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.     

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.     

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.     

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.     

Characteristics of flow from an oxy-fuel burner with separated jets: influence of jet injection angle

This paper presents an experimental study of flow development and structure on a separated jet burner in reacting and non-reacting flows. Effects of deflection jets in an aligned configuration of three round jets are emphasized. The idea is based on the confinement of a central jet of fuel by two side jets of oxygen to improve mixing, to control flame stability, and to reduce pollutant emissions. The fields of mean velocity and fluctuation intensity were measured using Particle Image Velocimetry. The deflection of jets has a considerable effect on the dynamic behavior and on the flame characteristics. Results showed that the deflection of jets favors mixing and accelerates merging and combining of jets to a single one. Measurements in reacting flow showed a high influence of combustion on dynamic fields. Compared to non-reactive case, in combustion, larger radial expansion and higher velocity were observed, particularly, above the stabilization point of the flame.     

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.     

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.     

An experimental investigation of the interaction of swirl flow with partially premixed disk stabilized propane flames

The present work describes the experimental investigation of reacting wakes established through fuel injection and staged premixing with air in an axisymmetric double cavity arrangement, formed along three concentric disks, and stabilized in the downstream vortex region of the afterbody. The burner assembly is operated with a co-flow of swirling air, aerodynamically introduced upstream of the burner exit plane, to allow for the study of the interaction between the resulting partially premixed recirculating afterbody flames with the surrounding swirl. At low swirl the primary afterbody disk stabilizes the partially premixed annular jet in the downstream reacting wake formation region. As swirl increases, a system of two successive vortices emerges along the axis of the developing wake; the primary afterbody vortex is cooperating with an adjacent, swirl induced, central recirculation zone and this combination further promotes turbulent mixing in the hot wake.Complementary measurements of the counterpart isothermal turbulent velocity fields provided important information on the near wake aerodynamics under the interaction of the variable swirl and the double cavity produced annular jet stabilized by the afterbody. Under reacting conditions, measurements of turbulent velocities, temperatures and statistics together with an evaluation of the exhaust emissions were performed using LDV, thin digitally-compensated thermocouples and gas analyzers. A selected number of lean and ultra-lean flames were investigated by regulating the injected fuel and the air supply ratio, while the influence of the variation of the imposed swirl on wake development, flame characteristics and emission performance was documented for constant fuel injections. The differences and similarities between the present partially premixed stabilizer and other types of axisymmetric configurations are also highlighted and discussed.     

An experimental study of turbulence intensities and non-uniformities in the exit flow from a porous combustor

In this study, experiments were performed on a two-section porous burner operated on propane and air. The burner consisted of an upstream section of reticulated yttria stabilized zirconia with 23.6 pores per centimeter (ppc) followed by a downstream section of 3.9 ppc, composed of the same material. The velocity and turbulence intensity of the exit flow for reacting and non-reacting conditions were measured. The velocity profiles for both reacting and non-reacting flow were very non-uniform. The turbulence intensity for the reacting flow increased with distance due to turbulence created by the non-uniformities. Blow-off occurred first on one side of the burner, but otherwise the flame in this burner was stable.     

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.     

Large-scale dynamics in high Reynolds number jets and jet flames

The far-field large-scale dynamics of a momentum-driven Re = 2 × 10⁸ non-reacting jet and a Re = 3 × 10⁷ jet diffusion flame are presented and compared. The results are derived from computer graphic volume rendering of a set of sequential images of each flow. When compared to conventional display techniques, volume rendering, by allowing many frames of a movie sequence to be presented simultaneously, more clearly shows the detailed flow evolution. For the non-reacting jet we see the passage and growth of large-scale organized structures up through the jet column, the axial velocity decay of the structures, the fluid entrainment patterns, and occasional pairing events. A rendering of a non-sequential set of images shows no discernible organized component. Volume rendering of the reacting jet shows a similar pattern of burning large-scale organized structures which convert over considerable axial distances but without the corresponding velocity decay, similar to observations of laboratory flames. The images presented here are believed to be some of the most direct visual evidence to date for large-scale organized motions in the far-field of high Reynolds number, fully developed jets and jet flames. Since conditional sampling techniques are not used, we believe that the volume renderings seen here are likely to be representative of the natural development of jet flows.     

High-resolution imaging of turbulence structures in jet flames and non-reacting jets with laser Rayleigh scattering

A comparative study of the length scales and morphology of dissipation fields in turbulent jet flames and non-reacting jets provides a quantitative analysis of the effects of heat release on the fine-scale structure of turbulent mixing. Planar laser Rayleigh scattering is used for highly resolved measurements of the thermal and scalar dissipation in the near fields of CH₄/H₂/N₂ jet flames (Re _d  = 15,200 and 22,800) and non-reacting propane jets (Re _d  = 7,200–21,700), respectively. Heat release increases the dissipation cutoff length scales in the reaction zone of the flames such that they are significantly larger than the cutoff scales of non-reacting jets with comparable jet exit Reynolds numbers. Fine-scale anisotropy is enhanced in the reaction zone. At x/d = 10, the peaks of the dissipation angle PDFs in the Re _d  = 15,200 and 22,800 jet flames exceed those of non-reacting jets with corresponding jet exit Reynolds numbers by factors of 2.3 and 1.8, respectively. Heat release significantly reduces the dissipation layer curvature in the reaction zone and in the low-temperature periphery of the jet flames. These results suggest that the reaction zone shields the outer regions of the jet flame from the highly turbulent flow closer to the jet axis.     

LES-CMC Simulations of Different Auto-ignition Regimes of Hydrogen in a Hot Turbulent Air Co-flow

Large-Eddy Simulation (LES) results in combination with first-order Conditional Moment Closure (CMC) are presented for a hydrogen jet, diluted with nitrogen, issued into a turbulent co-flowing hot air stream. The fuel mixes with the co-flow air, ignites and forms a lifted-like flame. Global trends in the experimental observations are in general well reproduced: the auto-ignition length decreases with increase in co-flow temperature and increases with increase in co-flow velocity. In the experiments, the co-flow temperature was varied, so that different auto-ignition regimes, including low Damköhler number situations, were obtained (no ignition, random spots, flashback and lifted flame). All regimes are recovered in the simulations. Auto-ignition is found to be the stabilizing mechanism. The impact of different detailed chemistry mechanisms on the auto-ignition predictions is discussed. With increasing air temperature, the differences between the mechanisms considered diminish. The evolution of temperature, H₂O, H, HO₂ and OH from inert to burning conditions is discussed in mixture fraction space.     

High-resolution imaging of dissipative structures in a turbulent jet flame with laser Rayleigh scattering

High-resolution 2-D imaging of laser Rayleigh scattering is used to measure the detailed structure of the thermal dissipation field in a turbulent non-premixed CH₄/H₂/N₂ jet flame. Measurements are performed in the near field (x/d = 5–20) of the flame where the primary combustion reactions interact with the turbulent flow. The contributions of both the axial and radial gradients to the mean thermal dissipation are determined from the 2-D dissipation measurements. The relative contributions of the two components vary significantly with radial position. The dissipation field exhibits thin layers of high dissipation. Noise suppression by adaptive smoothing enables accurate determination of the dissipation-layer widths from single-shot measurements. Probability density functions (PDF) of the dissipation-layer widths conditioned on temperature are approximately log-normal distributions. The conditional layer width PDFs are self-similar functions with the layer widths scaling with temperature to the 0.75 power. The high signal-to-noise ratio of the Rayleigh scattering images coupled with an interlacing technique for noise suppression enable fully resolved measurements of the mean power spectral density (PSD) of the temperature gradients. These spectra are used to determine the turbulence microscales by measuring a cutoff wavelength, λ _C , at 2% of the peak PSD. The Batchelor scale is estimated from λ _C , and the results are compared with estimates from scaling laws in non-reacting flows. At x/d = 20, the different approaches to determining the Batchelor scale are comparable on the jet centerline. However, the estimates from non-reacting flow scaling laws are significantly less accurate in off-centerline regions and at locations closer to the nozzle exit. Throughout the near field of the jet flame, the measured ratio of a characteristic dissipation-layer width to the local Batchelor scale is larger than values previously reported for the far field of non-reacting flows.     

Large Eddy Simulation of Stratified and Sheared Flames of a Premixed Turbulent Stratified Flame Burner Using a Flamelet Model with Heat Loss

This paper presents large eddy simulations (LES) of the Darmstadt turbulent stratified flame burner (TSF) at different operating conditions including detailed heat loss modeling. The target cases are a non-reacting and two reacting cases. Both reacting cases are characterized by stratification, while one flame additionally features shear. In the regime diagram for premixed combustion, the studied flames are found at the border separating the thin reaction zones regime and the broken reaction zones regime. A coupled level set/progress variable model is utilized to describe the combustion process. To account for heat loss, an enthalpy defect approach is adopted and reformulated to include differential diffusion effects. A novel power-law rescaling methodology is proposed to integrate the enthalpy defect approach into the level set/progress variable model which is extensively validated in two validation scenarios. It is demonstrated that the LES with the newly developed model captures the influence of heat loss well and that the incorporation of heat loss effects improves the predictions of the TSF-burner over adiabatic simulations, while reproducing the experimentally observed flame lift-off from the pilot nozzle.     

Effects of Turbulence, Evaporation and Heat Release on the Dispersion of Droplets in Dilute Spray Jets and Flames

The dispersion characteristics of a selection of non-evaporating non-reacting, evaporating non-reacting, and reacting dilute spray jets issuing in ambient air (Gounder et al, Combust Sci Technol 182:702–715, 2010; Masri and Gounder, Combust Flame 159:3372–3397, 2010) and in a hot coflow (Oloughlin and Masri, Flow Turbul Combust 89:13–35, 2012) are analysed. Other than the cases found in those contributions, two additional sprays of kerosene have been investigated in order to systematically study the effects of evaporation. The burners are well designed such that boundary conditions may be accurately measured for use in numerical simulations. The dynamics and dispersion characteristics are analysed by conditioning results on the droplet Stokes numbers and by systematically investigating changes in dispersion and dynamics as a function of carrier air velocity, liquid loading, ignition method, and location within the flame or spray jet. The tendency for droplet dispersion defined by the ratio of radial rms velocity to axial mean velocity varies significantly between reacting and non-reacting flows. However, dispersion is found to be largely unaffected by evaporation. The total particle concentration, or number density of droplets within the spray has also been used as a direct measure of spray dispersion with the effect of evaporation on a turbulent polydisperse spray being isolated by investigating acetone and kerosene sprays with similar boundary conditions. The rate of change of droplet size with radial position is almost identical for the kerosene and acetone cases. The dispersion characteristics, closely related to the ‘fan spreading’ phenomenon are dependant on the carrier air velocity and axial location within the spray.     

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.     

Heat Transfer Study of the Hencken Burner Flame

The Hencken burner flame is often used in combustion laser diagnostics as a calibration flame because of its near adiabatic condition. For a fast burning H₂ flame, it can tolerate high flow rate and the flame is indeed near adiabatic; however, for a slow burning CH₄ flame, the flow rate is not always high enough to maintain near adiabatic conditions. The heat transfer of the H₂ and CH₄ Hencken burner flames are studied numerically and experimentally. Three heat loss mechanisms are analyzed: the burner surface radiation, the hot gas radiation, and the convection heat transfer between the main flow and the co-flow. The surface radiation produces negligible temperature drop while the gas radiation and the convection heat loss contribute significant temperature drop. Reducing the co-flow rate can decrease the convection heat loss slightly. The temperature drop caused by the heat loss is inversely proportional to the main flow rate. Increasing the burner size and running the flame premixed mode can increase the flow rate and reduce the temperature deviation from the adiabatic equilibrium value. Based on the heat loss and temperature drop analysis, suggestions are given to maintain the flame at near adiabatic conditions.     

Effect of swirl intensity on the flow and combustion of a turbulent non-premixed flat flame

An experiment in a turbulent non-premixed flat flame was carried out in order to investigate the effect of swirl intensity on the flow and combustion characteristics. First, stream lines and velocity distribution in the flow field were obtained using PIV (Particle Image Velocimetry) method in a model burner. In contrast with the axial flow without swirl, highly swirled air induced streamlines going along the burner tile, and its backward flow was generated by recirculation in the center zone of the flow field. In the combustion, the flame shape with swirled air also became flat and stable along the burner tile with increment of the swirl number. Flame structure was examined by measuring OH and CH radicals intensity and by calculating Damkohler number (Da) and turbulence Reynolds number (Re _T ). It appeared that luminescence intensity decreased at higher swirl number due to the recirculated flue gas, and the flat flames were comprised in the wrinkled laminar-flame regime. Backward flow by recirculation of the flue gas widely contacted on the flame front, and decreased the flame temperature and emissions concentration as thermal NO. The homogeneous temperature field due to the widely flat flame was obtained, and the RMS in the high temperature region was rather lower at higher swirl number. Consequently, the stable flat flame with low NO concentration was achieved.     

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.