Global Flame Research Articles

Structure and NOx Emissions of Stratified Hydrogen-Air Flames Stabilized on a Coaxial Injector

Abstract In recent years, the need for low-carbon power has seen hydrogen emerge as a potential fuel to replace conventional hydrocarbons in combustion to limit CO2 emissions in several sectors, including aeronautics. The challenges posed by hydrogen combustion are similar to the issues of kerosene flames but more challenging, like nitrogen oxide (NOx) emissions and flame flashback. One potential solution to address these problems is to burn a rich mixture of hydrogen and air in globally lean conditions on a coaxial injector to obtain a stable and staged combustion and attempt to reduce emissions. In this article, the evolution of NOx production as more air is mixed into the fuel is studied, as well as the changes in flame size and structure. In particular, the appearance of a secondary flame front is observed and increasing the proportion of air in the fuel mixture both shortens the flame and reduces the NOx emission index. Additionally, the effect of the global equivalence ratio and flame thermal power is studied. Finally, existing models for NOx emission of hydrogen flames on a coaxial injector based on average flame residence time and strain rate are tested and shown to have promising results.

Fundamental Experiment of Ignition and Flame Development in Turbulent Jet Ignition Ignited Methane Jet Flame

ABSTRACT In heavy-duty natural gas engines, diesel is usually injected before natural gas to initiate methane combustion. However, such an approach requires additional fuel and precise injection strategies. In the present work, an alternative approach of pre-chamber turbulent jet ignition (TJI) ignited methane jet flame is performed experimentally. The underlying mechanism of TJI-HPDI jet combustion is investigated in an optically visualized constant-volume combustion chamber with an active pre-chamber. It is found that with the increase of the injection-ignition delay (ti), the combustion process undergoes several transitions. In addition, the success ignition region and ignition failure region separated by ti and T MI are discerned on the ti-T MI diagram. As ti increases, there is an initial rise in both the mean flame front velocity and peak pressure, followed by a subsequent decrease This behavior highlights the TJI-HPDI system’s notable advantages in expanding the lean combustion limit and enhancing overall combustion characteristics compared to premixed combustion. These advantages can be attributed to the mixture stratification and turbulence. Furthermore, pre-chambers fueled with hydrogen can further improve the intensity and energy of the turbulent hot jet, which further decreases the critical injection-ignition delay to initiate global flame propagation in the TJI-HPDI system. The present work deals with real engineering technologies, and it will provide new insights into the design and control of TJI-HPDI engines, especially for heavy-duty natural gas engines.

Research on influence of oxygen on flow/combustion characteristics in a scramjet combustor equipped with an O2-pilot strut

With the help of an O2-pilot strut, the stable supersonic combustion was achieved in a scramjet combustor fueled by liquid kerosene. Objective of this work is to investigate the effect of different mass flow rate of oxygen on the cold flow and combustion characteristics to explain the role of the supplied oxygen in supersonic combustion. A series of numerical simulations and experiments were carried out under the high inflow enthalpy condition (Ma = 2.8). High-speed photography and pressure measurement were used to record the flame images and pressure of combustor. A skeleton kerosene mechanism was used for combustion simulation for more details of combustion. Simulation results indicated that by adjusting the mass flow rate of oxygen, the severe rich-kerosene zone around the ignitor could be improved. Under the low mass flow rate of oxygen condition, limited by over-high equivalence ratio the ignition delay time was too long and heat release was too small to establish the global flame. On the contrary, under the high mass flow rate of oxygen condition, limited by over-low equivalence ratio, only little kerosene could be ignited with most of the kerosene carried downstream by high-speed flow resulting in a sharp increase in flame lift distance. The boundary of mass flow rate of supplied oxygen for combustion stabilization was obtained. The results in this paper would be valuable for reveling the ignition and flame stabilization mechanism in supersonic airflow.

Investigation of the mechanism of global and terminal flame uplifted in subsonic horizontal spray jet flame under sub-atmospheric pressure: From macroscopic to microscopic perspective

This work explored how subsonic horizontal spray flames behave under sub-atmospheric pressure and the mechanisms behind their combustion. For examine a horizontal spray flame with rated air volume flow and equivalence ratio, large eddy simulation (LES) and probability density function (PDF) were employed, based on the discrete phase (DPM) random trajectory model. The simulation results are in good agreement with the previous experiments. Through the micro-analysis of the burning time, burning path, and rate of the burn path of droplets under different atmospheric pressures, the combustion characteristics of the flame have been revealed; as well, a component change analysis was conducted to investigate the combustion mechanisms of the flame. Density distributions in buoyancy plumes can reveal the morphologic mechanisms in the corresponding flame regions. A detailed explanation of the varied mechanisms in horizontal spray flames in low-pressure environments is provided, in conjunction with the results of previous experiments on macroscopic flame morphology. There is also a proposed chain of influence across certain characteristics of horizontal spray flames under low pressure. This improves our understanding of mechanisms governing flame behavior in sub-atmospheric pressure environments by allowing us to theoretically design and regulate flames.

SAFT Regimes and Laminar Burning Velocities: A Comparative Study of NH3 + N2 + O2 and CH4 + N2 + O2 Flames

Super adiabatic flame temperature (SAFT) is a distinctive phenomenon in the adiabatic flame where the local maximum temperature exceeds the adiabatic flame temperature. The flame temperatures exhibiting the extent of SAFT are difficult to measure with low uncertainties in experiments, while the laminar burning velocity also represents global flame features, thus could possibly be related to the SAFT. The present study investigated the SAFT regimes, laminar burning velocities (SL), and their relationships for the CH4 + O2 + N2 and NH3 + O2 + N2 flames over large equivalence (ϕ) and oxygen ratio (xO2) ranges. The laminar burning velocities were experimentally measured using the heat flux method at ϕ = 1.4–1.8 and xO2 = 0.22–0.44, where some conditions have never been reported before in the literature. Comparisons were made with simulated SL results using five CH4 mechanisms and five NH3 mechanisms, and none of them well reproduce all of the experimental data. From the simulation results, three CH4 SAFT regimes (I, II, and III) and two NH3 SAFT regimes (I and II) have been identified, among which regime III for CH4 and regime II for NH3 were found for the first time. The kinetic origins of these regimes were discussed, and different flame features regarding the flame temperature and dominant species were clarified. The relationship between the SAFT extent and the laminar burning velocity is revealed by equation derivation based on the classical flame theories, proving that a mechanism reproducing well the SL and its temperature dependence can at the same time yield accurate predictions of the SAFT. The present study also provided the most sensitive reactions in the SAFT predictions accompanied by the rate constant uncertainties, which can be helpful for further mechanism development since none of the mechanisms reproduces well the present SL experimental data, let alone the SAFT extent.

MCGA-assisted ignition process and flame propagation of a scramjet at Mach 2.0

The ignition process and flame propagation with ethylene fuel in cavity-stabilized scramjet by a Multi-Channel Gliding Arc (MCGA) at Mach 2.0 were investigated. Effects of equivalence ratios on the MCGA-assisted ignition process and flame propagation of the scramjet were recorded by two high-speed cameras from different view angles. The discharge characteristics of MCGA are also collected synchronously with the high-speed cameras. The distributions of temperature, velocity, and equivalence ratios in non-reactive flows of the cavity were simulated by Reynolds Averaged Navier-Stokes (RANS) model. The results show that MCGA can achieve reliable ignition with the Global Equivalence Ratios (GER) between 0.06 and 0.17. The ignition process is composed of flame kernel generation, flame development, and stable combustion. The time from flame kernel generation to the establishment of global flame decreases as GER decreases from 0.17 to 0.08. In the streamwise direction, the flame first develops to the Cavity Leading Edge (CLE) because of the influence of the cavity recirculation zone and then uplifts into the cavity shear layer, and finally develops to the Cavity Trailing Edge (CTE). In the spanwise direction, the flame width is less than 50% of the width of the cavity before developing to CLE and begins to develop towards the two sides of the combustor after reaching CLE, which is affected by the angular recirculation zone on both sides of CLE. The ignition processes by MCGA in the scramjet combustor are significantly affected by local distributions of equivalence ratios and velocity in the cavity.

Soot structure and flow characteristics in turbulent non-premixed methane flames stabilised on a bluff-body

The soot properties of methane in turbulent regimes are not well characterised but are highly desirable. Methane is the main constituent of natural gas that is broadly used in many industrial combustors. Investigation of turbulent methane flames under well-defined boundary conditions is therefore useful for interpreting soot formation in practical burners and can be used for further model development. This study presents a joint experimental and numerical study of a series of turbulent non-premixed bluff-body flames fuelled with pure methane for three values of the momentum flux ratio of fuel jet to co-flowing air. Soot volume fraction (SVF) and flowfield are measured simultaneously using planar laser-induced incandescence (P-LII) and 2D-polarised particle image velocimetry (P-PIV). Additionally, time-averaged temperature, mixture fraction, OH and C2H2 concentrations are estimated numerically using RANS models. The global flame structure for all three flames features a recirculation zone with a double-vortex structure, a jet-propagating zone, and a neck zone connecting the two regions. The soot distribution within the recirculation zone shows clear distinct features, which is attributed to the mean mixture fraction distribution in this zone. Increasing the momentum flux ratio shifts the location of the mean stoichiometric mixture fraction to the rich inner vortex core, leading to a distinct peak of the total integrated soot in the inner vortex of the recirculation zone that is not observed in other cases. Also, it is deduced that the soot inception starts earlier in the recirculation zone for the flame with the highest momentum flux ratio and in the jet zone for the other two flames. Much higher soot concentration and lower intermittency are found with ethylene-based flames stabilised on the same burner and with the same operating conditions. In addition, the study has generated a database of soot and flowfield results, which can be helpful for future model validations.

The Effect of Rectangular Confinement Aspect Ratio on the Flame Transfer Function of a Turbulent Swirling Flame

Abstract The flame transfer function (FTF) has been experimentally investigated for a premixed, turbulent and highly swirled flame in rectangular enclosures with varying aspect ratio. A range of equivalence ratios and inlet bulk velocities are considered to vary the length of the flame. Increasing confinement is also shown to increase flame length. More confined flames become increasingly asymmetric, with significant changes to the mean flame shape. Characteristic peaks and dips are observed in the FTF gain for several configurations, caused by constructive and destructive interference between different sources of flow perturbations. Changes in the geometrical confinement distinctly affects these interactions, with FTFs in the most confined chamber containing more pronounced peaks and dips. Analysis of the phase-averaged dynamics has been conducted for two limiting operational conditions in the least and most confined enclosures: a short flame at a low bulk velocity and high equivalence ratio condition; and a long flame at a high velocity and low equivalence ratio condition. The analysis of the short flames shows similar behavior in both enclosures, both in terms of the global response and the local structure of the heat release rate oscillations. Small differences in local response symmetry in the flame due to the close confinement do not affect the global flame response. By comparison, close confinement significantly affects the symmetry of the flame dynamics in the long flame. However, the changes in symmetry do not significantly modify the response, and more important is the change in flame length, which significantly alters the cutoff frequency, reducing the gain at high frequencies.

Thermoacoustic instabilities with varying geometries of the main-stage exit in a centrally staged burner

The present study investigates the effect of the exit geometry of the main stage on thermoacoustic instabilities in a model combustor with a dual-swirl layout. In practical combustors, the main stage has a much larger airflow than the pilot stage, therefore dominating the aerodynamics of the global flame. In this paper, six different geometries of the main-stage exit are designed to cover the common styles in engineering applications. Their effects on thermoacoustic instability and flame shape are examined experimentally. Sudden transitions from the attached flame to the blurred flame are found in most of the exit geometries, excluding the outer-rough case. This transition of flame shape triggers the onset of strong thermoacoustic oscillation, leading to a jump in the pressure fluctuation amplitude. Frequency drop and the acoustic mode shift are also found at the transition point. Flame dynamics are further analyzed using the dynamic mode decomposition method. It is concluded that the outer-rough case has the best thermoacoustic stability within the tested range. The mechanisms of the observed phenomena are analyzed by measuring the reacting flow fields. It is found that the increased roughness of the outer wall can prevent the flame shape transition and the onset of thermoacoustic instability, possibly due to the weakened vortex structures and strain rate in the outer shear layer.

Influence of Incident Shocks on Flame Characteristics in the Strut-Equipped Supersonic Combustor

In this paper, under incident shock waves, the flame characteristics of the strut-equipped supersonic combustor fueled by liquid kerosene are investigated by numerical simulation and experimental means in operating cases of Mach . During this process, the Mach number at the combustor entrance varied from 2.0 to 2.8, and a 14 deg wedge was installed at the isolator inlet as an incident shock simulator. A position-adjustable thin strut was used to optimize the ignition efficiency, and flame propagation was captured and reproduced using a visualization window mounted on the combustor side wall. The conclusions drawn from the study could reduce potential hazards for flight experiments. The simulation found that with the addition of the incident shock wave, the asymmetric shock wave system and the discontinuous acceleration regions were formed in the initial flowfield, which caused the relative position between the strut and wedge to have a significant effect on ignition. Experiments showed that when the two were mounted on the same side, the incident shock wave weakened the ignition conditions, preventing the formation of a global flame within the combustor. After changing the wedge installation direction and reducing the inlet Mach number, the effect of the flame characteristics affected by the incident shock wave was easier to capture. The flame kernel appeared at the strut tail, the combustion heat release area became more concentrated, and the flame development was accelerated. The results showed that the incident shock wave kept the flame oscillation frequency around 170 Hz, and the combustor pressure rise performance in the steady state was almost unchanged. Overall, the ignition delay of the combustor is increased, but the engine thrust is hardly affected by comparison with the ideal situation.

Tomographic reconstruction of an azimuthally forced flame in an annular chamber

Azimuthally forced spinning modes on a single ethylene-air swirl flame in an annular combustor were investigated using phase-averaged three-dimensional (3D) Computed Tomography of Chemiluminescence. Strongly spinning azimuthal modes in the clockwise (CW) and anticlockwise (ACW) directions were introduced by means of carefully controlled acoustic forcing. Tomographic reconstructions were calculated based on 25 independent views for an unforced, and CW and ACW forced cases. Using only a single flame in an annular enclosure resulted in a flame shape that differed from previous multi-flame investigations. The flame experiences significantly less flame-wall and no flame-flame interactions. Results at high and low equivalence ratio indicate that in the absence of significant enclosure effects, the global heat release rate (HRR) response is the same for both forcing directions. The 3D HRR oscillation distributions were investigated in detail, and the local flame response to modes with opposite spin direction was shown to feature 180∘ rotational symmetry. High magnitude HRR oscillations are formed close to the flame base, but due to the combined azimuthal and axial excitation the response on one side of the flame is significantly higher in magnitude, and concentrations of HRR oscillations are seen close to the inner or outer annular walls during CW or ACW excitation respectively. Swirl also causes these high magnitude structures to rotate slowly with downstream distance, although the spatial redistribution of HRR oscillations through this effect is subtle. This improves our physical understanding of previous observations of differing global flame responses to CW and ACW azimuthal excitation.

Effects of molecular diffusion modeling on turbulent premixed NH[formula omitted]/H[formula omitted]/air flames

Ammonia (NH3) is a promising carbon-free energy carrier and can be used as a fuel in gas turbines for power generation. Using a blend of NH3 and hydrogen (H2) has the potential to address the poor combustion properties of NH3/air mixtures. Predictive modeling of the NH3/H2 combustion process requires a clear understanding of molecular diffusion due to the high diffusivity of H2. While the effects of molecular diffusion modeling (MDM) have been extensively investigated in laminar/turbulent hydrocarbon and pure H2 flames, comparatively less investigations have been made to explore the impact of MDM on turbulent premixed NH3/H2/air combustion characteristics as well as NO emission. To this end, four three-dimensional direct numerical simulations of temporally evolving turbulent premixed NH3/H2/air jet flames are performed in this work considering MDM methods of increased complexity: 1) unity Lewis number (UL); 2) unity Lewis number with Soret effect (UL+S); 3) mixture-averaged diffusion with Soret effect (MA+S); and 4) full multicomponent diffusion with Soret effect (MC+S), respectively. The emphasis is placed on assessing MDM effects on flame structure, NO production, and combined role of MDM and stretch rate. Comparing MA+S with UL+S under both laminar and turbulent conditions indicates that the differential diffusion effect is significantly suppressed in the turbulent NH3/H2/air flames considered here. While MDM can affect the local equivalence ratio, it is found that the choice of the MDM method has a negligible impact on the major species and NO distribution conditioned on progress variable, as well as on critical reactions. However, MDM still shows a notable impact on NO production and turbulent flame speed. Specifically, from the most detailed model (MC+S) to the most simplified model (UL), the NO production speed decreases by 30% for the fuel pathway but increases by 338% for the thermal pathway; simultaneously, the global turbulent flame speed decreases by 18%. Furthermore, it is found that both NO mass fraction and local displacement speed are very sensitive to flame curvature and strain, while MDM has a negligible impact on the responses of normalized NO mass fraction and local displacement speed to the change in flame curvature or strain. This suggests that the effects of curvature and strain should be included in flamelet-based models to accurately predict NO emission and NH3/H2/air flame propagation, even when using the assumption of unity Lewis number.

Flamelet modeling of thermo-diffusively unstable hydrogen-air flames

In order to reduce CO2 emissions, hydrogen combustion has become increasingly relevant for technical applications. In this context, lean H2-air flames show promising features but, among other characteristics, they tend to exhibit thermo-diffusive instabilities. The formation of cellular structures associated with these instabilities leads to an increased flame surface area which further promotes the flame propagation speed, an important reference quantity for design, control, and safe operation of technical combustors. While many studies have addressed the physical phenomena of intrinsic flame instabilities in the past, there is also a demand to predict such flame characteristics with reduced-order models to allow computationally efficient simulations. In this work, a H2-air spherical expanding flame, which exhibits thermo-diffusive instabilities, is studied with flamelet-based modeling approaches both in a-priori and a-posteriori manner. A recently proposed Flamelet/Progress Variable (FPV) model, with a manifold based on unstretched planar flames, and a novel FPV approach, which takes into account a large curvature variation in the tabulated manifold, are compared to detailed chemistry (DC) calculations. Both flamelet approaches account for differential diffusion utilizing a coupling strategy which is based on the transport of major species instead of transporting the control variables of the manifold. First, both FPV approaches are assessed in terms of an a-priori test with the DC reference dataset. Thereafter, the a-posteriori assessment contains two parts: a linear stability analysis of perturbed planar flames and the simulation of the spherical expanding flame. Both FPV models are systematically analyzed considering global and local flame properties in comparison to the DC reference data. It is shown that the new FPV model, incorporating large curvature variations in the manifold, leads to improved predictions for the microstructure of the corrugated flame front and the formation of cellular structures, while global flame properties are reasonably well reproduced by both models.

On Flame Extinction and Free-Floating Limits Using a Large Diameter Opposed Jet Burner

ABSTRACT This paper presents studies on flame extinction and associated flame structure using a large diameter (40 mm) opposed jet burner for the first time. The extinction experiments were carried out at various nozzle separation distance (L) to diameter (D) ratios (L/Ds) for methane-air non-premixed flames, while all similar experiments in the past literature are with smaller burner diameters (<25 mm). The extinction studies were used to determine the free-floating limits of the burner, and a detailed analysis of the effect of nozzle exit diameter and Applied Stress Rates (ASRs) on the free-floating limits are reported by comparing with similar works in the past. The Particle Image Velocimetry (PIV) technique was used to determine the axial velocity profiles and local extinction strain rates (Kl) of the flames. Based on the experiments, a correlation for the free-floating separation distance from the stagnation plane (zFF) is proposed, which was found to be proportional to the square of the ASRs. The reported free-floating limits in the literature using various diameters agree with the observed data trend. This solves the ambiguity regarding the free-floating limits of opposed jet burners and provides a general trend of the free-floating separation distance (LFF) even in the off-extinction operation regime. 2D axisymmetric simulations were carried out using realistic boundary conditions on the OpenFOAM® platform to predict flame extinction and, consequently, global flame extinction strain rates (Kg). The model predicts the Kg values and also its trend with L/Ds accurately. The model also captures the flame structure before extinction for various L/Ds.

Large Eddy Simulation of a Reacting Kerosene Spray in Hot Vitiated Cross-Flow

The evaporation and combustion characteristics of a kerosene spray injected perpendicularly into a cross-flow of high-temperature vitiated air is investigated. This fundamental flow configuration has wider implications for the future development of ultra-low emission aeronautical combustors, particularly with respect to technologies involving MILD combustion. Large eddy simulations with a Eulerian–Lagrangian framework are performed to investigate the spray evolution and the characteristics of the reaction zone for a range of conditions. For the closure of turbulence-chemistry interactions at the sub-grid scales, a transported probability density function approach solved by the Eulerian stochastic fields method is applied. A configuration based on the use of airblast atomisation is assessed first and compared with experimental observations. The effect of the atomiser air-to-liquid mass flow ratio is studied in greater detail, both in terms of the resulting gas-phase properties and the droplet evaporation process. Then, the effect of ambient pressure on the global spray flame behaviour is examined. For this part of the study, no atomising air is included in the simulation to separate the effects of ambient pressure on the spray from the interaction with the air jet. Analysis of the flame and spray properties at cross-flow operating pressures of 1 atm, 2 bar and 4 bar highlights the strong coupling between the reacting flow and droplet evaporation characteristics, which are highly affected by the penetration of the spray into a flow field characterised by relatively large gradients of temperature. The results reported in this work provide fundamental understanding for the development of novel low-emission combustion technologies and demonstrate the feasibility of applying large eddy simulation with detailed chemistry for the investigation of reacting aviation fuel sprays in hot vitiated cross-flow.

Dynamics of premixed hydrogen/air flames in unsteady flow

Fully resolved numerical simulations with finite rate chemical reactions and detailed molecular diffusion have been conducted for a series of laminar premixed hydrogen–air flames under atmospheric condition. The objective of the work is to study the influence of unsteadiness of the flow on the local and global flame dynamics. Two equivalence ratios with Φ=0.5 and 4 are considered, leading to a negative and a positive Markstein number Ma0 at steady-state condition. The flames are excited with oscillating inflows at pre-defined frequencies f to assess the effect of unsteady flame stretch on flame dynamics. The Damköhler number, defined by the ratio of the inverse frequency of the oscillations and flame transit time, is used to characterize the interactions between the flow and the chemical reactions based on their time scales. For both lean and rich flame conditions, the local flame speed Sl is less sensitive to the flame stretch in an unsteady flow, which results in a reduced magnitude of the Markstein number |Ma¯|. In addition, |Ma¯| is smallest when the time scale of the flow approaches the intrinsic time scale of the flame (Da≈1). The global consumption speed St, computed from integration of the fuel burning rate over the whole computational domain, yields a phase delay and a damped oscillation with respect to the unsteady inflow. The phase delay increases with f or decreasing Da, whereas the reverse trend has been found for the oscillation amplitude of St. The flame is not able to follow the unsteady flow or adjust its flame surface at high excitation frequencies with Da &amp;lt;1, and vice versa in the low frequency range with Da≫1. An efficiency factor E has been introduced to model the damped response of the flame due to flow unsteadiness, which reproduces the asymptotic behavior of E→0 at Da≪1 and E→1 at Da≫1. The simulation results reveal that the fluctuation time scale plays a significant role in elucidating the effect of flame–flow interaction, which should be considered for turbulent combustion modeling.

Flame characteristics of under-expanded, cryogenic hydrogen jet fire

In this work, cryogenic hydrogen fires at various pressures and initial temperatures were investigated experimentally. The flame length, width, thermal radiation fluxes and flame temperatures in the downstream region were measured for the scenarios with 1.6–3 mm jet nozzle, 133 to 273 K and 2–4 barabs. The results show that the flame size is related to not only the jet nozzle diameter but also the stagnation pressure and temperature. Under constant pressure, the flame size, total radiative power and radiation fraction increase with the decrease of temperature, due to lower choked flow velocity and higher density of cryogenic hydrogen. A correlation of normalized flame length Lf/dj is proposed based on the stagnation pressure of hydrogen p0 and the ratio of ambient temperature and hydrogen stagnation temperature Tatm/T0. The results verify that such correlation can be used to a priori prediction of flame size at a wide variety of hydrogen temperatures (133 K ∼ room temperature). Additionally, based on the reciprocal function, a correlation of the radiative fraction χrad is presented as a function of global flame residence time τf.

Dual-frequency plasma promoting flameholding in a supersonic combustion chamber

Facing the requirement of flameholding in a supersonic combustion chamber, a flameholding experiment under dual frequency excited plasma (DF excited plasma) was carried out in an axisymmetric supersonic combustion chamber. The flame establishment process was observed by using a high-speed-camera, and the flameholding characteristics with or without DF excited plasma were analyzed under different equivalence ratio(ER). The experimental results show that the flame will be enhanced by using dual-frequency excitation plasma in the supersonic combustion chamber: the flameholding mode was transformed from a global flame to a local flame, as well as the pressure of the combustion chamber was decreased significantly, when the DF excited plasma is switched off. In the ER range of 0.51 (0.18+0.33)∼0.60 (0.18+0.42), the DF excited plasma could induce a transformation from a local to a global flame, while if the ER is too high (ER>0.69 (0.18+0.51)) or too low (ER<0.18), the action of the DF excited plasma always couldn't. A basic feedback mechanism of the flameholding was proposed, with the consideration of the experimental results. The O radicals generated by the DF excited plasma was one important factors for flame reverse propagation, and the existence of the global flame was another necessary factor. Under the combined action of the O radicals of DF excited plasma and the global flame, the positive feedback process in the combustion chamber will be intensified, and the flame will propagate reversely.

The effect of hydrogen enrichment, flame-flame interaction, confinement, and asymmetry on the acoustic response of a model can combustor

To maximise power density practical gas turbine combustion systems have several injectors which can lead to complex interactions between flames. However, our knowledge about the effect of flame-flame interactions on the flame response, the essential element to predict the stability of a combustor, is still limited. The present study investigates the effect of hydrogen enrichment, flame-flame interaction, confinement, and asymmetries on the linear and non-linear acoustic response of three premixed flames in a simple can combustor. A parametric study of the linear response, characterised by the flame transfer function (FTF), is performed for swirling and non-swirling flames. Flame-flame interactions were achieved by changing the injector spacing and the level of hydrogen enrichment by power from 10 to 50%. It was found that the latter had the most significant effect on the flame response. Asymmetry effects were investigated by changing one of the flames by using a different bluff-body to alter both the flame shape and flow field. The global flame response showed that the asymmetric cases can be reconstructed using a superposition of the two symmetric cases where all three bluff-bodies and flames are the same. Overall, the linear response characterised by the flame transfer function (FTF) showed that the effect of increasing the level of hydrogen enrichment is more pronounced than the effect of the injector spacing. Increasing hydrogen enrichment results in more compact flames which minimises flame-flame interactions. More compact flames increase the cut-off frequency which can lead to self-excited modes at higher frequencies. Finally, the non-linear response was characterised by measuring the flame describing function (FDF) at a frequency close to a self-excited mode of the combustor for different injector spacings and levels of hydrogen enrichment. It is shown that increasing the hydrogen enrichment leads to higher saturation amplitude whereas the effect of injector spacing has a comparably smaller effect.

Flame characteristics of methane/air with hydrogen addition in the micro confined combustion space

This paper investigated methane/air flame characteristics with hydrogen addition in micro confined combustion space experimentally and computationally. The focus is on the effect of hydrogen addition on the methane/air flame stabilization, the onset of flame with repetitive extinction and ignition (FREI), and the global flame quenching in decreasing continuously combustion space. Furthermore, the effects of hydrogen addition on the flame temperature and the local equivalence ratio distribution were analyzed systematically using numerical simulations. In addition, the effects of hydrogen addition on the concentrations of OH and H radicals, and the critical scalar dissipation rate of local flame extinction were discussed. With a higher hydrogen ratio, the mixing is faster, and the flame is smaller. When the micro confined space is narrower, the heat loss to the combustor walls has a higher impact on the flames. The flames with higher hydrogen ratios have therefore lower peak flame temperatures and lower concentrations of H and OH radicals. The results show that hydrogen addition can effectively widen the stable combustion range of methane/air flames in the micro confined space by about 20% when the hydrogen addition ratio reaches 50%. The frequency and the maximum propagation velocity of FREI flames can be increased as well. The quenching distance of methane/hydrogen/air flames decreases nearly linearly with the increase of hydrogen ratio. This is attributed to the higher critical scalar dissipation rate of local flame extinction in flames with a higher hydrogen ratio.