Characteristics of strongly-forced turbulent jets and non-premixed jet flames

Previous researchers have demonstrated that strong pulsations of the fuel flow rate can significantly reduce the flame length and luminosity of laminar/transitional non-premixed jet flames. The physical mechanisms responsible for these changes are investigated experimentally in acoustically-forced jet flows where the peak velocity fluctuations are up to eight times the mean flow velocity. Both reacting and non-reacting flows were studied and Reynolds numbers, based on the mean flow properties, ranged from 800 to 10,000 (corresponding to peak Reynolds numbers of 1,450–23,000), and forcing frequencies ranged from 290 to 1,140 Hz. Both the first and second organ-pipe resonance modes of the fuel delivery tube were excited to obtain these frequencies. An analysis of the acoustic forcing characteristics within the resonance tube is provided in order to understand the source of the high amplitude forcing. Flow visualization of jets with first resonant forcing confirms the presence of large-scale coherent vortices and strong reverse flow near the exit of the fuel tube. With second-resonant forcing, however, vortices are not emitted from the tube as they are drawn back into the fuel tube before they can fully form. Increased fine-scale turbulence is associated with both resonant cases, but particularly at second resonance. The power spectra of the velocity fluctuations for a resonantly pulsed jet show the presence of an inertial subrange indicating that the flow becomes fully turbulent even for mean-Reynolds-number jets that are nominally laminar. It is shown that these pulsed jet flows exhibit strong similarities to synthetic jets and that the Strouhal number, based on the maximum velocity at the fuel tube exit, is the dominant parameter for scaling these flows. The Strouhal number determines the downstream location where the coherent vortices breakdown, and is found to provide better collapse of flame length data (both current and previous) than other parameters that have been used in the literature.     

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.     

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.     

Evaluation of fractal dimension for mixing and combustion by the schlieren method

The evaluation of fractal dimension values from schlieren flow images has been investigated. It was found for passive mixing heated jet flows that a value of 2.31, close to that obtained by tomographic imaging, was obtained. For turbulent diffusion flames a value of 2.40 was obtained, and this increased slightly with axial movement and with acoustic excitation of the flame. Broadly counteracting effects due to the transverse differentiation and integration along the imaging beam inherent in the schlieren method was found to arise.     

Effects of Small- and Large-Scale Structures in a Piloted Jet Diffusion Flame

Laser Doppler Anemometry (LDA) and Planar Laser-Induced Fluorescence (PLIF) measurements have been performed in a turbulent nonpremixed jet flame. One of the features of this configuration is a central co-axial fuel jet surrounded by a turbulent annular air flow. The whole is placed within a low-speed coflowing air stream. This three-flow system with turbulent primary air differs from flow systems used for nonpremixed jet flames reported in the literature and is very useful for obtaining information on the mixing process between fuel and primary air. Next to the characterization of the velocity field, special attention has been paid to the conditional seeding of the central fuel jet and of the annular air flow. Together with visualizations of the OH radical, an important combustion intermediate which is formed during combustion, and the NO radical, which is seeded to the central jet flow, the resulting statistics reveal the properties of small- and large-scale structures in the flame.     

3D-CT(Computer Tomography) Measurement of an Instantaneous Density Distribution of Turbulent Flames with a Multi-Directional Quantitative Schlieren Camera (Reconstructions of High-Speed Premixed Burner Flames with Different Flow Velocities)

In order to provide a suitable technique for 3D observation of high speed turbulent flames, non-scanning 3D-CT(Computer Tomography) technique using a multi-directional quantitative schlieren system with flash light source, is proposed for instantaneous density distribution of unsteady premixed flames. This “schlieren 3D-CT” is based on (i)simultaneous acquisition of flash-light schlieren images taken from numerous directions, and (ii) 3D-CT reconstruction of the images by an appropriate CT algorithm. In this paper, first, as a preliminary research, 3D-CT reconstruction of non-axisymmetric steady flame is made with a single-directional quantitative schlieren system. Next, with custom-made 20 directional schlieren camera, instantaneous density distributions of a high-speed turbulent flames of nozzle exit velocities of 8.0 and 10.0 m/s has been CT-reconstructed. The 3D-views of the reconstructed flame front shape clearly give the information of the flame structure with fine scale corrugations. Based on the distributions, area-enlargement rates of the flame front area are derived, and investigated.     

A Fully Implicit, Compact Finite Difference Method for the Numerical Solution of Unsteady Laminar Flames

Forced laminar diffusion flames form an important class of problems that can help to bridge the significant gap between steady laminar flames in simple burner configurations and the turbulent flames found in many practical combustors. Such flames offer a much wider range of interactions between convection, diffusion, and chemical reaction than can be examined under steady-state conditions, and yet detailed simulations of them should be feasible without having to resort to “modeling” any of the relevant physics, above all without having prematurely to reduce the large kinetic mechanisms typical of hydrocarbon fuels. Nevertheless, the computation of time-dependent laminar diffusion flames with conventional numerical methods is hindered by technical challenges that, while not new, are more troublesome to surmount than in the calculation of otherwise similar, unforced flames. First, the intricate spatiotemporal coupling between fluid dynamics and combustion thermochemistry ensures that spurious numerical diffusion or spatial under-resolution of the mixing process at any stage of the computation can lead to inaccurate prediction of flame characteristics for the remainder thereof. Second, relatively long simulated flow times and extremely short chemical time scales make many standard time integration algorithms impractical on all but the largest parallel computer clusters. This paper introduces a new numerical approach for time-varying laminar flames that addresses these challenges through the use of high order compact finite difference schemes within a robust, fully implicit solver based on a Jacobian-free Newton–Krylov method. The capabilities of this implicit-compact solver are demonstrated on a periodically forced axisymmetric laminar jet diffusion flame with one-step Arrhenius chemistry, and the results are compared to those of a conventional low order finite difference solver.     

Turbulent Combustion of Hydrogen–CO Mixtures

Laminar and turbulent burning velocities were measured in a closed-volume fan-stirred vessel for H₂–CO mixtures using two independent methods of flame definition. It has been shown that the unsteady flame development is an important factor and it needs to be taken into account for comparison of the burning rates obtained in different experiments. For the atmospheric pressure flames, the mixtures with faster laminar flame velocities burnt faster in turbulent flow despite the fact that the lean flames exhibit cellular structures. However, even a modest increase of the initial pressure promotes strongly cellularity and causes a significant acceleration of a lean laminar flame. The same lean flame burns faster in turbulent flow as well and this increase in the rate of combustion is greater that can be deduced from variation of the molecular heat diffusivity and laminar flame speed.     

Structures and stabilization of low calorific value gas turbulent partially premixed flames in a conical burner

Experiments are carried out on partially premixed turbulent flames stabilized in a conical burner. The investigated gaseous fuels are methane, methane diluted with nitrogen, and mixtures of CH₄, CO, CO₂, H₂ and N₂, simulating typical products from gasification of biomass, and co-firing of gasification gas with methane. The fuel and air are partially premixed in concentric tubes. Flame stabilization behavior is investigated and significantly different stabilization characteristics are observed in flames with and without the cone. Planar laser induced fluorescence (LIF) imaging of a fuel-tracer species, acetone, and OH radicals is carried out to characterize the flame structures. Large eddy simulations of the conical flames are carried out to gain further understanding of the flame/flow interaction in the cone. The data show that the flames with the cone are more stable than those without the cone. Without the cone (i.e. jet burner) the critical jet velocities for blowoff and liftoff of biomass derived gases are higher than that for methane/nitrogen mixture with the same heating values, indicating the enhanced flame stabilization by hydrogen in the mixture. With the cone the stability of flames is not sensitive to the compositions of the fuels, owing to the different flame stabilization mechanism in the conical flames than that in the jet flames. From the PLIF images it is shown that in the conical burner, the flame is stabilized by the cone at nearly the same position for different fuels. From large eddy simulations, the flames are shown to be controlled by the recirculation flows inside cone, which depends on the cone angle, but less sensitive to the fuel compositions and flow speed. The flames tend to be hold in the recirculation zones even at very high flow speed. Flame blowoff occurs when significant local extinction in the main body of the flame appears at high turbulence intensities.     

Experimental study of biogas combustion using a gas turbine configuration

The aim of the present work is to compare stability combustion domains, flame structures and dynamics between CH₄/air flames and a biogas/air flames (issued from waste methanisation) in a lean gas turbine premixed combustion conditions. Velocity profiles are obtained by Laser Doppler Anemometry measurements. CH* chemiluminescence measurements and temporal acquisition of chamber pressure are performed in order to describe flame structure and instabilities. Changes in flame structure and dynamics when fuel composition is varying are found to strongly depend on laminar flame speed. No clear correlation between the unstable flame and the reaction zone penetration in the corner recirculation can be found.     

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.     

Effects of Differential Diffusion on Predicted Autoignition Delay Times Inspired by H2/N2 Jet Flames in a Vitiated Coflow Using the Linear Eddy Model

Mixing and chemistry interactions in a H₂/N₂ jet flame into a vitiated coflow are considered key factors affecting autoignition. A 1-D numerical model under laminar flow condition first is simulated to reveal the effects of fuel species, pressure, and coflow properties on the autoignition with and without the consideration of preferential diffusion among species. Proper laminar reference autoignition delays are proposed and examined for different diffusion models. Next, the reference autoignition delays defined from laminar simulations are investigated in an example turbulent flow using the Linear Eddy Model (LEM). LEM is used to model the effect of turbulent mixing on autoignition, where we specifically investigate if the effect of turbulence on autoignition can be classified in two regimes, which are dependent on a proper reference laminar autoignition delay and turbulence time scale. The trend of the effect of differential diffusion on autoignition versus turbulence Reynolds is simulated and analyzed, and several tentative conclusions are drawn.     

Identification of vortex motions in turbulent mixing of choked jets

A specially adapted schlieren system is used to generate fluctuating signals which respond strongly to large scale coherent components of a turbulent mixing jet flow and which have a relatively reduced response to random disturbances. The schlieren signals also provide a direct indication of the presence of vortex-like structures in the turbulent mixing layers by virtue of the phase relationship of the schlieren signals to the pressure field. This system gives a clear resolution of the fluctuating periodic effects associated with vortex structures in the flow from a choked convergent nozzle. It has thus been possible to determine that vortex-like eddies are associated with the feedback screech mechanism, and also generate periodic disturbances due to their passage through the diamond shaped wave structure in the flow. The regular disturbances in the flow move at 0.77 of the fully expanded flow velocity. Phase spectral observations demonstrate clearly the vortex like structure of coherent disturbances in the flow by virtue of the quadrature phase relation between the schlieren and microphone signals. Movement of the sensing microphone in the pressure field external to the flow shows disturbance propagation at the acoustic velocity, and also shows that disturbances at Strouhal numbers above 0.7 emanating from the inner mixing zone can be identified by an additional time delay to reach the microphone and only influence the microphone when it is located downstream of the flow sensing schlieren system due to confinement of pressure disturbances within Mach cones of the flow.     

Instability of buoyant diffusion flames

Buoyant jet diffusion flames are known to exhibit large scale vortical flow structures strongly interacting with flame structures. In the present work, the formation and evolution of coherent flow structures is studied in a methane/ air coflow arrangement. This is accomplished by utilizing visualization techniques (planar laser induced hydroxyl fluorescence and Mie-scattering) and Laser Doppler Velocimetry. A striking repeatability and correlation of evolving coherent structures of the air co-flow and the reaction zone is observed. In the transitional region, flow and flame structures oscillate at very pure frequencies ranging from 10–15 Hz. A local absolutely unstable velocity profile close to the burner rim seems to be responsible. Self-excited axisymmetric wavelike structures propagate up- and downstream of this location. We study the influence of the exit velocities and the type of coflowing oxidizer (air or oxygen) on the location of transition to periodic flow structures and related frequencies. Conditional averages of image and velocity data are employed to describe the evolution of coherent flow structures and their interaction with flame structures.The authors wish to thank the Deutsche Forschungsgemeinschaft for financial support under contract Kn 118/22-2.     

Soot surface temperature measurements in pure and diluted flames at atmospheric and elevated pressures

Soot surface temperature was measured in laminar jet diffusion flames at atmospheric and elevated pressures. The soot surface temperature was measured in flames at one, two, four, and eight atmospheres with both pure and diluted (using helium, argon, nitrogen, or carbon dioxide individually) ethylene fuels with a calibrated two-color soot pyrometry technique. These two dimensional temperature profiles of the soot aid in the analysis and understanding of soot production, leading to possible methods for reducing soot emission. Each flame investigated was at its smoke point, i.e., at the fuel flow rate where the overall soot production and oxidation rates are equal. The smoke point was chosen because it was desirable to have similar soot loadings for each flame. A second set of measurements were also taken where the fuel flow rate was held constant to compare with earlier work. These measurements show that overall flame temperature decreases with increasing pressure, with increasing pressure the position of peak temperature shifts to the tip of the flame, and the temperatures measured were approximately 10% lower than those calculated assuming equilibrium and neglecting radiation.     

Direct measurement of mixture fraction in reacting flow using Rayleigh scattering

The mixture fraction variable, , is very useful in describing reaction zone structure in nonpremixed flames. Extinction limits and turbulent mixing are often described as a function of this variable. Experimental evaluation of is critical for improving our understanding of the influence of turbulent mixing on the chemistry process. Heretofore, the evaluation of mixture fraction in combusting flow required multiple simultaneous concentration measurements. In this paper we present a fuel designed to permit measurements of mixture fraction by Rayleigh scattering technique only. A Rayleigh intensity/mixture fraction correspondence has been obtained experimentally in a laminar coflow flame. The influence of strain rate and differential diffusion effects have been investigated using laminar counterflow diffusion flame and shifting equilibrium chemistry models. The results obtained from comparisons between experiments and these models are very encouraging and suggest that the Rayleigh/mixture fraction correspondence established is valid under both the turbulent mixing and laminar strained flamelet combustion regimes.     

Buoyancy effects on flow transition in low-density inertial gas jets

Effects of buoyancy on transition from laminar to turbulent flow are presented for a momentum-dominated helium jet injected into ambient air. The buoyancy was varied in a 2.2 s drop tower facility without affecting the remaining operating parameters. The jet flow in Earth gravity and microgravity was visualized using the rainbow schlieren deflectometry apparatus. Results show significant changes in the flow structure and transition behavior with change of buoyancy.     

Imaging of premixed flames in microgravity

A laser schlieren system which uses video recording and digital images analysis has been developed and applied successfully to microgravity combustion experiments performed in a drop-tower. The optical system and the experiment are installed within a small package which is subjected to free-fall. The images are recorded on video tape and are digitized and analyzed by a computer-controlled image processor. The experimental results include laminar and turbulent premixed conical flames in microgravity, normal positive gravity (upward), and reverse gravity (downward). The procedures to extract frequency information from the digitized images are described. Many gross features of the effects of gravity on premixed conical flames are found. Flames that ignite easily in normal gravity fail to ignite in microgravity. Buoyancy driven instabilities associated with an interface formed between the hot products and the cold surrounding air is the mechanism through which gravity influences premixed laminar and turbulent flames. In normal gravity, this causes the flame to flicker. In reverse gravity, -g, and microgravity, g, the interface is stable and flame flickering ceases. The flickering frequencies of +g flames vary with changing upstream boundary conditions. The absence of flame flickering in g suggest that g flames would be less sensitive to these changes.This work is supported by NASA Microgravity Sciences and Applications Divisions under contract No. C-32000-R through the U.S. Department of Energy Contract No. DE-AC03-76F00098. Technical support is provided by NASA Lewis Research Center. Project Scientist is Dr. Karen J. Weiland. The authors would like to acknowledge Dr. Liming Zhou for his contribution to early testing of the schlieren system, and to Mr. Gray Hubbard for writing the image analysis software     

Tomographic PIV measurements in a turbulent lifted jet flame

Measurements of instantaneous volumetric flow fields are required for an improved understanding of turbulent flames. In non-reacting flows, tomographic particle image velocimetry (TPIV) is an established method for three-dimensional (3D) flow measurements. In flames, the reconstruction of the particles location becomes challenging due to a locally varying index of refraction causing beam-steering. This work presents TPIV measurements within a turbulent lifted non-premixed methane jet flame. Solid seeding particles were used to provide the 3D flow field in the vicinity of the flame base, including unburned and burned regions. Four cameras were arranged in a horizontal plane around the jet flame. Following an iterative volumetric self-calibration procedure, the remaining disparity caused by the flame was less than 0.2 pixels. Comparisons with conventional two-component PIV in terms of mean and rms values provided additional confidence in the TPIV measurements.     

A Combined Experimental and Numerical Study of Laminar and Turbulent Non-piloted Oxy-fuel Jet Flames Using a Direct Comparison of the Rayleigh Signal

In the present study laminar and turbulent oxy-fuel jet flames are investigated both experimentally and numerically with emphasis on the direct comparison of the Rayleigh signal. The Rayleigh signal was measured for both flame setups, correcting for background light appropriately. Two downstream regions were recorded for the laminar flame and three for the turbulent flame. Equivalently, the signal was processed numerically based on the numerical species data and temperature. The laminar flame was used for validating the procedure of processing the Rayleigh signal. Both the numerical species data and the temperature are known from detailed simulations, so a predicted Rayleigh signal can easily be obtained. Further, the influence of the choice of the kinetic mechanism, radiation and diffusion model was investigated. In contrast, in the turbulent Large Eddy Simulation, the Rayleigh signal has to be computed using an appropriate turbulence-chemistry interaction model in order to obtain the Reynolds-filtered Rayleigh signal which is of non-linear nature. In the present investigation, the Rayleigh signal was incorporated in the flamelet/progress variable approach. The statistics of the experimental and numerical Rayleigh signal were then compared. The proposed procedure of directly comparing the experimental and predicted Rayleigh signal was shown to be advantageous in model validation especially in turbulent flame configurations. The procedure enables accurate model validation across an entire 2D field of view whilst using a realistic fuel-oxidizer combination and reducing experimental complexity.