Nonpremixed Methane-air Flames Research Articles

Assessment of a Differential Subgrid Stress Model for Large-Eddy Simulations of Turbulent Unconfined Swirling Flames

Swirling flames are widely used in engineering to intensify mixing and stabilize combustion in gas turbine power plants and industrial burners. Swirling induces new instability modes, leading to intensification of coherent structures, asymmetric geometry, vortex core precession, and flame oscillations. Large-Eddy Simulation (LES) has the capability to furnish more accurate and reliable results than the simulations based on Reynolds-averaged Navier–Stokes equations (RANS). Subgrid-scale models in LES need to describe the backscatter (local transfer of kinetic energy from small scales to larger scales) that is intensified in swirling flames. In this paper, the Differential Subgrid Stress Model (DSM), previously developed by the authors, is assessed using an experimental database from Sydney University on swirl-stabilized turbulent unconfined non-premixed methane-air flame. Regime without vortex precession is simulated numerically using the DSM and Smagorinsky subgrid-scale model. Experimental measurements of mean velocity, profiles of mass fractions, and temperature are used for comparison with the simulation data. The standard Smagorinsky model is considered the basic approach. Differences in the flow field statistics obtained in both subgrid-scale LES models are analyzed and discussed. The importance of taking the backscatter into account is highlighted.

Numerical investigation to evaluate the effects of gravity and pressure on flame structure and soot formation of turbulent non-premixed methane-air flame

In this study, a turbulent non-premixed (diffusion) methane-air flame has been investigated computationally to analyze the influences of pressure and gravity on flame structure and sooting characteristics between 1 and 10 atm. The simulation has been conducted in a 2-D axisymmetric computational domain using the finite volume-based computational fluid dynamics (CFD) code. The interaction of turbulence and chemistry is modeled by considering the steady laminar flamelet model (SLFM) and the GRI Mech 3.0 chemical mechanism. The radiative heat transfer calculation is carried out by considering the discrete ordinate (DO) method and the weighted sum grey gas model (WSGGM). The semi-empirical Moss-Brookes model is considered to calculate soot. The impact of gravity on flame and sooting characteristics are evaluated by comparing the normal-gravity flames with the zero-gravity flames. The effect of soot and radiation on flame temperature is also examined. The results show a close agreement with the measurement when both soot and radiation are included in the numerical modeling. The rates of soot formation, surface growth, and oxidation increase with increased operating pressure, regardless of gravity. Zero-gravity flames have a higher soot volume fraction, a wider soot-containing zone, a higher CO mass fraction, and a lower flame temperature than normal-gravity flames while maintaining constant pressure. In normal-gravity flames, the CO mass fraction decreases with pressure, whereas it increases with pressure rise in flames of zero gravity. Flames of zero gravity appear taller and broader compared to the flames of normal-gravity for a fixed pressure. An increase in pressure significantly reduces the flame length and width in normal-gravity flames. However, the pressure elevation has little effect on the shape of a zero-gravity flame. The outcomes of the present study will assist in fully understanding the combustion and sooting characteristics of turbulent diffusion flames that will help design and develop high-efficiency, pollutant-free combustion devices and fire suppression systems for space application.

Using a flamelet generated manifold method in transported probability density function modeling of soot formation and thermal radiation

The simple semi-empirical precursor soot model of Brookes and Moss based on the soot number density and soot mass concentration is adopted in a transported probability density function (PDF) method for turbulent diffusion flames. The gas phase chemistry is described by a flamelet generated manifold (FGM) based on the mixture fraction, progress variable and enthalpy loss. The accuracy of the FGM method is validated by using flamelet solutions that are not included in the generating set of the FGM. To account for the radiative heat transfer in the flames, we use a non-gray weighted sum of gray gases model for the gas radiation and a gray soot radiation model. Turbulence–radiation interaction is closed at the level of the optically thin fluctuation approximation and the Reynolds averaged radiative transfer equation is solved by means of a discrete transfer method. The proposed modeling approach is applied in simulations of two turbulent non-premixed methane–air flames at one bar and three bar pressure, respectively. Predictions of the mean temperature and mean soot volume fraction are in good agreement with the measurements in the one bar flame. In the higher pressure flame the mean soot volume fraction is over predicted. For this flame, simulation results using the semi-empirical model of Lindstedt provided better agreement with the measurements. The main difference between the Brookes and Moss model and the Lindstedt model is the nine-times increased soot particle agglomeration rate of the latter. When using the same increased agglomeration rate parameter in the Brookes and Moss model the results become virtually identical. The negligible molecular diffusion of the soot was accounted for by neglecting mean molecular diffusion of the soot variables and by greatly reducing their micro-mixing. The effect of this differential soot diffusion on the mean soot volume fraction is found to be small, but it is significant for the variance of the soot volume fraction.

Experimental Investigations on Non-premixed Methane-air Flames in Radial Microchannels with a Controlled Temperature Profile

ABSTRACT Experimental investigations were made on the stabilization and dynamics of non-premixed methane-air flame in a series of radial microchannels with a diameter of 60 mm and widths of 2.0, 2.5 and 3.0 mm with a controlled wall temperature profile. Various flame modes, termed periodically rotating flame, stable flame, broken and cellular flames, single broken flame, and periodically splitting flame were observed with high-speed imaging. Regime diagrams of all these stable and unstable flame modes were then drawn for the three channel gap widths. In addition, some critical flame characteristics, such as the mean radii of the stable flames, and the rotating frequencies of the periodically rotating flames were quantitatively assessed. Effects of the fuel/oxidizer inflow rates and the nominal equivalence ratios on these parameters were studied. Furthermore, numerical studies on the flow field were performed, revealing the inner mechanism of various flame modes to a certain extent. Disturbance of the fuel/oxidizer mixing layer for various flow conditions was considered to be greatly responsible for the formation of some flame patterns.

Chemistry computation without a sub-grid PDF model in LES of turbulent non-premixed flames showing moderate local extinction

The use of the Eulerian Stochastic Fields (ESF) method to model the sub-grid turbulence-chemistry interaction (TCI) in the LES context can be computationally expensive if detailed chemistry mechanisms are involved. This work aims to assess whether it is possible to neglect the modelling of the TCI on sufficiently refined meshes while using finite rate chemistry, provided that at least 80 % of the turbulent kinetic energy scales are resolved. Turbulent non-premixed methane-air flames showing a moderate degree of local extinction are selected as benchmark. Results obtained for the Sandia flame E with and without transporting the ESF on three different meshes are discussed. Sensible deviations are visible on the fuel-rich side from section x/D = 30, by reducing the grid refinement. The influence of three finite rate chemistry solvers is further investigated on flame D, without the sub-grid scale chemistry model. All simulations are in good agreement with the experimental data and show a weak dependence on the chemistry involved. A trade-off assessment between computational time and accuracy is provided, in order to extend the validation to a more severe extinction regime.

Effect of CO2 diluent on the formation of pollutant NOx in the laminar non-premixed methane-air flame

In this paper, the effects of changes in the concentration of diluent CO2 in methane and air fuels are investigated. A finite volume method (FVM) with staggered grids is used for numerical solution. Equations of continuity, momentum, energy, ideal gas state and kinetic equations with thermodynamic and thermo-chemical information of chemical species are solved using the numerical method of SIMPLE. The convective terms are discretized using Power Law scheme (PLS). The under-relaxation factor dependent on temperature has been utilized to solve the chemical kinetic equations. In this research, by increasing the diluent mass fraction, the mass fraction of the fuel is diminished to the same extent. Therefore, the sum of the diluent and fuel mass fraction values remains constant. The results depicted that as the amount of CO2 diluent in the fuel increases, the total lift-off height and flame length decreases linearly, and the maximum temperature in the combustion chamber becomes smaller than that at the inlet of the chamber. When the concentration of CO2 increases in the fuel, the temperature changes become negligible at the inlet of the combustion chamber. By using the CO2 diluent, shorter combustion chambers can be used. While the diluent is used, the increase in the length of the combustion chamber will not have any effect on the NOx emissions reduction. Therefore, the more concentration of the diluent is used, the smaller the combustion chamber can be selected. Moreover, by increasing the diluent, NOx pollutant decreases in the combustion chamber.

Large-Eddy Simulation of Sandia Flame D with Efficient Explicit Filtering

A uniform Gaussian filter has been applied explicitly to the LES conservation equations for mass, momentum and mixture to simulate a piloted non-premixed methane-air flame (Sandia Flame D). Using a basic property of exponential functions, the three dimensional Gaussian filter is decomposed into the product of three one dimensional filters, greatly reducing the cost of filtering. Seven simulations on three different grids have been performed to investigate the influence of grid refinement with a purely implicit filter, the effects of explicit filtering with increasing filter width and the effect of grid refinement at constant filter-size. Overall, consistent results have been achieved at a cost that is moderate with implicit or explicit filtering, so that explicit filtering can be applied in cases where the numerical error should be independent of the modelling error.

Study of polycyclic aromatic hydrocarbons (PAHs) in hydrogen-enriched methane diffusion flames

Polycyclic aromatic hydrocarbons (PAHs) are the carcinogenic components of soot. Detailed understanding of PAH formation characteristics is required for development of effective strategies to curtail PAH formation and reduce soot in combustion devices. This study presents an experimental methodology to analyse PAH formation characteristics of a non-premixed methane-air flame with and without hydrogen (H2) addition, using simultaneous planar laser induced fluorescence (PLIF) imaging of PAH and hydroxyl radical (OH). OH PLIF was used to represent peak temperature regions in the flame front. One-dimensional, opposed-jet laminar non-premixed flame simulations were also carried out for the same fuel mixture conditions. This work describes comparison of trends from both sets of studies. PAH fluorescence intensity values were observed to increase with increasing height above burner, however this rate of increase reduced with H2 addition. This observed rate of change in PAH fluorescence (that is, PAH growth characteristics) is indicative of the sooting potential of the fuel mixture. PAH fluorescence from experiments and PAH concentration from simulation show strong reduction with increase in H2 addition. The percentage reduction in PAH fluorescence signal with H2 addition closer to the burner tip was of a similar magnitude to that observed with flame simulations. The reduction in PAH with H2 addition could be attributed to the reduction in acetylene and propargyl concentrations, and reduced H-abstraction rates, which reduced the availability of active sites for PAH growth. The proposed experimental methodology for PAH measurements can be readily applied to any fuel mixtures.

Structure and behavior of water-laden CH4/air counterflow diffusion flames

A counterflow configuration was used to measure thermal and species structure in water-vapor diluted nonpremixed methane–air flames. The motivation is to understand the chemical and thermal effects that water has when it is introduced as a diluent into the fuel side. This work is relevant to combustion processes where water is incorporated naturally in the fuel; e.g., methane hydrates, and when water is added intentionally for emission reduction such as in flares and H2O/fuel emulsions combustion. Experimental data are compared to 1-D computations. The agreement is generally very good, but the one dimensional counterflow diffusion model overpredicts flame temperature and major radical, OH, concentration very near extinction in highly diluted H2O–methane/air diffusion flames. Changes in flame position, flame width, and peak temperature with the addition of water were measured. Flame temperatures were measured with thin filament pyrometry. OH-PLIF is used to characterize the flame reaction zone with water dilution; the OH distribution, flame position and thickness from the OH-PLIF images were measured. The results show that the OH intensity and reaction zone thickness decreases with the increase in water. Predictions and experiments demonstrate that water mainly acts thermally to lower the flame temperature until extinction. The OH maximum intensity shifts towards the air side of the counterflow burner with water addition. OH is also measured with CO2 dilution of the fuel stream, and the results are compared with H2O addition, including comparisons with the OH molar peak predictions obtained using the GRI 3.0 mechanism and the CHEMKIN Pro one dimensional counterflow model. The study indicates that water’s chemical effects are to change the production and depletion of OH, H and O radicals, especially near extinction. Chemical kinetics simulation of the flame demonstrates good agreement in OH and flame temperatures over a wide range of dilution away from extinction, particularly for CO2. An over prediction of the water carrying capacity near extinction is found for highly water-diluted flames.

Effects of CO2 Dilution and O2 Enrichment on Non-premixed Turbulent CH4-Air Flames in a Swirl Burner

ABSTRACTEffects of oxygen enrichment and CO2 dilution on the characteristics of non-premixed methane-air turbulent flames in a coaxial swirl burner are investigated in this article. The work primarily focuses on pollutant emissions (NOx and CO), flame structure, and stability investigations. The experiments are conducted using a 1-m high, 0.5-m wide, 25-kW parallelepiped combustion chamber, cooled by outside water. The burner configuration consists of two concentric tubes in which a swirler is placed inside the annular part for air or air-O2-CO2 supply that can allow for the rotation of the oxidant. Fuel is injected radially from the central tube. OH* chemiluminescence measurements are performed to describe the structure and stability of the flame providing information on the flame lengths and lift-off heights. The measurements are conducted with the oxygen concentrations ranging from 21% to 35%, CO2 concentrations ranging from 0 to 20%, swirl numbers ranging from 0.8 to 1.4, and global equivalence ratios ranging from 0.8 to 1. The lift-off heights, the fluctuations of the flame base, and the flame lengths are determined as a function of these parameters. The results show that oxygen enrichment stabilizes better the flame, promotes the formation of NOX and CO2, and decreases the formation of CO. The dilution by CO2 changes significantly the flame shape and its behavior. The flame becomes longer, less intense, and unstable. CO2 dilution reduces considerably the flame temperature which obviously reduces the NOx formation, but it is observed that CO2 and CO concentrations in the flue gases are increased.

A computational study of the effects of DC electric fields on non-premixed counterflow methane-air flames

Two-dimensional axisymmetric simulations for counterflow non-premixed methane-air flames were undertaken as an attempt to reproduce the experimentally observed electro-hydrodynamic effect, also known as the ionic wind effect, on flames. Incompressible fluid dynamic solver was implemented with a skeletal chemical kinetic mechanism and transport property evaluations. The simulation successfully reproduced the key characteristics of the flames subjected to DC bias voltages at different intensity and polarity. Most notably, the simulation predicted the flame positions and showed good qualitative agreement with experimental data for the current–voltage curve. The flame response to the electric field with positive and negative polarity exhibited qualitatively different characteristics. In the negative polarity of the configuration considered, a non-monotonic variation of the current with the voltage was observed, along with the existence of an unstable regime at an intermediate voltage level. With positive polarity, a typical monotonic current–voltage curve was obtained. This behavior was attributed to the asymmetry in the distribution of the positive and negative ions resulting from ionization processes. The present study demonstrated that the mathematical and computational models for the ion chemistry, transport, and fluid dynamics were able to describe the key processes responsible for the flame-electric field interaction.

NO Emission of a Non-premixed Oxygen-Enhanced Flame in Burned Gas Entrainment above the Self-Ignition Temperature (BEST) Conditions

ABSTRACTThe emission characteristics of non-premixed oxy-fuel and oxygen-enhanced flames in burned gas entrainment above the self-ignition temperature (BEST) conditions were experimentally investigated in a lab-scale slot-type combustor. The inside geometry of the furnace was width = 250 mm, depth = 250 mm, and height = 350 mm. A slot-type burner was located on the center of the furnace bottom. The widths of the fuel jet and oxidizer nozzle exit were 0.24 mm and 0.52 mm. The separation distance between the nozzle exits was 8 mm and the depth of the nozzle exit was 20 mm. Light emission data were obtained with a digital single lens reflex camera, spectrometer, and intensified charge-coupled device camera to visualize the flame appearance in a furnace. The distribution of the furnace temperature in the BEST conditions was measured with R-type thermocouples. The pollutant emission trend was analyzed when the inlet velocity for fuel jet and oxidizer is 10 m/s to 30 m/s and 10 m/s to 55 m/s, the volumetric mole fraction of oxygen in an oxidizer is 40% to 100%, and the global equivalence ratio is 0.55 to 1.03. The objectives of the present work were to study the flame spectra in normal temperature and pressure (NTP) and the BEST conditions and to determine the global production rate of nitrogen oxides in non-premixed oxy-fuel and oxygen-enriched flames. The visible flame of a non-premixed oxy-methane flame disappeared and temperature deviation in a furnace decreased by 300°C in the BEST conditions. The global production rate of nitrogen oxides ([NOx]/τG) level decreased while increasing the volumetric mole fraction of oxygen in an oxidizer. The slope of [NOx]/τG was similar to that of a non-premixed methane-air flame in NTP conditions. The level of [NOx]/τG was lower in oxygen-enriched flames than in CH4-air flames.

Dynamics and thermodynamics of co-flow non-premixed methane–air flame in a cylindrical micro combustor with heat recirculating wall

Numerical studies have been made on co-flow non premixed methane air flame in a cylindrical micro combustor with heat recirculating wall. An optimum Peclet number Peopt based on air flow velocity has been observed at which the flame location from the combustor inlet is minimum and the rate of heat recirculation to fresh charge is maximum. The optimum Peclet number is almost independent of the outer wall Nusselt number NuE and the thermal conductivity ratio ks/kg. The rate of heat recirculation shows contrasting increasing and decreasing trends with NuE, depending upon whether Pe < Peopt or Pe >Peopt respectively.A higher limit of Peclet number for flame blow-off and a lower limit of Peclet number for flame extinction have been identified for a given global equivalence ratio and given values of NuE, ks/kg and t/D (the ratio of wall thickness to tube diameter). The upper Pe limit decreases sharply with NuE, while the lower Pe limit is almost uninfluenced by both NuE and ks/kg. Heat diffusion and chemical reaction in the gas phase are found to be the dominant processes of exergy destruction. The second law efficiency shows a minimum value around 30% at an optimum value of Peclet number where flame stand-off becomes minimum.

Investigation on Effect of Air Velocity in Turbulent Non-Premixed Flames

Abstract In this study, the turbulent non-premixed methane-air flame is simulated to determine the effect of air velocity on the length of flame, temperature distribution and mole fraction of species. The computational fluid dynamics (CFD) technique is used to perform this simulation. To solve the turbulence flow, k-ε model is used. In contrast to the previous works, in this study, in each one of simulations the properties of materials are taken variable and then the results are compared. The results show that at a certain flow rate of fuel, by increasing the air velocity, similar to when the properties are constant, the width of the flame becomes thinner and the maximum temperature is higher; the penetration of oxygen into the fuel as well as fuel consumption is also increased. It is noteworthy that most of the pollutants produced are NOx, which are strongly temperature dependent. The amount of these pollutants rises when the temperature is increased. As a solution, decreasing the air velocity can decrease the amount of these pollutants. Finally, comparing the result of this study and the other work, which considers constant properties, shows that the variable properties assumption leads to obtaining more exact solution but the trends of both results are similar.

A Taguchi approach for numerical investigation of CO emission from a non-premixed methane–air flame

In this study, carbon monoxide pollutant emission from a non-premixed methane–air flame has been numerically investigated and optimum design condition which emitted the minimum amount of CO pollutant has been obtained using Taguchi method. A L9(33) Taguchi orthogonal array was utilized to design the experimental plan. The signal to noise (S/N) ratio was employed to obtain the optimal levels and to analyze the influence of the operation conditions on CO emission values. Furthermore, the analysis of variance was applied to find the predominant factors affecting CO emissions. The design factors and their levels were furnace length at 340, 400 and 460 mm, swirl number of 0, 0.2 and 0.4, air preheat temperature at 300, 350 and 400 K, and percent excess air of 0, 10 and 20 %. The results revealed that air preheat temperature and furnace length were the most effective factors on CO emissions, while swirl number and percent excess air had less effect on this pollutant emission. Confirmation tests with the optimal levels of design parameters were conducted to assess the predictive capability of the Taguchi optimization method. Results of confirmation runs showed good agreement with the predicted values based on Taguchi method. It was, thus, shown that the Taguchi approach is appropriate to study the problems that attempts to achieve minimum CO emissions from diffusion flames with the minimum experimentation.

Um estudo numérico da influência da radiação sobre a turbulência em uma chama turbulenta 2D axissimétrica

This paper presents a study of the influence of thermal radiation on turbulence in the simulation of a turbulent, non-premixed methane-air flame. In such a problem, two aspects need to be considered for a precise evaluation of the thermal radiation: the turbulence-radiation interactions (TRI), and the radiative properties of the participating species, which are treated here with the weighted-sum-of-gray-gases (WSGG) model based on recently obtained correlations from HITEMP2010 database. The chemical reactions rates were considered as the minimum values between the Arrhenius and Eddy Break-Up rates. A twostep global reaction mechanism was employed, while the turbulence modeling was considered via standard k–e model. The source terms of the energy equation consisted of the heat generated in the chemical reaction rates as well as in the radiation exchanges. The discrete ordinates method (DOM) was employed to solve the radiative transfer equation (RTE), including the TRI. Comparisons of simulations with/without radiation demonstrated that radiation influenced turbulent-properties (root mean square of velocity and temperature fluctuations, and turbulent kinetic energy of the velocity fluctuations). Radiation smoothed turbulent-properties fields. The influence of radiation was more important on the temperature fluctuations than on the velocity fluctuations. Keywords: thermal radiation, turbulence, TRI, CFD, non-premixed flame.

Three Exergy Zones of a Typical Non-Premixed Methane-Air Jet Flame

Three exergy zones are observed in this study by investigating the local percentages of kinetic exergy, chemical exergy, and thermal exergy of a typical non-premixed methane-air jet flame, the Sandia flame D. It is observed that the flame domain can be divided into three zones as (1) zone І, the fuel zone where methane sprays and spreads, (2) zone ІІ, the air zone where air sprays and spreads, and (3) zone ІІІ, the left area besides zone І and zone ІІ in the flame domain, and this is the place where combustion occurs and flame spreads. Results show that zone І and zone ІІ have higher local percentages of kinetic exergy and chemical exergy whereas zone ІІІ has higher local percentages of thermal exergy.

Application of the WSGG model for the calculation of gas–soot radiation in a turbulent non-premixed methane–air flame inside a cylindrical combustion chamber

This paper presents a study of the effect of soot on the radiative heat transfer in a turbulent, non-premixed methane–air flame inside a combustion chamber. In this problem, an important aspect to be considered is the steep variation of the radiative properties of the medium, which is treated with the weighted-sum-of-gray-gases (WSGG) model based on the superposition of correlations recently obtained for water vapor, carbon dioxide mixtures and soot. The discrete ordinates method (DOM) is employed for the angular integration of the radiative transfer equation (RTE), taking into account turbulence–radiation interactions (TRI). Calculations indicate that the global contribution of soot to the radiative heat transfer in the chamber was of 8%, which is compatible with methane flames. However, in the region with the highest concentration of soot, it was observed a local increase of 30% in the radiative volumetric source in the medium, and of 25% in the radiative heat flux on the wall. In the second part of the paper, the WSGG model is compared with the line-by-line (LBL) integration using the final temperature and concentrations fields obtained from the global flame calculation, leading to average and maximum normalized errors of 1.2% and 4.8%, respectively, and requiring only 1/7000 of the computation effort of the LBL integration. This result indicates that the WSGG model, in spite of its relative simplicity, can provide accurate results for the spectral integration of the radiative properties of gas–soot mixtures in combustion processes.

Numerical simulations of the radiative transfer in a 2D axisymmetric turbulent non-premixed methane–air flame using up-to-date WSGG and gray-gas models

This paper presents a study of the effect of thermal radiation in the simulation of a turbulent, non-premixed methane–air flame. In such a problem, modeling of radiative properties of the gaseous mixture is a fundamental aspect to be evaluated. In this work, such properties were modeled using three different models [a constant-ratio (CR-) and a non-constant-ratio (NCR-) weighted-sum-of-gray-gases (WSGG); and a gray-gas (GG) model], both based on newly obtained correlations from HITEMP 2010 database. The chemical reaction rates were considered as the minimum values between Arrhenius and Eddy Break-Up rates. A two-step global reaction mechanism was used, while the turbulence modeling was considered via standard k–e model. The source terms of the energy equation consisted of the heat generated in the chemical reaction rates as well as in the radiation exchanges. The discrete ordinates method (DOM) was employed to solve the radiative transfer equation (RTE), including the TRI. Comparisons of simulations with/without radiation demonstrated that the temperature, the radiative heat source, and the wall heat flux were importantly affected by thermal radiation, while the influence on species concentrations proved to be less important. The numerical results considering radiation in the analysis were closer to the experimental data from literature when compared to the case neglecting it; the computation of radiation with both the CR-WSGG and NCR-WSGG models provided better results than GG model, when compared with experimental data. For the combustion chamber and turbulence and combustion models employed in the present work, it was shown that the CR-WSGG is the recommended model, due to the acceptable agreement with experimental data and relatively low computational requirements in comparison to the NCR-WSGG model. The results show the importance of thermal radiation for an accurate prediction of the thermal behavior of a combustion chamber.

A comparative study of instabilities in forced reacting plumes of nonpremixed flames

A comparative study has been performed to investigate the flow instabilities and their interaction for nonpremixed methane-air flames using experimental results, numerical simulations and theoretical analyses. The effects of buoyancy and the strong external perturbations on the vortex dynamics and instabilities of forced reacting plumes are studied. Results suggest that the flame surface breaks in forced reacting plumes and two flame fronts are formed eventually due to the convective instability coupled with buoyancy-driven instability. Flame pinch-off could occur in a short time for the cases with the strong perturbations of low frequencies. This indicates that the nonpremixed flame system exhibits a low-pass characteristic, which is sensitive to the low frequency perturbations. In addition, the buoyancy instability can be observed from the comparisons, which is of an absolute unstable nature. Despite the fact that theoretical analyses are derived from the single transport equation for mixture fraction with a number of assumptions, the results are still in good agreement with those obtained from the experiments and numerical simulations.