Instability Of Flame Front Research Articles

Numerical investigation on the effect of porosity distribution on the flame characteristics

The low velocity filtration combustion of a lean methane/air mixture in the fixed bed reactors is numerically investigated using the system scale method. The effect of porosity on the flame characteristics, such as temperature profile, flame displacement speed and flame shape is examined in this study. For comparison, porosity is considered in two forms, i.e., volumetric average porosity (VAP) and porosity spatial distribution function (PSDF). The simulation results show that, local distribution of porosity primarily affects the flame shape, while having minimal impact on the peak flame temperature and mean flame displacement velocity. Quantitative analysis indicates that the flame displacement velocity obtained using VAP is closer to the experimental data compared to the PSDF. However, the simulation with PSDF explicitly captures the flame front bending caused by local porosity variation. Velocity and temperature fields suggests that flame front bending predominantly depends on the local flame displacement speed (LFDS), which is the vector sum of local interstitial airflow speed and the laminar flame speed of the combustible gas. The radial difference in LFDS, which is associated with uniformity in velocity and temperature fields, serves as the main driving force for flame front instability. The inclusion of PSDF enables a more accurate prediction of flame front development by capturing the preferential flow near the wall.

Study and characterization of the instabilities generated in expanding spherical flames of hydrogen/methane/air mixtures

In the present work an analysis of the origin and nature of intrinsic instabilities in combustion processes with different hydrogen/methane mixtures is developed. These expanding spherical flame front experiments have been developed in a cylindrical constant volume combustion bomb, which allows recording the process through Schlieren photography method.The stability study in combustion processes has a great importance to assure their security and control, since the understanding of flame instabilities is necessary for improving the internal combustion engines performance.To carry out this mentioned study, a review of the concepts and parameters used in spherical flame front instabilities research is first proposed, as well as a physical explanation of each concept and the relations among them.Additionally, a methodology that aims to determine the influence of the fuel mixtures in the origin and development of the flame front instabilities is suggested. Moreover, the intrinsic effects of the combustion process, such as the thermal-diffusive and the hydrodynamic effect, are separately studied, including their individual contributions to the growth rate of instabilities which allows to determine combustion nature and to obtain the instability peninsula of each fuel mixture.Finally, this methodology includes a qualitative study of the cellularity phenomenon (when the instabilities develop all over the flame front), considering the parameters which influence on this phenomenon.

Effect of hydrogen addition on the laminar burning velocity of n-decane/air mixtures: Experimental and numerical study

Hydrogen is a potential aviation alternative fuel. To explore the influence of hydrogen addition on the combustion of aviation kerosene, the spherical expanding flame method was used to measure the laminar burning velocity and Markstein length of n-decane/hydrogen/air mixtures at 1bar, 2bar and 470 K using experimental and numerical analysis. The experimental LBVs agreed well with the simulation at an initial pressure of 1 bar, but lower than the simulation at 2bar. The laminar burning velocity showed a linear relationship with the hydrogen addition ratio (RH) at the different effective fuel-air equivalence ratios (ϕF). The Markstein length decreased with increased RH and initial pressure, hence, increasing the susceptibility of flame front instability. Based on a one-step overall reaction assumption, sensitivity analysis of the premixture mechanism showed that the kinetic effect greatly influenced n-decane/hydrogen/air mixture combustion. Further kinetic analysis indicated that the maximum (H + OH) mole fraction of the chemical reaction is highly responsible for the linear relationship between laminar burning velocity and RH.

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.

Investigation of Laminar Burning Velocities and Cellular Instability for Dimethyl Carbonate at Elevated Pressures

A spherical propagating flame experiment was performed to study the laminar burning velocities (LBVs), Markstein lengths, and onset of instability of flame fronts for dimethyl carbonate (DMC) at 37...

Acoustic parametric instability, its suppression and a beating instability in a mesoscale combustion tube

The present work reports thermo-acoustic instability in a mesoscale tube of diameter 8 mm (> quenching diameter) and length 702 mm. Mixtures with Lewis number, Le~ 0.8 (rich C2H4/O2/CO2), 1.05 (lean C2H4/O2/CO2) and 1.34 (lean C2H4/O2/N2) are used. Several new flame responses are observed. For lower burning velocity mixtures, flame extinction is observed due to heat loss when primary instability transforms to secondary instability for all Le mixtures. However, parametric cellular structures which are characteristic of parametric instability are observed only for Le of 0.8. It is proposed based on calculations and experiments that parametric structures will be observed only when diameter of tube is two times larger than the characteristic wavelength of parametric instability. If the tube diameter doesn't allow formation of parametric structure whirling and counter-rotating flames are observed instead of parametric structures. Suppression of acoustic parametric instability is observed for higher burning velocity mixtures with Le>1 and its mechanism is discussed. For a range of SL, a beating instability is observed for CO2 diluted mixtures of Le>1, where pressure oscillation and flame motion show beating oscillations of frequency around 15 Hz. This beating instability is believed to be caused by non-linear interaction of acoustic instability with pulsating instability of flame front which is caused due to combined radiative and convective heat loss. Due to CO2 dilution, the radiative heat losses could play a significant role in inducing pulsating instability.

Experimental investigation of hydrous ethanol/air flame front instabilities at elevated temperature and pressures

The present work experimentally investigates hydrous ethanol/air flame stability. The experimental data were obtained using spherically expanding flames in a constant volume bomb with optical access for high-speed schlieren photography. It explores the effect of flame parameters, such as thermal expansion rate, flame thickness, activation energy, and effective Lewis numbers, on flame dynamics at elevated pressures (2 to 6 bar) and temperatures (380 and 450 K), at various equivalence ratios (0.6 to 1.3) and water dilution contents (0, 5, 20 and 30% in volume). Adding water to the ethanol/air mixture and increasing its content leads to a significant decrease in flame instability, reducing the thermal expansion ratio while increasing the flame thickness and therefore reducing the propensity of hydrodynamic instability appearance on the flame front. The equivalence ratio has a significant effect on flame stability as well. Slightly rich mixtures present the maximum thermal expansion ratio and minimum flame thickness, therefore, presenting the highest propensity for hydrodynamic instability of the flame front. Besides, the effective Lewis number significantly decreases with equivalence ratio, showing a higher propensity of diffusional-thermal instability for rich mixtures. The flame front instability significantly increases with the mixture initial pressure, which results from the enhancement of the hydrodynamic instability due to the significant decrease in the flame thickness for all equivalence ratios. The initial temperature has a weaker effect on flame stability compared to the other thermo-chemical properties investigated. However, the flame front instability slightly increases with temperature.

Intensification mechanisms of the lean hydrogen-air combustion via addition of suspended micro-droplets of water

The present study is devoted to the detailed numerical analysis of the combustion wave propagation in the confined vessel filled with the lean 15% hydrogen-air mixture containing suspended micro-droplets of water. Considered gaseous mixtures possesses a relatively low reactivity, and so the intensity of the combustion is moderate. Herewith, it is found that the flame can be noticeably accelerated in the presence of suspended micro-droplets with a diameter larger than 50 μm. Numerical analysis of the flame structure performed via in-house computational package developed by authors showed that the interaction between the flame and the droplets results in the intensification of the flame instability and small scale flame front wrinkling. It is demonstrated that along with the scales associated with intrinsic hydrodynamic and thermo-diffusive modes of flame front instability, smaller wrinkles are excited as a result of the local effect of micro-droplets on the flame front. The rapid growth of the corrugated flame surface leads to the combustion intensification, and in the considered system, the burnout process can proceed up to 2.2 times faster in the presence of micro-droplets of 200 μm diameter than in the pure gaseous mixture. It is expected that distinguished mechanisms of the influence of water droplets on the flame front structure are relevant for the other two-phase combustible systems, such as reactive gaseous mixtures with suspended fuel droplets or solid particles.

Instability of free interfaces in premixed flame propagation

<p style="text-indent:20px;">In this survey, we are interested in the instability of flame fronts regarded as free interfaces. We successively consider a classical Arrhenius kinetics (<em>thin flame</em>) and a stepwise ignition-temperature kinetics (<em>thick flame</em>) with two free interfaces. A general method initially developed for thin flame problems subject to interface jump conditions is proving to be an effective strategy for smoother thick flame systems. It relies on the elimination of the free interface(s) and reduction to a fully nonlinear parabolic problem. The theory of analytic semigroups is a key tool to study the linearized operators.

Flame Front Deformation Instabilities of Filtration Combustion for Initial Thermal Perturbation

AbstractThe flame front deformation instability of low‐velocity filtration combustion within an inert packed bed is studied based on the initial preheating non‐uniformity. Based on the experimental phenomena, an initial thermal perturbation model is numerically proposed so as to predict the deformation behaviors of the flame front instabilities. The numerical prediction indicates that the assumption of an initial thermal perturbation is a feasible explanation as the cause of the flame front inclination instability. As the initial thermal perturbation increases, the phenomena of the flame front break and shrinking instabilities could easily occur at high filtration velocity or low equivalence ratio. Moreover, the evolutions of the flame front break rate and the shrinking rate are quantitatively analyzed.

The role of instability mechanisms in gas flame acceleration

To study the role of individual mechanisms of the flame-front instability, a series of experiments was carried out on the propagation of a spherical flame. Transparent latex shells were filled with pre-prepared hydrogen–air mixture. In various series of experiments, the percentage of hydrogen was varied. The ignition of the flame was produced by a spark discharge with an energy of 1 mJ, a discharger located in the center of the shell. Experimentally, the scattering parameters of the spherical hydrogen–air flame acceleration was found with a constant mixture composition and combustion initiation energy. It was found that at the initial stage of propagation, both acceleration and deceleration of flame front occurs. The parameters of the flame front propagation experimentally obtained are supplemented by calculations carried out using analytical models.

Range of “complete” instability of flat flames propagating downward in the acoustic field in combustion tube: Lewis number effect

Downward propagating flames ignited at the open end of an open-closed tube exhibit thermo-acoustic instability due to interaction of combustion generated acoustic fluctuations with the flame front. At sufficiently high laminar burning velocity (SL) two regimes of thermo-acoustic instability are observed, namely, primary instability (where initial cellular flame transitions to a vibrating flat flame) and a secondary instability (where vibrating flat flame transitions to vibrating turbulent flame due to parametric instability of flame front). On further increasing SL to a particular value, “complete instability” of flat flames is observed meaning flat flame cannot be stabilized and initial cellular flame transitions directly to parametric instability. This particular SL introduced in this work is termed “critical SL”. In past experimental works, stability of flat flames in the acoustic field had only been studied in terms of acoustic velocity amplitude and a critical acoustic velocity amplitude had been measured at the onset of parametric instability. The novelty of this work is that boundary of unconditional instability of flat flame (flat flame is unstable irrespective of acoustic velocity amplitude) is determined in terms of mixture conditions, e.g., SL. Particularly for propagating flames, this critical SL can be measured more easily and accurately than the critical acoustic velocity. This work presents the effect of Le (Lewis number) on critical SL. Three different fuels, CH4, C2H4 and C3H8 are tested with two different dilution gases (N2 and CO2) for equivalence ratio of 0.8 (lean) and 1.2 (rich). Twelve different Le ranging from 0.7 to 1.9 are generated through these mixture combinations. Generally, larger Le mixtures show higher critical SL than lower Le mixtures for any fuel. Theoretical calculations are performed to predict critical SL by studying instability of planar flame fronts in presence of acoustic forcing. Theoretical calculations successfully captured the effect of Le as predicted stability region of planar flame is narrower for lower Le than that for higher Le. However, accurate quantitative predictions of critical SL couldn't be obtained from existing theory, particularly for non-unity Le. Hence, a correction (a function of Zeldovich number, β and Le) to width of stability region is proposed to obtain better quantitative agreement for critical SL between experiments and theory and performs significantly well. The correction factor acts to compensate for the inaccuracies in Markstein number obtained from an analytical relationship during calculation of stability region width.

Three-dimensional Instability of Flame Fronts in Type I X-Ray Bursts

Abstract We present the first realistic 3D simulations of flame front instabilities during type I X-ray bursts. The unperturbed front is characterized by the balance between the pressure gradient and the Coriolis force of a spinning neutron star (ν = 450 Hz in our case). This balance leads to a fast horizontal velocity field parallel to the flame front. This flow is strongly sheared in the vertical direction. When we perturb the front an instability quickly corrugates the front. We identify this instability as the baroclinic instability. Most importantly, the flame is not disrupted by the instability and there are two major consequences: the overall flame propagation speed is ∼10 times faster than in the unperturbed case and distinct flame vortices appear. The speedup is due to the corrugation of the front and the dynamics of the vortices. These vortices may also be linked to the oscillations observed in the light curves of the bursts.

Energetic valorization of the residual biomass produced during Jatropha curcas oil extraction

The Jatropha curcas fruit shells (JCFS) and Jatropha curcas seed cake (JCSC) have drawbacks when used as fuel, namely flame front instabilities and short combustion periods. In this work, a blend of 25 wt% Jatropha curcas pellets with 75 wt% of pruning residues (PR) resulted in a fuel mixture that promote a stable flame front and the combustion process could be sustained for periods larger than 60 min. The improvement in the combustion of the blend composed of Jatropha curcas pellets and PR, when compared to mono-combustion of Jatropha curcas pellets, can be explained in result of the better fuel properties given by PR to the fuel blend. It was observed that PR combustion is associated to higher temperatures in the flame gases and the CO concentration in the flue gas is less than 1500 mg/Nm3 (dry gases, 13% vol. O2), thus in accordance to the European standards for solid fuel stoves. Although the PR fraction in the biomass mixtures is high (75 wt%), the observed CO concentration in the flue gas during combustion of the fuel blends was higher than 1500 mg/Nm3 (dry gases, 13% vol. O2) especially in the fuel mixtures that incorporate JCFS in their composition.

Experimental and theoretical study of secondary acoustic instability of downward propagating flames: Higher modes and growth rates

Flames propagating in tubes open at the ignition end typically show two different kinds of thermo-acoustic instability namely, primary and secondary. Secondary acoustic instability is accompanied by parametric instability of flame front during which, cellular structures on the flame surface oscillate with half the acoustic frequency of excitation. The growth rates associated with secondary acoustic instability of flame structure are higher compared to primary instability of flames leading to very high peak pressures. In this work, we present experimental and theoretical study on parametric instability of downward propagating C2H4/O2/CO2 flames at two different Le of 1.0 and 0.8. Lower Le mixtures are found to be more unstable. Parametric instability of higher acoustic modes is reported for the first time for gaseous fuels. Higher modes of parametric instability transitioned successively to lower modes as the flame propagated downward. Growth rate of parametric instability is measured in experiments. Theoretical prediction of growth rate is done based on velocity coupling mechanism. Theoretical calculations provide good approximation of growth rates and its variation with frequency.

Numerical investigation of low-velocity filtration combustion instability based on the initial preheating non-uniformity

The effects of the initial preheating perturbation on the dynamical behaviors of FGC wave propagation instability for low-velocity FGC in packed bed are studied numerically. The behaviors of the flame front inclination, break, and shrinking instabilities are always observed in experiments. Based on the experimental phenomena, an initial thermal perturbation model is numerically proposed as to predict the deformation behaviors of the flame front instabilities. The typical flame shapes are obtained depending on filtration velocity, equivalence ratio, and initial preheating temperature difference. It is demonstrated that the development of flame front inclination instability is proportional to the magnitude of initial preheating perturbation. At a lower equivalence ratio, the initial thermal perturbation of 300 K leads to the evolution of flame front break. Increasing filtration velocity leads to the appearance of flame front break, due to the intensification of the hydrodynamic instability. In addition, a perculiar instability of flame front shifting is also confirmed with the initial thermal perturbation of 400 K, which results in a fuel leakage of incomplete combustion.

Flame front stability of low calorific fuel gas combustion with preheated air in a porous burner

The flame front stabilization of low calorific fuel gas (LCFG) combustion with preheated air was investigated numerically in a burner filled with Al2O3 ceramic foams. A two-dimensional model was implemented to simulate the flame propagation process of methane–preheated air combustion at an extra-low equivalence ratio. In addition to the wall heat loss, the effects of the preheated air temperature were analyzed by focusing on the flame front inclination and propagation velocity. The flame front instability for room temperature air agreed well with the results of the corresponding experiments. The results show that an increase in the preheated air temperature helps to improve the flame stabilization, such as inhibiting flame front inclination, decreasing its propagation velocity, and increasing the maximum combustion temperature. A stable combustion flame can be realized for LCFG by preheating the air to a critical temperature, which decreases with increase in the equivalence ratio and decrease in the inlet velocity. The wall heat loss promotes the flame front inclination, whereas it has little effect on its propagation velocity for stable combustion. The results are conducive to clean combustion of LCFG in porous media, and helpful for the design and operation of porous burners.

Role of Darrieus–Landau instability in propagation of expanding turbulent flames

In this paper we study the essential role of Darrieus–Landau (DL), hydrodynamic, cellular flame-front instability in the propagation of expanding turbulent flames. First, we analyse and compare the characteristic time scales of flame wrinkling under the simultaneous actions of DL instability and turbulent eddies, based on which three turbulent flame propagation regimes are identified, namely, instability dominated, instability–turbulence interaction and turbulence dominated regimes. We then perform experiments over an extensive range of conditions, including high pressures, to promote and manipulate the DL instability. The results clearly demonstrate the increase in the acceleration exponent of the turbulent flame propagation as these three regimes are traversed from the weakest to the strongest, which are respectively similar to those of the laminar cellularly unstable flame and the turbulent flame without flame-front instability, and thus validating the scaling analysis. Finally, based on the scaling analysis and the experimental results, we propose a modification of the conventional turbulent flame regime diagram to account for the effects of DL instability.

Numerical simulation of combustion instabilities under the alternating gravity conditions

The work is devoted to the analysis of the methane-air conical flame behaviour under conditions of an alternating gravitational field. Numerical simulation based on the software package FlowVision, has shown the possibility of modeling the flame front instabilities during the transition from the normal gravitational conditions to zero gravity. The appearance of the flame front oscillations is demonstrated under the such conditions. Further studies will provide a complete picture of the behavior of the flame in an alternating gravitational field.

Topological development of homogeneous-charge and spray-guided stratified-charge flames in an internal combustion engine

In this work, engine flame topological development was studied to improve the understanding of flame growth in direct-injection engines, in particular when operated under homogeneous-charge and spray-guided stratified-charge combustion conditions. Flame wrinkledness is one of the most important and poorly understood engine combustion phenomena. Generally, a flame may wrinkle for two reasons: (1) its own natural instabilities and/or (2) through interaction with turbulent flow. The relative contribution of these two causes toward flame wrinkledness in the engine environment was unclear, so targeted experiments were performed to provide some clarity. The development of flame wrinkledness within an optically accessible engine was measured with a combination of planar laser–induced fluorescence and stereo particle image velocimetry under homogeneous-charge and stratified-charge conditions. Using the resulting data, equivalence ratio, charge velocities, and flame wrinkledness were quantified and analyzed. For the iso-octane–toluene mixtures studied, flame wrinkling was insensitive to thermo-diffusive flame front instabilities. The relative contribution of wrinkles of various spatial scales toward overall flame wrinkledness was also measured. Homogeneous-charge flames generally had lower wrinkling factors than stratified-charge flames. Overall, flame wrinkledness increased with flame size under both modes of engine operation. Large flames demonstrated an ability to maintain more large-scale wrinkles than small flames, which contributed to their overall higher levels of wrinkledness.