Flame speed and tangential strain measurements in widely stratified partially premixed flames interacting with grid turbulence

Methane-air partially premixed flames subjected to grid-generated turbulence are stabilized in a two-slot burner with initial fuel concentration differences leading to stratification across the stoichiometric concentration. The fuel concentration gradient at the location corresponding to the flame base is measured using planar laser induced fluorescence (PLIF) of acetone in the non-reacting mixing field. Simultaneous PLIF of the OH radical and particle image velocimetry (PIV) measurements are performed to deduce the flow velocity and the flame front. These flames exhibit a convex premixed flame front and a trailing diffusion flame, with flow divergence upstream of the flame, as indicated by the instantaneous OH–PLIF, Mie scattering images, and PIV data. The mean streamwise velocity profile attains a global minimum just upstream of the flame front due to expansion of a gases caused by heat release. The flame speed measured just upstream of the flame leading edge is normalized with respect to the turbulent stoichiometric flame speed that takes into account variations in turbulent intensity and integral length scale. The turbulent edge flame speed exceeds the corresponding stoichiometric premixed flame speed and reaches a peak at a certain concentration gradient. The mean tangential strain at the flame leading edge locally peaks at the concentration gradient corresponding to the peak flame speed. The strain varies non-monotonically with the flame curvature unlike in a non-stratified curved premixed flame. The mechanism of peak flame speed is explained as the competition between availability of hot excess reactants from the premixed flame branches to the flame stretch induced due to flame curvature. The results suggest that the stabilization of lifted turbulent partially premixed flames occurs through an edge flame even at a relatively gentle concentration gradient. The strain is also evaluated along the flame front; it peaks at the flame leading edge and decreases gradually on either side of the leading edge. The present results also show qualitatively similar trends as those of laminar triple flames.     

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.     

Investigating the effect of local addition of hydrogen to acoustically excited ethylene and methane flames

While lean combustion in gas turbines is known to reduce NOx, it makes combustors more prone to thermo-acoustic instabilities, which can lead to deterioration in engine performance. The work presented in this study investigates the effectiveness of secondary injection of hydrogen to imperfectly premixed methane and ethylene flames in reducing heat release oscillations. Both acoustically forced and unforced flames were studied, and simultaneous OH and H atom PLIF (planar laser induced fluorescence) was conducted. The tests were carried out on a laboratory scale bluff-body combustor with a central V-shaped bluff body. Two-microphone method was used to estimate velocity perturbations from pressure measurements, flame boundary images were captured using high speed Mie scattering, while global heat release fluctuations were determined from OH* chemiluminescence.The results showed that hydrogen addition considerably reduced heat release oscillations for both methane and ethylene flames at all the forcing frequencies tested, with the exception of methane flames forced at 315 Hz, where oscillations increased with hydrogen addition. The addition of hydrogen reduced the extent of flame roll-up for both methane and ethylene flames, however, this reduction was larger for methane flames. NOx exhaust emissions were observed to increase with hydrogen addition for both methane and ethylene flames, with absolute NOx concentrations higher for ethylene flames, due to higher flame temperatures.     

Turbulence and combustion interaction: High resolution local flame front structure visualization using simultaneous single-shot PLIF imaging of CH, OH, and CH2O in a piloted premixed jet flame

High resolution planar laser-induced fluorescence (PLIF) was applied to investigate the local flame front structures of turbulent premixed methane/air jet flames in order to reveal details about turbulence and flame interaction. The targeted turbulent flames were generated on a specially designed coaxial jet burner, in which low speed stoichiometric gas mixture was fed through the outer large tube to provide a laminar pilot flame for stabilization of the high speed jet flame issued through the small inner tube. By varying the inner tube flow speed and keeping the mixture composition as that of the outer tube, different flames were obtained covering both the laminar and turbulent flame regimes with different turbulent intensities. Simultaneous CH/CH₂O, and also OH PLIF images were recorded to characterize the influence of turbulence eddies on the reaction zone structure, with a spatial resolution of about 40 μm and temporal resolution of around 10 ns. Under all experimental conditions, the CH radicals were found to exist only in a thin layer; the CH₂O were found in the inner flame whereas the OH radicals were seen in the outer flame with the thin CH layer separating the OH and CH₂O layers. The outer OH layer is thick and it corresponds to the oxidation zone and post-flame zone; the CH₂O layer is thin in laminar flows; it becomes broad at high speed turbulent flow conditions. This phenomenon was analyzed using chemical kinetic calculations and eddy/flame interaction theory. It appears that under high turbulence intensity conditions, the small eddies in the preheat zone can transport species such as CH₂O from the reaction zones to the preheat zone. The CH₂O species are not consumed in the preheat zone due to the absence of H, O, and OH radicals by which CH₂O is to be oxidized. The CH radicals cannot exist in the preheat zone due to the rapid reactions of this species with O₂ and CO₂ in the inner-layer of the reaction zones. The local PLIF intensities were evaluated using an area integrated PLIF signal. Substantial increase of the CH₂O signal and decrease of CH signal was observed as the jet velocity increases. These observations raise new challenges to the current flamelet type models.     

Investigation of local flame structures and statistics in partially premixed turbulent jet flames using simultaneous single-shot CH and OH planar laser-induced fluorescence imaging

We report on the application of simultaneous single-shot imaging of CH and OH radicals using planar laser-induced fluorescence (PLIF) to investigate partially premixed turbulent jet flames. Various flames have been stabilized on a coaxial jet flame burner consisting of an outer and an inner tube of diameter 22 and 2.2 mm, respectively. From the outer tube a rich methane/air mixture was supplied at a relatively low flow velocity, while a jet of pure air was introduced from the inner one, resulting in a turbulent jet flame on top of a laminar pilot flame. The turbulence intensity was controlled by varying the inner jet flow speed from 0 up to 120 m/s, corresponding to a maximal Reynolds number of the inner jet airflow of 13,200. The CH/OH PLIF imaging clearly revealed the local structure of the studied flames. In the proximity of the burner, a two-layer reaction zone structure was identified where an inner zone characterized by strong CH signals has a typical structure of rich premixed flames. An outer reaction zone characterized by strong OH signals has a typical structure of a diffusion flame that oxidizes the intermediate fuels formed in the inner rich premixed flame. In the moderate-turbulence flow, the CH layers were very thin closed surfaces in the entire flame, whereas the OH layers were much thicker. In the high-intensity-turbulence flame, the CH layer remained thin until it vanished in the upper part of the flame, showing local extinction and reignition behavior of the flame. The single-shot PLIF images have been utilized to determine the flame surface density (FSD). In low and moderate turbulence intensity cases the FSDs determined from CH and OH agreed with each other, while in the highly turbulent case a locally broken CH layer was observed, leading to a significant difference in the FSD results determined via the OH and CH radicals. Furthermore, the means and the standard deviations of CH and OH radicals were obtained to provide statistical information about the flames that may be used for validation of numerical calculations.     

Time-resolved blowoff transition measurements for two-dimensional bluff body-stabilized flames in vitiated flow

Flame holding and blowoff characteristics of bluff-body stabilized, turbulent flames were measured in an enclosed rectangular duct with a triangular flame holder in vitiated, premixed flows. Blowoff stability margins were characterized with chemiluminescence measurements performed by high-speed imaging to capture flame dynamics during the approach to flame blow off. As the equivalence ratio was decreased, local extinctions along the flames interacting with shear layers surrounding the bluff body recirculation zone occurred with greater frequency and proximity to the wake stagnation zone. Decreased equivalence ratio resulted in extinction events at the trailing edge of the stagnation zone, which allowed reactants to be convected into the recirculation zone and burned behind the bluff body. Increasing reactant dilution of the recirculation zone eventually resulted in flame lift-off or extinction of the flame in the neighboring shear layer. These near field shear layer flames convected to the wake stagnation zone, and were eventually quenched. Simultaneous particle imaging velocimetry (PIV) and OH planar laser-induced fluorescence (PLIF) measurements captured the flame edge location and aerodynamic behavior as blowoff was approached. Two-dimensional hydrodynamic stretch along the flame front and flow field vorticity maps were extracted from the combined PIV/OH PLIF data. The distribution of flame stretch shifted to greater values as the equivalence ratio decreased and is believed to be the cause of local flame extinction in the wake stagnation zone that starts the blowoff process.     

Effect of pulsed,sub-breakdown applied electric field on propane/air flame through simultaneous OH/acetone PLIF

The effects of chemi-ion current induced flow perturbations in a premixed, laminar propane/air flame at atmospheric pressure have been measured with 30 ms-wide applied pulsed voltages. Single-shot OH and acetone planar laser-induced fluorescence (PLIF) images have been collected to measure the spatio-temporal structural changes to a laminar flame with incoming flow speed of 2 m/s in response to positive polarity voltage pulses of 2.8 kV over a 20 mm electrode gap. OH and acetone PLIF are specifically chosen to measure reaction zone modification as the flame undergoes large-scale, stochastic changes. These large-scale changes of flame structure are observed after the flame becomes fully crushed and unstable behavior occurs lasting until the end of the applied voltage pulse. The experimental results of combined OH and acetone PLIF presented in this paper show a significant widening of the reaction zone observed during this unstable behavior. This widening of the reaction zone is indicative of a flame brush normally observed in turbulent flames, demonstrating the ability of the sub-breakdown applied voltage to cause a laminar flame to a transitioning-to-turbulent behavior.     

Spatially resolved heat release rate measurements in turbulent premixed flames

Heat release rate is a fundamental property of great importance for the theoretical and experimental elucidation of unsteady flame behaviors such as combustion noise, combustion instabilities, and pulsed combustion. Investigations of such thermoacoustic interactions require a reliable indicator of heat release rate capable of resolving spatial structures in turbulent flames. Traditionally, heat release rate has been estimated via OH or CH radical chemiluminescence; however, chemiluminescence suffers from being a line-of-sight technique with limited capability for resolving small-scale structures. In this paper, we report spatially resolved two-dimensional measurements of a quantity closely related to heat release rate. The diagnostic technique uses simultaneous OH and CH₂O planar laser-induced fluorescence (PLIF), and the pixel-by-pixel product of the OH and CH₂O PLIF signals has previously been shown to correlate well with local heat release rates. Results from this diagnostic technique, which we refer to as heat release rate imaging (HR imaging), are compared with traditional OH chemiluminescence measurements in several flames. Studies were performed in lean premixed ethylene flames stabilized between opposed jets and with a bluff body. Correlations between bulk strain rates and local heat release rates were obtained and the effects of curvature on heat release rate were investigated. The results show that the heat release rate tends to increase with increasing negative curvature for the flames investigated for which Lewis numbers are greater than unity. This correlation becomes more pronounced as the flame gets closer to global extinction.     

Effects of turbulence and carrier fluid on simple, turbulent spray jet flames

This paper presents simultaneous LIF images of OH and the two-phase acetone fuel concentration as well as detailed single-point phase-Doppler measurements of velocity and droplet flux in three turbulent spray flames of acetone. This work forms part of a larger program to study spray jets and flames in a simple, well-defined geometry, aimed at providing a platform for developing and validating predictive tools for such flows. Spray flames that use nitrogen or air as droplet carrier are investigated and issues of flow field, droplet dispersion, size distribution, and evaporation are addressed. The joint OH/acetone concentration images reveal a substantial similarity to premixed flame behavior when the carrier stream is air. When the carrier is nitrogen, the reaction zone has a diffusion flame structure. There is no indication of individual droplet burning. The results show that evaporation occurs close to the jet centerline rather than in the outer shear layer. Turbulence does not have a significant impact on the evaporation rates. A small fraction of the droplets escapes the reaction zone unburned along the centerline and persists far downstream of the flame tip. The proportion of this droplet residue increases with shorter residence times as observed for the higher velocity flame.     

Physical Insight and Modelling for Lewis Number Effects on Turbulent Heat and Mass Transport in Turbulent Premixed Flames

The effects of global Lewis number on the behavior of Reynolds heat and mass fluxes in turbulent premixed flames are studied based on three-dimensional direct numerical simulation (DNS) of a number of statistically planar turbulent premixed flames with a global Lewis number ranging from Le = 0.34 to 1.2. For the same values of initial turbulent flow field parameters and duration of flame-turbulence interaction, it has been found that both Reynolds heat and mass fluxes may exhibit countergradient transport for flames with a Lewis number significantly smaller than unity; whereas predominantly gradient-type transport is obtained for flames with a Lewis number closer to unity. It is demonstrated that strong flame normal acceleration due to greater heat release in the low Lewis number flames acts to promote countergradient transport, and that the magnitude of the flame normal acceleration decreases with increasing Lewis number. Algebraic models for Reynolds heat and mass fluxes are proposed in which the effects of the Lewis number on flame normal acceleration are explicitly taken into account. The predictions of the new models are compared with DNS data, and the models are found to capture the influence of the Lewis number on turbulent scalar flux in a satisfactory manner for all the flames considered in this study.     

A combined computational and experimental characterization of lean premixed turbulent low swirl laboratory flames: I. Methane flames

This paper is the first in a series that presents a combined computational and experimental study to investigate and characterize the structure of premixed turbulent low swirl laboratory flames. The simulations discussed here are based on an adaptive solution of the low Mach number equations for turbulent reacting flow, and incorporate detailed models for transport and thermo-chemistry. Experimental diagnostics of the laboratory flame include PIV and OH-PLIF imaging, and are used to quantify the flow field, mean flame location, and local flame wrinkling characteristics. We present a framework for relating the simulation results to the flame measurements, and then use the simulation data to further probe the time-dependent, 3D structure of the flames as they interact with the turbulent flow. The present study is limited to lean methane–air flames over a range of flow conditions, and demonstrates that in the regime studied, local flame profiles are structurally very similar to the flat, unstrained steady (“laminar”) flame. The analysis here will serve as a framework for discussing a broader set of premixed flames in this same configuration. Papers II and III will discuss corresponding analysis for pure hydrogen–air and hydrogen–methane mixed fuels, respectively.     

Experimental annular stratified flames characterisation stabilised by weak swirl

A burner for the investigation of lean stratified premixed flames propagating in intense isotropic turbulence has been developed. Lean pre-mixtures of methane at different equivalence ratios were divided between two concentric co-flows to obtain annular stratification. Turbulence generators were used to control the level of turbulence intensity in the oncoming flow. A third annular weakly swirling airflow provided the flame stabilisation mechanism. A fundamental characteristic was that flame stabilisation did not rely on flow recirculation. The flames were maintained at a position where the local mass flux balanced the burning rate, resulting in a freely propagating turbulent flame front. The absence of physical surfaces in the vicinity of the flame provided free access for laser diagnostics. Stereoscopic Planar Image Velocimetry (SPIV) was applied to obtain the three components of the instantaneous velocity vectors on a vertical plane above the burner at the point of flame stabilisation. The instantaneous temperature fields were determined through Laser Induced Rayleigh (LIRay) scattering. Planar Laser Induced Fluorescence (PLIF) of acetone was used to calculate the average equivalence ratio distributions. Instantaneous turbulent burning velocities were extracted from SPIV results, while flame curvature and flame thermal thickness were calculated using the instantaneous temperature fields. The PDFs of these quantities were analysed to consider the separate influence of equivalence ratio stratification and turbulence. Increased levels of turbulence resulted in the expected higher turbulent burning velocities and flame front wrinkling. Flames characterised by higher fuel gradients showed higher turbulent burning velocities. Increased fuel concentration gradients gave rise to increased flame wrinkling, particularly when associated with positive small radius of curvature.     

Experimental study of industrial gas turbine flames including quantification of pressure influence on flow field,fuel/air premixing and flame shape

A commercial swirl burner for industrial gas turbine combustors was equipped with an optically accessible combustion chamber and installed in a high-pressure test-rig. Several premixed natural gas/air flames at pressures between 3 and 6 bar and thermal powers of up to 1 MW were studied by using a variety of measurement techniques. These include particle image velocimetry (PIV) for the investigation of the flow field, one-dimensional laser Raman scattering for the determination of the joint probability density functions of major species concentrations, mixture fraction and temperature, planar laser induced fluorescence (PLIF) of OH for the visualization of the flame front, chemiluminescence measurements of OH^* for determining the lift-off height and size of the flame and acoustic recordings. The results give insights into important flame properties like the flow field structure, the premixing quality and the turbulence–flame interaction as well as their dependency on operating parameters like pressure, inflow velocity and equivalence ratio. The 1D Raman measurements yielded information about the gradients and variation of the mixture fraction and the quality of the fuel/air mixing, as well as the reaction progress. The OH PLIF images showed that the flame was located between the inflow of fresh gas and the recirculated combustion products. The flame front structures varied significantly with Reynolds number from wrinkled flame fronts to fragmented and strongly corrugated flame fronts. All results are combined in one database that can be used for the validation of numerical simulations.     

Assessment of a theoretical model of turbulent combustion by comparison with a simple experiment

In this study a turbulent combustion model for premixed flames with finite rate chemistry is implemented with a new calculation of the diffusion fluxes and turbulent kinetic energy taking into account the fluctuations of density. After the proposal of closure assumptions related to this phenomenon, the model is integrated and the results compared with a simple experimental study.This experimental study concerns an oblique plane premixed (hydrogenair) flame expanding in a turbulent confined flow. Measurements of mean and turbulent velocity field are made using an LDV technique. For mean temperature and concentration measurements, a thermocouple and an isokinetic sampling probe, respectively, are used.     

Effects of strain rate,turbulence, reactant stoichiometry and heat losses on the interaction of turbulent premixed flames with stoichiometric counterflowing combustion products

Intense strain, turbulence, heat transfer, and mixing with combustion products can affect premixed flames in practical combustion devices. These effects are systematically studied in turbulent premixed CH₄/N₂/O₂ flames using a reactant versus product counterflow system and independently varying bulk strain rate, turbulent Reynolds number, equivalence ratio of the reactant mixture, and temperature of the stoichiometric counterflowing combustion products. The flow field and the turbulent flames are investigated using particle image velocimetry (PIV) measurements and laser-induced fluorescence (LIF) imaging of OH. The OH-LIF images are used to identify the interface between the counterflowing streams, referred to here as the gas mixing layer interface (GMLI). The flame response for different flow conditions is compared in terms of the probability of localized extinction along the GMLI, the turbulent flame brush thickness, and flame position relative to the GMLI, by using an OH-LIF-based progress variable. The probability of localized extinction at the GMLI increases as the separation between the turbulent flame brush and the GMLI decreases. Flame fronts in the vicinity of the GMLI are more likely to extinguish as a result of heat losses, dilution of the reaction zone by the product stream, and large local strain rates. A higher probability of localized extinction at the GMLI is induced by either a larger bulk strain rate or a slower flame speed. As the turbulent Reynolds number increases, the corresponding increase in turbulent flame brush thickness enhances the interactions of the flame fronts with the GMLI. Heat losses are substantially less significant for cases in which the turbulent flame brush is sufficiently separated from the GMLI. For flames in close proximity to the GMLI, the effects of the product stream on the flame front differ for lean and rich reactant mixtures. These disparities are attributed in part to differences in the ignitibility of the reactant mixtures by the hot product stream.     

Radiation intensity imaging measurements of methane and dimethyl ether turbulent nonpremixed and partially premixed jet flames

Quantitative time-dependent images of the infrared radiation intensity from methane and dimethyl ether (DME) turbulent nonpremixed and partially premixed jet flames are measured and discussed in this work. The fuel compositions (CH₄/H₂/N₂, C₂H₆O/H₂/N₂, CH₄/air, and C₂H₆O/air) and Reynolds numbers (15,200–46,250) for the flames were selected following the guidelines of the International Workshop on Measurement and Computation of Turbulent Nonpremixed Flames (TNF Workshop). The images of the radiation intensity are acquired using a calibrated high speed infrared camera and three band-pass filters. The band-pass filters enable measurements of radiation from water vapor and carbon dioxide over the entire flame length and beyond. The images reveal localized regions of high and low intensity characteristic of turbulent flames. The peak mean radiation intensity is approximately 15% larger for the DME nonpremixed flames and 30% larger for the DME partially premixed flames in comparison to the corresponding methane flames. The trends are explained by a combination of higher temperatures and longer stoichiometric flame lengths for the DME flames. The longer flame lengths are attributed to the higher density of the DME fuel mixtures based on existing flame length scaling relationships. The longer flame lengths result in larger volumes of high temperature gas and correspondingly higher path-integrated radiation intensities near and downstream of the stoichiometric flame length. The radiation intensity measurements acquired with the infrared camera agree with existing spectroscopy measurements demonstrating the quantitative nature of the present imaging technique. The images provide new benchmark data of turbulent nonpremixed and partially premixed jet flames. The images can be compared with results of large eddy simulations rendered in the form of quantitative images of the infrared radiation intensity. Such comparisons are expected to support the evaluation of models used in turbulent combustion and radiation simulations.     

Computed Tomography of Chemiluminescence (CTC): Instantaneous 3D measurements and Phantom studies of a turbulent opposed jet flame

Time resolved 3D measurements are required to further the understanding of turbulent combustion and to support the development of advanced simulation techniques such as LES. The Computed Tomography of Chemiluminescence (CTC) technique reconstructs the 3D chemiluminescence field of a turbulent flame from a series of integral measurements (camera images). The resulting data can be analysed to obtain the flame surface density, wrinkling factor, flame normal direction and possibly heat release rate, and also to study transient phenomena. High resolution CTC requires measurements from many viewing angles, and the capabilities of recent machine vision cameras make this affordable. The present paper investigates CTC using such commodity cameras. CTC is implemented using five PicSight P32M cameras and mirrors to provide 10 simultaneous views of a premixed turbulent opposed jet (TOJ) flame. The reconstructions are then performed using a 3D Algebraic Reconstruction Technique (ART) algorithm. For the flame investigated, camera exposure times of only 250 μs were found to provide more than sufficient signal-to-noise ratios for ART reconstruction with still shorter exposures times possible. All reconstructions capture the main features of the TOJ flame and were found to provide a useful spatial resolution, even with just 10 views. Detailed Phantom studies were performed to assess the resolution available from ART. The resolution was found to be object dependent but a good working estimate was obtained from a relation by Frieder and Herman (1971) [64]. Reconstructions of realistic LES Phantom data have shown that high resolution reconstructions, which resolve wavelengths of 0.035 object diameters, can be a achieved from only 20 views, with each view costing less than $1000.     

Strategy for PLIF single-shot HCO imaging in turbulent methane/air flames

Formyl (HCO) has since long been recognized as a common intermediate species and a potential local indicator of the major heat release in hydrocarbon combustion. Consequently, the detection of HCO is desirable especially in turbulent flames of practical relevance. However, due to the low concentration and low fluorescence quantum yield, single-shot based detection of HCO with planar laser-induced fluorescence (PLIF) has been a real challenge for experimentalists. In the present paper, a series of systematic investigations have been performed in order to develop a strategy for single-shot HCO PLIF detection in methane/air premixed flames. Potential spectral interference and applicable combustion conditions were analyzed in stable laminar flames employing fluorescence detection with high spectral and spatial resolution for different laser wavelengths. The wavelength 259.004 nm was identified as optimum in giving the maximum signal and minimum spectral interference from other species (e.g., OH and hot O₂). Photolytically generated HCO from formaldehyde (CH₂O) was also observed, which restricts the applicable laser fluence to below 2.5 J/cm² in order to diminish the influence of CH₂O down to 5%. Besides, large hydrocarbon species generated in rich flames were found to contribute a considerable interference which can hardly be screened out. This limits the application of the HCO PLIF technique to lean premixed flames. Finally, by employing an optimized alexandrite laser system, single-shot HCO PLIF imaging in a turbulent methane/air flame is demonstrated, indicating the feasibility of further application of this technique to turbulent combustion systems.     

Simultaneous measurements of velocity and CH distributions. Part 1: jet flames in co-flow

Velocity field and CH distribution are measured simultaneously using particle-image velocimetry (PIV) and planar laser-induced fluorescence (PLIF) of CH in piloted, turbulent, jet flames in coflow. The CH distribution is found to correspond well with the location of the stoichiometric velocity, U_S, both instantaneously and on average. In addition, the CH distribution is observed to align with high-strain rate regions; however, significantly higher values of the maximum strain rates, compared to the mean value, are frequently observed. The residence time in the flame surface as represented by CH, τ_F, remains nearly constant with axial distance downstream and is found to scale as τ_F ∼ d/U_b, where d and U_b are nozzle exit diameter and bulk nozzle-exit velocity, respectively. The mean value of the compressive principal strain rate is observed to decrease along the axial direction and shows a good correlation to a S ∼ (x/d)^−0.7 relation for a wide range of jet Reynolds numbers. Finally, the two-dimensional dilatation is not seen to be a good marker of the flame position, unlike the case for premixed flames.     

Stability characteristics of lifted turbulent partially premixed jet flames

The stability characteristics of partially premixed turbulent lifted methane flames have been investigated and discussed in the present work. Mixture fraction and reaction zone behavior have been measured using a combined 2-D technique of simultaneous Rayleigh scattering, Laser Induced Predissociation Fluorescence (LIPF) of OH and Laser Induced Fluorescence (LIF) of C₂H_x. The stability characteristics and simultaneous mixture fraction-LIPF-LIF measurements in three lifted flames with originally partially premixed jets at different mean equivalence ratio and Reynolds number are presented and discussed in this paper. Higher stability of partially premixed flames as compared to non-premixed flames has been observed. Lifted, attached, blow-out and blow-off regimes have been addressed and discussed in this work. The data show that the mixture fraction field on approaching the stabilization region is uniquely characterized by a certain level of mean and rms fluctuations. This suggests that the stabilization mechanism is likely to be controlled by premixed flame propagation at the stabilization region. Triple flame structure has been detected in the present flames, which is likely to be the appropriate model at the stabilization point.