Liftoff of a Co-Flowing Non-Premixed Turbulent Methane Flame: Effect of the Fuel Nozzle Orifice Geometry

This paper reports an experimental study on the effect of the fuel nozzle orifice geometry on the stability of turbulent non-premixed methane flame. Different internal geometries (orifice equivalent diameter, length to diameter ratio and contraction angle) of a circular and a rectangular nozzle with an aspect ratio of 2 were examined. The strength of the co-airflow was also varied to assess its impact on the jet flame stability. The experimental data revealed that the level of turbulence in the jet near-field is, in general, higher for the rectangular nozzle in comparison with the circular nozzle. This high level of turbulence was found to accelerate the liftoff transition of the attached flame. The results revealed also that there is a clear interplay between the flame liftoff height and the jet flow characteristics. That is, a rectangular jet, which spreads faster along the minor axis and generates higher near-field turbulence, results in a flame base sitting closer to the nozzle exit in comparison with that of its circular nozzle counterpart. Finally, the presence of a moderate co-airflow resulted in a higher flame liftoff velocity and height. It also led to the appearance of a hysteresis phenomenon in the low jet velocity range regardless of the exit orifice shape of the fuel nozzle.     

Leading-Edge Reaction Zones in Lifted-Jet Gas and Spray Flames

An investigation of the leading edge characteristics in lifted turbulent methane-air (gaseous) and ethanol-air (spray) diffusion flames is presented. Both combustion systems consist of a central nonpremixed fuel jet surrounded by low-speed air co-flow. Non-intrusive laser-based diagnostic techniques have been applied to each system to provide information regarding the behavior of the combustion structures and turbulent flow field in the regions of flame stabilization. Simultaneous sequential CH-PLIF/particle image velocimetry and CH-PLIF/Rayleigh scattering measurements are presented for the lifted gaseous flame. The CH-PLIF data for the lifted gas flame reveals the role that ``leading-edge' combustion plays as the stabilization mechanism in gaseous diffusion flames. This phenomenon, characterized by a fuel-lean premixed flame branch protruding radially outward at the flame base, permits partially premixed flame propagation against the incoming flow field. In contrast, the leading edge of the ethanol spray flame, examined using single-shot OH-PLIF imaging and smoke-based flow visualization, does not exhibit the same variety of leading-edge combustion structure, but instead develops a dual reaction zone structure as the liftoff height increases. This dual structure is a result of the partial evaporation (hence partial premixing) of the polydisperse spray and the enhanced rate of air entrainment with increased liftoff height (due to co-flow). The flame stabilizes in a region of the spray, near the edge, occupied by small fuel droplets and characterized by intense mixing due to the presence of turbulent structures. This revised version was published online in July 2006 with corrections to the Cover Date.     

The stabilization mechanism of the lifted jet diffusion flame in the hysteresis region

Recent experimental efforts focused on near-field coherent vortex dynamics, and their impact on stabilization of a lifted jet diffusion flame in the hysteresis region are reported. Simultaneous jet flow and flame visualizations are conducted first to obtain a global feature of flow/flame interaction. The statistical liftoff heights are calculated by a DIP (digital image processing) method. The gas concentration and velocity distributions induced by the vortex evolution as well as the corresponding flame front motion are deduced from phase-averaged measurements of planar Mie-scattering gas concentration images, LDV and ion-signals, respectively. The planar gas concentration technique employed here extends our previous work (Chao et al. 1990, 1991 a) to include phase-averaging. Results of the experiments show that the most probable flame base locations in the hysteresis region are at the coherent vortex roll-up and pairing locations. The deeply entrained air lump caused by large-scale vortices during roll-up and pairing is the main obstruction to flame propagation back to the nozzle exit and causes the hysteresis phenomenon.     

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

The 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.     

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.     

Noise Reduction in Non-Premixed Lifted-Jet Flames

The characteristic changes in non-premixed lifted flames when excited by hole tones from a cavity, placed in the flow path of the fuel gas, were studied. A significant reduction of the sound pressure level was observed in the low-frequency noise at the flame base of the lifted flame when the hole tones were induced in the jet. The liftoff height and the mean diameter of the flame base decreased for a given jet Reynolds number. The blow-off velocities also increased suggesting improved flame stability in the presence of the hole tones induced by the cavity. Incorporation of the cavity upstream of a burner nozzle is demonstrated to give a quieter lifted flame with improved stability characteristics. This revised version was published online in July 2006 with corrections to the Cover Date.     

An Experimental Investigation of the Turbulence Structure of a Lifted H2/N2 Jet Flame in a Vitiated Co-Flow

Measurements of mean velocity components, turbulent intensity, and Reynolds shear stress are presented in a turbulent lifted H₂/N₂ jet flame as well as non-reacting air jet issuing into a vitiated co-flow by laser doppler velocimetry (LDV) technique. The objectives of this paper are to obtain a velocity data base missing in the previous experiment data of the Dibble burner and so provide initial and flow field data for evaluating the validity of various numerical codes describing the turbulent partially premixed flames on this burner. It is found that the potential core is shortened due to the high ratio of jet density to co-flow density in the non-reacting cases. However, the existence of flame suppressed turbulence in the upstream region of the jet dominates the length of potential core in the reacting cases. At the centreline, the normalized axial velocities in the reacting cases are higher than the non-reacting cases, and the relative turbulent intensities of the reacting flow are smaller than in the non-reacting flow, where a self-preserving behaviour for the relative turbulent intensities exists at the downstream region. The profiles of mean axial velocity in the lifted flame distribute between the non-reacting jet and non-premixed flame both in the axial and radial distributions. The radial distributions of turbulent kinetic energy in the lifted flames exhibit a change in distributions indicating the difference of stabilisation mechanisms of the two lifted flame. The experimental results presented will guide the development of an improved modelling for such flames.     

Active control of lifted diffusion flames with arrayed micro actuators

Active control of a lifted flame issued from a coaxial nozzle is investigated. Arrayed micro flap actuators are employed to introduce disturbances locally into the initial shear layer. Shedding of large-scale vortex rings is modified with the flap motion, and the flame characteristics such as liftoff height, blowoff limit, and emission trend, are successfully manipulated. Spatio-temporal evolution of large-scale vortical structures and fuel concentration is examined with the aid of PIV and PLIF in order to elucidate the control mechanisms. It is found that, depending on the driving signal of the flaps, the near-field vortical structures are significantly modified and two types of lifted flames having different stabilization mechanisms are realized.     

Lifted Diffusion Flame Stabilisation: Conditional Analysis of Multi-Parameter High-Repetition Rate Diagnostics at the Flame Base

Data from simultaneous 5?kHz OH-PLIF and Stereo-PIV at the stabilisation region of a propane/ argon lifted diffusion jet flame are presented for jet-exit Reynolds numbers of 10,000 and 15,000. The time history leading to the upstream appearance of flame islands is investigated for both flames. These flame islands are found to be preceded, on average, by a increased out-of-plane fluid velocity. Conditioning local flame statistics on the instantaneous flame base, as indicated by the OH image, permits analysis of upstream and downstream flame motions (in laboratory co-ordinates). The relative velocity is investigated by conditioning out the data with significant out-of-plane fluid velocity. This has introduced greater accuracy over previous attempts at estimating this quantity. No evidence is found for a correlation between increased turbulence intensity or the passage of large scale eddies with increased flame propagation speeds. Furthermore, divergence at the flame base is not found to correlate with upstream flame motion (as a combination of propagation and convection). The volume of the data investigated has led to the development of robust statistics for all quantities presented here.     

PIV Measurements in the Near and Intermediate Field Regions of Jets Issuing from Eight Different Nozzle Geometries

An experimental study was conducted to investigate the effect of nozzle geometry on the mixing characteristics and turbulent transport phenomena in turbulent jets. The nozzle geometry examined were round, square, cross, eight-corner star, six-lobe daisy, equilateral triangle as well as ellipse and rectangle each with aspect ratio of 2. The jets were produced from sharp linear contoured nozzles which may be considered intermediate to the more widely studied smooth contraction and orifice nozzles. A high resolution particle image velocimetry was used to conduct detailed velocity measurements in the near and intermediate regions. It was observed that the lengths of the potential cores and the growth rates of turbulence intensities on the jet centerline are comparable with those of the orifice jets. The results indicate that the decay and spreading rates are lower than reported for orifice jets but higher than results for smooth contoured jets. The jets issuing from the elliptic and rectangular nozzles have the best mixing performance while the least effective mixing was observed in the star jet. The distributions of the Reynolds stresses and turbulent diffusion clearly showed that turbulent transport phenomena are quite sensitive to nozzle geometry. Due to the specific shape of triangular and daisy jets, the profiles of mean velocity and turbulent quantities are close to each other in their minor and major planes while in the elliptic and rectangular jets are considerably different. They also exhibit more isotropic behavior compared to the elliptic and rectangular jets. In spite of significant effects of nozzle geometry on mean velocity and turbulent quantities, the integral length scales are independent of changes in nozzle geometry.     

Simultaneous OH and Schlieren visualization of premixed flames at the lean blow-out limit

The structure of an air-propane premixed flame was studied experimentally at the lean flammability limit, using Schlieren photography synchronized with OH-imaging done with the Planar Laser Induced Fluorescence (PLIF) technique. The flame was studied in a wide range of fuel equivalence ratios. Various steps in the process of the flame destabilization were investigated, including partial lift-off, stable lift-off, and final blow-out conditions. The flame structure was visualized for each stage showing the transition from a flame held at the nozzle to a flame held by the flow structures. In order to study the latter conditions in more detail the flame was acoustically excited at the preferred mode frequency generating large, stable, coherent structures in the core region. The modified flame structure was visualized to understand the interaction between the flame and vortical flow dynamics.It is shown that for the flow conditions when the flame cannot be stabilized at the nozzle, a new anchoring point is reached at the location of the initial vortex roll-up in the jet shear layer. At this point the flow reversal and transition to turbulence produce stagnation points with relatively low local velocities and velocity gradients where the flame can be stabilized. When the flame jet is being forced at the jet most unstable frequency, large coherent structures are formed and the flame is stabilized intermittently on these vortices.     

Flame liftoff height dependence on geometrically modified bluffbodies in a vitiated flow

In this study, the improvement of liftoff height of bluffbody-stabilized, partially premixed methane flames and the change of flow field in the recirculation zone of bluffbodies, of variously modified base geometries, are investigated in a high temperature (~1,315 K) vitiated flow. The basic geometry of the bluffbody consists of a two-dimensional rectangular body with a rounded nose with fuel jets being discharged from the body at several locations upstream of the base. Flame liftoff height measurements are characterized by CH chemiluminescence, while the three-dimensional flow field is determined using stereo particle image velocimetry (PIV). The lowest liftoff height is observed when the geometric modifications from the original rectangular bluffbody base are carried out such that the base has three-dimensional local cavities together with two-dimensionally modified geometries. PIV measurements show that the improvement of liftoff height is primarily attributed to an intense recirculation induced by multi-dimensional vortex structures in the presence of the two- and three-dimensionally modified base.     

Local curvature measurements of a lean,partially premixed swirl-stabilised flame

A swirl-stabilised, lean, partially premixed combustor operating at atmospheric conditions has been used to investigate the local curvature distributions in lifted, stable and thermoacoustically oscillating CH₄-air partially premixed flames for bulk cold-flow Reynolds numbers of 15,000 and 23,000. Single-shot OH planar laser-induced fluorescence has been used to capture instantaneous images of these three different flame types. Use of binary thresholding to identify the reactant and product regions in the OH planar laser-induced fluorescence images, in order to extract accurate flame-front locations, is shown to be unsatisfactory for the examined flames. The Canny-Deriche edge detection filter has also been examined and is seen to still leave an unacceptable quantity of artificial flame-fronts. A novel approach has been developed for image analysis where a combination of a non-linear diffusion filter, Sobel gradient and threshold-based curve elimination routines have been used to extract traces of the flame-front to obtain local curvature distributions. A visual comparison of the effectiveness of flame-front identification is made between the novel approach, the threshold binarisation filter and the Canny-Deriche filter. The novel approach appears to most accurately identify the flame-fronts. Example histograms of the curvature for six flame conditions and of the total image area are presented and are found to have a broader range of local flame curvatures for increasing bulk Reynolds numbers. Significantly positive values of mean curvature and marginally positive values of skewness of the histogram have been measured for one lifted flame case, but this is generally accounted for by the effect of flame brush curvature. The mean local flame-front curvature reduces with increasing axial distance from the burner exit plane for all flame types. These changes are more pronounced in the lifted flames but are marginal for the thermoacoustically oscillating flames. It is concluded that additional fuel mixture fraction and velocimetry studies are required to examine whether processes such as the degree of partial-premixedness close to the burner exit plane, the velocity field and the turbulence field have a strong correlation with the curvature characteristics of the investigated flames.     

Investigation of Lifted Flame Propagation Under Pulsing Conditions Using High-Speed OH-LIF and LES

Flame propagation in a lifted flame subjected to a transient velocity pulse is investigated using high-speed OH-LIF and Large Eddy Simulation (LES). The design of the burner, taking the requirements of the simulations into consideration, comprises an attached and lifted CNG jet flame in a mild air co-flow, forced to transition by a controlled mass flow pulse of fuel. The high-speed images taken at 5 kHz show a rapid lifting of the flames upon pulsation before the flame base propagates back towards the nozzle. The resulting steady state position differed from the initial lift-off position, consistent with the previously observed hysteresis concept. Calculations using LES along with detailed chemistry are shown to capture the basic features observed in the experiment.     

A numerical study of turbulent opposed impinging jets issuing from triangular nozzles with different geometries

In present research, two turbulent opposed impinging air jets issuing from triangular nozzles with fixed and variable exit velocity ratios and different nozzle-to-nozzle distances have been studied numerically and then compared with rectangular and circular nozzles. The finite volume method has been applied for solving mass and momentum equations. The turbulence model being used here is k-ε RNG. Distributions of pressure, turbulence, kinetic energy and its dissipation rate in various regions especially on the impingement regions have been obtained with high accuracy. Study of the nozzle geometries has shown the advantage of triangular nozzles over other geometries. First, the triangle’s base in nozzle geometry has an important role in our study case which, mixing two flows and regions with high turbulence intensity, directly depends on it. Second, our results show that circular and rectangular nozzles have less efficiency than triangular nozzles in mixing applications. Third and last, it was found that the radial jet being created by opposed jets has some similarities to free jets. In this investigation, air in standard atmospheric pressure has been applied as working fluid.     

Static pressure effects on calibration of velocity transducers at nozzle exits

Calibrations of velocity transducers are generally performed at nozzle exits where the turbulence level is minimal. Such nozzles require a high contraction ratio and are usually short with a consequent effect on stream static pressure due to formation of a vena contracta. Calibrations performed with a Pitot tube and no static pressure correction can lead to a significant velocity error. Measurements of static pressure, mean velocity and turbulence intensity are presented for a typical high contraction ratio nozzle. These show two distinct flow regimes which can be described very simply by the Reynolds form of the Navier-Stokes equations.     

Topology and Brush Thickness of Turbulent Premixed V-shaped Flames

Topology and brush thickness of turbulent premixed V-shaped flames were investigated using Mie scattering and Particle Image Velocimetry techniques. Mean bulk flow velocities of 4.0, 6.2, and 8.3 m/s along with two fuel-air equivalence ratios of 0.6 and 0.7 were tested in the experiments. Using a novel experimental turbulence generating apparatus, three turbulence intensities of approximately 2 %, 6 %, and 17 % were tested in the experiments. The results show that topology of the flame front is significantly altered by changing the turbulence intensity. Specifically, at relatively small turbulence intensities, the flame fronts feature wrinkles which are symmetric with respect to the vertical axis. At moderate values of turbulence intensities, the flame fronts form cusps. The formation of cusps is more pronounced at large mean bulk flow velocities. The results associated with relatively large turbulence intensity show that flame surfaces feature: mushroom-shaped structures, freely propagating sub-flames, pocket formation, localized extinction, and horn-shaped structures. Analysis of the results show that the flame brush thickness follows a linear correlation with the root-mean-square of the flame front position. The correlation is in agreement with the results of past experimental investigations associated with moderately turbulent premixed V-shaped flames, and holds for the range of turbulence conditions tested. This suggests that the underlying mechanism associated with the dynamics of moderately turbulent premixed V-shaped flames proposed in past studies can potentially be valid for the the wide range of turbulence conditions examined in the present investigation.     

Helical-mode excitation of lifted flames using piezoelectric actuators

 Experiments of helical excitation using piezoelectric actuators on jet flows and lifted flames are performed to enhance the understanding of the effects of vortical structures of various instability modes on the stabilization mechanism of the lifted flame. In addition to the common ring and braid structures, five or seven azimuthal fingers (or lobes) can be identified in the transverse image of the jet near field. Excitation with various helical modes enhances the azimuthal structures and entrainment in the near field. When helically excited with the asymmetric m=1 mode, one of the fingers is enhanced and may evolve into a strong streamwise vortex. The streamwise vortices generated in the braid region between the adjacent ring vortices may enhance fuel-air mixing due to additional azimuthal entrainment upstream of a lifted flame when helically excited with the m=1 mode. Therefore, the streamwise vortex serves as an additional path of high probability of premixed flammable layer for the upstream propagation of the lifted flame so that the flame base on one side of the lifted flame may extend farther upstream and the flame base is inclined. In addition to the inclined flame base, multiple-legs phenomenon is also observed in the flame base, which is strongly associated with fingers of the helical modes of the jet flow. Received: 21 August 1997/Accepted: 24 January 1999     

Schlieren measurement of turbulent structure in a diffusion flame

The regular and random mixing structures in a turbulent diffusion flame were investigated using the quantitative, dynamic crossed-beam schlieren method. Evidence was found close to the nozzle relating to the vortexlike structure of eddies surrounding the central fuel jet flow. The observations also make possible resolution of turbulent intensity, scales, convection, and spectra within the diffusion flame without the use of seeding or intrusion of measuring probes. It is found that length scales and other turbulence parameters in the diffusion flame progressively revert to values similar to those expected and observed in scalar passive mixing as the combustion reaction intensity reduces with axial distance from the nozzle system.