On flame kernel formation and propagation in premixed gases

Flame kernel formation and propagation in premixed gases have been studied experimentally and theoretically. The experiments have been carried out at constant pressure and temperature in a constant volume vessel located in a high speed shadowgraph system. The formation and propagation of the hot plasma kernel has been simulated for inert gas mixtures using a thermodynamic model. The effects of various parameters including the discharge energy, radiation losses, initial temperature and initial volume of the plasma have been studied in detail. The experiments have been extended to flame kernel formation and propagation of methane/air mixtures. The effect of energy terms including spark energy, chemical energy and energy losses on flame kernel formation and propagation have been investigated. The inputs for this model are the initial conditions of the mixture and experimental data for flame radii. It is concluded that these are the most important parameters effecting plasma kernel growth. The results of laminar burning speeds have been compared with previously published results and are in good agreement.     

Propagation speeds and stretch rates measured along wrinkled flames to assess the theory of flame stretch

Local propagation speeds and stretch rates were measured along a premixed flame that undergoes unsteady wrinkling in order to see if these two quantities correlate in the manner that is predicted by the theory of flame stretch. The Markstein number, which relates these two quantities, also was measured. Previous studies had considered the simple geometries of counterflow or spherical flames, but in this case a complex geometry was generated by interacting a flame with a vortex, such that both the strain and curvature components of the stretch rate are present. The diagnostics used were shadowgraph movies and simultaneous particle imaging velocimetry and OH planar laser-induced fluorescence. The overall conclusion is that the theory of flame stretch remains valid for these unsteady complex conditions, because the measured trends are found to be in agreement with trends predicted by the theory. That is, propagation speeds decrease at locations where positive stretch is applied to stable (lean propane-air and rich methane-air) flames. Conversely, propagation speeds increase where positive stretch was applied to unstable (lean methane-air) flames. The shape of the profiles of propagation speed along stable flames is opposite to that of unstable flames, as is predicted by the theory. However, values of Markstein number show large variations and are much larger than that of an outwardly propagating spherical flame. Negative strain regions are of particular interest because they previously had not been studied experimentally; these regions yield the largest propagation speeds for the stable cases and some negative speeds for the unstable cases.     

The characteristics of flame propagation in hydrogen/oxygen mixtures

The extreme explosiveness and high flame velocity of hydrogen challenge its application. Overcoming these challenges requires improving the fundamental flame characteristics of H₂/O₂ mixtures. In this study, the propagation characteristics of H₂/O₂ flames are investigated. The laminar burning velocity (LBV) is evaluated using nonlinear extrapolation. The empirical relations of LBV are given with the equivalence ratio (ER) and initial mixture pressure (IMP). The LBV increases first and then decreases as the ER increases and reaches its maximum value at the ER slightly higher than 1.0 (φ = 1.1–1.2). The LBV increases monotonically with increasing IMP. The critical instability radius and Markstein length increase as the ER increases, while decreasing with the IMP increase. The flame thickness decreases significantly with increasing IMP. The flame remains stable and smooth throughout the propagation process for all examined ERs only at the lower IMPs of 0.1 atm and 0.3 atm.     

Nonlinear effects in the extraction of laminar flame speeds from expanding spherical flames

Various factors affecting the determination of laminar flames speeds from outwardly propagating spherical flames in a constant-pressure combustion chamber were considered, with emphasis on the nonlinear variation of the stretched flame speed to the flame stretch rate, and the associated need to nonlinearly extrapolate the stretched flame speed to yield an accurate determination of the laminar flame speed and Markstein length. Experiments were conducted for lean and rich n-butane/air flames at initial pressure, demonstrating the complex and nonlinear nature of the dynamics of flame evolution, and the strong influences of the ignition transient and chamber confinement during the initial and final periods of the flame propagation, respectively. These experimental data were analyzed using the nonlinear relation between the stretched flame speed and stretch rate, yielding laminar flame speeds that agree well with data determined from alternate flame configurations. It is further suggested that the fidelity in the extraction of the laminar flame speed from expanding spherical flames can be facilitated by using small ignition energy and a large combustion chamber.     

Finite-rate evaporation and droplet drag effects in spherical flame front propagation through a liquid fuel mist

An evolution equation for a laminar flame front propagating into an air and liquid fuel mist cloud is derived for the first time, accounting for both the finite-rate evaporation of the fuel droplets and the slip velocity between them and their host environment. The asymptotic analysis employed in developing the equation exploits the usual inverse large activation energy parameter associated with chemical reaction in the flame and a small drag parameter. It is demonstrated that, in the no-slip velocity case, increasing the vaporization Damköhler number can produce flame extinction, presumably due to the more intense heat loss incurred due to droplet heat absorption necessary for vaporization. Droplet drag can also induce extinction due to the longer residence time of the droplets in any locale (than if there was no slip), leading to more vaporization with greater attendant heat loss. The predicted results for droplet velocity are compared to independent experimental data from the literature with good qualitative agreement.     

Effect of preferential diffusion and flame stretch on flame structure and laminar burning velocity of syngas Bunsen flame using OH-PLIF

By using OH-PLIF technique, experiments were conducted for laminar Bunsen flame of premixed CO/H₂/air mixtures with equivalence ratio ranging from 0.5 to 1.8. Reynolds number was varied from 800 to 2200, X_H2 = H₂/(H₂+CO) in the mixture was varied from 20% to 100% to study the effects of both preferential diffusion and flame curvature on flame structures and laminar flame burning velocities. Results showed that the combined effects of preferential diffusion and curvature gave an interesting phenomenon of the flame OH radical distributions on high hydrogen content flames. Furthermore, with the increase of H₂ fraction in fuel mixture, the effects of both preferential diffusion and flame curvature were increased. Interpretation of flame stretch effect on laminar burning velocity is also provided in this paper.     

Numerical investigation on the effects of reaction orders on the flame propagation dynamic behaviors for premixed gas in a closed tube

In this paper, the premixed flame propagation in a closed tube is surveyed using Computational Fluid Dynamics. The propagation characteristics of premixed flame are obtained coupling a single-step reaction mechanism with a laminar flame model. Three single-step reaction mechanisms are established with different reaction orders for hydrocarbon fuels. This study is to establish a wider range of reaction mechanisms and represent actual experimental conditions better. The numerical simulation results demonstrate that reaction orders can affect the tulip flame development. As the flame spreads, the tulip flame fronts become wrinkled. When the reaction order is 2, there are more wrinkles in the flame front and the degree of wrinkles is more obvious. Reaction orders also affect the flame tip velocity and the flame skirt velocity. The main reason is that laminar flame speeds are significantly different. When the reaction orders are 1.5 and 2, laminar flame speeds are mainly affected by temperature, which respectively increase by about 25% and 75%. When the reaction order is 1, the pressure is crucial for the variation of laminar flame speed. The laminar flame speed decreases by about 33%.     

Experimental investigation of unconfined spherical and cylindrical flame propagation in hydrogen-air mixtures

This paper presents results of experimental investigations on spherical and cylindrical flame propagation in pre-mixed H₂/air-mixtures in unconfined and semi-confined geometries. The experiments were performed in a facility consisting of two transparent solid walls with 1 m² area and four weak side walls made from thin plastic film. The gap size between the solid walls was varied stepwise from thin layer geometry (6 mm) to cube geometry (1 m). A wide range of H₂/air-mixtures with volumetric hydrogen concentrations from 10% to 45% H₂ was ignited between the transparent solid walls. The propagating flame front and its structure was observed with a large scale high speed shadow system. Results of spherical and cylindrical flame propagation up to a radius of 0.5 m were analyzed. The presented spherical burning velocity model is used to discuss the self-acceleration phenomena in unconfined and unobstructed pre-mixed H₂/air flames.     

Flame front evolution and laminar flame parameter evaluation of buoyancy-affected ammonia/air flames

For flames with very low burning speed, the flame propagation is affected by buoyancy. Flame front evolution and laminar flame parameter evaluation methods of buoyancy-affected flame have been proposed. The evolution and propagation process of a center ignited expanding ammonia/air flame has been analyzed by using the methods. The laminar flame parameters of ammonia/air mixture under different equivalence ratio (ER) and initial pressure have been studied. At barometric pressure, with the increase of ER, the laminar burning velocity (LBV) of ammonia/air mixture undergoes a first increase and then decrease process and reaches its maximum value of 7.17 cm/s at the ER of 1.1, while the Markstein length increases monotonously. For ammonia/air flames with ER less than unity, the flame velocity shows a decreasing trend with stretch rate, resulting in the propensity to flame instability, but no cellular structure was observed in the process of flame propagation. As the initial pressure increases, the LBV decreases monotonously as well as the Markstein length. The flame thicknesses of ammonia/air mixtures decrease with initial pressure and are much thicker than those of hydrogen flames, which makes a stronger stabilizing effect of curvature on the flame front. The most enhancement of LBV is contributed by the dehydrogenation reaction of NH₃ with OH. The NO concentration decreases significantly with the increase of ER.     

Simulations of laminar flame propagation in droplet mists

In order to clarify the conditions conducive to propagation of premixed flames in quiescent sprays, a one-dimensional code with detailed chemistry and transport was used. n-Heptane and n-decane, distinguished by their volatility, were studied under atmospheric and low temperature, low pressure conditions. The effects of initial droplet diameter, overall equivalence ratio ?₀ and droplet residence time before reaching the flame front were examined. Increasing the residence time had an effect only for n-heptane, with virtually no evaporation occurring before the flame front for n-decane. The trends were only marginally correlated with the local gaseous equivalence ratio ?_eff at the location of maximum heat release rate. ?_eff could be as low as 0.4 (beyond the lean flammability limit), but the flame speed could still be 40% of the gaseous stoichiometric flame speed S_L,0. For n-heptane, ?_eff increased towards ?₀ with smaller droplets while high flame speeds occurred when ?_eff was near 1. This implied that the highest flame speed was achieved with small droplets for ?₀ ? 1 and with relatively large droplets for ?₀ > 1. In the latter case, the oxidiser was completely consumed in the reaction zone and droplets finished evaporating behind the flame where the fuel was pyrolysed. The resulting small species, mainly C₂H₂, C₂H₄ and H₂, diffused back to the oxidation zone and enhanced the reaction rate there. Ultimately, this could result in flame speeds higher than S_L,0 even with ?₀ = 4. For n-decane, the same trends were followed but smaller droplets were needed to reach the same ?_eff due to the slow evaporation rate. Under low pressure and low temperature, the effects of pressure and temperature on ?_eff and the flame speed were competitive and resulted in values close to the ones at atmospheric conditions.     

The influence of preferential diffusion and stretch on the burning intensity of a curved flame front with fuel spray

In our most recent paper on Bunsen spray flames, only a completely prevaporized mode of a normal Bunsen flame was considered; inverted Bunsen flame and droplet size effects had not been examined yet. In the present study, we consider two flame structures: normal and inverted Bunsen flames, and two spray modes: completely and partially prevaporized burning, by the method of large activation energy asymptotics. In this way, a complete parametric study of flame tip intensification or extinction (opening) can be conducted. Four parameters are used in the analysis. The first two are the droplet size and amount of liquid-fuel loading, which indicate internal heat loss for a rich spray but heat gain for a lean spray. The other two are the stretch and Lewis number (Le). Stretch is negative for a normal Bunsen flame but positive for an inverted Bunsen flame. Stretch strengthens (or weakens) the burning intensity of the Le>1 (or Le<1) normal Bunsen flame but decreases (or increases) the burning intensity of the Le>1 (or Le<1) inverted Bunsen flame. Burning intensity of the flame tip intensifies (or weakens) when the lean (or rich) spray has a smaller droplet size or a larger amount of liquid loading. For lean and rich ethanol-spray normal Bunsen flames with Le>1 or a rich methanol-spray inverted Bunsen flame with Le<1, closed-tip solutions are obtained. Conversely, stretch weakens the burning intensities of lean and rich ethanol-spray inverted Bunsen flames with Le>1, or rich methanol-spray normal Bunsen flames with Le<1, eventually leading to tip opening. Moreover, the opening becomes wider (or narrower) as the droplet size decreases or liquid loading increases for the rich (or lean) sprays. Note that for lean ethanol-spray normal (or inverted) Bunsen flame with Le>1, if liquid loading is large enough and droplet size is sufficiently small, there exists flame transition from normal (or inverted) Bunsen through planar to inverted cone (or normal Bunsen) flame. Finally, the critical value of droplet size, at which there exists a planar flame rather than a normal (or an inverted) Bunsen flame, increases with increasing liquid loading.     

Unstretched unburned flame speed and burned gas Markstein length of diluted hydrogen/air mixtures

Fundamental combustion characteristics of H₂/air flames with the addition of actual H₂/air combustion residuals (a mixture of 65% N₂ + 35% H₂O by mole) are examined experimentally and numerically at 1–2 bar, 373–473 K, equivalence ratio of 0.7, and dilution ratios of 0–40%. Spherically expanding flame measurements at constant pressure show that flame speed and adiabatic flame temperature drop almost linearly with increasing diluent level. Detailed numerical simulations and analyses of sensitivity coefficients reveal that this is because of the low chemical reactivity of the dilution mixture. On the other hand, the change in burned gas Markstein length with the dilution mixture addition is found more complex and cannot be represented with a linear trend. Experimental flame speed data are compared with results of chemical kinetic analyses obtained by several chemical mechanisms in order to assess the accuracy of these models.     

Generation of PDFS for flame curvature and for flame stretch rate in premixed turbulent combustion

Experimentally derived pdfs of turbulent, premixed, flame curvatures from a variety of sources, for a wide range of conditions are surveyed and a suitable expression sought to generalize these. This proves to be one based on the Damköhler number, Da. This is tantamount to normalizing the curvature by multiplying it by the Taylor scale of turbulence. It enables the distribution of flame curvature when normalized by the laminar flame thickness, to be expressed in terms of the Karlovitz stretch factor, K, and the turbulent Reynolds number, R_l. The value of the pdf at zero curvature is linearly related to Da^1/2.The pdf expressions of Yeung et al. [3] obtained from numerical simulations are used for the strain rate distribution and, on the assumption that these and that for flame curvature are statistically independent, values of flame stretch rate pdfs are generated numerically. It is necessary to define an appropriate surface to define the burning velocity, flame stretch rate, and appropriate Markstein numbers. Two surfaces are considered and employed in the computations, one located at the start of the preheat zone, the other at the start of the reaction zone. The latter seems more rational and gives the better generalisation of the pdfs of flame stretch rate.An assumed linearity of laminar burning velocity with flame stretch rate, extending over both positive and negative stretch rates, enables flame stretch rate pdfs to be generated. It is concluded that negative values of burning velocity are unlikely and that burning velocities should tend to zero rather than attain negative values. This modifies the derivation of flame stretch rate pdfs. These depend on the Markstein number, Karlovitz stretch factor and turbulent Reynolds number. Computations suggest that, for values of K above 0.1 and of R_l above 100, the pdf of stretch rate is similar to that of strain rate. At very low values of K and negative values of Markstein number, pronounced flamelet instability might be anticipated.     

Numerical analysis on the combined transport effects of H2 and CO2 on the laminar premixed flame characteristics of biogas-hydrogen blends

The combined and respective transport effects of H₂ and CO₂ on the flame structure, laminar flame speed and radical pool of the BG40H60 blends at different equivalence ratios are investigated quantitatively with the numerical simulation in this study. The results show that H₂ transport dominates the decrease and enhancement of HRR and mole fractions of minor species at the fuel-lean and fuel-rich conditions. However, H₂ or CO₂ transport hardly affects concentrations of major species expect for H₂ and CO₂. Besides, the dominated H₂ transport contributes to the decreased/increased laminar flame speed at the fuel-lean/fuel-rich condition, while the OH radical can reflect the laminar flame speed variation caused by the H₂ and CO₂ transport. Based on the rate-of-production (ROP) analysis of OH radical, the most sensitive reactions to H₂ and CO₂ transport are OH + H₂H₂O + H/H + O₂O + OH and OH + CH₂OHCO + H₂O at the fuel-lean and fuel-rich conditions respectively. The major production reactions (H + O₂O + OH, H + HO₂ = 2OH, O + H₂H + OH, 2OH = O + H₂O) of OH radical are suppressed or improved more significantly with the H₂ and CO₂ transport at the fuel-lean or fuel-rich condition, leading to the suppressed or improved OH radical pool and the flame propagation at the fuel-lean or fuel-rich condition. Furthermore, it is demonstrated that CO₂ transport suppresses the reaction of OH + H₂H₂O + H considerably to improve the OH radical pool at the fuel-rich condition and cannot be neglected when investigating the flame propagation of biogas-hydrogen blends.     

Premixed edge flame in a counterflow field with a stretch rate gradient

An edge flame was established in a counterflow field with a stretch rate gradient using twin rectangular burners which were misaligned by a few degrees. The stretch rate gradient was quantitatively defined as a function of the angle between the two burners and the distance from the edge of the burner, and thus the effect of stretch rate gradient on the behavior of the edge flame was investigated. The local chemical reaction rate at the edge of a CH₄/air flame was stronger than that at other parts of the flame. On the other hand, the reaction rate at the edge of a C₃H₈/air flame was weaker than that of other parts of the flame. The curvature of the flame edge of the CH₄/air flame was much larger than that of the C₃H₈/air flame. These results are thought to be due to the effect of the Lewis number. The ratios of the local stretch rate at the flame edge to the extinction stretch rate for planar twin flames with the same composition as the edge flame were 0.5 to 0.7 for the CH₄/air flame and 0.6 to 0.8 for the C₃H₈/air flame. These values were midway between those in the numerical simulation by Daou and Linan and those in the experiment by Liu and Ronney. Moreover, it was shown that an increase in the stretch rate gradient resulted in a lower local stretch rate at the flame edge. Behavior of the edge flames did not depend on the Lewis number of the mixture.     

Laminar flame speeds of primary reference fuels and reformer gas mixtures

The laminar flame speeds of neat primary reference fuels (PRFs), n-heptane and iso-octane, PRF blends, reformer gas, and reformer gas/iso-octane/air mixtures are measured over a range of equivalence ratios at atmospheric pressure, using counterflow configuration and digital particle image velocimetry (DPIV). PRF blends with various octane numbers are studied. The synthetic reformer gas mixture employed herein has a composition that would be produced from the partial oxidation of rich iso-octane/air mixture into CO and H₂, namely, 28% H₂, 25% CO, and 47% N₂. Computationally, the experimentally determined laminar flame speeds are simulated using the detailed kinetic models available in the literature. Both experimental and computational results demonstrate that the flame speeds of hydrocarbon/air mixtures increase with addition of a small amount of reformer gas, and the flame speeds of reformer gas/air mixtures are dramatically reduced with addition of a small amount of hydrocarbon fuel. Furthermore, the number density effect of seeding particles on flame speed measurement is assessed, and the experimental uncertainties associated with the present DPIV setup as well as the linear extrapolation method employed herein are discussed.     

天然气-氢气-空气混合气的层流燃烧速度测定

在定容燃烧弹内研究了常温常压下天然气-氢气-空气混合气的火焰传播规律,得到了不同掺氢比例（氢气在天然气中的体积掺混比例为0％～100％）和燃空当量比（0．6～1．4）下混合气的层流燃烧速率和马克斯坦长度,通过对马克斯坦长度的测量,分析了拉伸对火焰传播的影响。结果表明,随着天然气中掺氢比例的增加,混合气的燃烧速率呈指数规律增加,马克斯坦长度值减小,火焰的稳定性下降。各掺氢比例下,随当量比的增加,马克斯坦长度值增加,火焰的稳定性增强。通过对试验结果的数据拟合,得到了计算天然气-氢气-空气混合气层流燃烧速率的关系式。     

Uncertainties in interpretation of high pressure spherical flame propagation rates due to thermal radiation

It has been suggested that radiation heat loss may be a large source of experimental uncertainty in flame speed measurements using the outwardly propagating spherical flame method. Thermal radiation is usually not considered in interpretation of these experiments, yet it may contribute significantly to uncertainty especially for model-constraining conditions at low flame temperature and high pressure. In the present work, a conservative analytical estimate of the effects of radiation heat loss is derived and validated against detailed numerical simulations. A solver with a graphical interface is provided in the Supplemental material to allow implementation of these analytical results. The analytical estimate considers the radiation induced burned gas motion as well as the decreasing flame temperature due to conduction to the radiating burned gas and radiation loss from the flame zone. The results show that previous measurements of hydrogen flame speeds at low flame temperature by Burke et al. (2010) [3] are minimally affected by radiation, but flames with low flame speeds can be strongly inhibited by radiative loss. Future laminar spherical flame measurements and interpretation of existing determinations with low adiabatic flame speeds must include consideration of radiation effects to avoid large uncertainties.     

Assessment of elliptic flame front propagation characteristics of hydrogen in an optically accessible spark ignition engine

The absence of carbon content of hydrogen fuel makes it an attractive candidates for future energy carriers. Hydrogen or dual fuelled engines are a practical alternative to pure hydrocarbon fuelling modes. However, fine tuning of current engines is necessary. In this study premixed hydrogen flame propagation is investigated in a single-cylinder, spark-ignited, four-stroke optically accessible spark ignition test engine using high-speed imaging. Ellipses were fitted on the flame contours during the analysis to obtain flame speeds and flame centre motion. The test conditions covered a range of engine speeds from 1000 rpm to 2000 rpm with 100 rpm increments using a lean mixture, ? = 0.67. The fine temporal resolution allowed the time, at which spark governed kernel formation becomes a function of engine parameters to be determined. The few data that have been published in the literature regarding hydrogen flame speeds were compared with the finding of this study.