Flame Stabilization Mechanism Research Articles

Transient and steady-state behavior of auto-igniting propane and dimethyl ether fuel jets in high-temperature vitiated coflows

In the current study, the auto-ignition dynamics of cold fuel jets issuing into a high-temperature, vitiated environments is investigated. Due to the short time scale of these events, high-speed measurements are used to resolve the coupled spatio-temporal behavior. The present study uses high-speed (20-kHz) OH* chemiluminescence imaging to identify the location and timing of the formation of the initial ignition kernels, providing visualization of the ignition dynamics and a detailed statistical evaluation of ignition heights and ignition delay times across a broad parameter space which includes variations in fuel type, dilution levels, coflow temperature, and coflow oxidizer content. The auto-ignition location and ignition delay times show a strong sensitivity to coflow temperature with increased sensitivities at lower coflow temperatures. Comparisons between kernel formation location for the transient jet and the fluctuating flame base of the subsequent, steady-state flame is presented, highlighting the role of flame propagation on flame stabilization. Results indicate that at lower temperatures the flame stabilization mechanism is dominated by auto-ignition, but at higher coflow temperatures, flame propagation plays a key role. The effects of variations in the hot, coflow oxidizer content on ignition properties were found to be noticeable, but still significantly less than variations in the temperature.

Experimental and Numerical Investigations of Novel Natural Gas Low NOx Burners for Heavy Duty Gas Turbine

A novel dry low-NOx gas turbine technology requires well-balanced assessments since the early development phases. The weak knowledge of often conflicting aspects, such as operability and manufacturability, make any roadmap difficult to be drawn. Introduction of innovative manufacturing technologies such as the direct metal laser sintering (DMLS) process allows rapid manufacturing of components and test them in dedicated facilities to support real-time development of new products. The use of such a manufacturing process allows the adoption of designed experiment-based development strategies, which are still uncommon at industrial level, due to the reduced time from drawings to test.The paper describes a reactive test campaign performed by BHGE in cooperation with University of Florence, aimed at the exploration of capabilities of different innovative burners in terms of pollutant emissions containment and blow-out margin. In particular, the test campaign has been conceived to provide a robust estimate of the effects of key geometrical parameters on principal burner performances. The flame stabilization mechanism of the investigated burners is based on the swirling flow generated by different setup of two internal channels: corotating and counter-rotating radial and axial swirlers. The effect of both the shape and the size of the internal air passages, as well as of the swirler characteristics, has been matter of investigation. Burners were tested in a single-cup test rig operated at moderate pressure conditions (up to 6 bar), with two levels of preheated air temperature (300 °C and 400 °C). Each burner was equipped with two natural gas feeding lines representing the diffusion (pilot) and premixed (main) fuel supplies: both lines were regulated during tests to assess the effect of fuel split on emissions and to identify a stable low-NOx operating window, within which a lean blow-out test was performed. Dynamic pressure probes were used to evaluate the onset of combustion instabilities. The burner development was supported by computational fluid dynamics (CFD) investigations with the purpose to have a detailed understating of the flow-field and flame structure and to perform a preliminary screening to select the most promising solutions for the testing phase. The post process of the experimental results has allowed to correlate the main design parameters to burner performance variables discovering possible twofold optimizations in terms of emissions and operability.

Optical and laser diagnostic investigation of flame stabilization in a novel, ultra-lean, non-premixed model GT burner

The flame dynamics and stabilization mechanism in a novel, ultra-lean, non-premixed, model GT burner is experimentally investigated with optical and laser diagnostics. The burner operating with methane as fuel exhibits a convergent-divergent flow field and is capable of stabilizing a non-sooty flame at ultra-low global equivalence ratio (ϕglob) down to 0.1. At 1.0 > ϕglob > 0.6, the flame stabilizes in the diverging section where the flow field is characterized predominantly by the swirl. The flame stabilizes at locations of low velocity and low turbulence where the mean flow strain rates are found to be well below the extinctions strain rates for turbulent non-premixed flames. With decreasing ϕglob, the flame is located progressively near the burner lip, where a large recirculation zone of the central bluff body in the burner exists. The transition from swirl stabilized to bluffbody stabilized region occurs between 0.6 > ϕglob > 0.4 and bi-stability of the flame with low-frequency oscillation between the two stabilization locations was observed. The upstream marching of the flame, despite a decreasing ϕglob, and a corresponding decrease in heat input, is explained in terms of the interaction of the flame front with the evolution of the scalar profile upstream, in presence of the recirculating hot gases. The results support flame stabilization theories based on partial premixing and flow modification through heat release upstream of the flame. For the leaner conditions, 0.4 > ϕglob > 0.1, the flame is stabilizing in a region characterized by bluffbody induced toroidal flow structures of high strain rates, where the presence of recirculating hot burned gases aid in flame stabilization by the ignition of flammable mixture.

Structure and propagation of two-dimensional, partially premixed, laminar flames in diesel engine conditions

We investigate the influence of inflow velocity (Vin) and scalar dissipation rate (χ) on the flame structure and stabilisation mechanism of steady, laminar partially premixed n-dodecane edge flames stabilised on a convective mixing layer. Numerical simulations were performed for three different χ profiles and several Vin (Vin = 0.2 to 2.5m/s). The ambient thermochemical conditions were the same as the Engine Combustion Network’s (ECN) Spray A flame, which in turn represents conditions in a typical heavy duty diesel engine. The results of a combustion mode analysis of the simulations indicate that the flame structure and stabilisation mechanism depend on Vin and χ. For low Vin the flame is attached. Increasing Vin causes the high-temperature chemistry (HTC) flame to lift-off, while the low-temperature chemistry (LTC) flame is still attached. A unique speed SR associated with this transition is defined as the velocity at which the lifted height has the maximum sensitivity to changes in Vin. This transition velocity is negatively correlated with χ. Near Vin=SR a tetrabrachial flame structure is observed consisting of a triple flame, stabilised by flame propagation into the products of an upstream LTC branch. Further increasing the inlet velocity changes the flame structure to a pentabrachial one, where an additional HTC ignition branch is observed upstream of the triple flame and ignition begins to contribute to the flame stabilisation. At large Vin, the LTC is eventually lifted, and the speed at which this transition occurs is insensitive to χ. Further increasing Vin increases the contribution of ignition to flame stabilisation until the flame is completely ignition stabilised. Flow divergence caused by the LTC branch reduces the χ at the HTC branches making the HTC more resilient to χ. The results are discussed in the context of identification of possible stabilisation modes in turbulent flames.

Autoignition-controlled flame initiation and flame stabilization in a reacting jet in crossflow

This paper describes an analysis of the mechanisms of autoignition-controlled flame initiation and flame stabilization in a nonpremixed jet in crossflows, using simultaneous high-speed (10 kHz) tomographic particle image velocimetry, OH-PLIF and line-of-sight flame emissions. Measurements are conducted on a turbulent, transverse, reacting propane jet issued into a crossflow generated by combustion of natural gas at an equivalence ratio of 0.4 with the crossflow velocity of 10 m/s, the crossflow temperature of 1350 K and the jet momentum flux ratio of 41. While several prior studies have analyzed the lifted character of the flame in similar configurations, we show that several dynamic processes precede the leading edge of the lifted diffusion flame, including formation and evolution of “autoignition kernels”, “flame kernels” and “flame fragments”. “Autoignition kernels”, i.e., discrete compact reaction zones with the peak hydroxyl (OH) fluorescence intensity below that of the diffusion flame, initiate preferably at bulges along the jet periphery where the strain rates and the scalar dissipation rates are lower. The autoignition kernel grows in both size and the OH-fluorescence intensity as it convects downstream. An autoignition kernel transitions into a propagating flame kernel, which quickly gets distorted and elongated in the direction of the principal expansion strain rate to form a flame fragment. Neighboring flame fragments merge with each other and with the downstream diffusion flame via edge-flame propagation. Merging of upstream flame fragments with the downstream diffusion flame results in an upstream advancement of the diffusion-flame front. The diffusion flame front is intrinsically unsteady because of the rather random formation and evolution of autoignition kernels, flame kernels and flame fragments, presumably due to the stochastic velocity, the strain rate and mixture-fraction oscillations.

A Pressurized Vitiated Co-Flow Burner and Its Preliminary Application for a Methane Lifted Flame

A new pressurized vitiated co-flow burner (PVCB) was designed and built for the investigation of lifted flame properties and was supported by a vitiated co-flow of hot combustion products from a lean H2/air flame at a controllable pressure; its preliminary application for a methane lifted flame was tested. The distribution of the co-flow temperature, oxygen mole fraction, flow rate, and pressure of the PVCB was measured and calculated. The research results show that the co-flow temperature range is from 300 to 1300 K, the background pressure range is from 1 to 1.5 bar, the stable temperature field of the PVCB is wider, and the background pressure of the PVCB can be controlled. The simulation results show that the PVCB provides a controllable, pressurized co-flow of hot and vitiated gases, which makes it possible to investigate flame stabilization mechanisms. The PVCB has the advantages of controllable background pressure and a stable temperature field. The well-defined uniform boundary conditions and simplified flow of the PVCB simplify the establishment of a numerical model and decouple the turbulent chemical kinetics from the complex recirculating flow. It can be widely used in the research on lifted flames. A lifted flame of methane was recorded under conditions of a co-flow temperature of 1133 K and pressure from 1 to 1.043 bar. The lift-off height decreased and stabilized with the increase in the background pressure. The laminar flame speed and the autoignition delay time were tested and simulated at the same time by Chemkin; the influence of background pressure on the lift-off height, laminar flame speed, and autoignition delay were analyzed. The results show that the autoignition, as well as the flame propagation, dominated the stabilization mechanism of the lifted flame in the PVCB.

Experimental and numerical study on bluff-body and swirl stabilized diffusion flames

Bluff-body and swirl flow are commonly utilized to stabilize diffusion flames in industrial applications, such as gas turbines, ramjets and furnaces. Flame stabilization mechanisms of these two kinds of burners are similar with each other: the interaction between the recirculation zone and the fuel jet. In the present paper, flow fields within flames stabilized by combinations of swirl flow and bluff-body were captured using high-speed PIV; while the flame structures were visualized by high-speed CH2O PLIF, CH∗ chemiluminescence and broadband chemiluminescence. The global CO emissions from the flames were captured as well. In addition, based on the CFD software OpenFOAM, simulations were adopted to better understand the interactions between flames and flow structures. Flames stabilized by bluff-bodies with different diameters (db = 14 mm and 20 mm), or only by swirl flow without a bluff-body, were studied. All reacting experiments were carried out with a constant mass flow rate of the central fuel jet (with thermal power 2.01 kW) and a constant mass flow rate of the total air flow (m = mt + ma = 200 ln/min). The swirl strength was controlled by the mass flow rate ratio of the tangential to the axial air flow. The geometrical swirl number was altered between Sg = 0 and Sg = 4.08. Simulation results matched well with experimental data, especially in predicting the spatial distribution of CH2O. The position of the outer recirculation zone would be affected by the size of the bluff-body and the swirl strength. In addition, the recirculation zone determined the flame structures and the global CO emission levels. With a larger bluff-body, the air driven recirculation zone located more upstream near the burner exit. Flame prone to be more stable with a larger bluff-body and/or a stronger swirl flow. Flame was observed propagating into the upstream region in cases without a bluff-body or in cases with the small bluff-body (db = 14 mm), when the swirl strength was sufficiently strong. The mechanism for the diffusion flame ‘flashback’ was proposed. Flames in cases with a larger swirl number were shorter while its CO emission levels were higher.

High Momentum Jet Flames at Elevated Pressure: Detailed Investigation of Flame Stabilization With Simultaneous Particle Image Velocimetry and OH-LIF

A model FLOX® combustor, featuring a single high momentum premixed jet flame, has been investigated using laser diagnostics in an optically accessible combustion chamber at a pressure of 8 bar. The model combustor was designed as a large single eccentric nozzle main burner (Ø 40 mm) together with an adjoining pilot burner and was operated with natural gas. To gain insight into the flame stabilization mechanisms with and without piloting, simultaneous particle image velocimetry (PIV) and OH laser-induced fluorescence (LIF) measurements have been performed at numerous two-dimensional (2D) sections of the flame. Additional OH-LIF measurements without PIV particles were analyzed quantitatively resulting in absolute OH concentrations and temperature fields. The flow field looks rather similar for both the unpiloted and the piloted cases, featuring a large recirculation zone next to the high momentum jet. However, flame shape and position change drastically. For the unpiloted case, the flame is lifted and widely distributed. Isolated flame kernels are found at the flame root in the vicinity of small-scale vortices. For the piloted flame, on the other hand, both pilot and main flame are attached to the burner base plate, and flame stabilization seems to take place on much smaller spatial scales with a connected flame front and no isolated flame kernels. The single-shot analysis gives rise to the assumption that for the unpiloted case, small-scale vortices act like the pilot burner flow in the opposed case and constantly impinge and ignite the high momentum jet at its root.

Investigation of flameholding mechanisms in a kerosene-fueled scramjet combustor

Laser-induced fluorescence and high-speed photography were employed to investigate the kerosene flame stabilization mechanism in a cavity-based scramjet combustor with an inlet condition corresponds to flight Mach number of 4. Pilot hydrogen was used to ignite the kerosene fuel. The PLIF results of kerosene distribution in the reacting cases showed that the mixing process was dramatically enhanced compared to the non-reacting cases. Sharp OH gradients were observed in the shear layer and the aft region of cavity, which indicated that the flame was located at these positions. A portion of hot products participated in the recirculation of the cavity and preheated the kerosene-air mixture in the leading edge. The heated mixture was ignited in the mid-cavity and the reaction zone spread into the mainstream flow. Due to the competition between the local flame speed and the local flow speed, the high-speed images showed that the spreading location was in fluctuation. This movement was observed to cause a low-frequency wall pressure fluctuation.

Direct estimation of edge flame speeds of lifted laminar jet flames and a modified stabilization mechanism

The flame stabilization mechanism of a lifted flame in a laminar fuel jet has been explained based on the edge flame concept. Previous studies have employed a similarity solution between velocity and fuel concentration, and showed that a lifted flame can be stabilized when the Schmidt number, Sc, is within a range of either Sc > 1 or Sc < 0.5. However, two unsolved problems remained, and they were mainly answered in this study. First, the edge flame speed could not be determined from the similarity solution using the experimental results of stable lifted flames. To resolve this, the experimental relationship between the fuel flow rate and the liftoff height was measured with a higher resolution, and a new method employing an effective Schmidt number was suggested. As a result, the relationship between the edge flame speed and the fuel concentration gradient could then be directly estimated from the simple experimental values for flow rate and the liftoff height. This new method was validated for various experimental parameters including the tube diameter, air-premixing ratio, and nitrogen-dilution ratio. Second, the reason why a stable lifted flame was not obtained when Sc < 0.5 could not be explained theoretically. Here, the existence of a unique criterion of Sc > 1, for a stable lifted flame was clarified theoretically. This study will advance understanding of the characteristics and stabilization mechanism of lifted edge flames in laminar non-premixed fuel jets.

Investigation of flame establishment and stabilization mechanism in a kerosene fueled supersonic combustor equipped with a thin strut

The flame establishment and propagation processes in a supersonic combustor fueled by liquid kerosene at Mach 6 condition were observed by high speed photography. In this paper, a thin strut acted as the flame holder equipped in the center of the combustor. In the experiment condition, the Mach number in the inlet of the combustor is 2.8, with the stagnation state Tt=1680 K, Pt=1.87 MPa. The combustion establishment process and the flame characteristics in the supersonic combustor were well captured and reproduced by the high-speed camera, with the camera parameter of 5000 frames per second. Experimental results show that initial flame appears around the plasma jet torch in the recirculation zone at the tailing edge of the strut, and grows to form a steady flame in the center of main inflow. The attribute of the flame is partially premixed flame in the experimental conditions, and the flame could be divided into three main parts in accordance with the flame characteristics. By the analysis of the high speed images, different flame propagation patterns in the flame establishment processes are found in different equivalence ratios, the mechanism of which is explained in this paper.

Flame stabilization mechanism study in a hydrogen-fueled model supersonic combustor under different air inflow conditions

Flame stabilization mechanisms of a hydrogen-fueled laboratory model scramjet combustor are numerically studied. The combustor consists of a sonic fuel jet injected paralleling into a supersonic air flow at the base of a wedge-shaped strut. The numerical calculations are based on an Open Source Field Operation and Manipulation (OpenFOAM) solver using the large eddy simulation method. A skeletal mechanism for hydrogen-air chemistry including 9 species 27 reactions is used to depict the combustion dynamics. Three cases with air stagnation temperature of 568 K, 460 K, and 960 K, are respectively studied under the same equivalence ratio and entrance Mach number. Damköler number based on the subgrid turbulent mixing time scale and the instantaneous chemical time scale is used to distinguish the role of mixture self-igniting from flame propagation in different air inflows. Flame stabilization in such strut-based combustor is compared with that of cavity-based one. It is found that the basic working principles for the two types of combustor are similar. However, the strut shows more simplicity in flame stabilization locations.

Numerical and Experimental Studies on a Syngas-Fired Ultra Low NOx Combustor

Simulations and exhaust measurements of temperature and pollutants in a syngas-fired model trapped vortex combustor for stationary power generation applications are reported. Numerical simulations employing Reynolds-averaged Navier–Stokes (RANS) and large eddy simulations (LES) with presumed probability distribution function (PPDF) model were also carried out. Mixture fraction profiles in the trapped vortex combustor (TVC) cavity for nonreacting conditions show that LES simulations are able to capture the mean mixing field better than the RANS-based approach. This is attributed to the prediction of the jet decay rate and is reflected on the mean velocity magnitude fields, which reinforce this observation at different sections in the cavity. Both RANS and LES simulations show close agreement with the experimentally measured OH concentration; however, the RANS approach does not perform satisfactorily in capturing the trend of velocity magnitude. LES simulations satisfactorily capture the trend observed in exhaust measurements which is primarily attributed to the flame stabilization mechanism. In the exhaust measurements, mixing enhancement struts were employed, and their effect was evaluated. The exhaust temperature pattern factor was found to be poor for baseline cases, but improved with the introduction of struts. NO emissions were steadily below 3 ppm across various flow conditions, whereas CO emissions tended to increase with increasing momentum flux ratios (MFRs) and mainstream fuel addition. Combustion efficiencies ∼96% were observed for all conditions. The performance characteristics were found to be favorable at higher MFRs with low pattern factors and high combustion efficiencies.

Temporal evolution of auto-ignition of ethylene and methane jets propagating into a turbulent hot air co-flow vitiated with NOx

In this paper, the temporal evolution of auto-ignition (AI) of C2H4 and CH4 jets propagating into a NOx vitiated hot co-flow at high velocity and turbulence was studied. Simultaneous temporally-resolved planar laser-induced fluorescence (PLIF) experiments of OH and CH2O were carried out in a recently developed test rig for auto-ignition studies of turbulent non-premixed flows. Flame stabilization mechanisms were analyzed for both fuels at several operating conditions. The reaction progress of AI kernels as well as their apparent growth rate were evaluated. Results revealed that the stabilization mechanism (i.e. lifted flame or isolated kernel) strongly depends on the turbulent mixing, co-flow temperature and fuel composition. A further statistical analysis of the heat release rate (HRR) zones, calculated as the product of the CH2O- and OH-PLIF signals, delivered an indication on the different AI characteristics of C2H4 and CH4. An evaluation of the temporal evolution of the HRR for C2H4 jets provided a deeper insight about the reaction progress at different conditions of the co-flow, when AI was initiated by isolated kernels. Finally, estimations of the apparent growth rate of AI kernels indicated a faster propagation of C2H4 kernels when compared with those of CH4.

Large eddy simulation of spark ignition of a bluff-body stabilized burner using a subgrid-ignition model coupled with FGM-based combustion models

PurposeSafety improvement and pollutant reduction in many practical combustion systems and especially in aero-gas turbine engines require an adequate understanding of flame ignition and stabilization mechanisms. Improved software and hardware have opened up greater possibilities for translating basic knowledge and the results of experiments into better designs. The present study deals with the large eddy simulation (LES) of an ignition sequence in a conical shaped bluff-body stabilized burner involving a turbulent non-premixed flame. The purpose of this paper is to investigate the impact of spark location on ignition success. Particular attention is paid to the ease of handling of the numerical tool, the computational cost and the accuracy of the results.Design/methodology/approachThe discrete particle ignition kernel (DPIK) model is used to capture the ignition kernel dynamics in its early stage of growth after the breakdown period. The ignition model is coupled with two combustion models based on the mixture fraction-progress variable formulation. An infinitely fast chemistry assumption is first done, and the turbulent fluctuations of the progress variable are captured with a bimodal probability density function (PDF) in the line of the Bray–Moss–Libby (BML) model. Thereafter, a finite rate chemistry assumption is considered through the flamelet-generated manifold (FGM) method. In these two assumptions, the classical beta-PDF is used to model the temporal fluctuations of the mixture fraction in the turbulent flow. To model subgrid scale stresses and residual scalars fluxes, the wall-adapting local eddy (WALE) and the eddy diffusivity models are, respectively, used under the low-Mach number assumption.FindingsNumerical results of velocity and mixing fields, as well as the ignition sequences, are validated through a comparison with their experimental counterparts. It is found that by coupling the DPIK model with each of the two combustion models implemented in a LES-based solver, the ignition event is reasonably predicted with further improvements provided by the finite rate chemistry assumption. Finally, the spark locations most likely to lead to a complete ignition of the burner are found to be around the shear layer delimiting the central recirculation zone, owing to the presence of a mixture within flammability limits.Research limitations/implicationsSome discrepancies are found in the radial profiles of the radial velocity and consequently in those of the mixture fraction, owing to a mismatch of the radial velocity at the inlet section of the computational domain. Also, unlike FGM methods, the BML model predicts the overall ignition earlier than suggested by the experiment; this may be related to the overestimation of the reaction rate, especially in the zones such as flame holder wakes which feature high strain rate due to fuel-air mixing.Practical implicationsThis work is adding a contribution for ignition modeling, which is a crucial issue in various combustion systems and especially in aircraft engines. The exclusive use of a commercial computational fluid dynamics (CFD) code widely used by combustion system manufacturers allows a direct application of this simulation approach to other configurations while keeping computing costs at an affordable level.Originality/valueThis study provides a robust and simple way to address some ignition issues in various spark ignition-based engines, namely, the optimization of engines ignition with affordable computational costs. Based on the promising results obtained in the current work, it would be relevant to extend this simulation approach to spray combustion that is required for aircraft engines because of storage volume constraints. From this standpoint, the simulation approach formulated in the present work is useful to engineers interested in optimizing the engines ignition at the design stage.

A numerical study on heat-recirculation assisted combustion for small scale jet diffusion flames at near-extinction condition

In order to prove the existence of heat-recirculation (through burner wall) assisted flame stabilization mechanism for miniaturized jet diffusion flames, a series of two-dimensional axisymmetric numerical computations are performed for methane–air flames over a sub-millimeter (constant inner diameter of 0.8mm) jet. A skeletal mechanism including 17 species and 58 steps is employed for the chemical kinetics and detailed transport properties are considered. Burner is considered as thermally conductive and chemically inert. Heat fluxes responsible for heat recirculation (feedback) (Qre) and heat loss (Qloss) through the burner surfaces are calculated respectively from the numerical results. An effective excess-enthalpy (H) is defined and used to measure the global thermal effect of the burner on the flame. A wide range of fuel jet velocities (V, from the minimum value to sustain a steady state flame to 3.2m/s) for different burners with various thermal conductivities (kb, from 1.0W/m-K to 100W/m-K, which covers most of the realistic burner materials) and wall thicknesses (cb, from 0.2mm to 0.8mm) are considered in this work. The results show that, as V increases, the ratio of Qre to Qloss monotonically increases, resulting in a critical fuel jet velocity Vc, above which Qre exceeds Qloss and thus the heat recirculation effect could cancel out the heat loss effect by the presence of the burner. Further, it is suggested that the Vc can be reduced by reducing kb and cb, hence, heat-recirculation assisted combustion is found to be promoted at near extinction condition under such burner system. Flame temperatures are significantly elevated as kb or cb decreases at near extinction conditions, revealing that the extinction limit in fuel jet velocity can be extended under specific condition. Computations are then conducted for flames of hydrogen and dimethyl ether to examine the fuel dependence. It is indicated that the flame–burner thermal interaction is influenced by both physical properties of the fuel (Sc number) and chemical kinetics. For the fuels with Sc > 1, such as dimethyl ether (Sc = 1.2), flame is readily to be lifted, resulting in negligible flame–burner interaction (H ≈ 0). For the fuels with Sc < 1, such as methane (Sc = 0.7) and hydrogen (Sc = 0.2), chemical kinetics that dominating the flame base structure becomes the key factor. Due to the flame–burner attachment, an extremely high H can be expected for a hydrogen jet diffusion flame to achieve excess-enthalpy combustion at certain condition. Overall, this study implies that, given the narrow operation flow rate range restrained by the quenching and the blow off limits in a premixed combustion system, heat recirculation assisted stable combustion could be possible in a relatively wider flow rate (power output) range by choosing a diffusion combustion mode.

Effect of Air Pressure and Gutter Angle on Flame Stability and DeZubay Number for Methane-Air Combustion

AbstractThe combustion at high speed reactants requires a flame holding characteristics to sustain the flame in the afterburner. The flame holding characteristics of the combustor is carried out by the bluff-body stabilizers. The range of conditions of parameters influencing the flame stabilization is to be identified and the effects on the flame sustainability have to be investigated. DeZubay used the concept of DeZubay number and flame stability envelope to determine the stabilization and blowout range. In the present work, the effect of air pressure and the angle of apex of the V-gutter on flame stabilization and blowout mechanism have been experimentally investigated for six different apex angles and four different air pressure conditions. The value of DeZubay number at each condition has been calculated and verified with DeZubay stability chart for flame stabilization. The results show that stable flame is obtained for the entire pressure range when the apex angle of the V gutter is in 60° and 90°.

Proper Orthogonal Decomposition for Analysis of Plasma-Assisted Premixed Swirl-Stabilized Flame Dynamics

The primary aim of this paper is to establish the effectiveness of microwave plasma discharges to improve combustor flame dynamics and stability through minimizing heat release fluctuations. A continuous, volumetric, direct coupled, nonequilibrium, atmospheric microwave plasma discharge was applied to a swirl-stabilized premixed methane-air flame to minimize combustion instabilities. Proper orthogonal decomposition (POD) is used to postprocess data and extract information on flame dynamics that are usually lost through classical statistical approaches. POD analysis carried out on OH planar laser-induced fluorescence images reveals that even at coupled plasma powers corresponding to less than 5% of the thermal power output, significant improvement in mean energy content of flames (~23%) was observed. The corresponding decrease in heat release fluctuations resulted in improved combustor flame dynamics and flame stability, which was found to be in good agreement with acoustic pressure measurements. In the presence of plasma discharge, an effective decoupling between the flame oscillations and the fluid unsteadiness was established due to the differences in flame stabilization mechanisms resulting in up to 47% reduction in root-mean-square pressure fluctuations. Thus, effective fluid-acoustic decoupling in addition to the accelerated combustion chemistry due to the nonthermal effects of plasma led to significantly improved combustor dynamics, namely, decreased heat release and pressure fluctuations.

Flame base structures of micro-jet hydrogen/methane diffusion flames

In this study, a comparison of flame base structures of hydrogen/methane–air diffusion flames formed over a tiny-jet is made numerically for both isothermal and thermal conductive burner conditions, in order to clarify the fuel dependent flame stabilization mechanisms. It is found that, unlike a methane flame, the flame base of a hydrogen flame always attaches to the burner. The analyses indicate that the dominant intermediate-consumption steps have significantly lower activation energies for the hydrogen flame as compared to a methane flame. More importantly, one of the HO2 production reactions (R43f: H+O2+M→HO2+M), which has a dominant role in sustaining reactivity at the flame base, shows a negative temperature dependence, causing the heat release rate in the flame base kernel to increase as the burner wall temperature decreases. With a thermal conductive burner (thermal conductivity of 16W/m-K) over a wide range of fuel jet velocities (0.5–4.0m/s), it is found that the burner tip is heated to a significantly higher temperature by a hydrogen flame due to its unique stabilization mechanism. The mixing effects of hydrogen and methane are then considered. It is found that the burner tip temperature can be reduced by adding methane into the fuel flow. This is because, according to the investigation of the structures of the hydrogen/methane jet diffusion flames, the reaction rate of R43f is suppressed due to the included intermediates (e.g., CH3, CH2O) consumption steps of methane. It is expected that the flame attachment feature associated with the flame base structure can be easily controlled by mixing hydrogen and methane, making it possible to control the burner tip temperature in advance.

Flame stabilization analysis of a premixed reacting jet in vitiated crossflow

The flame stabilization behavior of a premixed ethylene–air jet injected normal to a hot vitiated crossflow (JICF) was studied experimentally using simultaneous hydroxyl (OH) planar laser induced fluorescence (PLIF), formaldehyde (CH2O) PLIF, and particle image velocimetry (PIV). Pixel-by-pixel multiplication of OH and CH2O fluorescence signals was conducted to estimate the reacting JICF flame front. The simultaneous PLIF-PIV measurements allowed for an in-depth study of the interaction between the flame and the flowfield. The flame structure was divided into two branches, a windward and leeward flame branch. The unsteady windward flame exhibited both attached and lifted flame behavior, while the leeward flame branch remained consistently attached at the jet exit. In some cases, formaldehyde signal was observed upstream of the windward flame base, suggesting the build-up of a radical pool due to mixing between the jet reactants and hot crossflow. Both flame branches were anchored in the jet shear layer, but with increasing distance from the jet exit the flames moved inside the shear layers. Small scale vortices caused local wrinkling of the flame front. The windward flame was observed to wrap around the large-scale vortices that formed along the jet shear layer. The large-scale structures distorted the flame front but the associated strain-rate was typically lower than that imparted by the small-scale structures. The leeward flame edge aligned with regions of high principal extensive strain-rate and high dilatation. On the other hand, the windward flame edge was located in regions where principal extensive and principal compressive strain rate magnitudes were high and dilatation was low. The results suggest that auto-ignition is the dominant flame stabilization mechanism for the unsteady windward flame and premixed flame propagation is the more dominant stabilization mechanism for the leeward flame branch.