The nonlinear heat release response of stratified lean-premixed flames to acoustic velocity oscillations

This paper analyzes the forced response of swirl-stabilized lean-premixed flames to high-amplitude acoustic forcing in a laboratory-scale stratified burner operated with CH₄ and air at atmospheric pressure. The double-swirler, double-channel annular burner was specially designed to generate high-amplitude acoustic velocity oscillations and a radial equivalence ratio gradient at the inlet of the combustion chamber. Temporal oscillations of equivalence ratio along the axial direction are dissipated over a long distance, and therefore the effects of time-varying fuel/air ratio on the response are not considered in the present investigation. Simultaneous measurements of inlet velocity and heat release rate oscillations were made using a constant temperature anemometer and photomultiplier tubes with narrow-band OH^∗/CH^∗ interference filters. Time-averaged and phase-synchronized CH^∗ chemiluminescence intensities were measured using an intensified CCD camera. The measurements show that flame stabilization mechanisms vary depending on equivalence ratio gradients for a constant global equivalence ratio (?_g = 0.60). Under uniformly premixed conditions, an enveloped M-shaped flame is observed. In contrast, under stratified conditions, a dihedral V-flame and a toroidal detached flame develop in the outer stream and inner stream fuel enrichment cases, respectively. The modification of the stabilization mechanism has a significant impact on the nonlinear response of stratified flames to high-amplitude acoustic forcing (u′/U ∼ 0.45 and f = 60, 160 Hz). Outer stream enrichment tends to improve the flame’s stiffness with respect to incident acoustic/vortical disturbances, whereas inner stream stratification tends to enhance the nonlinear flame dynamics, as manifested by the complex interaction between the swirl flame and large-scale coherent vortices with different length scales and shedding points. It was found that the behavior of the measured flame describing functions (FDF), which depend on radial fuel stratification, are well correlated with previous measurements of the intensity of self-excited combustion instabilities in the stratified swirl burner. The results presented in this paper provide insight into the impact of nonuniform reactant stoichiometry on combustion instabilities, its effect on flame location and the interaction with unsteady flow structures.     

Response of partially premixed flames to acoustic velocity and equivalence ratio perturbations

This article describes an experimental investigation of the forced response of a swirl-stabilized partially premixed flame when it is subjected to acoustic velocity and equivalence ratio fluctuations. The flame’s response is analyzed using phase-resolved CH^* chemiluminescence images and flame transfer function (FTF) measurements, and compared with the response of a perfectly premixed flame under acoustic perturbations. The nonlinear response of the partially premixed flame is manifested by a partial extinction of the reaction zone, leading to rapid reduction of flame surface area. This nonlinearity, however, is observed only when the phase difference between the acoustic velocity and the equivalence ratio at the combustor inlet is close to zero. The condition, Δφ_Φ^′-V^′≈0°, indicates that reactant mixtures with high equivalence ratio impinge on the flame front with high velocity, inducing large fluctuations of the rate of heat release. It is found that the phase difference between the acoustic velocity and equivalence ratio nonuniformities is a key parameter governing the linear/nonlinear response of a partially premixed flame, and it is a function of modulation frequency, inlet velocity, fuel injection location, and fuel injector impedance. The results presented in this article will provide insight into the response of a partially premixed flame, which has not been well explored to date.     

Stability characteristics of non-premixed turbulent jet flames of hydrogen and syngas blends with coaxial air

The stability characteristics of attached hydrogen (H₂) and syngas (H₂/CO) turbulent jet flames with coaxial air were studied experimentally. The flame stability was investigated by varying the fuel and air stream velocities. Effects of the coaxial nozzle diameter, fuel nozzle lip thickness and syngas fuel composition are addressed in detail. The detachment stability limit of the syngas single jet flame was found to decrease with increasing amount of carbon monoxide in the fuel. For jet flames with coaxial air, the critical coaxial air velocity leading to flame detachment first increases with increasing fuel jet velocity and subsequently decreases. This non-monotonic trend appears for all syngas composition herein investigated (50/50 → 100/0% H₂/CO). OH^∗ chemiluminescence imaging was performed to qualitatively identify the mechanisms responsible for the flame detachment. For all fuel compositions, local extinction close to the burner rim is observed at lower fuel velocities (ascending stability limit), while local flame extinction downstream of the burner rim is observed at higher fuel velocities (descending stability limit). Extrema of the non-monotonic trends appear to be identical when the nozzle fuel velocity is normalized by the critical fuel velocity obtained for the single jet cases.     

Investigation of the nonlinear response of turbulent premixed flames to imposed inlet velocity oscillations

Acoustically forced lean premixed turbulent bluff-body stabilized flames are investigated using turbulent combustion CFD. The calculations simulate aspects of the experimental investigation by Balachandran et al. [R. Balachandran, B. Ayoola, C. Kaminski, A. Dowling, E. Mastorakos, Combust. Flame 143 (2005) 37-55] and focus on the amplitude dependence of the flame response. For the frequencies of interest in this investigation an unsteady Reynolds-averaged Navier-Stokes (URANS) approach is appropriate. The combustion is represented using a modified laminar flamelet approach with an algebraic representation of the flame surface density. The predictions are compared with flame surface density (FSD) and OH^∗ chemiluminescence measurements. In the experiments the response of the flame has been quantified by means of a number of single-frequency, amplitude-dependent transfer functions. The predicted flame shape and position are in good agreement with the experiment. The dynamic response of the flame to inlet velocity forcing is also well captured by the calculations. At moderate frequencies nonlinear behavior of the transfer functions is observed as the forcing amplitude is increased. In the experiments this nonlinearity was attributed in part to the rollup of the reacting shear layer into vortices and in part to the collision of the inner and outer flame sheets. This transition to nonlinearity is also observed in the transfer functions obtained from the predictions. Furthermore, the vortex shedding and flame-sheet collapse may be seen in snapshots of the predicted flow field taken throughout the forcing cycle. The URANS methodology successfully predicts the behavior of the forced premixed turbulent flames and captures the effects of saturation in the transfer function of the response of the heat release to velocity fluctuations.     

Numerical investigation on mixing performance and diffusion combustion characteristics of H2 and air in planar micro-combustor

In the present work, the effects of inlet velocity and channel height (H₀ = 0.6 mm, 1.0 mm and 1.4 mm) on the mixing performance, flame stability limit and combustion efficiency of H₂ and air in a 2D planar micro-combustor with a separating plate were studied numerically. The results demonstrate that improved mixing can be achieved with a decrease in inlet velocity and channel height. Moreover, the flame blow-off limit is the largest for a micro-combustor with H₀ = 0.6 mm; the flame becomes inclined at a high velocity and the direction varies with the inlet velocity. Furthermore, a micro-combustor with a medium height (H₀ = 1.0 mm) can achieve the largest blowout limit among the three cases. Finally, for identical inlet velocities, the combustion efficiency increases with decreasing combustor height. In summary, these findings can provide a guideline for the optimal design of such micro-combustors.     

Spatially distributed flame transfer functions for predicting combustion dynamics in lean premixed gas turbine combustors

The present paper describes a methodology to improve the accuracy of prediction of the eigenfrequencies and growth rates of self-induced instabilities and demonstrates its application to a laboratory-scale, swirl-stabilized, lean-premixed, gas turbine combustor. The influence of the spatial heat release distribution is accounted for using local flame transfer function (FTF) measurements. The two-microphone technique and CH^∗ chemiluminescence intensity measurements are used to determine the input (inlet velocity perturbation) and the output functions (heat release oscillation), respectively, for the local flame transfer functions. The experimentally determined local flame transfer functions are superposed using the flame transfer function superposition principle, and the result is incorporated into an analytic thermoacoustic model, in order to predict the linear stability characteristics of a given system. Results show that when the flame length is not acoustically compact the model prediction calculated using the local flame transfer functions is better than the prediction made using the global flame transfer function. In the case of a flame in the compact flame regime, accurate predictions of eigenfrequencies and growth rates can be obtained using the global flame transfer function. It was also found that the general response characteristics of the local FTF (gain and phase) are qualitatively the same as those of the global FTF.     

Stratified jet flames in a heated (1390 K) air cross-flow with autoignition

Measurements are reported of the heat release profiles, the flame lengths, flame structure and other properties of a reacting jet-in-cross-flow (JICF) for two fuels. The air was heated to a static temperature of 1390 K, which is above the autoignition temperature, and the air velocity was 468 m/s, which is much larger than values that were considered previously. Aerodynamic strain rates are so large that the flame was expected to fall into either the “distributed reaction”, “thickened flamelet”, or “shredded flamelet” regimes. Fluorescence images of CH, OH and formaldehyde identified the flame structure. The jet-in-cross-flow is a unit physics problem that occurs in turbojets and scramjets. While scaling relations are known for the non-reacting case, more information about the reacting case is needed, especially when autoignition and strain rates become important. Three regions were identified. In the liftoff region autoignition reactions occur which create a strong formaldehyde PLIF signal. However, flames and heat release do not occur in the liftoff region since CH and CH¹ signals were negligible. The second region is the lifted flame base, which has the character of a premixed flame, as evidenced by a very rapid rise in the heat release rate as indicated by the CH¹ and OH¹ signals. The third region contains a turbulent non-premixed flame and the CH images indicate the presence of thickened and shredded flamelets. The 2–3 mm thickness of each CH layer is more than 10 times the laminar flamelet thickness. In the third region the heat release rate decays slowly downstream, which is typical of a non-premixed flame. Because both upstream autoignition and downstream thickened flamelets were observed, we classify this combustion to be an “autoignition-assisted flame”. Flame lengths increase linearly with fuel mass flow rate, indicating that mixing is controlled by the air velocity rather than the fuel velocity.     

Investigation of OH1 chemiluminescence with lift-off characteristic in methane-oxygen inverse diffusion flame

OH¹ chemiluminescence is a reliable indication of the combustion state and can express characteristic parameters of flame. In this work, OH¹ chemiluminescence in the lift-off process of methane-oxygen inverse diffusion flame was investigated by spectrum diagnostic technology. The experimental results show that OH¹ chemiluminescence is mainly concentrated in the root of the flame and the fully developed region of the flame, and two OH¹ signal peaks appear obviously, which indicates that there are two different reaction regions during in the lift-off process. In addition, a simulation based on CFD combining with GRI-Mech 3.0 mechanism was performed about the OH¹ generation pathway of flame lift-off process. The simulation results show that the reaction of CH + O₂OH¹+CO (R2) is the main pathway to generate OH¹ in the root of the flame (zone I). Reaction H + O + M=OH¹+M (R1) is the dominant reaction in the fully developed region of the flame (zone II), and its reaction rate to generation OH¹ radical is significantly higher than reaction R2.     

Numerical investigations on flashback dynamics of premixed methane-hydrogen-air laminar flames

Injecting hydrogen into the natural gas network to reduce CO₂ emissions in the EU residential sector is considered a critical element of the zero CO₂ emissions target for 2050. Burning natural gas and hydrogen mixtures has potential risks, the main one being the flame flashback phenomenon that could occur in home appliances using premixed laminar burners. In the present study, two-dimensional transient computations of laminar CH₄ + air and CH₄ + H₂ + air flames are performed with the open-source CFD code OpenFOAM. A finite rate chemistry based solver is used to compute reaction rates and the laminar reacting flow. Starting from a flame stabilized at the rim of a cylindrical tube burner, the inlet bulk velocity of the premixture is gradually reduced to observe flashback. The results of the present work concern the effects of wall temperature and hydrogen addition on the flashback propensity of laminar premixed methane-hydrogen-air flames. Complete sequences of flame dynamics with gradual increases of premixture velocity are investigated. At the flame flashback velocities, strong oscillations at the flame leading edge emerge, causing broken flame symmetry and finally flame flashback. The numerical results reveal that flashback tendency increase with increasing wall temperature and hydrogen addition rate.     

The effects of aspect ratio on CH4/air flame stability in rectangular mesoscale combustors

The effects of aspect ratio (α = 1, 2, 3 and 4) of mesoscale rectangular channels on CH₄-air flame stability were numerically studied. The results show that at relatively high inlet velocity, the flame is convex (with respect to the upstream) when α ≤ 3; instead it shifts to a W-shaped appearance at α = 4. In addition, the flame blowout limit exhibits a non-monotonic variation and it peaks at α = 3. Our analysis reveals that when α is raised from 1 to 3, both of the heat loss ratio and hydrodynamic perturbation rate are reduced, which favors a larger blowout limit. However, for the channel with α = 4, the perturbation rate of hydrodynamic instability is significantly increased at the flame tip because of the increased wave number, which results from the W-shaped flame front. Consequently, the intensified hydrodynamic instability will lead to a reduction in the flame blowout limit of the channel with α = 4.     

Large eddy simulation of laser ignition and compressible reacting flow in a rocket-like configuration

The control of ignition in a rocket engine is a critical problem for combustion chamber design. Delayed ignition may lead to high-amplitude pressure fluctuations that can damage the burner (strong ignition), whereas early ignition may fail. This paper describes a numerical study of a strong ignition sequence observed in a laboratory-scale single-injector rocket chamber ignited by a laser and fueled with gaseous oxygen and hydrogen. OH-emission images, Schlieren pictures, and pressure measurements make it possible to follow the flame propagation experimentally. The present large eddy simulation (LES) approach includes shock treatment, a six species-seven reaction chemical scheme for H₂-O₂, and a model for the energy deposition by a laser. Flame/turbulence interaction is modeled with the thickened flame concept. LES is used to compute both the filling phase (during which the gaseous hydrogen and oxygen mix) and the ignition phase. The flame location and structure, as well as the temporal evolution of the chamber pressure obtained numerically, are in good agreement with the experiment. The use of complex chemistry in the computation also allows the comparison of LES data with experimental OH-images and shows that the sensitivity of the CCD camera used to record the spontaneous emission of the OH^∗ radical is not high enough to properly locate the flame front in rich regions. The combined experimental and numerical results lead to a more detailed analysis of the ignition processes and its coupling with flow rate oscillations in the H₂ and O₂ feeding lines.     

On the similitude between lifted and burner-stabilized triple flames: a numerical and experimental investigation

We have investigated lifted triple flames and addressed issues related to flame stabilization. The stabilization of nonpremixed flames has been argued to result due to the existence of a premixing zone of sufficient reactivity, which causes propagating premixed reaction zones to anchor a nonpremixed zone. We first validate our simulations with detailed measurements in more tractable methane–air burner-stabilized flames. Thereafter, we simulate lifted flames without significantly modifying the boundary conditions used for investigating the burner-stabilized flames. The similarities and differences between the structures of lifted and burner-stabilized flames are elucidated, and the role of the laminar flame speed in the stabilization of lifted triple flames is characterized. The reaction zone topography in the flame is as follows. The flame consists of an outer lean premixed reaction zone, an inner rich premixed reaction zone, and a nonpremixed reaction zone where partially oxidized fuel and oxidizer (from the rich and lean premixed reaction zones, respectively) mix in stoichiometric proportion and thereafter burn. The region with the highest temperatures lies between the inner premixed and the central nonpremixed reaction zone. The heat released in the reaction zones is transported both upstream (by diffusion) and downstream to other portions of the flame. Measured and simulated species concentration profiles of reactant (O₂, CH₄) consumption, intermediate (CO, H₂) formation followed by intermediate consumption and product (CO₂, H₂O) formation are presented. A lifted flame is simulated by conceptualizing a splitter wall of infinitesimal thickness. The flame liftoff increases the height of the inner premixed reaction zone due to the modification of the upstream flow field. However, both the lifted and burner-stabilized flames exhibit remarkable similarity with respect to the shapes and separation distances regarding the three reaction zones. The heat-release distribution and the scalar profiles are also virtually identical for the lifted and burner-stabilized flames in mixture fraction space and attest to the similitude between the burner-stabilized and lifted flames. In the lifted flame, the velocity field diverges upstream of the flame, causing the velocity to reach a minimum value at the triple point. The streamwise velocity at the triple point is ≈0.45 m s⁻¹ (in accord with the propagation speed for stoichiometric methane–air flame), whereas the velocity upstream of the triple point equals 0.7 m s⁻¹, which is in excess of the unstretched flame propagation speed. This is in agreement with measurements reported by other investigators. In future work we will address the behavior of this velocity as the equivalence ratio, the inlet velocity profile, and inlet mixture fraction are changed.     

Flame dynamics and unsteady heat release rate of self-excited azimuthal modes in an annular combustor

This paper presents an experimental study into the structure and dynamics of the phase-averaged heat release rate during self-excited spinning and standing azimuthal modes in an annular combustion chamber. The flame response was characterised using two methods: high-speed OH^∗ chemiluminescence imaged above the annulus to investigate the structure of the phase-averaged fluctuations in heat release rate, and high-speed OH-PLIF measured across the centreline of two adjacent flames to investigate phase-averaged flame dynamics. Two-microphone measurements were obtained at three circumferential locations to determine the modes and the amplitude of the velocity fluctuations. It was found that the flame responds differently to spinning and standing wave modes. During standing wave modes, the amplitude of the unsteady heat release rate of each flame (sector) varied according to its location in the mode shape with maximum fluctuations occurring at the pressure anti-nodes and minimum fluctuations occurring at the pressure nodes. At the pressure anti-nodes, peak fluctuations result from the production of flame surface area by axisymmetric flame motions caused by the modulation of flow at the burner inlet by the pressure fluctuations. However, at the pressure nodes, an anti-symmetric, transverse flapping motion of the flame occurred producing negligible unsteady heat release rate over the oscillations cycle via the mechanism of cancellation. During spinning modes, the structure of the heat release rate was found to be asymmetric and characterised by the preferential suppression of shear layer disturbances depending on the spin direction.     

Effects of CO addition on the characteristics of laminar premixed CH4/air opposed-jet flames

The effects of CO addition on the characteristics of premixed CH₄/air opposed-jet flames are investigated experimentally and numerically. Experimental measurements and numerical simulations of the flame front position, temperature, and velocity are performed in stoichiometric CH₄/CO/air opposed-jet flames with various CO contents in the fuel. Thermocouple is used for the determination of flame temperature, velocity measurement is made using particle image velocimetry (PIV), and the flame front position is measured by direct photograph as well as with laser-induced predissociative fluorescence (LIPF) of OH imaging techniques. The laminar burning velocity is calculated using the PREMIX code of Chemkin collection 3.5. The flame structures of the premixed stoichiometric CH₄/CO/air opposed-jet flames are simulated using the OPPDIF package with GRI-Mech 3.0 chemical kinetic mechanisms and detailed transport properties. The measured flame front position, temperature, and velocity of the stoichiometric CH₄/CO/air flames are closely predicted by the numerical calculations. Detailed analysis of the calculated chemical kinetic structures reveals that as the CO content in the fuel is increased from 0% to 80%, CO oxidation (R99) increases significantly and contributes to a significant level of heat-release rate. It is also shown that the laminar burning velocity reaches a maximum value (57.5 cm/s) at the condition of 80% of CO in the fuel. Based on the results of sensitivity analysis, the chemistry of CO consumption shifts to the dry oxidation kinetics when CO content is further increased over 80%. Comparison between the results of computed laminar burning velocity, flame temperature, CO consumption rate, and sensitivity analysis reveals that the effect of CO addition on the laminar burning velocity of the stoichiometric CH₄/CO/air flames is due mostly to the transition of the dominant chemical kinetic steps.     

Numerical study of methane and air combustion inside a small tube with an axial temperature gradient at the wall

A numerical study on CH₄ and air premixed combustion inside a small tube with a temperature gradient at the wall was undertaken to investigate the effects of inlet velocity, equivalence ratio and combustor size on combustion characteristics. The simulation results show that the inlet velocity has a significant influence on the reaction zone, and the flame front shifts downstream as the inlet velocity increases. The results also show that, the inlet velocity has no obvious effects on the flame temperature. The highest flame temperature is obtained if the equivalence ratio is set to 1. It is disclosed that the combustor size strongly influences the combustion characteristics. The smaller the combustor size is, the more difficult it is to maintain the steady combustion. The smallest combustor size that the stable flame can be sustained is determined mainly by the wall temperature of the micro-combustor under the given conditions. The higher the wall temperature is, the smaller the smallest combustor size. Therefore increasing wall temperature is an effective way to realize flame stabilization for a given combustor size.     

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.     

Local measurements of the time-dependent heat release rate and equivalence ratio using chemiluminescent emission from a flame

Chemiluminescent emissions from OH^⋅, CH^⋅, and C^⋅₂ and continuous emissions from CO^⋅₂ have been measured in natural-gas-fuelled, premixed, counterflow flames operating with an equivalence ratio between 0.7 and 1.3 and strain rates between 80 and 400 s⁻¹, achieved by varying the exit velocity of the jet between 1 and 5 m s⁻¹. Cassegrain receiving optics coupled to a high-performance spectroscopic unit allowed local, temporally resolved measurements of intensity of chemiluminescence in flat flames, which were compared with measurements along a line of sight obtained from flame spectra. This study allowed independent evaluation of the effects of strain rate and equivalence ratio on intensity of chemiluminescence and the ability of intensity of chemiluminescence to indicate heat release rate. Results suggest that intensities of chemiluminescence from OH^⋅ and CH^⋅ and background intensity from CO^⋅₂ are good indicators of heat release rate, whereas that from C^⋅₂ is not. The study also evaluated the ability to measure equivalence ratio of the reacting mixture using intensity of chemiluminescence and found that the intensity ratio OH^⋅/CH^⋅ has a monotonic decrease with equivalence ratio for lean and stoichiometric mixtures, while remaining independent of flame strain rate. The results indicate that the intensity ratio OH^⋅/CH^⋅ can measure with uncertainties of 5% the equivalence ratio up to values of 1.1 and with uncertainties of 20% for richer mixtures due to the low sensitivity of the intensity ratio OH^⋅/CH^⋅ for rich mixtures. The intensity ratios of C^⋅₂/OH^⋅ and C^⋅₂/CH^⋅ have a non-monotonic dependence on equivalence ratio and a dependence on strain rate and are thus not suitable for measurements of equivalence ratio. An approach to measuring the time-dependent equivalence ratio of the reacting mixture is suggested and data processing methods and associated uncertainties are presented. The potential of the technique for local measurements in practical burners is discussed and further evaluation of the spatial resolution is required in such flames. However, suggestions have been provided on how the spatial resolution can be improved in practical flames.     

Numerical evaluation of equivalence ratio measurement using OH and CH chemiluminescence in premixed and non-premixed methane-air flames

This work presents results from detailed chemical kinetics calculations of electronically excited OH (A²Σ, denoted as OH^∗) and CH (A²Δ, denoted as CH^∗) chemiluminescent species in laminar premixed and non-premixed counterflow methane-air flames, at atmospheric pressure. Eight different detailed chemistry mechanisms, with added elementary reactions that account for the formation and destruction of the chemiluminescent species OH^∗ and CH^∗, are studied. The effects of flow strain rate and equivalence ratio on the chemiluminescent intensities of OH^∗, CH^∗ and their ratio are studied and the results are compared to chemiluminescent intensity ratio measurements from premixed laminar counterflow natural gas-air flames. This is done in order to numerically evaluate the measurement of equivalence ratio using OH^∗ and CH^∗ chemiluminescence, an experimental practise that is used in the literature. The calculations reproduced the experimental observation that there is no effect of strain rate on the chemiluminescent intensity ratio of OH^∗ to CH^∗, and that the ratio is a monotonic function of equivalence ratio. In contrast, the strain rate was found to have an effect on both the OH^∗ and CH^∗ intensities, in agreement with experiment. The calculated OH^∗/CH^∗ values showed that only five out of the eight mechanisms studied were within the same order of magnitude with the experimental data. A new mechanism, proposed in this work, gave results that agreed with experiment within 30%. It was found that the location of maximum emitted intensity from the excited species OH^∗ and CH^∗ was displaced by less than 65 and 115 μm, respectively, away from the maximum of the heat release rate, in agreement with experiments, which is small relative to the spatial resolution of experimental methods applied to combustion applications, and, therefore, it is expected that intensity from the OH^∗ and CH^∗ excited radicals can be used to identify the location of the reaction zone. Calculations of the OH^∗/CH^∗ intensity ratio for strained non-premixed counterflow methane-air flames showed that the intensity ratio takes different values from those for premixed flames, and therefore has the potential to be used as a criterion to distinguish between premixed and non-premixed reaction in turbulent flames.     

Combustion behavior and stability map of hydrogen-enriched oxy-methane premixed flames in a model gas turbine combustor

The article describes an experimental study and comparison of the combustion behavior and determines the stability map of turbulent premixed H₂-enriched oxy-methane flames in a model gas turbine combustor. Static stability limits, in terms of flashback and blow-out limits, are recorded over a range of hydrogen fraction (HF) at a fixed oxygen fraction (OF) of 30% and a particular inlet bulk velocity, and the results are compared with the non-enriched case (HF = 0%). The static stability limits are also recorded for different inlet bulk velocity (4.4, 5.2, and 6 m/s) and the results are compared to explore the effect of flow dynamics on operability limits of H₂-enriched flames. The stability maps are presented as a function of equivalence ratio (0.3–1.0) and HF (0%–75%) plotted on the contours of adiabatic flame temperature (AFT), power density (PD), inlet Reynolds number (Re) and reacting mixture mass flow rate ($\dot{m}$) to understand the physics behind flashback and blow-out phenomena. The results indicated that both the flashback and blow-out limits tend to move towards the leaner side with increasing HF due to the improved chemical kinetics. The stability limits are observed to follow the Reynolds number indicating its key role in controlling flame static stability limits. The results showed that H₂ enrichment is effective in the zone from HF = 20% up to HF = 50%, and O₂ enrichment is also effective in a similar zone from OF = 20% up to 50%, with wider stability boundaries for H₂ enrichment. Axial and radial temperature profiles are presented to explore the effect of HF on the progress of chemical reactions within the combustor and to serve as the basis for validation of numerical models. Flame shapes are recorded using a high-speed camera and compared for different inlet velocities to explore the effects of H₂-enrichment and equivalence ratio on flame stability. The equivalence ratio at which a transition of flame stabilization from the inner shear layer (ISL) to the outer recirculation zone (ORZ) occurs is determined for different inlet bulk velocities. The value of the transition equivalence ratio is found to decrease while increasing the inlet bulk velocity. Flame shapes near flashback limit, as well as near blow-out limit, are compared to explore the mechanisms of flame extinctions. Flame shapes are compared at fixed adiabatic flame temperature, fixed inlet velocity and fixed flow swirl to isolate their effects and investigate the effect of kinetic rates on flame stability. The results showed that the adiabatic flame temperature does not govern the flame static stability limits.