Lifted Jet Flame Research Articles

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.

Flame Structure of a Jet Flame with Penetration of Side Micro-jets

In this paper, an innovative jet lifted flame with side micro-jets has been proposed and its effects on the flame structure have also been investigated. Due to the changes of the initial combustion conditions, mixing and aerodynamics which resulted from the perturbation of the side micro-jets, such a lifted jet flame has different flame structure compared with the common premixed flame. Results demonstrate that use of the micro-jets can control, to a certain extent, the flame structure, including the flame length, lift-off distance and blow-off limit. With the same fuel and air flow rate, the flame length with the side micro-jets will decrease about 5%-40% as the air volume ratio α increases from 58%-76%. Compared with the common diffusion flame, the jet flame with the side micro-jets demonstrates to be easier to be a momentum-dominated flame. The flame length with 2 micro-jets is about 5% less than with 6 micro-jets under the same fuel and air flow rate. With the same α, the fewer number of the controlled jets lead to the flame with relatively shorter length, not easier to be blown off and higher NO x emission. With certain fuel flow rate, the critical air volume ratio is largest for the flame with 3 micro-jets, which is more difficult to be blown off than the cases with 2, 4 or 6 micro-jets.

Simultaneous three-component PIV/OH-PLIF measurements of a turbulent lifted, C3H8-Argon jet diffusion flame at 1.5 kHz repetition rate

Planar laser-induced fluorescence (PLIF) and stereoscopic particle image velocimetry (PIV) were simultaneously applied at 1.5kHz repetition-rate to acquire time-resolved, cinematographic planar measurements of the flamebase stabilization region of a turbulent lifted jet flame. The current work focuses on the development and demonstration of the diagnostic system and presents key examples of the new capabilities it provides. OH-PLIF is used to identify and track the spatial position and shape of the flamefront near the flamebase, while the stereoscopic PIV is used to acquire three-component planar velocity measurements over the same region. Initial results indicate that the significant temporal histories and experimental access to the Vz-velocity component afforded by this system are frequently essential for accurate interpretation of planar measurements in the flow field of a lifted turbulent jet flame.

동축공기 수소 난류확산화염에서의 화염안정성에 대한 실험적 연구

It was experimentally studied that the stabilization mechanism of turbulent, lifted jet flames in a non-premixed condition to reveal the newly found liftoff height behavior of hydrogen jet. The objectives are to report the phenomenon of a liftoff height decreasing as increasing fuel velocity, to analyse the flame structure and behavior of the lifted jet, and to explain the mechanisms of flame stability in hydrogen turbulent non-premixed jet flames. The hydrogen jet velocity was changed from 100 to 300m/s and a coaxial air velocity was fixed at 16m/s with a coflow air less than 0.1m/s. For the simultaneous measurement of velocity field and reaction zone, PIV and OH PLIF technique was used with two Nd:Yag lasers and CCD cameras. As a result, it was found that the stabilization of lifted hydrogen diffusion flames is correlated with a turbulent intensity and Karlovitz number.

Level‐set based flamelet approach for simulating turbulent lifted jet flame

AbstractThe present study focuses on numerically investigating the flame structure, flame liftoff, and stabilization in a lifted turbulent H2/N2 jet flame with a vitiated coflow. To realistically represent the turbulent partially premixed nature in the flow region between nozzle exit and flame base, the level‐set approach coupled with the conserved scalar flamelet model has been applied. The unstructured‐grid level‐set approach has been developed to allow the geometric flexibility and computational efficiency for the solution of the physically and geometrically complex reacting flows. The pressure–velocity coupling is handled by the multiple pressure‐correction method. The predicted flame pattern is in good conformity with the measured one. In terms of the liftoff height, the agreement between prediction and experiment is quite good. Even if there are noticeable deviations in a certain region, the predicted profiles for the overall flame structure agree reasonably well with the experimental data. These numerical results indicate that the present level‐set‐based flamelet approach in conjunction with the unstructured‐grid finite‐volume method is capable of realistically predicting the essential features and precise structure of the turbulent‐lifted jet flame with computational efficiency. Copyright © 2008 John Wiley & Sons, Ltd.

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.

Transport budgets in turbulent lifted flames of methane autoigniting in a vitiated co-flow

Autoignition of hydrocarbon fuels is an outstanding research problem of significant practical relevance in engines and gas turbine applications. This paper presents a numerical study of the autoignition of methane, the simplest in the hydrocarbon family. The model burner used here produces a simple, yet representative lifted jet flame issuing in a vitiated surrounding. The calculations employ a composition probability density function (PDF) approach coupled to the commercial CFD package, FLUENT. The in situ adaptive tabulation (ISAT) method is used to implement detailed chemical kinetics. An analysis of species concentrations and transport budgets of convection, turbulent diffusion, and chemical reaction terms is performed with respect to selected species at the base of the lifted turbulent flames. This analysis provides a clearer understanding of the mechanism and the dominant species that control autoignition. Calculations are also performed for test cases that clearly distinguish autoignition from premixed flame propagation, as these are the two most plausible mechanisms for flame stabilization for the turbulent lifted flames under investigation. It is revealed that a radical pool of precursors containing minor species such as CH 3, CH 2O, C 2H 2, C 2H 4, C 2H 6, HO 2, and H 2O 2 builds up prior to autoignition. The transport budgets show a clear convective–reactive balance when autoignition occurs. This is in contrast to the reactive–diffusive balance that occurs in the reaction zone of premixed flames. The buildup of a pool of radical species and the convective–reactive balance of their transport budgets are deemed to be good indicators of the occurrence of autoignition.

Direct numerical simulation of hydrogen turbulent lifted jet flame in a vitiated coflow

The direct numerical simulation (DNS) method with 16 steps detailed chemical kinetics was applied to a lifted turbulent jet flame with H2/N2 fuel issuing into a wide hot coflow of lean combustion products, at temperature of 1045 K and low oxygen concentrations. The chemical reactions were handled by the library function of CHEMKIN which was called by the main program in every time step. Parallel computational technology based on message passing interface method (MPI) was used in the simulation. All the cases were run by 12 CPUs on a high performance computer system. Faver-averaged DNS results were obtained by long time averaging the transient profile and compared with the experimental data. The roll-up and evolution of the vortices in jet flame were well captured. The vortices in the same rotating direction attracted each other and those in different rotating directions repulsed each other. Through complex interactions between vortices, the original symmetrical vortex structure could be converted into nonsymmetrical and more complex structures by combination, distortion and splitting of the vortices. The transient profiles of H, OH and H2O mass fraction at 5.76 ms showed the flame structure in jet flame, especially the autoignition regions clearly. The lift-off height was about 9 d–11 d, in agreement with the experimental observation. At the corner point of the flame sheet indicated by OH and H profiles, the combustion was always enhanced by the flame curvature and extended resident time. The profiles of turbulence intensities show that the flames were diffused from the original two outside flame sheets into the core. The DNS results can be considered in developing more accurate and more universal turbulence models.

Observations on Upstream Flame Propagation in the Ignition of Hydrocarbon Jets

A variety of investigators have attempted to characterize the mechanisms of how reaction zones stabilize, or propagate, against incoming reactants, particularly in stable lifted jet flames both laminar and turbulent. In this paper, experiments are described that investigate the characteristics of upstream flame propagation in turbulent hydrocarbon jet flames. An axisymmetric, gaseous turbulent jet mixing in air has been selectively ignited at downstream positions to assess the upstream propagation of the bulk reaction zone. The farthest axial position that permitted the reaction zone to propagate upstream after application of the ignition source, referred to as the "upper propagation limit", or UPL, is determined for a variety of jet and air co-flow parameters. There is an inverse relationship between the upper propagation limit position and the jet Reynolds number. Conversely, there is a direct relationship between the upper propagation limit and the co-flow velocity. Interpretation of the results is related to the velocity at the stoichiometric surface. Global discussion is made as to what these results imply about the stabilization and propagation of turbulent lifted jet flames.

Plasma-Discharge Stabilization of Jet Diffusion Flames

The authors examine three different types of plasma discharges in their ability to stabilize a lifted jet diffusion flame in coflow. The three discharges include a single-electrode corona discharge, an asymmetric dielectric-barrier discharge (DBD), and a repetitive ultrashort-pulsed discharge. The degree of nonequilibrium of this pulsed discharge is found to be higher than that for the DBD. Furthermore, this pulsed discharge causes the most significant improvement in the flame stability. The optimal placement of the discharge electrodes is investigated, and it is found that there is a close relation between this placement and the emission spectra, suggesting use of the emission spectra as a possible indicator of fuel/air mixture fraction. The optimal placement is mapped into mixture-fraction space by use of a fully premixed flame experiment of known mixture fraction. The result shows that the mixture fraction, which corresponds to the optimal placement, is much leaner than that of a conventional lifted jet flame

Stabilization of turbulent lifted jet flames assisted by pulsed high voltage discharge

To reduce fuel consumption or the pollutant emissions of combustion (furnaces, aircraft engines, turbo-reactors, etc.), attempts are made to obtain lean mixture combustion regimes. These lead to poor stability of the flame. Thus, it is particularly interesting to find new systems providing more flexibility in aiding flame stabilization than the usual processes (bluff-body, stabilizer, quarl, swirl, etc.). The objective is to enlarge the stability domain of flames while offering flexibility at a low energy cost. Evidence is presented that the stabilization of a turbulent partially premixed flame of more than 10 kW can be enhanced by pulsed high-voltage discharges with power consumption less than 0.1% of the power of the flame. The originality of this work is to demonstrate that very effective stabilization of turbulent flames is obtained when high-voltage pulses with very short rise times are used (a decrease by 300% in terms of liftoff height for a given exit jet velocity can be reached) and to provide measurements of minimum liftoff height obtained with discharge over a large range of the stability domain of the lifted jet flame.

Lifted methane–air jet flames in a vitiated coflow

The present vitiated coflow flame consists of a lifted jet flame formed by a fuel jet issuing from a central nozzle into a large coaxial flow of hot combustion products from a lean premixed H 2/air flame. The fuel stream consists of CH 4 mixed with air. Detailed multiscalar point measurements from combined Raman–Rayleigh–LIF experiments are obtained for a single base-case condition. The experimental data are presented and then compared to numerical results from probability density function (PDF) calculations incorporating various mixing models. The experimental results reveal broadened bimodal distributions of reactive scalars when the probe volume is in the flame stabilization region. The bimodal distribution is attributed to fluctuation of the instantaneous lifted flame position relative to the probe volume. The PDF calculation using the modified Curl mixing model predicts well several but not all features of the instantaneous temperature and composition distributions, time-averaged scalar profiles, and conditional statistics from the multiscalar experiments. A complementary series of parametric experiments is used to determine the sensitivity of flame liftoff height to jet velocity, coflow velocity, and coflow temperature. The liftoff height is found to be approximately linearly related to each parameter within the ranges tested, and it is most sensitive to coflow temperature. The PDF model predictions for the corresponding conditions show that the sensitivity of flame liftoff height to jet velocity and coflow temperature is reasonably captured, while the sensitivity to coflow velocity is underpredicted.

Determination of schmidt number of mixed fuels by the characteristics of laminar lifted jet flames

A method to determine the Schmidt number of fuel is proposed from the behavior of laminar lifted jet flames. Based on the observation of a laminar lifted flame edge, the flame stabilization point is located along the stoichiometric contour in the mixing layer of fuel and air in laminar jets, since a tribrachial (triple) flame structure exists which is composed of a diffusion flame, a rich premixed flame and a lean premixed flame, all extending from a single location. For the flame edge to be stationary, the axial velocity at the edge should balance with the propagation speed of the tribrachial flame. Since the region between the flame stabilization point and the nozzle exit can be treated as a cold jet, the jet theory of momentum and species can be applied to obtain the correlation of liftoff height with jet velocity and nozzle diameter of H L ∝ d 2 u o ( 2 S c F − 1 ) / ( S c F − 1 ) . Using this relation, the mixture of fuels having Sc<1 and Sc>1 are tested. The dependence of liftoff height on jet velocity is curve-fitted to extract the effective Schmidt number of mixed fuels. Experimentally determined Schmidt numbers agree satisfactorily with the theoretical predictions from the kinetic theory.

Simulations of turbulent lifted jet flames with two-dimensional conditional moment closure

Simulations of H 2 air lifted jet flames are presented, obtained in terms of two-dimensional, first-order conditional moment closure (CMC). The unsteady CMC equation with detailed chemistry is solved without the need for operator splitting, while the accompanying flow field is determined using commercial CFD software employing a k − ε turbulence model. Computed lift-off heights and Favre-averaged species mole fractions are found to be very close to values obtained experimentally for a wide range of jet velocities and fuel–air mixtures. Simulations for which the initial condition is an attached flame and the jet velocity gradually increased do not result in lift-off, a result fully consistent with experimental observation and capturing the hysteresis behaviour of lifted flames. The stabilisation mechanism is explored by quantifying the balance of terms comprising the CMC in the lift-off region. In line with experimental data, it is found that the scalar dissipation rate at the stabilisation height is well below the extinction value, and that axial transport and molecular diffusion play a major role. The radial components of spatial convection and diffusion are always small, fully justifying the alternative approach of employing a cross-stream averaged CMC.

Numerical modeling of turbulent nonpremixed lifted flames

The present study has focused on numerical investigation on the flame structure, flame lift-off and stabilization in the partially premixed turbulent lifted jet flames. Since the lifted jet flames have the partially premixed nature in the flow region between nozzle exit and flame base, level set approach is applied to simulate the partially premixed turbulent lifted jet flames for various fuel jet velocities and co-flow velocities. The flame stabilization mechanism and the flame structure near flame base are presented in detail. The predicted lift-off heights are compared with the measured ones.

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.

Simultaneous two-shot CH planar laser-induced fluorescence and particle image velocimetry measurements in lifted CH 4/air diffusion flames

Joint two-shot CH planar laser-induced fluorescence (PLIF) and particle image velocimetry (PIV) measurements near the stabilization region of lifted methane/air diffusion flames stabilized under different flow conditions are presented. The simultaneous technique allows for a determination of the propagation rate of the CH zone relative to the fuel flow. Simultaneous single-shot CH-PLIF and PIV techniques have been used in the past to examine lifted jet flames: however, the double-shot technique of the current study is desirable because it yields information on flame dynamics—as indicated by sequential CH-PLIF—relative to the unburned mixture. Three flow conditions were examined corresponding to three different liftoff heights. While the average velocity at the stabilization point varies between 0.83 m/s for the lowest flow condition (Red=4800) and 1.28 m/s for the highest (Red=8300), the velocity at the stabilization point during instances of zero CH movement (during the time interval of the CH pulses) is constant for all three flow conditions (1.14±0.4 m/s). Furthermore, the flame is able to stabilize itself against the incoming unburned mixture only when the gas velocity is below a certain limit, above which the flame is convected downstream with the flow.

The role of secondary instabilities in the stabilization of a nonpremixed lifted jet flame

A nonpremixed lifted jet flame is studied dynamically in the hysteresis zone. High-speed laser tomography images show clearly that, in the case of an organized jet, the flame is located on streamwise counter-rotating vortex filaments generated to secondary instabilities and ejected towards ambient air. Particle image velocimetry is used to evaluate the amplitude of the translational and rotational velocity of these filaments. The use of an acoustic field to force jet instabilities shows that the flame, following large filament ejections, moves back upstream very close to the nozzle without anchoring at it. The role of streamwise vortices in the stabilization mechanism of the lifted flame is confirmed by measurements obtained with a disordered jet, from a straight tube burner. From these results, it is proposed that secondary vortices at the flame base are sufficiently strong to create a premixed zone and to oppose flame propagation.

K-1502 レーザー診断による乱流浮上がり拡散火炎の基部構造の解明(S21-1 燃焼の光学計測(熱工学))(S21 燃焼の光学計測)

Flame base structures of lifted diffusion flames have been investigated experimentally by using a simultaneous Rayleigh and Raman scattering method. The rations of signal to noise are estimated approximately 50 for the Rayleigh scattering method and 15 for the Raman scattering one, respectively. Experiments have been conducted in the configuration of a coaxial burner consisting of an inner tube of 3.2 ID and an outer tube of 56 ID. As a result, it is found that flames seem to limit the turbulent mixing and moreover at the flame base of lifted flames the nitrogen concentration increases near the center region of the jet, due to the turbulent mixing in the unburned region at the flame base, although decreasing at the flame region.

Visualization of multiple scalar and velocity fields in a lifted jet flame

The stabilization of lifted jet diffusion flames has long been a topic of interest to combustion researchers. The flame and flow morphology, the role of partial premixing, and the effects of large scale structures on the flame can be visualized through advanced optical imaging techniques. Many of the current explanations for flame stabilization can benefit from the flow and flame information provided by laser diagnostics. Additionally, the images acquired from laser diagnostic experiments reveal features invisible to the eye and line-of-sight techniques, thereby allowing a deeper insight into flame stabilization. This paper reports visualizations of flame and flow structures from Particle Image Velocimetry (PIV), Planar Laser-Induced Fluorescence (PLIF) and Rayleigh scattering. The techniques are surveyed and the success of visualization techniques in clarifying and furthering the understanding of lifted-jet flame stabilization is discussed.