Propane-air Flames Research Articles

Numerical Study of High-g Combustion Characteristics in a Channel with Backward-Facing Steps

High gravity (high-g) combustion can significantly increase flame propagation speed, thereby potentially shortening the axial length of aero-engines and increasing their thrust-to-weight ratio. In this study, we utilized the large eddy simulation model to investigate the combustion characteristics and flame morphology evolution of premixed propane–air flames in a channel with a backward-facing step. The study reveals that both the increase in centrifugal force and flow velocity can enhance pressure fluctuations during combustion and increase the turbulence intensity. The presence of centrifugal force promotes the occurrence of Rayleigh–Taylor instability (RTI) between hot and cold fluids. The combined effects of RTI and Kelvin–Helmholtz instability (KHI) enhance the disturbance between hot and cold fluids, shorten the fuel combustion time, and intensify the dissipation of large-scale vortices. The increase in fluid flow velocity can raise the flame front’s hydrodynamic stretch rate, thereby enhancing the turbulence level during combustion to a certain extent and increasing the fuel consumption rate. When a strong centrifugal force is applied, the global flame propagation speed can be more than doubled. Within a certain range, the increase in high-g field strength can enhance the intensity of RTI and accelerate the transition of RTI to the nonlinear stage.

A comprehensive investigation on flame flickering characteristics of premixed conical flame at different inclination angles

To thoroughly investigate how the inclination angle affects the flickering of premixed conical flames, a series of experiments and simulations were carried out. Propane-air conical flames were established on a Bunsen burner, and various inclination angles were tested, resulting in a wide range of flame flickering frequency data. The findings revealed that the further the inclination angle deviates from 0°, the lower the flickering frequency. Consequently, an empirical correlation that factors in the inclination angle was developed. To gain a more comprehensive understanding of the flame flickering behavior under different gas inclination angles, researchers utilized Particle Image Velocimetry (PIV) and numerical simulation to visualize the velocity fields of a typical flame. The results demonstrated that deviated inclination angles not only alter the shear strength between the burnt gas and surrounding gas but also significantly change the flame flickering mode. These factors lead to various Kelvin-Helmholtz instability patterns and vortex shedding modes, ultimately impacting flickering frequency.

Comparative Analysis Of Cycle-To-Cycle Variabilities And Combustion Development In An Optical Spark-Ignition Engine Fueled By Pure Hydrogen And Propane: Insights From Chemiluminescence and PI

Hydrogen, derived from renewable sources and devoid of carbon emissions, is a pivotal energy carrier for the future. Representing a viable substitute for fossil fuels in internal combustion engines (ICEs), contemporary studies advocate using very-lean and ultra-lean hydrogen-air mixtures, with a fuel-air equivalence ratio below 0.5, as a potent strategy to reduce NOx emissions in hydrogen-fueled ICEs. An experimental setup with an optical spark-ignition engine was devised to investigate the primary factors influencing cycle-to-cycle variations in H2ICEs by optimizing mixture homogeneity and focusing the study on flame interaction with in-cylinder aerodynamics. Simultaneous in-cylinder pressure, chemiluminescence, and PIV analyses were performed to characterize and compare the behaviors of ultra-lean hydrogen-air and propane-air flames, assessing flame development and cyclic variation features. Results indicate that the flame development of hydrogen and propane is significantly different. Propane exhibited increased in-cylinder pressure trace variability at lean and rich flammability limits with a high flame development difference between fast and slow cycles. In contrast, hydrogen-air mixtures under various equivalence ratios (from very-lean to ultra-lean) presented much more stable in-cylinder pressures. Furthermore, fast propane flames were mainly advected to the cylinder's symmetrical axes, while fast hydrogen flames started further away from the spark plug. Horizontal and vertical PIV measurements showed a single structure flow field over the studied conditions with mainly intensity variations over the measured cycles. Fast flames were associated with more intense tumble motion. The differences between fast and slow flames for hydrogen are attributed to the early flame development. However, after initial differences in flame development over several crank angles, there are similarities in all cycles, suggesting that the low variability in in-cylinder pressures is mostly due to the robustness of hydrogen flame ignition and its independence from in-cylinder flow small variations at the early development phase.

A numerical analysis of multi-dimensional iron flame propagation using boundary-layer resolved simulations

Iron combustion is emerging as a topic of immediate interest, given the need for the decarbonization of heat and power generation. Opposite to other solid fuels, iron particles burn predominantly in a non-volatile, heterogeneous combustion mode. For iron dust flames, two modes of flame propagation have been observed i.e. the discrete mode, when the heat transfer time scale is larger than the particle burn-time, and the continuous mode, when the heat transfer timescale is smaller than the burn-time of a single particle. The flame speed of such a flame primarily depends on the heat and mass transfer which is characterized by the distance between the particles for a dispersed mixture. Depending on the variation in particle distance and local distribution, planar or curved flames can form and propagate. The characteristics of flame propagation motivate this study and for the first time, three-dimensional particle boundary layer resolved simulations are used to analyse the effect of curvature and variable particle distances on iron–air flame propagation. Simulations are carried out for (1) planar flame propagation with constant particle spacing in the direction of propagation, and (2) curved flame propagation with varying particle spacing in the direction of propagation. The analysis of the flame speed for the planar flame propagation later guides the analysis of curved flames propagating through 900 boundary-layer resolved particles distributed in a 30 by 30 grid. The curved flame propagates with a uniform but decreased flame speed compared to the planar flame propagation with equivalent particle spacing. It is shown that positive curvature decreases the heat transfer from the burnt to the unburnt side while a variable spatial arrangement of particles allows for a local re-distribution of oxygen which causes some regions to burn richer or leaner than the corresponding planar flames with similar particle spacing.

Phase denoising and unwrapping method for moiré fringes based on multiresolution analysis

Phase unwrapping for moiré fringes plays a crucial role in moiré tomography, and the precision of its outcomes deeply influences the detection results concerning high-temperature complex flow fields. In this paper, a phase denoising and unwrapping technique grounded in multiresolution analysis (MRA) is presented, and it is effectively applied to extract phase information from moiré fringes for propane-air flame and argon arc plasma flow fields. Firstly, the wrapped phase undergoes a wavelet packet decomposition for overall denoising. Subsequently, MRA is conducted to refine the denoising process and unwrap phase, involving layered wavelet packet decomposition and iterative unwrapping on different levels of approximate components. Following the hierarchical structure of MRA, the results from different levels are systematically combined and summarized. In order to ascertain the feasibility and applicability of this approach, a comparison is conducted between the results obtained from our proposed method and those obtained from three representative traditional methods. And then, the outcomes obtained from various unwrapping methods in scenarios involving noise and discontinuity, as well as the results between diverse wavelet basis functions, different decomposition layers and distinct iterations, are meticulously analyzed and compared. The findings indicate that our proposed method outperforms traditional phase unwrapping techniques in terms of continuity and smoothness, particularly when exposed to severe noise. Unlike the three classic methods, our proposed method maintains consistent results even in the presence of noise, thereby highlighting its advantages in terms of resilience against noise. In a word, this research serves as a crucial reference for the accurate detection of complex high-temperature flow fields by moiré tomography.

On the mechanism of “tulip flame” formation: The effect of ignition sources

The initial stages of hydrogen–air flame propagation in tubes and the mechanism of tulip flame formation are investigated using a high-order numerical code to solve the fully compressible reactive Navier–Stokes equations for a spark or planar igniting flame at the closed end of a tube and propagating to the opposite closed or open end. It is shown that the mechanism of tulip flame formation is universal for both sparked and planar ignited flames in tubes with both ends closed. Flame front inversion results from the tulip-shaped profile of the unburned gas axial velocity near the flame front, which is the result of the superposition of the unburned gas flow generated by the accelerating flame and the reverse flow generated by the rarefaction wave during flame deceleration. In a half-open tube, this mechanism is valid for spark ignited flames. In the case of planar ignition, there is no rarefaction wave, but the growth of bulges on the sidewalls leads to the formation of a tulip flame.

Study on Gasoline–Air Mixture Explosion Overpressure Characteristics and Flame Propagation Behaviors in an Annular Cylindrical Confined Space with a Circular Arch

Gasoline–air mixture explosions mostly occur in buried tank rooms, which are annular cylindrical confined spaces with circular arches. In this paper, explosion experiments at different gasoline–air mixture volume fractions are carried out in an annular cylindrical steel bench with a circular arch curvature radius of 900 mm and an annular half-perimeter to radial width ratio of 12π. The results show that the development process of explosion overpressure is clearly divided into four stages after first-order differentiation treatment. Compared with other types of confined spaces, 1.70% is still the most dangerous gasoline–air mixture volume fraction. However, this type of confined space has a larger inner surface area in the same volume condition, which will inevitably increase the heat absorption rate, reduce the chemical reaction rate, and slow down the flame propagation speed. Meanwhile, this spatial structure will inevitably make the explosion flames collide, which will promote positive feedback coupling between explosion flames and pressure waves, making the explosion more violent and dangerous. These results can provide theoretical and technical support for the explosion prevention design of buried tank rooms.

Experimental measurements of laminar burning velocity of premixed propane-air flames at higher pressure and temperature conditions

This paper presents laminar burning velocity (LBV) measurements of premixed propane-air flames simultaneously at higher mixture pressure (1-5 atm) and temperature (350-630 K) conditions over mixture conditions (ϕ = 0.7-1.3) utilizing the externally heated diverging channel (EHDC) method. The maximum LBV was observed at ϕ = 1.1 for all pressure and temperature conditions. The non-monotonic behavior of the temperature exponents was noted with the minima at ϕ = 1.1. The pressure exponent (β) variation was observed to be parabolic with the maxima at ϕ = 1.0. The current measurements are then compared with the literature results, and the detailed kinetic model predictions of Qin mech, San Diego mech, and USC mech II. The present LBV measurements are in a better match with the mechanism predictions of San Diego mech at the majority of mixture and pressure conditions. The current measurement suggests the variation of temperature exponent (α) as a function of pressure ratio, and pressure exponent (β) as a function of temperature ratio for different mixture conditions (ϕ). A revised power-law correlation for α and β variations is also suggested as, Su=Su0TuTu0∝0+∝11-PuPu0PuPu0β0+β11-TuTu0. The sensitivity analysis reveals a significant increase (≈ 45%) in negative sensitivity for the chain termination reaction H + CH3 (+M) ↔ CH4 (+M) (R56), with a rise in pressure and temperature at all mixture conditions. The reaction pathway analysis indicates a maximum increment in elemental flux (≈ 305 %) in the formation of ethane (C2H6) from its predecessor methyl radical (CH3), due to enhanced pressure and temperature conditions.

Structure and extinction of methane and propane nonpremixed counterflow flames at extremely low stretch rates in buoyant flowfields

This study experimentally investigated the structures and extinction behaviors of methane-air and propane-air counterflow nonpremixed flames influenced by individually controlled fuel injection velocity uF at extremely low stretch rates, which were induced by natural convection alone. A sooting limit fuel injection velocity (SLFIV) index was introduced, which increased with stretch rate in the low stretch rate region. Methane had higher SLFIV than propane. Quasi-one-dimensional blue flames appeared for both methane and propane within a certain uF range. The range for methane was higher than that for propane. The steady flame standoff distances yf had a relationship with uF and low stretch rate ab of yf∝uFabλ∞1/2, which differed from high-stretch flames. Different flame instabilities occurred for methane and propane by changing uF only without fuel or oxidizer dilutions. At low uF near extinction limit of methane, periodic flame holes appeared, which was due to large heat loss. This instability finally caused flame extinction by keeping retreating edges. At high uF, simultaneous cellular and wave flame instability appeared for propane, which differed from cellular flames occurred alone in highly diluted oxy-fuel flames. By modulating uF alone in this experiment, the cellular/wave flame was due to Rayleigh-Taylor and hydrodynamic instabilities. Flame extinction at the lowest uF for propane was through outer edge shrinking, where the displacement speed increased with stretch rate. Flammability maps of limit uF versus low ab were established for both methane and propane. Both maps were divided into four regions by sooting, instability and extinction limits. The three limits uF all increased with ab. This changing tendency was opposite with high-stretch flames reported previously. For propane, the instability limit uF approached infinity at higher ab. This work provided knowledge of extremely low stretch flame combustion, which can be applied to microgravity flames, because the combustion characteristics of microgravity forced low stretch flames were identical to those of low stretch flame induced by natural convection alone.

Stretch Effects in Large Eddy Simulation of Turbulent Premixed Bunsen Flames

ABSTRACT The stretch effects in large eddy simulation (LES) of a turbulent lean propane-air premixed Bunsen flame in the wrinkled flamelet regime are investigated. A simple approach to model the subgrid-scale flame stretch is proposed. It is based on the analysis which shows that, when the surface-filtered strain rate in a subgrid-scale stretch model is approximated using the volume-filtered velocity field solved for in LES, heat release and associated gas expansion in the filtered flame brush tend to artificially alter the wrinkling of resolved flame fronts. To minimize such artifacts, it is suggested that the volume-filtered strain rate on the unburned side of the filtered flame brush be used to approximate the surface-filtered strain rate and projected through the filtered flame brush. The results show the importance of mitigating the artificial heat release effects when considering the strain effects in LES. The relative importance of the curvature stretch is also investigated in terms of the mean and local effects. For a Bunsen flame with a positive Markstein number, the mean flame curvature effect tends to increase the total burning rate by enhancing the laminar flame speed near the flame tip, while the local flame curvature effect tends to decrease the total burning rate by suppressing the wrinkling of the resolved flames. It is found that the competition between the two makes the overall curvature effects not influence the total burning rate much for the flame investigated here, as compared with the strain effects.

Distributed heat release effects on entropy generation by premixed, laminar flames

This article studies the generation of entropy disturbances by laminar premixed flames. The total entropy generation equals the integrated ratio of the local heat release rate and the local temperature, namely, [Formula: see text]. Due to this path dependency, evaluating this integral requires an understanding of how the heat release is distributed in the temperature space. Several studies evaluate the local entropy generation as [Formula: see text], where [Formula: see text] refers to the burned gas temperature, implicitly assuming all the heat release occurs at [Formula: see text]. Such an approximation is motivated by the high activation energy nature of combustion chemistry. This work evaluates this assumption by comparing it to results from one-dimensional premixed flame calculations for hydrogen, methane, and propane-air flames over a range of pressures, equivalence ratios, and preheat temperatures, quantified via the ratio [Formula: see text]. We show that this assumption is quite reasonable for methane and propane-air flames (with errors ranging from 5% to 25%) but deviates significantly from the exact results for hydrogen flames (where errors can be as high as 50%). In general, the peak heat release moves to lower temperatures as preheat temperature is increased. Noting that the temperature sensitivity of heat release is directly related to the activation energy, we use Law's approach to extract global activation energies and show that the deviations of [Formula: see text] from unity can be approximately correlated with [Formula: see text]. Finally, we show that significant improvements in entropy generation calculations can be obtained by estimating [Formula: see text] using [Formula: see text], where [Formula: see text] is the temperature at which the reaction rate peaks. This estimation leads to predictions of ∼5% within the exact value for the hydrocarbon cases but can still be in significant error for hydrogen at certain conditions.

Hydrodynamic effect of nanosecond repetitively pulsed discharges produced throughout a laminar stagnation flame

Plasma-assisted combustion has the potential to improve flame stability and combustion performance. Among all the different types of plasma source, nanosecond repetitively pulsed (NRP) discharges have received a lot of attention since they can produce nonequilibrium plasma containing excited species, ionized species, and radicals. These species can then accelerate combustion processes by opening new kinetic pathways. NRP discharges can also affect combustion through thermal and hydrodynamic pathways. This study investigates the hydrodynamic effect of NRP discharges produced throughout the surface of a lean premixed methane-air flame at atmospheric pressure. Time-resolved imaging was used to study the effect of the NRP discharges pulse repetition frequency (PRF). The imaging showed that the flame stabilizes further upstream as the PRF increases and that the flame remains steady and does not relax between consecutive discharges. The flame relaxation was also studied using time-resolved imaging after abruptly turning off the NRP discharges. This revealed that the flame takes about 50ms to completely relax to its unactuated position. Particle image velocimetry showed that the NRP discharges substantially reduce the flow velocity in their vicinity, which causes the flame to move upstream and stretch. The induced stretch was found to increase flame speed though nonequidiffusive effects. The experiment was repeated with a lean premixed propane-air flame. In those conditions, the flame speed was unaffected by the NRP discharges, as expected for a mixture that is less sensitive to nonequidiffusive effects. Hence, the hydrodynamic effect of NRP discharges appears to dominate over the thermal and kinetic pathways for the conditions investigated.

Premixed Propane–Air Flame Propagation in a Narrow Channel with Obstacles

Flame interaction with obstacles can affect significantly its behavior due to flame front wrinkling, changes in the flame front surface area, and momentum and heat losses. Experimental and theoretical studies in this area are primarily connected with flame acceleration and deflagration to detonation transition. This work is devoted to studying laminar flames propagating in narrow gaps between closely spaced parallel plates (Hele–Shaw cell) in the presence of internal obstacles separating the rectangular channel in two parts (closed and open to the atmosphere) connected by a small hole. The focus of the research is on the penetration of flames through the hole to the adjacent channel part. Experiments are performed for fuel-rich propane–air mixtures; combustion is initiated by spark ignition near the far end of the closed volume. Additionally, numerical simulations are carried out to demonstrate the details of flame behavior prior to and after penetration into the adjacent space. The results obtained may be applicable to various microcombustors; they are also relevant to fire and explosion safety where flame propagation through leakages may promote fast fire spread.

Особенности формирования пламени пропано-воздушной смеси в узком канале

This paper presents a numerical study of the features of propane-air mixture flame propagation in a narrow cylindrical channel. The main purpose of the study is to determine the effect of the channel width on the combustion characteristics of a propane-air mixture with a composition close to stoichiometric. The problem is formulated using the methods of reactive gas dynamics. The solution method is based on the Van Leer method for determining flows on the faces of computational cells. The peculiarities of the combustion front formation and its propagation along the channel are revealed and analyzed. The formation of the curved flame front is shown to have a cyclical nature. The visible flame velocity is obtained as a function of the channel radius. The proposed physical and mathematical model can be used to determine the thermal conditions of operating cylindrical burners.

Laminar burning velocity of hydrogen, methane, ethane, ethylene, and propane flames at near-cryogenic temperatures

The primary objective of this study is to measure the laminar burning velocity of perfectly premixed hydrogen-air, methane-air, ethane-air, ethylene-air, and propane-air flames, at near-cryogenic temperatures and atmospheric pressure. Initial fuel-air mixture temperatures as low as 160 K were investigated. The experimental methodology was validated by comparing the results obtained with those from previous studies available in the literature and with numerical simulations using five different chemical mechanisms. First, for all fuels, the laminar burning velocity evolution as a function of the equivalence ratio followed the same trend at 295 K and 240 K. Regardless of the equivalence ratio and mixture composition, the laminar burning velocity decreased by 22 to 44% for hydrogen, methane, ethane, ethylene, and propane flames when the temperature was decreased from 295 K to 240 K. Second, for stoichiometric conditions, the laminar burning velocity decreased by about 50% for all fuels, when the temperature was decreased by 100 K, from 295 to 195 K. These experimental results were in excellent agreement with laminar burning velocities calculated by a power law of the ratio of unburned gas temperature to ambient temperature, with exponent values from the literature, obtained for temperatures above the ambient. All five chemical mechanisms provided a very good agreement, within or near experimental uncertainties, for most of the fuels and conditions investigated, even at low temperatures. Overall, this study provides valuable information on the laminar burning velocities of various hydrocarbon and hydrogen fuels at near-cryogenic temperatures, which can be useful for the design of cryogenic storage systems and the validation of chemical kinetic models for conditions below ambient temperatures.

Four-Line C2*/CH* Optical Sensor for Chemiluminescence Based Imaging of Flame Stoichiometry

In the present work, an optical sensor was developed and calibrated for the purpose of non-intrusive equivalence ratio measurements in combustion systems. The sensor incorporates a unique four-line, single-sensor chemiluminescence imaging-based approach, which relies on the ratio of C2* and CH* radical-species intensities to obtain measurements of equivalence ratios. The advantage of the four-line sensor is the use of additional filtering to mitigate broadband luminescence signals, and its improvements over conventional two-line chemiluminescence diagnostics are discussed. The sensor was calibrated using a premixed bluff-body jet burner with a propane–air flame operating over a wide range of equivalence ratios. The results showed that the four-line processing technique improved the signal-to-noise ratio of the chemiluminescence images for all test cases. Calibrations of C2*/CH* intensity ratio to equivalence ratio were developed for both the four-line and two-line techniques. The calibrations were then used to create maps of local equivalence ratios in the flame-holding region. The maps revealed a non-uniform field of equivalence ratios due to the nature of the radical-species intensity profiles within the flame. Therefore, special consideration is required for calibration in order to accurately quantify equivalence ratios and apply these to diffusion flames.

Experimental investigation of OH*/CH* ratio variations in turbulent, disk stabilized, lean propane-air flames with inlet mixture preheat and stratification

An experimental investigation of OH* and CH* chemiluminescence emissions from bluff-body stabilized, stratified, lean propane-air flames, operated under elevated inlet temperatures has been performed in this work. Simultaneous band-pass filtered emissions, at inlet mixture temperatures ranging from 300 to 573 K, have been acquired to evaluate the OH*/CH* ratio in an effort to identify a possible correlation between the chemiluminescence ratio and the equivalence ratio, for these flame configurations. Moreover, the impact of preheat on the disposition of the radially stratified equivalence ratio gradient, issuing from an upstream premixing section has been investigated. The employed premixer/burner set up consists of three concentric disks that form two consecutive premixing cavities and lead to a radially stratified equivalence ratio gradient, characterized by its peak profile value, at the inlet to the burner zone. Band-passed filtered chemiluminescence measurements and Fourier Transform Infrared Spectroscopy (FTIR) analysis have been performed to assess the OH*/CH* ratio and the mixing fields supporting the flames. The results revealed an altered premixedness behavior of the employed arrangement with elevated inlet preheat, when compared to a non-preheated case. Additionally, both maximum and spatially averaged OH*/CH* ratio values have been studied in an effort to assess the equivalence ratio by using line of sight chemiluminscence measurements. The maximum chemiluminscence ratio values have been proven to better compensate for the spatial inhomogenieties encountered in the turbulent, stratified set ups studied, thus better estimating the effective equivalence ratio value that is burned in the reacting shear layer, closely corroborating counterpart laminar flame investigations.

An Investigation of Axisymmetric Disk Stabilized Propane-Air Flames Operating under Inlet Mixture Preheat and Stratification

ABSTRACT Turbulent, recirculating, lean propane-air flames with inlet mixture stratification and preheat have been simulated under stable and limiting burning conditions. The modeled burner setup comprises a supply tube enclosing three sequential disks producing two consecutive premixing cavities. Fuel is injected in the first cavity and is partially premixed with primary air flowing through this cavity system, resulting in a radially stratified equivalence ratio profile at the inlet of the afterbody disk flame stabilizer. Detailed velocity, turbulence, fuel-air mixing, and imaging data have been previously reported for inlet preheats from 300 to 573 K and for a range of fuel flow rates. The simulations were carried out with a finite-volume-based method, using the dynamic Smagorinsky subgrid model coupled with two combustion methodologies, a quasi-laminar reaction rate approach and the Thickened Flame Model approach. Propane oxidation was modeled with a 22-species skeletal scheme. OH* chemiluminescence distributions were also computed by post-processing quasi-steady state derived algebraic expressions, exploiting directly simulated species thus enabling comparisons with experimental images. The simulations were evaluated against velocity, turbulence, and temperature measurements as well as chemiluminescence images. These comparisons allowed for an assessment of the methodologies’ capability to reproduce important trends such as the notable extension of stable operation to ultra-lean mixtures, the appreciable effect on the near flame aerodynamic stretch and the variations in the local flame structure.

Impacts of Rectangular Obstacle Lengths on Premixed Methane–Air Flame Propagation in a Closed Tube

Impacts of Rectangular Obstacle Lengths on Premixed Methane–Air Flame Propagation in a Closed Tube

The Effect of Swirl Intensity on the Flow Behavior and Combustion Characteristics of a Lean Propane-Air Flame

The effect of swirl number (Sn) on the flow behavior and combustion characteristics of a lean premixed propane Flame Ф = 0.5 in a swirl burner configuration was numerically verified in this study. Two-dimensional numerical simulations were performed using ANSYS-Fluent software. For turbulence closure, a standard K-ε turbulence model was applied. The turbulence-chemistry interaction scheme was modeled using the Finite Rate-Eddy Dissipation hybrid model (FR/EDM) with a reduced three-step reaction mechanism. The P1 radiation model was used for the flame radiation inside the combustion chamber. Four different swirl numbers were selected (0, 0.72, 1.05, and 1.4) corresponding to different angles (0°, 39°, 50°, and 57.8°). The results show that the predicted model agrees very well with the experimental data, especially with respect to the axial and radial velocity and temperature profiles. An outer recirculation zone (ORZ) is present in the combustor corner at Sn = 0 and an inner recirculation zone (IRZ) appears at the combustor centerline inlet at a critical Sn = 0.72. When the Sn reaches an excessive value, the IRZ moves toward the premixing tube, leading to a flame flashback. The flame structure and its length are strongly affected by changes in the Sn as well as the formation of NOx and CO at the combustor exit.