Flame-vortex interaction and mixing behaviors of turbulent non-premixed jet flames under acoustic forcing

This study examines the effect of acoustic excitation using forced coaxial air on the flame characteristics of turbulent hydrogen non-premixed flames. A resonance frequency was selected to acoustically excite the coaxial air jet due to its ability to effectively amplify the acoustic amplitude and reduce flame length and NO_x emissions. Acoustic excitation causes the flame length to decrease by 15% and consequently, a 25% reduction in EINO_x is achieved, compared to coaxial air flames without acoustic excitation at the same coaxial air to fuel velocity ratio. Moreover, acoustic excitation induces periodical fluctuation of the coaxial air velocity, thus resulting in slight fluctuation of the fuel velocity. From phase-lock PIV and OH PLIF measurement, the local flow properties at the flame surface were investigated under acoustic forcing. During flame-vortex interaction in the near field region, the entrainment velocity and the flame surface area increased locally near the vortex. This increase in flame surface area and entrainment velocity is believed to be a crucial factor in reducing flame length and NO_x emission in coaxial jet flames with acoustic excitation. Local flame extinction occurred frequently when subjected to an excessive strain rate, indicating that intense mass transfer of fuel and air occurs radially inward at the flame surface.     

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.     

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.     

Structure of laminar sooting inverse diffusion flames

The flame structure of laminar inverse diffusion flames (IDFs) was studied to gain insight into soot formation and growth in underventilated combustion. Both ethylene-air and methane-air IDFs were examined, fuel flow rates were kept constant for all flames of each fuel type, and airflow rates were varied to observe the effect on flame structure and soot formation. Planar laser-induced fluorescence of hydroxyl radicals (OH PLIF) and polycyclic aromatic hydrocarbons (PAH PLIF), planar laser-induced incandescence of soot (soot PLII), and thermocouple-determined gas temperatures were used to draw conclusions about flame structure and soot formation. Flickering, caused by buoyancy-induced vortices, was evident above and outside the flames. The distances between the OH, PAH, and soot zones were similar in IDFs and normal diffusion flames (NDFs), but the locations of those zones were inverted in IDFs relative to NDFs. Peak OH PLIF coincided with peak temperature and marked the flame front. Soot appeared outside the flame front, corresponding to temperatures around the minimum soot formation temperature of 1300 K. PAHs appeared outside the soot layer, with characteristic temperature depending on the wavelength detection band. PAHs and soot began to appear at a constant axial position for each fuel, independent of the rate of air flow. PAH formation either preceded or coincided with soot formation, indicating that PAHs are important components in soot formation. Soot growth continued for some time downstream of the flame, at temperatures below the inception temperature, probably through reaction with PAHs.     

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.     

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.     

Hysteresis and transition in swirling nonpremixed flames

Strongly swirling nonpremixed flames are known to exhibit a hysteresis when transiting from an attached long, sooty, yellow flame to a short lifted blue flame, and vice versa. The upward transition (by increasing the air and fuel flow rates) corresponds to a vortex breakdown, i.e. an abrupt change from an attached swirling flame (unidirectional or with a weak bluff-body recirculation), to a lifted flame with a strong toroidal vortex occupying the bulk of the flame. Despite dramatic differences in their structures, mixing intensities and combustion performance, both flame types can be realised at identical flow rates, equivalence ratio and swirl intensity. We report here on comprehensive investigations of the two flame regimes at the same conditions in a well-controlled experiment in which the swirl was generated by the rotating outer pipe of the annular burner air passage. Fluid velocity measured with PIV (particle image velocimetry), the qualitative detection of reaction zones from OH PLIF (planar laser-induced fluorescence) and the temperature measured by CARS (coherent anti-Stokes Raman spectroscopy) revealed major differences in vortical structures, turbulence, mixing and reaction intensities in the two flames. We discuss the transition mechanism and arguments for the improved mixing, compact size and a broader stability range of the blue flame in comparison to the long yellow flame.     

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.     

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.     

Numerical and experimental study on silica generating counterflow diffusion flames

Temperatures and concentrations of OH radicals in silica generating counterflow oxy-hydrogen diffusion flames are measured using a broadband coherent anti-Stokes Raman spectroscopy (CARS) and a planar laser induced fluorescence (PLIF) techniques to study thermo-chemical effects of SiCl₄ addition to flames. Numerical analysis considering detailed chemical reactions including silica generating reactions is also conducted. The experimental results demonstrate that temperatures decrease in preheated zone due to the increase in specific heat of the gas mixture while the decrease is mitigated in particle formation zone due to the heat release through hydrolysis and oxidation reactions of SiCl₄. Also, OH concentrations significantly decrease in silica formation flame, which can be attributed to the consumption of oxidative radicals during the silica generating reactions of SiCl₄ and depletion of OH by HCl. The numerical simulation agrees well for flames having relatively low flame temperatures of 1750 K but underestimates the decrease in OH concentration for high temperature flame over 2700 K. The disagreement for the high temperature flames would imply possible OH consumption via direct reactions between OH radicals and silicon chlorides, which is expected to be highly sensitive to temperature.     

Acoustic excitation effect on NOx reduction and flame stability in a lifted non-premixed turbulent hydrogen jet with coaxial air

The effects of acoustic excitation on the reduction in nitric oxidant (NO_x) emission were experimentally investigated in non-premixed lifted hydrogen jet flames with coaxial air. The purpose of the present work was to analyze the acoustic forcing effect on the flow field, the reaction zone, and NO_x emission, and to study the mechanisms of NO_x reduction and flame stabilization. To analyze of the flow field, a PIV method was used that incorporated two Nd-YAG lasers and a CCD camera. The reaction zone was visualized by taking OH* chemiluminescence images with a 307.1 ± 5 nm narrow band pass filter and an ICCD camera. A flow condition was carefully selected at u_F = 150, 200, 250 m/s and u_A = 12, 16, 20 m/s, which was sustainable for acoustic excitation in a lifted flame region. The frequency was swept from 150 to 1000 Hz in 5 Hz steps. From the measurements of the flow field, the reaction zone, and NO_x emission, we concluded that NO_x emission was reduced and minimized at the resonance frequency. The vortex that was generated by acoustic forcing promoted air entrainment and enhanced the fuel-air mixing rate. This premixing effect resulted in a lower flame temperature, and thus lower NO_x emissions. In addition, the liftoff height periodically fluctuated due to the stretch effect as the vortex interacted with the flame base.     

Stable negative edge flame formation in a counterflow burner

In nonpremixed combustion, edge flames can form as a region of flame propagation or flame recession. Forwardly propagating edge flames, as occur in lifted flames, have a local gas velocity at the flame edge that is from unburned partially premixed fuel and air into the flame. These flames represent an ignition process, and permit the flame itself to either stabilize against an incoming gas stream or propagate into unburned fuel and air. Negative edge flames represent the opposite case of a local gas velocity from burned products through the flame edge. The negative edge flame represents a local extinction process, and occurs, for example, during vortex-induced extinction of a nonpremixed flame sheet. A technique for generating steady negative edge flames in a standard counterflow burner is presented, which permits detailed examination of their properties. A coannular counterflow burner is used to create a strain gradient that quenches a central diffusion flame. Unlike previous research on strain-induced flame edges, the axisymmetric flow field ensures gas flow from products through the edge. Measurements of the edge flame's sensitivity to global strain rates and fuel mixtures are presented, along with measurements of the edge flame structure using OH fluorescence and CH emission imaging.     

Transient response of premixed methane flames

The response of premixed methane-air flames to transient strain and local variations in equivalence ratio is studied during isolated interactions between a line-vortex pair and a V-flame. The temporal evolution of OH and CH is measured with planar laser-induced fluorescence for N₂-diluted flames with equivalence ratios ranging from 0.8 to 1.2. One-dimensional laminar flame calculations are used to simulate the flame response to unsteady strain and variations in reactant composition. When the reactant composition of the vortex pair and the V-flame are identical, the measurements and predictions show that the peak mole fractions of OH and CH decay monotonically in lean, stoichiometric, and rich flames. We also investigate the effects of a vortex pair with a leaner composition than the V-flame. In a stoichiometric flame, the leaner vortex enhances the decay of both OH and CH. In a rich flame, we observe an abrupt increase in OH-LIF signal and a disappearance of CH-LIF signal that are consistent with a previous experimental investigation. Our results indicate that the previously observed OH burst and CH breakage were caused by a difference in the equivalence ratios of the vortex pair and the main reactant flow. A numerical study shows that N₂ dilution enhances the response of premixed flames to unsteady strain and variations in stoichiometry. Reaction-path and sensitivity analyses indicate that the peak OH and CH mole fractions exhibit significant sensitivity to the main branching reaction, H + O₂ ↔ OH + O. The sensitivity of OH and CH to this and other reactions is enhanced by N₂ dilution. As a result, N₂-diluted flames provide a good test case for studying the reliability of chemical kinetic and transport models.     

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.     

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.     

Experimental study on the small‐scale diffusion flame of ethanol and the wall temperature field

An experimental investigation was carried out on the diffusion flames of liquid ethanol burning in air. Ceramic tubes with different inner diameters were used as burners. Three different flame structures at different flow rates were identified and different combustion regions were divided based on the experimental results. Some relevant factors which may affect the flame height and width were discussed. The outer surface wall temperature field of the tubes was measured in the combustion process. The results showed that both flame height and flame width all increased proportionally with an increasing flow rate in the steady flame region. Both flame height and flame width decreased with a decreasing tube inner diameter at the same flow rate. By decreasing the tube inner diameter, the quenching flow rate was decreased, the flow rate according to the produced periodic explosive flame decreased, and the flow rate range of the steady flames decreased. The outer wall temperature field presented an exponential distribution, and the wall temperature reached the greatest value at the outlet of the ceramic tube. © 2009 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20282     

Flame structure and global flame response to the equivalence ratios of interacting partially premixed methane and hydrogen flames

The interacting partially premixed methane and hydrogen flames established in a one-dimensional counterflow field were investigated numerically with the OPPDIF code and GRI-v3.0 was used to consider both fuels. The flame structure and response of the maximum flame temperature, heat-release rate, and flame speed to the equivalence ratios (Φ) and global strain rate (a_g) were investigated. The maximum temperature decreased with increasing a_g. The maximum temperature for cases with a stoichiometric hydrogen-side flame was higher than for other cases with the same a_g.The hydrogen-side flame played a key role in determining the maximum temperature. The maximum heat-release rates (MHRRs) for all cases show different trends. The MHRR of the methane-side flame was affected considerably by the interacting flame structure and hydrogen-side flame condition. However, the MHRRs of the hydrogen were independent of methane-side flame condition. For the cases where Φ of the methane-side flame was varied while the hydrogen-side flame was kept stoichiometric (Var-S), the MHRR and flame speed of the hydrogen-side flame were independent of the methane-side flame conditions. However, the methane-side flames had a negative flame speed except near-stoichiometric conditions. On the other hand, in the cases where Φ of the hydrogen-side flame was varied while the methane-side flame was kept stoichiometric (S-Var), the hydrogen-side flames had the MHRR and flame speed similar to those of an unstretched partially premixed hydrogen flame.     

Parametric and statistical investigation of the behavior of a lifted flame over a turbulent free-jet structure

Partially premixed combustion is involved in many practical applications, due to partial premixing of combustible and oxidant gases before ignition, or due to local extinctions, which lead to mixing of reactants and burned gases. To investigate some features of flames in stratified flows, the stabilization processes of lifted turbulent jet flames are studied. This work offers a large database of liftoff locations of flames stabilized on turbulence-free jets for different fuels and nozzle diameters studied over their flame stability domains. Methane, propane, and ethylene flames are investigated for nozzle diameters of 2, 3, 4, and 5 mm. Blowout velocities are measured and compared with an approach based on large-scale structures of the jet. The axial and radial locations of the flame base are measured by planar laser-induced fluorescence (PLIF) of the OH radical through high sampling (at least 5000 points). From this large database the average locations of the flame base are analyzed for the fuels investigated. The pdfs exhibit an evolution of their shapes according to the region of the turbulent jet where the flame stabilizes (potential core, transition to turbulence, or fully developed turbulence regions). This dependence is probably due to the interaction of the flame with the jet structures. This is confirmed by the comparison between the amplitude of the height fluctuations and the local size of the large-scale structures deduced from particle image velocimetry measurements and self-similarity laws for velocity. The results show the flame can be carried over a distance equal to the local diameter of the jet within the region of fully developed turbulence for propane and ethylene, and over a slightly larger distance for methane.     

Hydrogen addition effects in a confined swirl-stabilized methane-air flame

The effect of hydrogen addition in methane-air premixed flames has been examined from a swirl-stabilized combustor under confined conditions. The effect of hydrogen addition in methane-air flame has been examined over a range of conditions using a laboratory-scale premixed combustor operated at 5.81 kW. Different swirlers have been investigated to identify the role of swirl strength to the incoming mixture. The flame stability was examined for the effect of amount of hydrogen addition, combustion air flow rates and swirl strengths. This was carried out by comparing adiabatic flame temperatures at the lean flame limit. The combustion characteristics of hydrogen-enriched methane flames at constant heat load but different swirl strengths have been examined using particle image velocimetry (PIV), micro-thermocouples and OH chemiluminescence diagnostics that provided information on velocity, thermal field, and combustion generated OH species concentration in the flame, respectively. Gas analyzer was used to obtain NO_x and CO concentration at the combustor exit. The results show that the lean stability limit is extended by hydrogen addition. The stability limit can reduce at higher swirl intensity to the fuel-air mixture operating at lower adiabatic flame temperatures. The addition of hydrogen increases the NO_x emission; however, this effect can be reduced by increasing either the excess air or swirl intensity. The emissions of NO_x and CO from the premixed flame were also compared with a diffusion flame type combustor. The NO_x emissions of hydrogen-enriched methane premixed flame were found to be lower than the corresponding diffusion flame under same operating conditions for the fuel-lean case.     

Extinction limit extension of unsteady counterflow diffusion flames affected by velocity change

The unsteady extinction limit of (CH₄ + N₂)/air diffusion flames was investigated in terms of the time history of the strain rate and initial strain rates. A spatially locked flame in an opposed-jet counterflow burner was perturbed using linear velocity variation, and time-dependent flame luminosity and unsteady extinction limits were measured with a high-speed intensified CCD (ICCD) camera. In addition, the transient maximum flame temperature and hydroxyl (OH) radical were measured as a function of time using Rayleigh scattering and OH laser-induced fluorescence, respectively. In this experiment, unsteady flames survive at strain rates that are much higher than the extinction limit of steady flames and unsteady extinction limits increase as the slope of the strain rate increases or as the initial strain rate decreases. We found that the equivalent strain rate represents well the unsteady behavior in the outer convective-diffusive layer of the flame. By using the equivalent strain rate, we were able to accurately estimate the contribution of the unsteady effect in the outer convective-diffusive layer to the extinction limit extension, and we also identified the unsteady effect in the inner diffusive-reactive layer of the flame. Consequently, the extension of unsteady extinction limits results from the unsteady effects of both the convective-diffusive layer and the diffusive-reactive layer. The former effect is dominant at the beginning of the velocity change, and the latter effect is dominant near the extinction limit.