Intrinsic Flame Instability Research Articles

Intrinsic instability of lean hydrogen/ammonia premixed flames: Influence of Soret effect and pressure

The addition of hydrogen in ammonia/air mixtures can lead to the onset of intrinsic flame instabilities at conditions of technical relevance. The length and time scales of intrinsic instabilities can be estimated by means of linear stability analysis of planar premixed flames by evaluating the dispersion relation. In this work, we perform such linear stability analysis for hydrogen-enriched ammonia/air flames (50%H2-50%NH3 by volume) using direct numerical simulation with a detailed chemical kinetic mechanism. The impact of pressure and the inclusion of the Soret effect in the governing equations is assessed by comparing the resulting dispersion relation at atmospheric pressure and 10 atm. Our data indicate that both pressure and the Soret effects promote the onset of intrinsic instabilities. Comparisons with available numerical literature data as well as theoretical models are also discussed.

Chemical mechanism analysis on flame propagation properties of methane/air deflagration mix-diluted by CO2 + N2

To investigate the inhibitive behaviors methane/air deflagration using mix-dilutions (i.e., CO2 + N2), a series of deflagration experiments were conducted in a cylindrical chamber. Furthermore, various proportions of CO2 were employed in the 10% and 20% mix-dilutions to assess the requisite conditions for the CO2 chemical effect to effectively restrain methane deflagration. Additionally, the inhibition mechanism of methane deflagration by mix-diluents was elucidated by isolating the CO2 chemical effect and analyzing the chemical kinetics involved. The results show that an inversion directly forms in the lower flame front of spherical flame without undergoing the four evolution stages of “Tulip flame”, and buoyancy effect is the dominant factor leading to the inversion by theoretical calculations that exclude the influence of the intrinsic flame instabilities. Moreover, the flame inversion created by the buoyancy effect can reduce the chamber destruction by weakening the quench expansion wave. Additionally, the increasing CO2 proportion in 20% and 10% mix-dilutions exhibits significant differences in its inhibitory efficiency on methane detonation pressure, indicating the requirement of certain critical conditions for the CO2 chemical inhibition to take effect. Therefore, chemical pathway analysis reveals that the CO2 chemical effect significantly inhibits the rate of R99 through the reduction in the generation of H radicals only under thicker flame front conditions. Hence, the chemical inhibition of methane deflagration by CO2 necessitates the provision of temporal scales by reducing the temperature rate and the thickening the flame thickness through inerting effect.

A-posteriori analysis of a data-driven filtered wrinkled flamelet model for thermodiffusively unstable premixed flames

State-of-the-art turbulent combustion models do not account for the effects of intrinsic flame instabilities, which are known to have a non-negligible effect, under technically relevant conditions, in thermodiffusively unstable flames such as those of hydrogen-based fuels. In this work, a data-driven modeling framework for filtered, laminar, self-wrinkling flames subject to intrinsic instabilities is presented and proposed as a stepping stone for the development of turbulent combustion models for large eddy simulation. In this context, data-driven refers to the use of small- and medium-sized resolved simulations, explicitly filtered in physical space, as data sources for the development of filtered tables. For consistency, all the unclosed terms of the filtered governing equations are then written according to the filtered tabulated chemistry formalism. The a-posteriori analysis of the modeling framework indicates that medium-scale simulations, which conversely to the small-scale ones feature at least one characteristic flame-finger, are needed for the generation of the data-driven tables. Furthermore, a laminar wrinkling factor, accounting for the sub-filter cellular wrinkling, should also be used. The presented results show promise for the extension of this modeling framework to the simulation of turbulent flames, as well as to multi-step chemistry and detailed thermo-transport modeling. Statement of SignificanceIt is nowadays well-known that intrinsic flame instabilities have a leading order impact on the propagation of premixed flames in thermodiffusively unstable mixtures, such as lean hydrogen-air premixed flames. This work presents the first a-posteriori analysis of a combustion model for filtered, albeit laminar, large-scale, intrinsically unstable premixed flames. The novelty of the presented data-driven model lies in the use of data obtained from filtering direct numerical simulations performed for paradigmatic small and medium-scale cases, instead of standard one-dimensional flamelets.

Analysis of methyl pentanoate/air mixtures spherically expanding flame intrinsic instabilities

Recently, methyl pentanoate (MPe) has been considered an alternative fuel or additive in gasoline and diesel. Therefore, before its adoption in combustion devices, its combustion characteristics have to be thoroughly understood. Intrinsic flame instability and cellularity are combustion characteristics that influence the performance of combustion devices. In this light, the intrinsic instability and cellularity of MPe/air flames were investigated at the initial pressures of 1, 2, and 4 bar, temperatures of 453 K and 483 K, and equivalence ratios of 0.7–1.4 in a constant volume combustion chamber to understand the various combustion dynamics of MPe. The instability and cellularity were examined using physicochemical characteristics such as the stretch rate, Markstein length, Lewis number, thermal expansion ratio, and flame thickness. There were slight differences in the Markstein length at the various pressures, but generally, according to the Markstein length, MPe flames were unstable at higher pressures. The Lewis number revealed that MPe flames were thermal-diffusively stable, whereas the flame thickness showed MPe flames were more unstable at 483 K and 4 bar. Also, the MPe premixed flame growth rate and stability curves were obtained using theoretical analysis. The stability curves showed that at high initial pressures and temperatures, MPe flame instability occurs early at a smaller flame radius. Moreover, the experimental and theoretical critical radii agreed well. The research results provide a concrete foundation for advancing the knowledge of methyl pentanoate combustion.

Effects of density ratio and differential diffusion on flame accelerative propagation of H2/O2/N2 mixtures

The effects of density ratio and differential diffusion on premixed flame propagation of H2/O2/N2 mixtures are investigated by constant volume combustion chamber. The density ratio and differential diffusion are controlled independently by adjusting the O2/N2 ratio and equivalence ratio. Results show that the density ratio has no effect on turbulent burning velocity while the differential diffusion has a promotion effect on turbulent burning velocity. The onsets of laminar flame acceleration are promoted by both density ratio and differential ratio. The turbulent flames perform a continuous acceleration propagation and the dependence between flame propagation speed and flame radius can be characterized as (dR/dt)/(σ·SL) ∼ R0.33∼0.37, which is lower than the 1/2 power law. The acceleration parameters of laminar flames and turbulent flames (u’/SL = 1) are around 0.17 and 0.36 respectively, and both of them are not affected by density ratio and differential diffusion. The empirical formula m = 0.19·(u’/SL)0.4+0.17 is concluded to quantitatively describe the accelerative characteristics of laminar and turbulent flames. The current study indicates that the acceleration of laminar flames is mainly induced by flame intrinsic instability, and the latter can affect the acceleration onset but not affect the fractal excess. The acceleration of turbulent flames is dominated by turbulent stretch, while the effects of density ratio and differential diffusion can be ignored.

Flamelet modeling of thermo-diffusively unstable hydrogen-air flames

In order to reduce CO2 emissions, hydrogen combustion has become increasingly relevant for technical applications. In this context, lean H2-air flames show promising features but, among other characteristics, they tend to exhibit thermo-diffusive instabilities. The formation of cellular structures associated with these instabilities leads to an increased flame surface area which further promotes the flame propagation speed, an important reference quantity for design, control, and safe operation of technical combustors. While many studies have addressed the physical phenomena of intrinsic flame instabilities in the past, there is also a demand to predict such flame characteristics with reduced-order models to allow computationally efficient simulations. In this work, a H2-air spherical expanding flame, which exhibits thermo-diffusive instabilities, is studied with flamelet-based modeling approaches both in a-priori and a-posteriori manner. A recently proposed Flamelet/Progress Variable (FPV) model, with a manifold based on unstretched planar flames, and a novel FPV approach, which takes into account a large curvature variation in the tabulated manifold, are compared to detailed chemistry (DC) calculations. Both flamelet approaches account for differential diffusion utilizing a coupling strategy which is based on the transport of major species instead of transporting the control variables of the manifold. First, both FPV approaches are assessed in terms of an a-priori test with the DC reference dataset. Thereafter, the a-posteriori assessment contains two parts: a linear stability analysis of perturbed planar flames and the simulation of the spherical expanding flame. Both FPV models are systematically analyzed considering global and local flame properties in comparison to the DC reference data. It is shown that the new FPV model, incorporating large curvature variations in the manifold, leads to improved predictions for the microstructure of the corrugated flame front and the formation of cellular structures, while global flame properties are reasonably well reproduced by both models.

Flame propagation in narrow horizontal channels: Impact of the gravity field on the flame shape

In this paper, we study the effect of gravity, or buoyancy forces, on the structure of flames propagating in horizontal channels. It is shown that there are two mechanisms for the appearance of non-symmetric flames. The first, more obvious, is related to buoyancy, when a hotter gas of lower density tends to be located in the upper half of the channel. However, there is a second mechanism associated with the intrinsic flame instabilities, which also can cause the loss of flame symmetry. This mechanism can, at certain values of the parameters, act in the opposite direction, when hotter gases are enclosed in the lower part of the channel. In this case, two stable non-symmetric solutions may exist in the presence of gravitational forces and the establishment of one or another configuration depends on the initial conditions. The stability of these solutions is demonstrated by time-dependent computations.

Laminar combustion characteristics of ethyl acetate/hydrogen/air at elevated pressures

Ethyl acetate/hydrogen/air (EA/H2/air) premixed flame combustion characteristics were investigated at the initial pressures of 1 bar, 2 bar, and 3 bar and a temperature of 358 K using H2 in air volumetric fractions of 0 %, 4 %, 8 %, and 12 %. Specifically, the laminar burning speed (LBS) and intrinsic flame instabilities of the EA/H2/air premixed flames were investigated. The laminar burning speed follows the order of 0 % < 4 % < 8 % < 12 %. The maximum laminar burning speeds of 0.85 m/s, 0.78 m/s, and 0.70 m/s were measured at the initial pressures of 1, 2, and 3 bar, respectively, when a 12 % concentration of hydrogen was added to EA. Reactions R98: H + O2 ⟺ O + OH and R136: CH3 + H (+M) ⟺ CH4 (+M) significantly promoted and inhibited EA/H2/air LBS, respectively. Furthermore, the decreased flame thickness or curvature effect mainly caused the instability and cellularity of the EA/H2/air premixed flames when the concentration of hydrogen and the initial pressure increased. Theoretically, EA/H2/air premixed flames were thermal-diffusively stable and hydrodynamically unstable. This study provides a profound understanding of the combustion characteristics of EA/H2/air relevant to its application in combustors.

Investigations on cellularization instability of 2-ethylfuran

2-ethylfuran (2 EF) has potential as a biofuel for combustion devices. To apply it efficiently in combustion devices, it is necessary to study the combustion characteristics such as the intrinsic flame instability of 2 EF. The intrinsic flame instabilities of 2 EF outwardly propagating spherical premixed flames were theoretically and experimentally studied at initial temperatures of 373, 403, and 433 K, pressures of 1, 2, and 4 bar, and equivalence ratios of 0.7–1.4. The flame topography at different stages of flame development was examined. When large cracks appeared on the flame front surface, resulting in small cracks and new cell generation, the flame became unstable. Linear stability theory was used to investigate the effects of hydrodynamic instability and thermal-diffusion instability on the flame. The critical stability curves were plotted, and the theoretical calculations were consistent with the experimental results. The results showed that the best fits for the theoretical and experimental results were n = 2πPe/48 and n = 2πPe/80, respectively. The critical radius and Peclet number are calculated and discussed. The critical Peclet number decreased as the equivalence ratio increased.

Investigation on hydrogen/ethanol intrinsic flame instability

The instability of laminar premixed hydrogen–ethanol flame was studied under different initial pressure, hydrogen ratio, and equivalence ratio conditions by using a constant volume combustion bomb. The unstable laminar premixed flame was measured using its thermal expansion ratio, flame thickness, effective Lewis number, and logarithmic growth rate of perturbation. The results demonstrated that the thermal expansion ratio and effective Lewis number were insensitive to initial pressure variation, but hydrogen ratio variation was significantly affected. The linear stability analysis indicated that when the hydrogen ratio increased, the premixed flame became hydrodynamically unstable; thermal diffusion effects later stabilized the flame as the hydrogen ratio increased. The critical radius and Peclet number decreased with the increased equivalence ratio of a 20% hydrogen mixture, but both parameters increased with an increased equivalence ratio under an 80% hydrogen ratio. Finally, the growth rate of perturbation decreased when the equivalence ratio increased.

The effect of intrinsic instability on the surface topography of spherical 2-acetylfuran flame

The intrinsic instability of 2-acetylfuran (2-AF) expanding flame has been investigated at initial temperatures of 453–493 K, pressures of 1–3 bar, and equivalence ratios of 0.8–1.4. The linear stability theory was used to analyze the influence of the initial temperatures, initial pressures, and equivalence ratios on the 2-AF flame intrinsic instability. Moreover, an image processing technique was applied to obtain the experimental cell-wavenumber from the topography of 2-AF cellular flame to quantitatively analyze the local cell proliferation. The topography of the 2-AF expanding flame shows that the initial pressures and equivalence ratios are the two influential factors to its intrinsic instability. More specifically, the higher the initial pressures, as well as the equivalence ratios, the earlier the time of the flame becomes unstable. Furthermore, according to the comparison between the local experimental and theoretical wavenumbers, it is found that the average local experimental wavenumber falls within the unstable zone predicted by the linear stability theory. Last but not least, the cell-wavenumber distribution in the different regions on the 2-AF expanding flame surface indicates that the initial crack has a weaker shaping effect on the flame surface than the formation of small spontaneous cells.

Effects of Intrinsic Flame Instabilities on Thermoacoustic Oscillations in Lean Premixed Gas Turbines

Abstract Thermoacoustic instabilities are commonly encountered in the development of aeroengines and rocket motors. Research on the fundamental mechanism of thermoacoustic instabilities is beneficial for the optimal design of these engine systems. In the present study, a thermoacoustic instability model based on the lean premixed gas turbines (LPGT) combustion system was established. The longitudinal distribution of heat release caused by the intrinsic instability of flame front was considered in the model. Effects of different heat release distributions and characteristic parameters (Lewis number Le, Zeldovich Number β, and Prandtl number Pr) of the premixed gas on thermoacoustic instability behaviors of the LPGT system were investigated based on the established model. Results show that the LPGT system features have two kinds of unstable thermoacoustic modes. The first one corresponds to the natural acoustic mode of the plenum and the second one corresponds to that of the combustion chamber. The characteristic parameters of the premixed gases have a large impact on the stability of the system and can even change the system from stable to unstable state.

Outwardly growing premixed flames in turbulent media

The propagation of outwardly expanding premixed flames in turbulent media is examined within the context of the hydrodynamic theory wherein the flame, treated as a surface of density discontinuity separating fresh combustible mixture from burned products, is propagating at a speed dependent upon local geometric and mixture/flow characteristics. An embedded manifold approach, one adept at handling multi-valued and disjointed surfaces which are frequently observed in real flames, is used to couple the flow and flame evolution. A sensitivity analysis, based on mixtures with different Markstein numbers, is performed to investigate early flame kernel development in addition to its long-term evolution. The focus is to understand the effect of turbulent flow characteristics, distinguished by the intensity of velocity fluctuations and its integral length scale, in addition to intrinsic flame instabilities (predominantly the Darrieus-Landau instability) on flame propagation. The overarching objective is to quantify their influence on the flame morphology and burning rate and to construct scaling laws for the turbulent flame speed through appropriate modifications of Damköhler’s first hypothesis. Flame-turbulence interactions are inferred from statistical quantities based on its developing flame topology, including local flame curvature and hydrodynamic strain, and their combined effects integrated into the flame stretch rate experienced by the flame and the local flame speed deviation from the laminar flame speed.

Turbulent burning velocity and its unified scaling of butanol isomers/air mixtures

Turbulent expanding flames of butanol isomers/air mixtures are investigated in a wide range of conditions (T = 393 K, P = 1, 2 bar, ϕ = 0.7, 1.0, 1.3 and u’=0, 0.89, 1.77, 2.66 m/s). Turbulent burning velocity (ST) of four butanol isomers are determined, compared, and analyzed in detail. A unified scaling of ST is presented to consider the effects of molecular transport, during which different mathematical correlations are evaluated and modified. Results show that ST is increased with u’ and P. The maximum ST will shift to richer side compared to laminar condition, and the shifting trend will be more evident at high u’. The order of ST between four butanol isomers is similar to that of laminar burning velocity (SL) and it is: n-butanol > i-butanol≈s-butanol > t-butanol. The discrepancy in ST is mainly induced by SL, while turbulence stretch and flame intrinsic instability play an amplifying role. Turbulent Reynolds number (ReT), Karlovitz number (Ka), and Kobayashi model can all give a reasonable correlation of current experimental data while an acceptable scatter exists in Damkohler number (Da) correlation. The primary ST models are modified by Len and the value of n is unified to −0.35 for different correlations, which ensures the molecular transport effects at the quantitative level. It appears that these correlations are applicable for hydrocarbons and alcohols, which can be more simplified and suitable ST correlations for a wide range of conditions.

Thermo-acoustic instability in the process of flame propagation and transition to detonation

The paper is devoted to the numerical analysis of the reactive mixture explosion during the accidents at a launch site. The particular scenario concerns the explosion in the unconfined volume after accidental fuel release into the atmosphere. It is shown that the development of intrinsic flame instability results in the generation of compression waves, which occur to be intensified via the mechanism of thermo-acoustic instability. In turn, intensification of the compression waves leads to the formation of strong shock waves and even detonation in the case of a highly reactive mixture. The leading role in this process belongs to the transversal waves generated in the folds of the developed flame surface and propagated along the flame surface. It is the localization of the compression wave inside the reaction zone that causes amplification of the compression wave due to energy release.

Inherent instabilities in ethyl acetate premixed flames

The intrinsic flame instability of ethyl acetate (ETAC) spherically expanding premixture flame has been investigated and discussed using experimental and theoretical analysis at the initial temperature and pressure of 358 K, 388 K, 418 K and 1 bar, 2 bar, and 4 bar as well as the equivalence ratio of 0.8–1.6 using the constant volume combustion chamber (CVCC). When a premixture flame becomes unstable and cellularized the flame surface is wrinkled which further facilitates the rapid accelerative motion of the flame. Considering this, the wrinkling factor of the premixture was assessed. It was revealed that the wrinkling factor increases as the Peclet number increases. Comparative analysis of the experimental and theoretical Peclet number showed that the experimental data was smaller than the theoretical data, however, their trend was consistent. Flame instability was also investigated using the critical Peclet number (Pec) and the Markstein number (Mb) and empirical correlation Pec=52.31Mb+260.43was attained. In conclusion, an empirical correlation Kac=0.04704e(-0.33466Mb) and Kacl=0.04959e(-0.25775Mb) was established in terms of the Karlovitz numbers (Ka) and the Markstein number (Mb).

Subgrid modeling of intrinsic instabilities in premixed flame propagation

This work is devoted to the investigation and subgrid-scale modeling of intrinsic flame instabilities occurring in the propagation of a deflagration wave. Such instabilities, of hydrodynamic and thermodiffusive origin, are expected to be of particular relevance in recent technological trends such as in the use of hydrogen as a clean energy carrier or as a secondary fuel in hydrogen enriched combustion. A dedicated set of direct numerical simulations is presented and used, in conjunction with coherent literature results, in order to develop scaling arguments for the propagation speed of self-wrinkled flames which are also supported by the outcomes of a weakly non-linear model, namely the Sivashinsky equation. The observed scaling is based on the definition of the number of unstable wavelengths in a reference hydrodynamic lengthscale, in other words the ratio between the neutral or cutoff lengthscale of intrinsic instabilities and the lateral domain of a planar flame. The scalings are then employed to develop an algebraic model for the wrinkling factor in the context of a flame surface density closure approach. An a-priori analysis shows that the model correctly captures the flame wrinkling caused by intrinsic instability at sub grid level. A strategy to include the developed self-wrinkling model in the context of a turbulent combustion model is finally discussed on the basis of the turbulence induced cut-off concept.

Role of H2/CO Addition to Flame Instabilities and Their Control in a Stepped Microcombustor

ABSTRACT The effect of H2/CO addition on flame dynamics of propane-air mixtures in a stepped microcombustor with heat recirculation was investigated experimentally. Unstable flame propagation regimes and their transition modes were observed for a range of mixture velocities (5–8 m/s) and equivalence ratios (0.6–0.95). Up to 60% H2 addition to lean propane-air mixtures helps improve the flame stability range in the combustor. The suppression of these flame instabilities with H2 addition is attributed to the increase in the overall mixture reactivity and burning velocity. The enhancement in OH intensity with H2 addition indicated a change in the elementary reactions, thereby affecting the flame dynamics. Contrary to H2 addition, CO addition does not show any noticeable effect on the formation of stable flame modes. The change in mixture properties and burning velocity leads to a change in the flame behavior and alters the heat release rate due to H2 addition to propane-air mixtures. A higher percentage of H2 addition led to the reappearance of the unstable (rotating) flames in the microcombustor. Intrinsic flame instabilities appeared at higher H2 percentages due to differential diffusion and increased thermal-wall coupling effects.

Laminar Burning Velocity and Markstein Length of CH4/CO2/Air Premixed Flames at Various Equivalence Ratios and CO2 Concentrations Under Elevated Pressure

ABSTRACT Biogas is a renewable fuel predominantly composed of carbon dioxide (CO2) and methane (CH4) in varying proportions. The effects of the varying CO2 proportion need to be clarified for the development of engines. The laminar burning velocity and the burned gas Markstein length of premixed CH4/CO2/Air were measured with CO2 concentration ranging from 0.3 to 0.7 dilution ratios. The equivalence ratio was varied from 0.8 to 1.2, the initial pressure was set at 0.5 MPa, and the temperature was set to 298 K. The experiment was performed using a high-pressure constant volume combustion chamber. One-dimensional simulation of the flames was conducted using GRI-Mech 3.0. The results showed a reduction in the laminar burning velocity of CH4/CO2/Air mixtures with an increase in CO2 dilution ratio. A non-monotonic relationship was discovered between measured Markstein length and CO2 dilution ratio with different equivalence ratios. It was found that an increase in the CO2 dilution increased the response of the flames to stretch. For the lean and stoichiometric flames, the Markstein length was nearly constant with CO2 dilution of 0–0.5 and decreased with CO2 dilution of 0.7, suggesting an increase in susceptibility of the flame to the intrinsic flame instability. This was found to be mainly due to an increase in the Zel’dovich number and a decrease in the effective Lewis number with CO2 dilution. The Markstein length of the rich flame increased with CO2 dilution as it was more sensitive to CO2 dilution. Thermo-diffusive effects and pure stretch effects had similar influences on the burning velocity of the rich flames with an increase in stretch rate.

Turbulent flame structure characteristics of hydrogen enriched natural gas with CO2 dilution

This paper investigated the hydrogen enriched methane/air flames diluted with CO2. The turbulent premixed flame was stabilized on a Bunsen type burner and the two dimensional instantaneous OH profile was measured by Planar Laser Induced Fluorescence (PLIF). The flame front structure characteristics were obtained by extracting the flame front from OH-PLIF images. And the turbulence-flame interaction was analyzed through the statistic parameters. The role of hydrogen addition as well as CO2 dilution on the features of turbulent flame were revealed by those parameters. In this work, hydrogen fractions of 0, 0.2 and CO2 dilution ratios of 0, 0.05 and 0.1 were studied. Results showed that hydrogen addition can enhance turbulent burning velocity ST/SL through decreasing the scale of the finer structure of the wrinkled flame front, caused by the smaller flame instability scale. In contrast, CO2 dilution decreased turbulent burning velocity ST/SL due to its inactive response to turbulence perturbation and larger flame wrinkles. For all flames, the probability density function (PDF) profile of the local curvature radius R shows a bias to positive value, resulted from the flame intrinsic instability. The PDF profile of R decreases with CO2 dilution, while the value of local curvature radius corresponding to the peak PDF is larger. This indicates that larger wrinkles structure was generated due to CO2 dilution, which leads to the decrease in ST/SL as a consequence. Hydrogen addition increases the flame volume and results in more intense combustion. CO2 dilution has a decrease effect on flame volume for both XH2 = 0 and XH2 = 0.2 while the decrease is obvious at XH2 = 0.2, ZCO2 = 0.1. In all, hydrogen enrichment improves the combustion while CO2 can moderate combustion. Therefore, adding hydrogen and CO2 in natural gas can be a potential method for adjusting the combustion intensity in combustion chamber during the combustor design.