Enhancement of turbulent jet diffusion flame blowout limits by annular counterflow

The effects of a proposed combustion technique, named as annular counterflow, on the enhancement of jet diffusion flame blowout limits were investigated by a series of experiments conducted for the present study. Annular counterflow was formed in a concentric annulus, in which fuel jet was ejected from a nozzle and air was sucked into an outer cylinder encompassing the nozzle. Three fuel nozzles and outer cylinders of different sizes were utilized to perform the experiments. Schlieren technique and normal video filming were employed for the visualization of diverse flame morphologies triggered by the said flow. Gas samplings were taken and scrutinized by the use of a gas chromatograph. Results showed that the blowout limits can be enhanced dramatically by an increase in volume flow rates of air‐suction. Mixing enhancement is achieved with frequent and strong outward ejection of fluids from the cold jet when this technique is applied. The blowout limits are further extended when the diameter of outer cylinders becomes smaller and/or that of the fuel nozzle becomes larger. The base widths of lifted flames were found to be narrower in the interim of annular counterflow application. The rates of increase in flame lift‐off heights and base widths along with an increase in fuel flow velocities become sluggish when the volume flow rates of air are increased. The amount of fuel that was sucked into the outer cylinder was found to be negligible and trivial. A model based on annular and coaxial jet was developed to predict the lifted flame base width and blowout limits. The coincidence between the prediction and experimental results unambiguously validates that the momentum of air‐suction dominates the beneficial effect. Copyright © 2001 John Wiley & Sons, Ltd.     

Time-averaged characteristics of a reacting fuel jet in vitiated cross-flow

This paper describes an experimental study of reacting jets in a high-temperature (1775 K) vitiated crossflow at 6 atm. We present an extensive data set based on high speed chemiluminescence imaging and exhaust gas sampling showing the characteristics of the time-averaged trajectory, width of the flame, flame standoff (or ignition) location, and NO_x emissions over a momentum flux ratio range of 0.75 < J < 240. Key observations are: (1) Depending upon ignition times, reaction can initiate uniformly around the jet, initiate on the leeward side of the jet and spread around to the windward side farther downstream, or initiate further downstream. (2) The time-averaged trajectory generally follows nonreacting trajectories, but penetrates further in the far-field than for what would be expected of a nonreacting jet. (3) The width of heat release zone increases monotonically with downstream location, J, and flame flapping amplitude, but seems to be dominated by the size of the counter-rotating vortex pair. (4) The measured ignition locations were of the same order of magnitude as values based on calculated ignition time scales and mean jet exit velocities, but with some additional variability. (5) The incremental NO_x emissions were controlled primarily by the global temperature rise associated with burning the jet fuel (for the fixed crossflow conditions studied here), and the NO_x emissions increased roughly linearly with the temperature rise.     

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

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

Analysis of flame stabilization mechanism in a hydrogen-fueled reacting wall-jet flame

In this study, the novel conservative representation of chemical explosive mode analysis is augmented to analyze the key flame features in the Burrows-Kurkov flames simulated by both Reynolds-Averaged Navier-Stokes (RANS) and large eddy simulation (LES). Subtle difference are revealed in flame stabilization mechanisms resulting from the difference in modeling and spatial resolution. RANS shows that, ahead of the flame onset location, the composition diffusion and shock wave compression play dominant roles in chemical explosion indicating that the flame is stabilized by the assisted-ignition combustion mode. In contrast, LES shows that the flame is stabilized by the auto-ignition mode since the nonchemical contribution counteracts chemical reaction during the development of ignited flame kernels. For RANS, the radical pool builds up through the unphysical back diffusion near the flame stabilization front, which reveals the limitation of RANS method in the resolution and characterization of the key flame features in Burrows-Kurkov flames.     

Large eddy simulation of reacting flow in a hydrogen jet into supersonic cross-flow combustor with an inlet compression ramp

Large eddy simulation (LES) has been performed to investigate transverse hydrogen jet mixing and combustion process in a scramjet combustor model with a compression ramp at inlet to generate shock train. Partially Stirred Reactor (PaSR) sub-grid combustion model with a skeleton of 19 reactions and 9 species hydrogen/air reaction mechanism was used. The numerical solver is implemented in an Open Source Field Operation and Manipulation (OpenFOAM) and validated against experimental data in terms of mean wall pressure. Effects of a shock train induced by the inlet compression ramp on the flame stabilization process are then studied. It can be observed that the interaction of the oblique shock and the jet mixing layer enhance the combustion and stabilize the flame. Symmetrical recirculation zone, which contributes to the flame anchoring of the supersonic transverse jet combustion, is observed in the near wall region of 10 < x/D < 20. The hydrogen fuel is transported from the center of jet plume to the near wall region on both sides of the central plane (z/D = 0) and thus intense combustion near the wall is observed due to the enhanced mixing and shock compression heating. Besides, the jet penetration in the reacting field is different from that in non-reacting case with the influence of the interaction between the reflected oblique shock and the jet shear layer on the windward side.     

The blowout mechanism of turbulent jet diffusion flames

The complicated flame stabilization mechanisms and flame/flow interactions in the blowout of turbulent nonpremixed jet flames are experimentally studied using phenomenological observation, 2D Rayleigh scattering, 2D laser-induced predissociative fluorescence (LIPF) images of OH, and particle image velocimetry (PIV) techniques. The blowout process may be categorized into four characteristic regions: pulsating, onset of receding, receding, and extinction. Based on experimental findings, a blowout mechanism is proposed. The maximum “waistline” point of the stoichiometric contour, defined as the point where the radial distance between the elliptic stoichiometric contour and the jet axis reaches a maximum value, can be regarded as the dividing point separating the unstable and stable regions for the lifted flame in the blowout process. If the flame base is pushed beyond the maximum “waistline” point, the flame will step into the pulsating region and become unstable, triggering the blowout process. The triple flame structure is identified and found to play an important role in flame stabilization within the stable liftoff and pulsating regions. In the pulsating region, the stabilization point of the triple flame moves along the stoichiometric contour, stabilizing the flame where the flame base is bounded by the contours of lean and rich limits. If the flame is pushed beyond the tip of the stoichiometric contour, the stabilization point and triple flame structure vanish and the flame becomes lean. The flame then recedes downstream continuously and finally extinguishes.     

Large-eddy simulation of turbulent autoigniting hydrogen lifted jet flame with a multi-regime flamelet approach

In this work, the combustion model is focused on to describe a multitude of reaction regimes that are deemed to affect the flame stabilization. For this purpose, an efficient flame indicator is formulated to differentiate the differing flame structures and make use of flamelet chemistry that accounts for autoignition and multi-regime reactions. The large eddy simulation with this methodology is carried out to compute a turbulent lifted hydrogen-nitrogen flame in vitiated coflow. The canonical flame models of a laminar premixed flame and an unsteady counterflowing flame have been used to simulate the flamelet structure at different regimes. Present model improves the prediction of mean and rms profiles for temperature and species mass fraction in the comparison with experiments and a reference simulation, adopting the single-regime flamelet. The computed results also demarcate the formation of a triple flame structure at the flame base, where combustion develops into the premixed reaction that extends to the fuel-lean and rich branches. The counterflow mixing mode with autoignition is identified as the major mechanism for stabilization and is responsible for the propagating premixed zone above the liftoff height. The developed multi-regime flamelet approach properly accounts for the interactive different modes of burning.     

Auto-ignition and flame stabilization of hydrogen/natural gas/nitrogen jets in a vitiated cross-flow at elevated pressure

The influence of natural gas (NG) on the auto-ignition behavior of hydrogen (H₂)/nitrogen (N₂) fuel jets injected into a vitiated cross-flow was studied at conditions relevant for practical combustion systems (p = 15 bar, T_cross-flow = 1173 K). In addition, the flame stabilization process following auto-ignition was investigated by means of high-speed luminosity and shadowgraph imaging. The experiments were carried out in an optically accessible jet in cross-flow (JICF) test section. In a H₂/NG/N₂ fuel mixture, the fraction of H₂ was stepwise increased while keeping the N₂ fraction approximately constant. Two different jet penetration depths, represented by two N₂ fraction levels, were investigated. The results reveal that auto-ignition kernels occurred even for the lowest tested H₂ fuel fraction (XH_2/NG=XH₂/(XH₂+XNG)=80%)$\left(X_{{\text{H}}_2/\text{NG}}=X_{{\text{H}}_2}/\left(X_{{\text{H}}_2}+X_{\text{NG}}\right)=80\%\right)$, but did not initiate a stable flame in the duct. Increasing XH_2/NG$X_{{\text{H}}_2/\text{NG}}$ decreased the distance between the initial position of the auto-ignition kernels and the fuel injector, finally leading to flame stabilization. The H₂ fraction for which flame stabilization was initiated depended on jet penetration; flame stabilization occurred at lower H₂ fractions for the higher jet penetration depth (XH_2/NG$X_{{\text{H}}_2/\text{NG}}$ = 91% compared to 96%), revealing the influence of different flow fields and mixing characteristics on the flame stabilization process. It is hypothesized that the flame stabilization process is related to kernels extending over the duct height and thus altering the upstream conditions due to considerable heat release. This enabled subsequent kernels to occur close to the fuel injector until they could finally stabilize in the recirculation zone of the jet lee.     

Large eddy simulation of a lifted turbulent jet flame

The flame index concept for large eddy simulation developed by Domingo et al. [P. Domingo, L. Vervisch, K. Bray, Combust. Theory Modell. 6 (2002) 529–551] is used to capture the partially premixed structure at the leading point and the dual combustion regimes further downstream on a turbulent lifted flame, which is composed of premixed and nonpremixed flame elements each separately described under a flamelet assumption. Predictions for the lifted methane/air jet flame experimentally tested by Mansour [M.S. Mansour, Combust. Flame 133 (2003) 263–274] are made. The simulation covers a wide domain from the jet exit to the far flow field. Good agreement with the data for the lift-off height and the mean mixture fraction has been achieved. The model has also captured the double flames, showing a configuration similar to that of the experiment which involves a rich premixed branch at the jet center and a diffusion branch in the outer region which meet at the so-called triple point at the flame base. This basic structure is contorted by eddies coming from the jet exit but remains stable at the lift-off height. No lean premixed branches are observed in the simulation or and experiment. Further analysis on the stabilization mechanism was conducted. A distinction between the leading point (the most upstream point of the flame) and the stabilization point was made. The later was identified as the position with the maximum premixed heat release. This is in line with the stabilization mechanism proposed by Upatnieks et al. [A. Upatnieks, J. Driscoll, C. Rasmussen, S. Ceccio, Combust. Flame 138 (2004) 259–272].     

DQMOM based PDF transport modeling for turbulent lifted nitrogen-diluted hydrogen jet flame with autoignition

The Direct-Quadrature Method of Moment (DQMOM) based PDF transport approach has been adopted to simulate turbulent lifted nitrogen-diluted hydrogen jet flame with vitiated coflow. In this study, the joint composition PDF is approximated using the multi-environment PDF consisting of the combination of weights and abscissas on composition and physical space. The micro mixing is represented by the IEM model. To numerically simulate the auto-ignition process and conditional fluctuations on composition space, all composition vector including species mass fraction and enthalpy are directly integrated with DVODE solver. In terms of the lift-off height, mean and rms of temperature, species mass fractions, and probability density function, the predicted profiles are reasonably well agreed with experimental data. Numerical results clearly indicate that the present DQMOM based PDF transport model has the capability to predict the autoignition, flame lift-off and stabilization process in the turbulent H₂/N₂ lifted jet flame.     

Effects of turbulent combustion modeling errors on soot evolution in a turbulent nonpremixed jet flame

Due to the long time scales associated with soot evolution and its sensitivity to the background thermochemical state, even small errors in a turbulent combustion model have the potential to lead to large errors in soot evolution. For example, in turbulent jet flames, small upstream errors in the temperature and species concentrations could lead to large errors in soot volume fraction downstream. In this work, an algorithm is developed for propagating upstream errors in the thermochemical state, specifically, the temperature, into soot predictions downstream. The algorithm is based on a stochastic collocation approach that perturbs the reaction progress variable in the flamelet model at an upstream location and lets this error passively propagate downstream in the soot and combustion models (i.e., the hydrodynamic field is unaffected). The approach is applied to the simulation of Delft Flame III, a natural gas turbulent nonpremixed piloted jet flame for which both upstream temperature measurements and downstream soot volume fraction measurements are available. The results indicate that upstream errors in temperature, which are within the experimental uncertainty, can lead to errors in the soot volume fraction downstream up to 30%; the downstream error in the temperature is comparable in magnitude to the upstream perturbation. Further analysis reveals that the primary source of the downstream error in soot volume fraction is the accumulation of errors in the soot precursor mass fraction with downstream distance.     

Large eddy simulation of soot formation in a turbulent non-premixed jet flame

A recently developed subgrid model for soot dynamics [H. El-Asrag, T. Lu, C.K. Law, S. Menon, Combust. Flame 150 (2007) 108-126] is used to study the soot formation in a non-premixed turbulent flame. The model allows coupling between reaction, diffusion and soot (including soot diffusion and thermophoretic forces) processes in the subgrid domain without requiring ad hoc filtering or model parameter adjustments. The combined model includes the entire process, from the initial phase, when the soot nucleus diameter is much smaller than the mean free path, to the final phase, after coagulation and aggregation, where it can be considered in the continuum regime. A relatively detailed but reduced kinetics for ethylene-air is used to simulate an experimentally studied non-premixed ethylene/air jet diffusion flame. Acetylene is used as a soot precursor species. The soot volume fraction order of magnitude, the location of its maxima, and the soot particle size distribution are all captured reasonably. Along the centerline, an initial region dominated by nucleation and surface growth is established followed by an oxidation region. The diffusion effect is found to be most important in the nucleation regime, while the thermophoretic forces become more influential downstream of the potential core in the oxidation zone. The particle size distribution shows a log-normal distribution in the nucleation region, and a more Gaussian like distribution further downstream. Limitations of the current approach and possible solution strategies are also discussed.     

Hysteresis in flame stabilization in a hydrogen fueled supersonic combustor

Hysteresis in flame stabilization mode transitions in a hydrogen-fueled strut-stabilized supersonic combustion test rig was experimentally observed and studied. Air was vitiated using H₂–O₂ combustion products to stagnation conditions of 8.65 bar and 1350 K and was expanded through a rectangular nozzle to Mach number 2.5. H₂ fuel was injected transversely using a strut positioned at the center of the combustor. The equivalence ratio (ER) was changed in time to study its effects on flame stabilization modes. Shadowgraph and wall pressure measurements were used to study the shock system generated by the strut in the supersonic combustor. High-speed OH1 chemiluminescence and high-speed flame imaging were used to study the heat release zones and flame structure of different combustion modes and transitions between them. Three different combustion modes were observed, namely: divergent section flame (CM1), strut wake stabilized flame (CM2), and jet stabilized flame (CM3). CM1 was observed at a very low ER, where the H₂ was ignited by the normal shock positioned in the divergent section. At this point, the weak shock system at the strut is unable to ignite the fuel. At higher ER, CM2 was observed, as a stronger shock system ignites the richer mixture at the wake of the strut. It was observed that the mixture auto-ignites in the strut wake and doesn't flashback from the divergent section. When the ER is further increased, the stronger injection shock reduces the local velocity and increases the static temperature, enhancing the flame speed of the richer mixture. Thus, the flame flashes back to the fuel jet. Two hysteresis were observed in the supersonic combustor based on ER as a time-varying input. The flame stabilization mode has two solutions based on the history of the change in ER, hence indicating hysteresis. The hysteresis between CM1 and CM2 is because of the retention of the temperature and radicals in the recirculation zone at the wake of the strut. The hysteresis between CM3 and CM2 is because of the retention of the temperature and radicals in the horseshoe vortices around the fuel jets. Understanding hysteresis will help design scramjets with wider operability.     

The blowout limit of a jet diffusion flame in a coflowing stream of lean gaseous fuel-air mixtures

The blowout limit of a circular jet diffusion flame in a low velocity coflowing stream of air is extended significantly by the introduction of a small amount of some fuel vapor in a surrounding flow. The jet fuels employed were methane or hydrogen, while the coflowing stream contained, in turn, methane, hydrogen, propane, or ethylene. The widening of the flame blowout limit could be correlated to the concentration of the fuel in the surrounding stream relative to the corresponding concentration that caused a flame flashback within the surrounding stream in the presence of the jet flame.Moreover, the blowout limit of the flame of a jet of methane containing significant proportions of a diluent such as nitrogen was also extended markedly by the presence of fuel in the surrounding stream. As expected, when carbon dioxide was the diluent in the central jet instead of nitrogen, relatively higher fuel concentrations were needed in the coflowing stream to provide the same jet blowout velocities.     

Autoignition of turbulent hydrogen jet in a coflow of heated air

Autoignition of hydrogen, leading to flame development under turbulent flow conditions is numerically investigated including a detailed chemical mechanism. The chosen configuration consists of a turbulent jet of hydrogen diluted with nitrogen which is issued into a coflow of heated air. Numerical simulations are performed with the Conditional Moment Closure model, to capture the transient evolution of the flow. Turbulence closure is achieved using the k–? model. Simulations revealed that the injected hydrogen mixes with coflowing air, autoignites and a stable diffusion flame is established. Sometimes, flashback of the ignited mixture is observed, whereby the flame travels upstream and stabilizes. It is found that the constants assumed in various modeling terms can severely influence the degree of mixing. Hence, certain modifications to these constants are suggested, and improved predictions are obtained. The sensitivity of autoignition length to the coflow temperature is investigated. The predicted autoignition lengths show a reasonable agreement with the experimental data and LES results.     

Correlation of optical emission and turbulent length scale in a coaxial jet diffusion flame

This article investigates the correlation between optical emission and turbulent length scale in a coaxial jet diffusion flame. To simulate the H₂O emission from an H₂/O₂ diffusion flame, radiative transfer is calculated on flame data obtained by numerical simulation. H₂O emission characteristics are examined for a one-dimensional opposed-flow diffusion flame. The results indicate that H₂O emission intensity is linearly dependent on flame thickness. The simulation of H₂O emission is then extended to an H₂/O₂ turbulent coaxial jet diffusion flame. Time series data for a turbulent diffusion flame are obtained by Large Eddy Simulation, and radiative transfer calculations are conducted on the LES results to simulate H₂O emission optical images. The length scales of visible structures in the simulated emission images are determined by the procedure proposed by Ivancic and Mayer (2002) [8]. The length scales of emission intensity are compared with the integral length scales of velocity and temperature evaluated from LES flowfield data. The results clearly indicate that the length scale of emission intensity agrees well with the integral length scale of temperature, and is also close to that of the radial velocity component. Finally, the explanation as to why the integral length scale of temperature can be extracted from emission intensity distributions is stated.     

Measurements of fuel mixture fraction oscillations of a turbulent jet non-premixed flame

This work describes new type of combustion instability for which the 3-way coupling between mixing, flame heat release, and acoustics is modified by local buoyancy effects. Measurements of fuel mixture fraction are made for a non-premixed jet flame in a combustion chamber to assess the dynamics of mixing under imposed acoustic oscillations (22-55 Hz). Infrared laser absorption and phase resolved acetone-planar laser induced fluorescence are used to measure the fuel mixture fraction and then the degree of fuel/air mixing is calculated by determining the unmixedness. Results show acoustic excitation causes oscillations in the degree of fuel/air mixing at the driving frequency, which results in oscillatory flame behavior. This oscillatory flame behavior couples to the buoyancy and this in turn affects the mixing. Results also show that the mixing becomes less effective when the excitation frequency is increased or when the flame is present, compared to the non-reacting case. This work describes a key coupling mechanism that occurs when buoyancy is a significant factor in the flow field.     

Detailed numerical simulation of radiative transfer in a nonluminous turbulent jet diffusion flame

Radiative transfer in a turbulent jet diffusion flame has been calculated using the discrete ordinates and the ray tracing. The radiative properties of the medium were computed using the correlated k-distribution method and the statistical narrow-band model. The interaction between turbulence and radiation was examined and ways to account for this interaction were compared. Calculations using a stochastic semicausal model were carried out to accurately simulate that interaction, and to provide reference solutions for evaluating the precision of simpler approaches. The models were applied to decoupled radiative transfer calculations in a flame, using experimental fields for temperature and species' concentrations as an input. The correlated k-distribution method, along with the full turbulence/radiation interaction, gave results in very good agreement with the statistical narrow band along with the stochastic model, but the total measured radiative heat loss was underestimated by ∼11.5%. This is likely to be mainly due to the need to extrapolate the data downstream of the last measured radial profile. Enhancement of the radiative heat loss due to turbulent fluctuations was almost 50% in this flame; this exceeds a previous estimate based on a simpler model for the radiative properties of a gas.