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.     

Effect of Partial Premixing on Stabilization and Local Extinction of Turbulent Methane/Air Flames

The stabilization characteristics and local extinction structures of partially premixed methane/air flames were studied using simultaneous OH-PLIF/PIV techniques, and large eddy simulations employing a two-scalar flamelet model. Partial premixing was made in a mixing chamber comprised of two concentric tubes, where the degree of partial premixing of fuel and air was controlled by varying the mixing length of the chamber. At the exit of the mixing chamber a cone was mounted to stabilize the flames at high turbulence intensities. The stability regime of flames was determined for different degree of partial premixing and Reynolds numbers. It was found that in general partially premixed flames at low Reynolds numbers become more stable when the level of partial premixing of air to the fuel stream decreases. At high Reynolds numbers, for the presently studied burner configuration there is an optimal partial premixing level of air to the fuel stream at which the flame is most stable. OH-PLIF images revealed that for the stable flames not very close to the blowout regime, significant local extinction holes appear already. By increasing premixing air to fuel stream successively, local extinction holes grow in size leading to eventual flame blowout. Local flame extinction was found to frequently attain to locations where locally high velocity flows impinging to the flame. The local flame extinction poses a future challenge for model simulations and the present flames provide a possible test case for such study.     

Investigation of Jet-Flame Blowout with Lean-Limit Considerations

The current study utilizes digital image sequences of flames to better understand the blowout phenomenon. Methane flames are studied near blowout conditions to determine if the disappearance of the diffusion flame prior to extinguishment signifies the leading edge of the reaction zone reaching the lean-limit. Various concentrations of nitrogen are used to dilute methane flames. The axial position of the flames is compared with the calculated position of the lean flammability limit to determine the role of the diffusion flame. The blowout limits of these flames are established and a blowout parameter is empirically determined from the data. Results from flames in co-flow show agreement with the blowout parameter previously published; however, the analysis shows that, the disappearance of the bulk diffusive reaction zone occurs at the lean flammability limit and is an accurate predictor of blowout for diluted and non-diluted methane 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.     

Simultaneous particle image velocimetry and chemiluminescence visualization of millisecond-pulsed current�Cvoltage-induced perturbations of a premixed propane/air flame

The effects of millisecond-wide, pulsed current?Cvoltage-induced behavior in premixed laminar flames have been investigated through the simultaneous collection of particle image velocimetry (PIV) and chemiluminescence data with particular attention paid to the onset mechanisms. Disturbances caused by applied voltages of 2?kV over a 30-mm gap to a downward propagating, atmospheric pressure, premixed propane/air flame with a flow speed near 2?m/s and an equivalence ratio of 1.06 are investigated. The combined PIV and chemiluminescence-based experimental data show the observed disturbance originates only in or near the cathode fall region very close to the burner base. The data also suggest that the coupling mechanism responsible for the flame disturbance behavior is fluidic in nature, developing from the radial positive chemi-ion distribution and an ion-drift current-induced net body force that acts along the annular space discharge distribution in the reaction zone in or near the cathode fall. This net body force causes a reduction in flow speed above these near cathodic regions causing the base of the flame to laterally spread. Also, this effect seems to produce a velocity gradient leading to the transition of a laminar flame to turbulent combustion for higher applied current?Cvoltage conditions as shown in previous work (Marcum and Ganguly in Combust Flame 143:27?C36, 2005; Schmidt and Ganguly in 48th AIAA aerospace sciences meeting. Orlando, 2010).     

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.     

Laser-based investigations in gas turbine model combustors

Dynamic processes in gas turbine (GT) combustors play a key role in flame stabilization and extinction, combustion instabilities and pollutant formation, and present a challenge for experimental as well as numerical investigations. These phenomena were investigated in two gas turbine model combustors for premixed and partially premixed CH₄/air swirl flames at atmospheric pressure. Optical access through large quartz windows enabled the application of laser Raman scattering, planar laser-induced fluorescence (PLIF) of OH, particle image velocimetry (PIV) at repetition rates up to 10 kHz and the simultaneous application of OH PLIF and PIV at a repetition rate of 5 kHz. Effects of unmixedness and reaction progress in lean premixed GT flames were revealed and quantified by Raman scattering. In a thermo-acoustically unstable flame, the cyclic variation in mixture fraction and its role for the feedback mechanism of the instability are addressed. In a partially premixed oscillating swirl flame, the cyclic variations of the heat release and the flow field were characterized by chemiluminescence imaging and PIV, respectively. Using phase-correlated Raman scattering measurements, significant phase-dependent variations of the mixture fraction and fuel distributions were revealed. The flame structures and the shape of the reaction zones were visualized by planar imaging of OH distribution. The simultaneous OH PLIF/PIV high-speed measurements revealed the time history of the flow field–flame interaction and demonstrated the development of a local flame extinction event. Further, the influence of a precessing vortex core on the flame topology and its dynamics is discussed.     

Flow Structure of Swirling Turbulent Propane Flames

Flow structure of premixed propane–air swirling jet flames at various combustion regimes was studied experimentally by stereo PIV, CH* chemiluminescence imaging, and pressure probe. For the non-swirling conditions, a nonlinear feedback mechanism of the flame front interaction with ring-like vortices, developing in the jet shear layer, was found to play important role in the stabilisation of the premixed lifted flame. For the studied swirl rates (S = 0.41, 0.7, and 1.0) the determined domain of stable combustion can be divided into three main groups of flame types: attached flames, quasi-tubular flames, and lifted flames. These regimes were studied in details for the case of S = 1.0, and the difference in the flow structure of the vortex breakdown is described. For the quasi-tubular flames an increase of flow precessing above the recirculation zone was observed when increased the stoichiometric coefficient from 0.7 to 1.4. This precessing motion was supposed to be responsible for the observed increase of acoustic noise generation and could drive the transition from the quasi-tubular to the lifted flame regime.     

Effect of Nozzle Exit Turbulence on the Column Trajectory and Breakup Location of a Transverse Liquid Jet in a Gaseous Flow

This study examines the effect of fully developed turbulent flow at the exit of nozzle/injector on the trajectory and column breakup location of a liquid jet injected transverly into a gaseous crossflow. Liquid jet trajectory and column breakup for different nozzle geometries at different velocities of liquid jet and crossflow are analytically and experimentally Investigated. Shadowgraph imaging technique is used to determine the jet trajectory and breakup location of a transverse liquid jet in a uniform airflow. Particle image velocimetry (PIV) is used to measure the near-field velocity profile of a liquid jet discgarged into a quiescent atmosphere. The experimental results show a higher penetration and breakup height for the liquid jet ensuing from a nozzle with a smaller length to diameter ratio. This is due to the surface irregularities of the liquid column of a turbulent jet, which breaks up and consequently follows the cross airflow sooner. In order to capture the effect of turbulence, the analytical trajectory correlation developed in our previous studies is modified to account for the discharge coefficient of a nozzle. The discharge coefficient is estimated indirectly by comparing the liquid column trajectory predicted by the modified analytical correlation with that determined experimentally. The indirectly determined discharge coefficient is then used in the analytical correlation for predicting the breakup height of a transverse liquid jet. The results predicted using this approach are in good agreement with the experimental data of the present study at standard temperature and pressure (STP) test conditions.     

Simultaneous measurement of temperature and fuel mole fraction using acetone planar induced fluorescence and Rayleigh scattering in stratified flames

The use of acetone as a tracer for planar laser induced of fluorescence (PLIF) measurements is very popular both for mixing investigations and for premixed or partially premixed combustion systems when evaluating the local mixture fraction (or equivalence ratio) in the fresh gases. The local structure of a flame front can be investigated by using Rayleigh scattering, and this technique has been quite frequently used in combustion. We present here an application of simultaneous imaging of temperature and fuel mole fraction with both acetone PLIF and Rayleigh scattering techniques. The strong influence of temperature on fluorescence signals can be corrected if the local temperature is known. Simultaneously, the contribution of the acetone Rayleigh cross-section can be evaluated through the local value of acetone mole fraction. An iterative process enables the fuel mole fraction (in the limit of the preheat zone) and temperature fields to be obtained in a reactive configuration. The technique is limited by the maximum temperature that can be corrected and by the tracer specificities. Tests in laminar homogeneous stabilized flames and in stratified stabilized flames demonstrate the ability to record the instantaneous flame structure and fuel mole fraction field. Finally, the paper presents correlations of the local flame thickness with the local methane mole fraction, which underline the strong influence of large scales of the equivalence ratio on the local flame structure.     

A Comparison of the Blow-Off Behaviour of Swirl-Stabilized Premixed, Non-Premixed and Spray Flames

Confined short turbulent swirling premixed and non-premixed methane and heptane spray flames stabilized on an axisymmetric bluff body in a square enclosure have been examined close to the blow-off limit and during the extinction transient with OH* chemiluminescence and OH-PLIF operated at 5 kHz. The comparison of flames of different canonical types in the same basic aerodynamic field allows insights on the relative blow-off behaviour. The flame structure has been examined for conditions increasingly closer to blow-off. The premixed flame was seen to change from a cylindrical shape at stable burning condtions, with the flame brush closing across the flow at conditions close to blow-off. The PLIF images show that for the gaseous non-premixed flame, holes appear along the flame sheet with increasing frequency as the blow-off condition is approached, while the trend is less obvious for the spray flame. Non-premixed and spray flames showed randomly-occurring lift-off, which is further evidence of localised extinction. The mean lift-off height increased with increasing fuel jet velocity and decreased with increasing air velocity and approaches zero (i.e. the flame is virtually attached) just before the blow-off condition, despite the fact that more holes were evident in the flame sheet as extinction was approached. It was found that the average duration of the blow-off event, when normalised with the characteristic flow time d/U _b (d being the bluff-body diameter and U _b the bulk velocity) was in the range 9–38 with the spray flame extinction lasting a shorter time than the gaseous flames. Finally, it was found that correlations based on a Damköhler number collapse the blow-off velocity data for all flames with reasonable accuracy. The results can help the development of advanced turbulent combustion models.     

Central recirculation zone analysis in an unconfined tangential swirl burner with varying degrees of premixing

Swirl-stabilised combustion is one of the most widely used techniques for flame stabilisation, uses ranging from gas turbine combustors to pulverised coal-fired power stations. In gas turbines, lean premixed systems are of especial importance, giving the ability to produce low NOx systems coupled with wide stability limits. The common element is the swirl burner, which depends on the generation of an aerodynamically formed central recirculation zone (CRZ) and which serves to recycle heat and active chemical species to the root of the flame as well as providing low-velocity regions where the flame speed can match the local flow velocity. Enhanced mixing in and around the CRZ is another beneficial feature. The structure of the CRZ and hence that of the associated flames, stabilisation and mixing processes have shown to be extremely complex, three-dimensional and time dependent. The characteristics of the CRZ depend very strongly on the level of swirl (swirl number), burner configuration, type of flow expansion, Reynolds number (i.e. flowrate) and equivalence ratio. Although numerical methods have had some success when compared to experimental results, the models still have difficulties at medium to high swirl levels, with complex geometries and varied equivalence ratios. This study thus focuses on experimental results obtained to characterise the CRZ formed under varied combustion conditions with different geometries and some variation of swirl number in a generic swirl burner. CRZ behaviour has similarities to the equivalent isothermal state, but is strongly dependent on equivalence ratio, with interesting effects occurring with a high-velocity fuel injector. Partial premixing and combustion cause more substantive changes to the CRZ than pure diffusive combustion.     

Comparison of Combustion Models and Assessment of Their Applicability to the Simulation of Premixed Turbulent Combustion in IC-Engines

In order to simulate the turbulent combustion process occurring in spark-ignition (IC) engines, it is necessary to provide suitable and numerically economical models, the latter being particularly important in the application to industrial problems. Moreover, these models must deliver sufficiently accurate results for the unsteady operation of spark combustion engines, concerning variable geometries, temperatures, pressures and charge development in different configurations. In this work different turbulent combustion models for premixed hydrocarbon combustion are compared with respect to their ability to accurately predict the propagation of turbulent perfectly premixed flames. As a first configuration a cylinder of constant volume was studied. Transient calculations were used to simulate the propagation of the turbulent flame and to evaluate the resulting turbulent burning velocity. These calculations were performed for a perfect mixture of air and hydrocarbons at stoichiometric mixture and different initial conditions concerning pressure, temperature and turbulence intensity. As a second configuration a stationary turbulent bunsen-type flame with methane fuel was used to validate the turbulent combustion model of [Lindstedt and Vaos, Combust. Flame 116 (1999) 461] at different pressures. Calculated results were then compared to experimental data of [Kobayashi, Tamura, Maruta and Niioka. In: Proceedings of the 26th Symposium on Combustion, 1996, p. 389] and show excellent agreement for the turbulent burning velocity at several pressure levels using only a single set of model parameters.     

Measurement of the conditioned turbulence and temperature field of a premixed Bunsen burner by planar laser Rayleigh scattering and stereo particle image velocimetry

The turbulence and temperature field of Bunsen-type turbulent lean methane/air flames has been investigated using planar laser Rayleigh scattering (PLRS) and stereo particle image velocimetry (stereo PIV). Temporally averaged reaction progress variable plots have been computed from PLRS measurements in order to provide a basis with regards to the verification of computational fluid dynamics (CFD) models. Turbulence was characterised by stereo PIV in one plane for all three velocity components. Averaged velocity fields have been calculated, as well as Reynolds-decomposed fluctuation vector fields. Conditioned root mean square (RMS) values of the turbulent fluctuations in terms of unburnt and burnt gas could be determined by making use of the information gained from a threshold setting procedure in the PIV raw images. Furthermore, several length scales were measured indirectly from PIV vector plots. In this context, all integral length scales being accessible with stereo PIV were computed separately for the burnt and unburnt regions and were compared to each other. It could be observed that all integral length scales increased in the burnt zone. Additionally, the conditioned Taylor and Kolmogorov lengths have been extracted from the PIV field data, derived either from the zero-radius curvature of the correlation function or from common turbulence theory relations.     

Comparison of PIV data using multiple configurations and processing techniques

Particle image velocimetry (PIV) data have been acquired using three different experimental configurations in the far-field of the interaction created by a transverse supersonic jet exhausting from a flat plate into a transonic crossflow. The configurations included two-component PIV in the centerline streamwise plane at two overlapping stations, as well as stereoscopic PIV in both the same streamwise plane and in the crossplane. All measurement planes intersected at a common line. Data from both two-component measurement stations and the stereoscopic streamwise configuration agreed to within the estimated uncertainty, but data from the crossplane exhibited reduced velocity and turbulent stress magnitudes by a small but significant degree. Subsequent reprocessing of the data in nominally the same manner using a newer software package brought all values into close agreement with each other, but produced substantially higher turbulent stresses. The error source associated with the choice of software was traced to the use of image deformation in the newer software to treat velocity gradients, which is shown by synthetic PIV tests to yield a more accurate result for turbulence measurements even for gradients within the recommended limits for classical PIV. These detailed comparisons of replicate data suggest that routine methods of uncertainty quantification used for a turbulent PIV experiment may not fully capture the actual error sources.     

Effects of the Local Flow Topologies Upon the Structure of a Premixed Methane-air Turbulent Jet Flame

Local flow topologies have been identified and their interactions with the iso-scalar surfaces geometries have been investigated using the results of a three-dimensional direct numerical simulation (DNS) of a turbulent premixed methane-air flame in a piloted Bunsen burner configuration with tabulated chemistry. The universal teardrop shape of the joint probability density function (jpdf) of the second and third invariants of the velocity-gradient tensor disappears in the different flame regions under study. A ‘canonical’ vortex, which affects the fine-scale structure of the turbulent premixed flame, has been identified and analyzed at three times, differing by increments of the order of the Kolmogorov time micro-scale.     

Spatial and Temporal Characteristics of OH in Turbulent Opposed-Jet Double Flames

Simultaneous high repetition-rate, two-point hydroxyl (OH) time-series measurements with associated PLIF/PIV measurements are employed to investigate spatio–temporal scales and flame-velocity interactions in turbulent opposed jets sustaining methane-air double flames. For a fuel-side equivalence ratio, ϕ _B  = 1.2, a rich premixed flame exists on the fuel side while a diffusion flame exists on the air side of the stagnation plane. The bulk Reynolds number (Re) and strain rate (SR) can be adjusted to generate flames at ϕ _B  = 1.2 with both well separated and completely merged flame fronts. Simultaneous PLIF/PIV measurements highlight distinct spatial OH structures of the premixed and diffusive fronts corresponding to variations in the flow field. The self-propagating tendency of the rich premixed front causes large-scale wrinkling, thereby enhancing the OH contour length by 15% as compared to the diffusive front. Two-point OH time-series measurements are implemented to quantify both spatial and temporal fluctuations via study of radial length and time scales. In general, these integral length and time scales follow similar trends and reach a minimum at the axial location of peak [OH]. In comparison to merged double flames having higher Re and SR, greater OH fluctuations are observed in the rich-premixed front as compared to the diffusive front for a well separated double flame. Because of the developing turbulence, the OH length scales exhibit reduced axial gradients across the reaction zone for higher Re in comparison to lower Re. A stochastic time-series simulation, using a state relationship based on a joint mixture fraction and progress variable, is utilized to extract estimated scalar time scales from those of measured OH. The simulations indicate that the hydroxyl fluctuations in double flames are only twice those of the underlying conserved scalar. “Turbulent Opposed-Jet Double Flames” is submitted for consideration as a full length article to Flow Turbulence and Combustion.     

Near-Equidiffusion Disturbed Premixed Flames with Wider Reaction Zones

Most of the asymptotic considerations of the interaction of premixed flames with a general flow, i.e. curved and stretched flames subjected to time dependent flow, are dedicated to high activation energy asymptotes. Therefore, in these considerations the reaction zone is thin and the temperature within the reaction zone is constant to the leading order of approximation. Here we consider an order unity activation energy for near-equidiffusion flames and show that the flame speed relations obtained are distinct from those obtained by high activation energy asymptotes. The flame is assumed to be thin in comparison with the flow scales but the reaction zone is no longer thin in comparison with the flame width. Although obtaining analytical solutions is problematic even for undisturbed flames with wide reaction zones, we found that the propagation speed of disturbed premixed flames with wide reaction zones is determined by analytical integrals involving the temperature profile of the undisturbed flame. We also found independent effects of curvature and stretch for the flames with wider reaction zones. This revised version was published online in July 2006 with corrections to the Cover Date.     

Dynamics of PIV seeding particles in turbulent premixed flames

Particle image velocimetry (PIV) estimates the fluid velocity field measuring the displacement of small dispersed particles between two successive instants separated by a small time interval. The accuracy of the measurements depends on the ability of the particles to accommodate their velocity to the fluid fluctuations. When the fluid is subjected to extreme accelerations, the small but finite inertia prevents the particles from following the fluid, originating a substantial relative velocity. This effect is shown to be crucial for applications of PIV to turbulent premixed combustion, particularly in the product region at locations just behind the instantaneous flame front. The issuing inaccuracy may easily spoil the estimate of certain statistical observables which are of crucial importance in the theory of turbulent premixed combustion. By exploiting the direct numerical simulation of a model air/methane flame, a suitable criterion for proper particle seeding is validated and compared with the corresponding experiments with a combined PIV/OH-LIF (laser-induced fluorescence) system. The proposed parameter, the flamelet Stokes number, depends on particle properties and thermochemical conditions of the flame and substantially restricts the particle dimensions required for a reliable estimate of the relevant flow statistics.     

Finite Rate Chemistry Effects in Highly Sheared Turbulent Premixed Flames

Detailed scalar structure measurements of highly sheared turbulent premixed flames stabilized on the piloted premixed jet burner (PPJB) are reported together with corresponding numerical calculations using a particle based probability density function (PDF) method. The PPJB is capable of stabilizing highly turbulent premixed jet flames through the use of a small stoichiometric pilot that ensures initial ignition of the jet and a large shielding coflow of hot combustion products. Four lean premixed methane-air flames with a constant jet equivalence ratio are studied over a wide range of jet velocities. The scalar structure of the flames are examined through high resolution imaging of temperature and OH mole fraction, whilst the reaction rate structure is examined using simultaneous imaging of temperature and mole fractions of OH and CH₂O. Measurements of temperature and mole fractions of CO and OH using the Raman–Rayleigh–LIF-crossed plane OH technique are used to examine the flame thickening and flame reaction rates. It is found that as the shear rates increase, finite-rate chemistry effects manifest through a gradual decrease in reactedness, rather than the abrupt localized extinction observed in non-premixed flames when approaching blow-off. This gradual decrease in reactedness is accompanied by a broadening in the reaction zone which is consistent with the view that turbulence structures become embedded within the instantaneous flame front. Numerical predictions using a particle-based PDF model are shown to be able to predict the measured flames with significant finite-rate chemistry effects, albeit with the use of a modified mixing frequency.