Stabilization Of Lifted Flames Research Articles

The Role of Flame–flow Interactions on Lean Premixed Lifted Flame Stabilization in a Low Swirl Flow

ABSTRACT Swirling flows have been widely used to stabilize lean premixed combustion in various gas turbines and furnaces. In such flows, understanding and characterizing the flame stabilization are of both practical and fundamental interests. It is known that the swirling motion decreases the flow velocity at the burner outlet, which contributes to flame stabilization. In low swirl flows, such a deceleration stabilizes a premixed flame aerodynamically. The present investigations, using large eddy simulations, study flame–flow interactions in lean premixed lifted flame stabilized in a low swirl flow. The results show that in addition to the swirling motion, combustion heat release reduces axial velocity in the reactant stream by reducing dilatation, vortex stretching, and baroclinic vorticity production terms. The analyses of vorticity production source terms show that besides the flow deceleration induced by the swirling motion, the dominant mechanism for the flow deceleration upstream of the low swirl lifted flame is the baroclinic torque. However, unlike the dilatation term, the effects of the vortex stretching and baroclinic diminish further upstream of the flame front.

Large eddy simulation of a supersonic lifted hydrogen flame with sparse-Lagrangian multiple mapping conditioning approach

The Multiple Mapping Conditioning / Large Eddy Simulation (MMC-LES) approach is used to simulate a supersonic lifted hydrogen jet flame, which features shock-induced autoignition, shock-flame interaction, lifted flame stabilization, and finite-rate chemistry effects. The shocks and expansion waves, shock-reaction interactions and overall flame characteristics are accurately reproduced by the model. Predictions are compared with the detailed experimental data for the mean axial velocity, mean and root-mean-square temperature, species mole fractions, and mixture fraction at various locations. The predicted and experimentally observed flame structures are compared through scatter plots of species mole fractions and temperature against mixture fraction. Unlike most past MMC-LES which has been applied to low-Mach flames, in this supersonic flame case pressure work and viscous heating are included in the stochastic FDF equations. Analysis indicates that the pressure work plays an important role in autoignition induction and flame stabilization, whereas viscous heating is only significant in shear layers (but still negligibly small compared to the pressure work). The evolutions of particle information subject to local gas dynamics are extracted through trajectory analysis on representative fuel and oxidizer particles. The particles intermittently enter the extinction region and may be deviated from the full burning or mixing lines under the effects of shocks, expansion waves and viscous heating. The chemical explosive mode analysis performed on the Lagrangian particles shows that temperature, the H and OH radicals contribute dominantly to CEM respectively in the central fuel jet, fuel-rich and fuel-lean sides. The pronounced particle Damköhler numbers first occur in the fuel jet / coflow shear layer, enhanced at the first shock intersection point and peak around the flame stabilization point.

Attached and lifted flame stabilization in a linear array of swirl injectors

Effective flame anchoring maximizes the low-emissions operating window of gas-turbine systems featuring closely spaced, multiple lean flames on single injector units. The complex flame-flow interactions inherent to such designs are not well understood, in particular, at part-load gas turbine conditions. In the present work, large eddy simulation (LES) was used to investigate flames in an optically accessible, confined, linear array of five industrial swirl injector elements. Well-anchored, partially attached and fully lifted flames were observed and three cases corresponding to these were selected for modeling. The dynamically thickened flame (DTF) model was used with a newly developed reduced chemical kinetics scheme that contains low temperature reaction pathways. Chemical explosive mode analysis (CEMA) shows that the flames exhibit thin ignition zones near their base, while a more distributed region of strongly positive modes occur downstream when a transition to partial blow-off takes place. Extinctions are frequent in this region and create broken reaction zones which seem to exhibit both autoignition as well as flamelet characteristics. Further decrease in reactivity leads to a fully lifted flame. It is clear that part-load conditions are best investigated by models that are combustion regime independent.

Investigation of the Effects of Plasma Discharges on Methane Decomposition for Combustion Enhancement of a Lean Flame

The present work focuses on the impact of dielectric barrier discharge (DBD) plasma actuators (PAs) on non-premixed lifted flame stabilization in a methane CH4-air Bunsen burner. Two coaxial DBD-PA configurations are considered. They are composed of a copper corona, installed on the outer surface of a quartz tube and powered with a high voltage sinusoidal signal, and a grounded needle installed along the burner axis. The two configurations differ in the standoff distance value, which indicates the positioning of the high frequency/high voltage (HV) electrode’s upper edge with respect to the needle tip. Experimental results highlight that flame reattachment is obtained at a lower dissipated power when using a negative standoff distance (i.e., placing the needle upstream with respect to the corona). At 11 kV peak-to-peak voltage and 20 kHz frequency, plasma actuation allowed for reattaching the flame with a very low dissipated power (of about 0.05 W). Numerical simulations of the electrostatic field confirmed that this negative standoff configuration has a beneficial effect on the momentum sources, which oppose the flow and show that the highest electric field extends into the inner quartz tube, as confirmed by experimental visualization close to the needle tip. The modeling predicted an increase in the gas temperature of about 21.8 °C and a slight modification of the fuel composition at the burner exit. This impacts the flame speed with a 10% increase close to the stoichiometric conditions with respect to the clean configuration.

Effect of H<sub>2</sub>O dilution on lifted flame stabilization

Effect of H<sub>2</sub>O dilution on lifted flame stabilization

Specific features of the diffusion flame in the transition from the laminar to turbulent regime of combustion

In studying lifted diffusion flames in the transitional regime from laminar to turbulent combustion, structures with a reduced intensity of own luminescence and a clearly expressed contour are observed in the combustion front. The emergence of such structures in the flame has a spontaneous character and may occur at significant distances from the ignition points (region of lifted flame stabilization).

Experimental investigation of the flame extinction processes of nonpremixed methane flames inside an air coflow diluted with CO2, N2, or Ar

The flame extinction processes and lifted flame stabilization features of nonpremixed flames issuing in diluted coflowing air were systematically examined under a wide range of aerodynamic conditions in order to reveal the competition between aerodynamics and dilution. Four diluents (CO2, N2, Ar, and CO2+Ar) were used in preparation for discriminating between the effects, namely, dilution, thermal action, and chemistry, induced by diluent addition. Flame extinction limits, ignition diagrams, liftoff height, and its associated apparent radius were investigated to provide key elements involved in lifted flame stabilization and extinction mechanisms with a diluted air stream. Flame extinction limits have been expressed as a 3D surface Eextinction in the physical space (Qdiluent/Qair, Uair, UCH4). This surface is composed of two parts, a pure extinction surface where flame extinction is necessarily attained from lifted flames, and a common surface between flame lifting and extinction where flame extinction is achieved directly from attached flames. This distinct behavior is due to the presence of the semithick burner rim that protects the flame base. The flame ignition diagrams confirm that the pure extinction surface is independent of UCH4 for flames initially lifted without dilution. Furthermore, (Qdiluent/Qair)/Kdiluent, where Kdiluent characterizes the capacity of a diluent to act on the flame destabilization process, has proved to be the affine parameter leading to a unified extinction surface for the tested chemically weak diluents. Moreover, by using (Qdiluent/Qair)/Kdiluent, liftoff height HL/HL∘ and flame radius RP/RP∘ reduced by the no-dilution measures merge to unique self-similar curves whatever the diluents and aerodynamic conditions. The key element is the flame leading-edge burning velocity, which was found to be identical for diluted flames when the diluents were added into the air stream in the relative proportions given by Kdiluent. In this way, it is possible to estimate flame behavior based on the results for CO2, once Kdiluent is known.

Simulation of a supersonic hydrogen–air autoignition-stabilized flame using reduced chemistry

A three-step mechanism for H2-air combustion (Boivin et al., Proc. Comb. Inst. 33 (2010)) was recently designed to reproduce both autoignition and flame propagation, essential in lifted flame stabilization. To study the implications of the use of this reduced chemistry in the context of a turbulent flame simulation, this mechanism has been implemented in a compressible explicit code and applied to the simulation of a supersonic lifted co-flowing hydrogen–air flame. Results are compared with experimental measurements (Cheng et al., C&F (1994)) and simulations using detailed chemistry, showing that the reduced chemistry is very accurate. A new explicit diagnostic to readily identify autoignition regions in the post-processing of a turbulent hydrogen flame simulation is also proposed, based on variables introduced in the development of the reduced chemical mechanism.

Petascale direct numerical simulation of turbulent combustion—fundamental insights towards predictive models

The advent of petascale computing applied to direct numerical simulation (DNS) of turbulent combustion has transformed our ability to interrogate fine-grained ‘turbulence-chemistry’ interactions in canonical and laboratory configurations. In particular, three-dimensional DNS, at moderate Reynolds numbers and with complex chemistry, is providing unprecedented levels of detail to isolate and reveal fundamental causal relationships between turbulence, mixing and reaction. This information is leading to new physical insight, providing benchmark data for assessing model assumptions, suggesting new closure hypotheses, and providing interpretation of statistics obtained from lower-dimensional measurements. In this paper the various roles of petascale DNS are illustrated through selected examples related to lifted flame stabilization, premixed and stratified flame propagation in intense turbulence, and extinction and reignition in turbulent non-premixed jet flames. Extending the DNS envelope to higher Reynolds numbers, higher pressures, and greater chemical complexity will require exascale computing in the next decade. The future outlook of DNS in terms of challenges and opportunities in this regard are addressed.

Flame stabilization in a lifted non-premixed turbulent hydrogen jet with coaxial air

To understand hydrogen jet liftoff height, the stabilization mechanism of turbulent lifted jet flames under non-premixed conditions was studied. The objectives were to determine flame stability mechanisms, to analyze flame structure, and to characterize the lifted jet at the flame stabilization point. Hydrogen flow velocity varied from 100 to 300 m/s. Coaxial air velocity was regulated from 12 to 20 m/s. Simultaneous velocity field and reaction zone measurements used, PIV/OH PLIF techniques with Nd:YAG lasers and CCD/ICCD cameras. Liftoff height decreased with increased fuel velocity. The flame stabilized in a lower velocity region next to the faster fuel jet due to the mixing effects of the coaxial air flow. The non-premixed turbulent lifted hydrogen jet flames had two types of flame structure for both thin and thick flame base. Lifted flame stabilization was related to local principal strain rate and turbulent intensity, assuming that combustion occurs where local flow velocity and turbulent flame propagation velocity are balanced.

Improvement of Laminar Lifted Flame Stability Excited by High-Frequency Acoustic Oscillation

A high-frequency (20kHz) standing wave was applied to the unburned mixture upstream of a methane-air lifted jet flame using a bolt-clamped Langevin transducer (BLT) to improve stability. The flow field near the flame was visualized using acetone planar-laser-induced fluorescence (PLIF). The standing wave decreased the lifted flame height and increased the blow-off limit. The upstream flow field of the center jet then bent. This phenomenon appeared when there was a density difference between the center jet and the surrounding secondary flow. When the density of the center jet was less than that of the co-flow, the center jet was redirected to the pressure anti-node side. Conversely, when the density of the center jet was greater than that of the co-flow, the center jet was redirected to the pressure node side. This redirection tended to stabilize the laminar lifted flame.

Stability of a Turbulent Jet Methane Flame Issuing from an Asymmetrical Nozzles with Sudden Expansion

The effect of quarl (i.e., a sudden expansion) on the stability of turbulent diffusion flame of methane jet issuing from asymmetric nozzles and discharging into still air environment is examined experimentally. The study covers the flame liftoff height and velocity, as well as the reattachment and blowout velocities. Five nozzles with different internal geometries but with similar equivalent exit diameter are tested. The results reveal that the jet flame liftoff height is reduced further when quarl is attached to the exit of the nozzle. Moreover, the stability range of the lifted flame is found to expand when quarl is attached to the nozzle. Quarl leads to an increased flame blowout velocity, which is the upper stability limit, and decreasing the liftoff velocity, which is the lower lifted flame stability limit. This might be of great interest for industrial applications in which a wide stability range of the lifted flame is desired.

Influence of a Horizontal Magnetic Field on a Co-Flow Methane/Air Diffusion Flame

Influence of magnetic fields on the liftoff and blow out properties of the methane/air co-flow diffusion flame has been investigated experimentally. The present experiments showed that magnetic gradients are able to minimize the liftoff height and increase the flow rate above which the flame blows out. This observed increase of stability of lifted flames is attributed to the magnetic force that develops on paramagnetic oxygen in magnetic gradients. Two mechanisms are provided to explain the extension of the lifted flame stability. By decreasing locally the air velocity upstream of the flame, the magnetic force, resulting from an upward decreasing magnetic field, modifies the fuel mixture fraction at the flame edge, reducing the liftoff height of the flame. The second mechanism is related to the variation of the magnetic susceptibility with temperature and oxygen mass fraction in which originates a magnetic convection phenomenon in air in the flame vicinity.

Non-Premixed Lifted Flame Stabilization Coupled with Vortex Structures in a Coaxial Jet

An experiment concerning the non-premixed lifted flame stabilization studies the coupling between the flame behavior and the vortices developed in the inner mixing layer of a coaxial air/methane jet. It completes a previous global approach in which we show that “macro parameters” (potential cores, inner jet spreading) were efficient to describe the overall lifted flame behavior. Here, the jet dynamics is locally determined in vertical and transverse planes by means of particle image velocimetry and time resolved laser tomography. A systematic identification of vortices, obtained from PIV fields, is developed from a criterion proposed by Graftieaux et al. The analysis relies on the quantification of the Kelvin-Helmholtz vortices and the secondary streamwise vortices. Local parameters characterizing the vortices are quantified as functions of the air flow rate velocity, Uo. They contribute to formulate a scenario explaining how a small change of Uo produces a great change in the flame behavior.

Improvement of Laminar Lifted Flame Stability Excited by High Frequency Acoustic Oscillation

A high-frequency (20kHz) standing wave was applied to the unburned mixture upstream of a methane-air lifted jet flame using a bolt-clamped Langevin transducer (BLT) to improve stability. The flow field near the flame was visualized using acetone planar-laser-induced fluorescence (PLIF). The standing wave decreased the lifted flame height and increased the blow-off limit. The upstream flow field of the center jet then bent. This phenomenon appeared when there was a density difference between the center jet and the surrounding secondary flow. When the density of the center jet was less than that of the co-flow, the center jet was redirected to the pressure anti-node side. Conversely, when the density of the center jet was greater than that of the co-flow, the center jet was redirected to the pressure node side. This redirection tended to stabilize the laminar lifted flame.

On scalar dissipation and partially premixed flame propagation

Measurements of the scalar dissipation rate in the region immediately upstream of a lifted jet flame are presented. The scalar dissipation is determined in this isothermal region from a planar measurement of a two-dimensional conserved scalar (jet fluid) using laser Rayleigh scattering. Fields of the scalar dissipation rate are presented in addition to tabulated values for three different liftoff heights ( Re d =4800, 6400, and 8300). Scalar dissipation rates do not reach levels thought to cause extinction of the leading edge based on comparison with extinction data for counterflow diffusion flames. Additionally, results are presented on the axial flame propagation velocities relative to the jet flow. The data indicate that over the three flow conditions, the flame velocity relative to the flow is approximately constant during the case of a quasi-stationary lifted flame. In light of these findings, it is suggested that concepts involving partially premixed flame propagation, rather than those of critical scalar dissipation rate, are central to modern lifted flame stabilization models.

Lifted flame stabilization in developing and developed regions of coflow jets for highly diluted propane

Stabilization of lifted flames has been investigated experimentally for highly diluted propane with nitrogen in coflow. For various fuel mole fractions and jet velocities, three distinct flame types are observed: nozzle attached flames, stationary lifted flames, and oscillating lifted flames. When fuel jet velocity is much smaller than coflow velocity, the base of a nozzle attached flame is observed to have a tribrachial structure. Based on the balance mechanism of the propagation speed of a tribrachial flame with flow velocity, jet velocity is scaled with stoichiometric laminar burning velocity. Results show two distinctive lifted flame stabilization modes—stabilization in the developing region and stabilization in the developed region of jets—depending on the initial fuel mole fraction. It has been found that a lifted flame can be stabilized for a fuel velocity even smaller than the stoichiometric laminar burning velocity. This behavior has been attributed to the buoyancy effect, and flow visualization supports it. Oscillating lifted flames are observed with the frequency range of 3–4.5 Hz by the repetitive action of buoyancy due to increased and decreased burning rates during falling and rising periods of flame oscillation.

Observations on the leading edge in lifted flame stabilization

Observations on the leading edge in lifted flame stabilization

Studies on Lifted Jet Flames in Coflow: The Stabilization Mechanism in the Near- and Far-Fields

This paper presents the results of a parametric study concerning the phenomenon of liftoff of a nonpremixed jet flame. The dependence of liftoff height on jet exit velocity and coflow velocity is described. It is shown that lifted flames become less sensitive to jet exit velocity as the stabilization point recedes from the burner exit. The results reveal that in cases of extreme liftoff height, increases in jet exit velocity with a constant coflow cause some ethylene flames to stabilize closer to the burner. The success of current theories on lifted flame stabilization in comparison to the experimental results of this study are assessed. The existence of multiple regimes for flame stabilization, incorporating aspects of both premixed and nonpremixed combustion, is proposed.

Measurements of the stabilization zone of a lifted jet flame under acoustic excitation

The turbulence and concentration characteristics in the stabilization zone of a lifted jet flame with and without acoustic excitation are measured by a time-resolved Rayleigh scattering, a LDV, and a hot vire anemometry system together with other probes. Both amplification and suppression of the flow can be achieved by acoustic excitation. By careful comparison of the turbulence and concentration characteristics in the stabilization zone for the natural, amplification and suppression cases, it is found that the key parameters of lifted flame stabilization in the stabilization zone are the integral length scale, the F probability of the presence of a flammable premixture, and the G probability of the presence of a fluid with a temperature reaching the ignition temperature. Amplification excitation enhances the large-scale coherent vortices and the vortical entrainment, thus enhancing the length scale, theF probability, and the G probability of having a combustibltoe premixture in the stabilization zone. In this case, the flame shifts upstream to a higher gas speed location and restabilizes there. Suppression excitation shows the opposite results. Practical need calls for a new model capable of predicting the stabilization zone structure of excited lifted flames where the large length scale, theF andG probabilities are suggested to be important parameters.