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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]

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]

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]

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]

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]

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]

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]

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]

A variety of different flame configurations and heat release distributions exist in high swirl, annular flows, due to the existence of inner and outer shear layers as well a vortex breakdown bubble. Each of these different configurations, in turn, has different thermoacoustic sensitivities and influences on combustor emissions, nozzle durability, and liner heating. This paper presents findings on the sensitivities of the outer shear layer- stabilized flames to a range of parameters, including equivalence ratio, bulkhead temperature, flow velocity, and preheat temperature. There is significant hysteresis for flame attachment/detachment from the outer shear layer and this hysteresis is also described. Results are also correlated with extinction stretch rate calculations based on detailed kinetic simulations. In addition, we show that the bulkhead temperature near the flame attachment point has significant impact on outer shear layer detachment. This indicates that understanding the heat transfer between the edge flame stabilized in the shear layer and the nozzle hardware is needed in order to predict shear layer flame stabilization limits. Moreover, it shows that simulations cannot simply assume adiabatic boundary conditions if they are to capture these transitions. We also show that the reference temperature for correlating these transitions is quite different for attachment and local blow off. Finally, these results highlight the deficiencies in current understanding of the influence of fluid mechanic parameters (e.g. velocity, swirl number) on shear layer flame attachment. For example, they show that the seemingly simple matter of scaling flame transition points with changes in flow velocities is not understood. [ABSTRACT FROM AUTHOR]

The influence of Lewis number on turbulent premixed flame interactions is investigated using automatic feature extraction (AFE) applied to high-resolution flame simulation data. Premixed turbulent twin V-flames under identical turbulence conditions are simulated at global Lewis numbers of 0.4, 0.8, 1.0, and 1.2. Information on the position, frequency, and magnitude of the interactions is compared, and the sensitivity of the results to sample interval is discussed. It is found that both the frequency and magnitude of normal type interactions increases with decreasing Lewis number. Counternormal type interactions become more likely as the Lewis number increases. The variation in both the frequency and the magnitude of the interactions is found to be caused by large-scale changes in flame wrinkling resulting from differences in the thermo-diffusive stability of the flames. During flame interactions, thermo-diffusive effects are found to be insignificant due to the separation of time scales. [ABSTRACT FROM AUTHOR]

Multiple flame–flame interactions in premixed combustion are investigated using direct numerical simulations of twin turbulent V-flames for a range of turbulence intensities and length scales. Interactions are identified using a novel automatic feature extraction (AFE) technique, based on data registration using the dual-tree complex wavelet transform. Information on the time, position, and type of interactions, and their influence on the flame area is extracted using AFE. Characteristic length and time scales for the interactions are identified. The effect of interactions on the flame brush is quantified through a global stretch rate, defined as the sum of flamelet stretch and interaction stretch contributions. The effects of each interaction type are discussed. It is found that the magnitude of the fluctuations in flamelet and interaction stretch are comparable, and a qualitative sensitivity to turbulence length scale is found for one interaction type. Implications for modeling are discussed. [ABSTRACT FROM AUTHOR]

Lean limit methane-air flame propagating upward in a 24 mm diameter tube was studied experimentally and by numerical simulations. Gas velocity field was measured using Particle Image Velocimetry (PIV) method. The experimental measurements are compared with the results of numerical simulations and with previous similar measurements performed for a standard flammability tube. The experimentally determined lean flammability limit in 24 mm diameter tube was 4.9±0.03% CH4 by volume, against determined earlier flammability limit 5.1-5.2% CH4 in a standard tube. Maximum stretch rate value was found to be at flame leading point and was higher in a 24 mm diameter tube than in a standard flammability tube. The observed extension of the lean flammability limit is attributed to the strengthening effect of positive flame stretch on lean methane-air flames, characterized by Lewis number, Le < 1. Numerical simulations of limit methane/air flame in a 24 mm diameter tube demonstrated presence of negative flame speed at the flame leading edge. [ABSTRACT FROM AUTHOR]

Based on the tubular burner, the burning velocities, flame stretch and inhibition rules influenced by co-flow water mists were studied using a high-speed schlieren system. Moreover, the variation rules of the flame critical extinction in our burner equipment were also obtained by analyzing the process and mechanism of flame extinction and inhibition. It is shown that the flame stretch is related to the fuel concentration, co-flow fluxes and water mist diameters. For droplets with a larger diameter, the smaller the co-flow fluxes, the more obvious the flame stretch. When the water mist loading rate is rather smaller, for fuel-rich premixed flame with Le>1, the flame with larger burning rate tends to backfire more easily. Under the same water mist conditions, for fuel-lean premixed flame with Led, the smaller the gas concentration, the easier the flame is extinct. [ABSTRACT FROM AUTHOR]

The paper is concerned with an extension of the classical relation between the flame speed and the curvature-flow stretch, valid only for high Lewis numbers (diffusively stable flames). At low Lewis numbers the corresponding flame-flow system suffers short- wavelength instability, making the associated initial value problem ill-posed. In this study the difficulty is resolved by incorporation of higher-order effects. As a result one ends up with a reduced model based on a coupled system of second-order dynamic equations for the flame interface and its temperature. As an illustration the new model is applied for description of diffusively unstable stagnation-point flow flames. [ABSTRACT FROM AUTHOR]

This numerical study modifies Tsa and Chen's preliminary combustion model (2003) by using a multi-block grid to investigate further the interaction of flames over binary Tsuji burners. The effects of inter-cylinder spacing (L) and inflow velocity (U in ) are investigated. A wider inter-cylinder spacing generally corresponds to lower flame transition velocity, associated with the transformation of the envelope flame into the wake flame. However, the combustion efficiency increases with L. The twin envelope diffusion flames merge into a larger envelope diffusion flame completely when L is equal to or less than 1.5D. Only one vortex is present behind each burner when L = 1.5D or 2D. However, no vortex is present when L = 1.2D. When L is equal to or greater than 3.5D, no interference occurs between the two flames. The mechanism of control of the interaction between twin counterflow diffusion flames involves oxygen deficiency between the dual flames. In the case of varying U in at fixed L = 3D, the dual envelope diffusion flames transform into dual wake flames as U in increases to 0.79 m/sec. The dual wake flames are extinguished as U in increases further to 1.96 m/sec. Increasing the inflow velocity can enhance the interaction between dual envelope flames. A larger inflow velocity yields a lower flame temperature due to the flame stretch effect. Three vortices are present behind each cylinder when the flames are at the near-extinction velocity. For a fixed inter-cylinder spacing, the dual flames tend to attract each other normally. However, the dual flames repel each other as the inflow velocity increases to the near-extinction limit. [ABSTRACT FROM AUTHOR]

The propagation speed of a thin premixed flame disturbed by an unsteady fluid flow of a larger scale is considered. The flame may also have a general shape but the reaction zone is assumed to be thin compared to the flame thickness. Unlike in preceding publications, the presented asymptotic analysis is performed for a general multistep reaction mechanism and, at the same time, the flame front is curved by the fluid flow. The resulting equations define the propagation speed of disturbed flames in terms of the properties of undisturbed planar flames and the flame stretch. Special attention is paid to the near-equidiffusion limit. In this case, the flame propagation speed is shown to depend on the effective Zeldovich number Z f , and the flame stretch. Unlike the conventional Zeldovich number, the effective Zeldovich number is not necessarily linked directly to the activation energies of the reactions. Several examples of determining the effective Zeldovich number for reduced combustion mechanisms are given while, for realistic reactions, the effective Zeldovich number is determined from experiments. Another feature of the present approach is represented by the relatively simple asymptotic technique based on the adaptive generalized curvilinear system of coordinates attached to the flame (i.e., intrinsic disturbed flame equations [IDFE]). [ABSTRACT FROM AUTHOR]

The characteristics and extinction of the partially-premixed diffusion flame are studied both experimentally adopting counter-flow burner system and theoretically using matched asymptotic expansion techniques. Results show that the partially-premixed diffusion flame exists at high rate of strain when the degree of partial premixing is low. The results for the practical partially premixed diffusion system indicate that the partially premixed diffusion flame plays an important role in charaterizing the turbulent flames. [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. [ABSTRACT FROM AUTHOR]

Many countries consider hydrogen as a promising energy source to resolve the energy challenges over the global climate change. However, the potential of hydrogen explosions remains a technical issue to embrace hydrogen as an alternate solution since the Hindenburg disaster occurred in 1937. To ascertain safe hydrogen energy systems including production, storage, and transportation, securing the knowledge concerning hydrogen flammability is essential. In this paper, we addressed a comprehensive review of the studies related to predicting hydrogen flammability by dividing them into three types: experimental, numerical, and analytical. While the earlier experimental studies had focused only on measuring limit concentration, recent studies clarified the extinction mechanism of a hydrogen flame. In numerical studies, the continued advances in computer performance enabled even multi-dimensional stretched flame analysis following one-dimensional planar flame analysis. The different extinction mechanisms depending on the Lewis number of each fuel type could be observed by these advanced simulations. Finally, historical attempts to predict the limit concentration by analytical modeling of flammability characteristics were discussed. Developing an accurate model to predict the flammability limit of various hydrogen mixtures is our remaining issue. [ABSTRACT FROM AUTHOR]