time variations in δ/rf and gf, and the magnitude of these values.

Natural Gas (NG) is a promising alternative fuel. Historically, the slow burning velocity of NG poses significant challenges for its utilisation in energy efficient Spark Ignited (SI) engines. It has been experimentally observed that a binary blend of NG and gasoline has the potential to accelerate the combustion process in an SI engine, resulting in a faster combustion even in comparison to that of the base fuels. The mechanism of such effects remains unclear. In this work, an optical diagnosis has been integrated with in-cylinder pressure analysis to investigate the mechanism of flame velocity and stability with the addition of NG to gasoline in a binary Dual Fuel (DF) blend. Experiments were performed under a sweep of engine load, quantified by the engine intake Manifold Air Pressure (MAP) (0.44, 0.51. 0.61 bar) and equivalence air to fuel ratio (ε = 0.8, 0.83, 1, 1.25). NG was added to a gasoline fuelled engine in three different energy ratios 25%, 50% and 75%. The results showed that within the flamelet combustion regime, the effect of Markstein length dominates the lean burn combustion process both from a stability and velocity prospective. The effect of the laminar burning velocity on the combustion process gradually increases as the air fuel ratio shifts from stoichiometric to fuel rich values. [ABSTRACT FROM AUTHOR]

High swirl flows with vortex breakdown are frequently used to create recirculating flow regions for flame stabilization. This paper experimentally characterizes flame stabilization in such flows, motivated by the fact that conditions under which they enable (or do not enable) aerodynamically stabilized flames are not well understood. Such flames are substantially more difficult to experimentally characterize than shear layer stabilized flames, as the flame leading edge moves around significantly and may or may not coincide with a diagnostic laser sheet, in contrast to the near-fixed leading edge of shear layer stabilized flames. This paper characterizes the leading edge conditions, based upon simultaneous stereo Particle Image Velocimetry (sPIV), OH Planar Laser-Induced Fluorescence (PLIF) and OH* chemiluminescence data. Frames are conditionally analyzed when the flame's most upstream point was captured in the laser sheet. These conditioned frames were used to determine the coordinates, local flow velocity and full, three-dimensional, hydrodynamic strain component of flame stretch at the dynamically evolving flame leading edge. Results are taken at two conditions, 70 m/s and 45 m/s axial velocity, referred to as case 1 and 2, respectively. These data show that the leading edge of the flame precesses off-axis, with additional uncorrelated radial and axial motions. The conditioned mean flame stretch rate is positive in both cases, due to the bulk flow deceleration from the high velocity nozzle flow into the larger diameter combustor. Mean strain or velocity values conditioned on the leading flame edge are substantially different from mean values calculated at the same location. For example, the case 1 mean conditioned axial velocity at the flame leading edge is 9 m/s, while the mean axial velocity at the same location is 4 times higher, 36 m/s. In addition, the case 1 mean hydrodynamic strain rate is 3.6 times higher than the corresponding laminar flame extinction stretch rate, κ ext , calculated from detailed kinetics for the high velocity case, while less than half of κ ext in case 2. The mean strain rate predicted from bulk velocity scaling changes by a factor of 70/45 between the two cases, a value confirmed by measurements. However, the conditioned mean stretch values differ by a factor of 14. This result illustrates that additional work is needed to understand stretch scalings in such flames. Furthermore, they show that current correlations for flame stability do not capture much of the intermittency that is present in swirl flows with precessing flow features, and illustrates the need for continued work in very basic physics of how and when flames are stabilized aerodynamically. [ABSTRACT FROM AUTHOR]

The characteristics of local extinction on the tip of syngas premixed Bunsen flames with different hydrogen fractions have been investigated experimentally with OH-PLIF technique and numerical simulation. Coupled effects of stretch and diffusion on the syngas Bunsen flame with highly negative curvature were studied. Local Karlovitz number was introduced to indicate the onset of local extinction at the Bunsen flame tip. Results showed that the onset equivelance ratio of local extinction at the syngas Bunsen flame tip does not change with the outlet velocity. The coupled effects of stretch and diffusion result in the decrease of flame reaction rate along the syngas Bunsen flame at fuel-lean condition. When the flame reaction rate decrease to a critical extent, local extinction of Bunsen flame tip occures. The local Karlovitz number can be considered as a parameter of the critical condition. The syngas Bunsen flame tip will extinguish when the local Karlovitz number reaches around 4, irrespective of hydrogen fractions and outlet velocity. This means that Bunsen flame tip starts to appear local extinction when the tangential flow time is comparable to normal chemical reaction time, which is the same as counterflow flames with positive stretch. Furthermore, results also showed that there is no H 2 leakage even the syngas Bunsen flame tip extinguishes at the numerical study conditions. [ABSTRACT FROM AUTHOR]

Experimental flame trajectories of hydrogen/air and n-butane/air spherically expanding flames were measured over a wide range of equivalence ratios at pressures of 0.1 MPa and 0.05 MPa. An experimental radius range selection method was developed to determine the critical initiation and end radii for extraction of laminar burning velocity. The results showed that both flame radius range selection and nonlinear flame stretch have significant effects on laminar burning velocity extraction. As the selected initiation radius increases, the flame radius range effect increases. Thus, the widest unaffected radius range should be processed using a proper method to reduce the extraction induced uncertainty. Additionally, the extraction method, which reflects a linear relationship between the flame propagation speed and the flame curvature, well fits the experimental data for a wide radius range when Lewis number, Le > 1. And the performances of all tested extraction methods are almost equal when Le < 1, especially for Le slightly smaller than unity. The nonlinear method is still recommended in extraction for very small Le. The critical initiation radius depends strongly on the Lewis number and initial pressure, and increases linearly with the absolute value of Markstein length. At large and small Le, the initiation radius used in measurements should be large enough to ensure reliable extraction results. Furthermore, the critical initiation radius is affected by transition and spark-assisted propagation regimes for Le > 1 and Le < 1, respectively. It is concluded that the proper flame radius range for extraction is narrowed primarily by the strong flame stretch effects, which increase the critical initiation radius for stable flames. [ABSTRACT FROM AUTHOR]

Experimental study was performed to reveal the response characteristics of a stretched cylindrical diffusion flame to sinusoidal oscillation of air flow velocity. The cylindrical flame used in this study has a convex curvature with respect to air stream and is formed in air stream. The fuel is methane, diluted with nitrogen, and the oxidizer is air. Oscillation frequency f is varied from 5 Hz to 250 Hz. Velocity at the outlet of air supplying nozzle was changed sinusoidally with four speakers. Velocity at the fuel nozzle outlet was kept constant. The air velocity at the nozzle outlet is measured using particle image velocimetry. Flame radius rf, flame thickness δ, and flame luminosity Lf are obtained by using high speed video camera. Results are summarized as follows: Though fluctuation amplitude of the velocity gradient of air stream Δga is constant with increasing f, that of the fuel stream Δgf increases. The fluctuation amplitude of rf varies to quasi-steady at low frequency, while the fluctuation amplitude is reduced with increasing f. Lf does not respond to quasi-steady at low frequency. The fluctuation amplitude of Lf has maximum value at 50 Hz and is larger than that for the steady flame corresponding to the velocity fluctuation. It is considered that this complex change of the flame luminosity with respect to f is related to δ/rf which is associated with the flame curvature effect, ga which affects the flame stretch effect, and gf which impacts fuel transport.

It is difficult to understand turbulent combustion, because the combustion process is very complex and unsteady. One of the useful parameters for discussion of the unsteady flame behaviors is the flame stretch rate, which is evaluated by local flame curvature and strain rate. In this study, we conducted simultaneous OH-PLIF/Stereo PIV measurements of turbulent premixed flames. Flame shape and flow field were obtained. Then, local flame curvature and strain rate were discussed. The turbulent flame was established by a cyclone-jet combustor for propane/air mixtures. The main jet velocity was Um = 10, 30 m/s, and the equivalence ratio was Φm = 0.75, 0.90. The equivalence ratio of the cyclone jet was the same as that of the main jet, but the velocity of the cyclone jet was the constant of 10 m/s. It was found that the variation of flame curvature was large when the main jet velocity was high. Difference between curvature convex to unburned gas and curvature convex to burned gas was relatively small. Frequency of larger strain rate was high when the main jet velocity was large and equivalence ratio was small. It is suggested that the local flame extinction could be caused by the large strain rate because the local flame extinction occurs more frequently. At all conditions, compared with the positive strain rate, the frequency of negative strain rate was high.

To safely mine coal, engineers must prevent gas combustion and explosions, as well as seek feasible and reasonable techniques to control for these types of incidents. This paper analyzes the causes and characteristics of methane combustion and explosions. Water mist is proposed to prevent and control methane combustion in an underground confined space. We constructed an experiment platform to investigate the suppression of methane combustion using water mist for different conditions. The experimental results showed that water mist is highly effective for methane flame inhibition. The flame was extinguished with water mist endothermic cooling. However, the annular regions of water vapor around the fire played a vital role in flame extinction. Water from the evaporating mist replaces the oxygen available to the fuel. Additionally, the time required for fuel ignition is prolonged. For these reasons, the water particle action to flame surface is reinforced and the fuel's reaction with air is delayed. As a result, flame stretching and disturbances occur, which serve to extinguish the flame. Engineering application tests were carried out in the goaf, drill hole and upper-corner to investigate the prevention and control of methane gas combustion, with the results showing a good application effect. [ABSTRACT FROM AUTHOR]

Laminar flamelets are often used to model premixed turbulent combustion. The libraries of rates of conversion from chemical to thermal enthalpies used for flamelets are typically based on counter-flow, stained laminar planar flames under steady conditions. The current research seeks further understanding of the effect of stretch on premixed flames by considering laminar flame dynamics in a cylindrically-symmetric outward radial flow geometry (i.e., inwardly propagating flame). This numerical model was designed to study the flame response when the flow and scalar fields align (i.e., no tangential strain on the flame) while the flame either expands (positive stretch) or contracts (negative stretch, which is a case that has been seldom explored) radially. The transient response of a laminar premixed flame has been investigated by applying a sinusoidal variation of mass flow rate at the inlet boundary with different frequencies to compare key characteristics of a steady unstretched flame to the dynamics of an unsteady stretched flame. An energy index (EI), which is the integration of the source term in the energy equation over all control volumes in the computational domain, was selected for the comparison. The transient response of laminar premixed flames, when subjected to positive and negative stretch, results in amplitude decrease and phase shift increase with increasing frequency. Other characteristics, such as the deviation of the EI at the mean mass flow rate between when the flame is expanding and contracting, are non-monotonic with frequency. Also, the response of fuel lean flames is more sensitive to the frequency of the periodic stretching compared to a stoichiometric flame. An analysis to seek universality of transient flame responses across lean methane–air flames of different equivalence ratios (i.e., 1.0–0.7) using Damköhler Numbers (i.e., the ratio of a flow to chemical time scales) had limited success. [ABSTRACT FROM AUTHOR]