Lifted Jet Flame Research Articles

Investigation on the effect of the leading angle of the strut on the stabilization characteristics of lifted jet flames

To achieve stable combustion within supersonic combustors, the strut-based mixing augmentation techniques are commonly employed. Large Eddy Simulations (LES) are employed to capture the intricate details of the combustion process in this study, focusing on the influence of shock waves induced by the leading edge of the strut and recirculation zones formed at the trailing edge on flame structure and stability. The effects of five struts with different leading edge angles on the stabilization characteristics of lifted jet flames were investigated, and a novel wedge-shaped strut was proposed. The results reveal that variations on the leading-edge angle of the struts lead to significant differences in the morphology of the jet flames. Under conditions permitting stable combustion, a strut with a 5° leading edge half-angle exhibits the highest combustion efficiency, though the combined effects of shock waves and recirculation zones in the 7° leading edge half-angle strut are also noteworthy. Based on these findings, a novel wedge-shaped strut design is introduced, which combines the characteristics of both strut types. The novel wedge-shaped strut achieves a combustion efficiency of 97 % at the slight cost of additional total pressure loss, providing a foundation for further optimization.

Effects of Partial Premixing and Coflow Temperature on Flame Stabilization of Lifted Jet Flames of Dimethyl Ether in a Vitiated Coflow Based on Stochastic Multiple Mapping Conditioning Approach

The Reynolds-averaged Navier–Stokes (RANS)-based stochastic multiple mapping conditioning (MMC) approach has been used to study partially premixed jet flames of dimethyl ether (DME) introduced into a vitiated coflowing oxidizer stream. This study investigates DME flames with varying degrees of partial premixing within a fuel jet across different coflow temperatures, delving into the underlying flame structure and stabilization mechanisms. Employing a turbulence k-ε model with a customized set of constants, the MMC technique utilizes a mixture fraction as the primary scalar, mapped to the reference variable. Solving a set of ordinary differential equations for the evolution of Lagrangian stochastic particles’ position and composition, the molecular mixing of these particles is executed using the modified Curl’s model. The lift-off height (LOH) derived from RANS-MMC simulations are juxtaposed with experimental data for different degrees of partial premixing of fuel jets and various coflow temperatures. The RANS-MMC methodology adeptly captures LOH for pure DME jets but exhibits an underestimation of flame LOH for partially premixed jet scenarios. Notably, as the degree of premixing escalates, a conspicuous underprediction in LOH becomes apparent. Conditional scatter and contour plots of OH and CH2O unveil that the propagation of partially premixed flames emerges as the dominant mechanism at high coflow temperatures, while autoignition governs flame stabilization at lower coflow temperatures in partially premixed flames. Additionally, for pure DME flames, autoignition remains the primary flame stabilization mechanism across all coflow temperature conditions. The study underscores the importance of considering the degree of premixing in partially premixed jet flames, as it significantly impacts flame stabilization mechanisms and LOH, thereby providing crucial insights into combustion dynamics for various practical applications.

Experimental observation of the TJI-initiated HPDI gas combustion: Vertically crossed flame jet and methane jet

In the field of natural gas marine engines, the high-pressure direct injection (HPDI) technology is widely used to achieve emission reduction while maintaining equivalent thermal efficiency and power output compared to diesel engines. In these engines, the in-cylinder combustion is primarily initiated by the diesel spray flame near the top dead center (TDC). In the present work, pre-chamber turbulent jet ignition (TJI) is employed to substitute diesel injection to initiate the combustion of HPDI methane jets. The optical experiments of ignition and flame development in the TJI-HPDI system with various injector configurations and injection/ignition control parameters are conducted in a constant-volume combustion chamber (CVCC). It is revealed that with the increase of injection-ignition delay(ti), three ignition modes are observed sequentially: methane jet suppresses ignition, methane jet first suppresses ignition then promotes flame propagation, and direct ignition and promoted flame propagation. The effect of various injection-ignition delays and injection pulse width were explored. The injection-ignition delay significantly influences the combustion characteristics such as heat release rate, while the injection pulse width influences the duration of the lifted jet flame. The flame lift-off length first decreases and then increases due to variations in thermodynamic conditions and oxygen concentration. With a larger-nozzle injector, the critical injection-ignition delay to initiate the main chamber combustion is significantly reduced. Moreover, the use of different nozzle diameters leads to varying levels of turbulent intensity in the methane jet, which in turn affects the combustion behavior.

An Experimental Study Of Natural Gas Jet-Flames In Crossflow With High Hydrogen Admixture At Elevated Pressure Conditions

A series of high hydrogen-content jet flames in crossflow are investigated experimentally using high-speed (10 kHz) stereoscopic particle image velocimetry, dual-plane laser-induced fluorescence of the hydroxyl radical, and OH* chemiluminescence imaging. The flames were fueled with natural gas enriched to 70-, 80-, 90- and 100% H2, by volume at conditions approaching those found in the mixing duct of a modern gas turbine engine, specifically in close confinement, at 7 bars pressure and with a crossflow preheat of 511 K. The measurements indicate the recirculation zone on the leeward side of the jet plays a critical role in stabilization of the lifted jet flame in crossflow. The measurements show that local heat-release from a flame in this region acts as a self-reinforcing feedback mechanism, strengthening the recirculation zone and increasing local residence time there. This, in turn, creates conditions more favorable to flame stabilization. The measurements also showed how close confinement of the JICF by the flow channel walls affects jet-penetration and growth of the counter-rotating vortex pair.

The evolution of soot morphology for the maturation of nascent particle in a turbulent lifted jet flame

In order to understand the soot formation in Diesel engine, a turbulent jet flame is used to simulate the combustion in the cylinder. The experimental study is performed to investigate the evolution of soot morphology for the maturation of nascent particle in a turbulent lifted jet flame fueled by n-heptane/toluene mixtures. An ultrasonic atomizer is used to evenly spread the fuel droplets to acquire single primary particles. Transmission electron microscopy is used to study the morphology of the particle sampled from the flame at different heights. Small soot aggregates are acquired from all the samples. Particle maturation such as agglomeration is accelerated under a high temperature by comparing the particle morphology sampled on the centerline and the wings of the flame. The precursors of nascent particles are depicted as dark nucleation dispersed to semitransparent polycyclic aromatic hydrocarbons. The nanostructure of nascent particles transforms from an amorphous state to an onion structure with the maturation of particles. Surface growth initially dominates the maturation of nascent particles in the direction of outside to inside for single particles. Agglomeration begins to emerge with the increased probability of collision at the end of maturation. Surface growth and agglomeration increase the mean particle diameter as the flame height increases. The oxidability of particles that undergo surface growth and agglomeration notably increases. The structure of nascent particles is inclined to be ordered and the mean particle diameter decreases with oxidation dominating the combustion reaction.

On the flame stabilization of turbulent lifted hydrogen jet flames in heated coflows near the autoignition limit: A comparative DNS study

Three-dimensional direct numerical simulations of turbulent lifted hydrogen jet flames in heated coflows are performed with a detailed H2/air chemical mechanism to understand their ignition dynamics and stabilization mechanisms. Turbulent lifted jet flames with four different coflow temperatures, Tc, between 750 K and 1100 K are investigated by examining the instantaneous/time-averaged values and conditional means of heat release rate and species critical to ignition, and by performing a displacement speed analysis and a local combustion mode analysis with an indicator, α. Although Tc at 950 K is higher than the autoignition limit, the flame is primarily stabilized by flame propagation rather than autoignition, while at 1100 K, flame stabilization is found to be highly affected by autoignition. The local combustion mode analysis further reveals that at 950 K, even if a local ignition mode with |α|<1 first appears in the near field of the jet, it develops into a local extinction mode with α<−1 as local temperature decreases due to the excessive mixing of heated coflow and cold H2 within vortical structures, which inhibits the ignition kernel development upstream of the flamebase. At 1100 K, however, a local ignition mode prevails upstream of the flamebase. To further identify the effect of a vortex on the early development of an ignition kernel in a mixing layer between the heated coflow and cold H2, a series of two-dimensional DNSs are performed, varying several vortex parameters and air temperature, as a reference for the more complicated corresponding 3-D turbulent DNS cases. The results substantiate that the development of a vortex in the mixing layer tends to retard the autoignition within the vortex, especially when its temperature is slightly above the autoignition limit.

Large-Eddy Simulation of a Lifted High-Pressure Jet-Flame with Direct Chemistry

ABSTRACT A large-eddy simulation is presented of a challenging high-pressure jet flame case that is representative of state of the art, dry low-NO x and low-CO real gas turbine combustion. A reaction scheme is developed for lifted lean premixed high pressure methane jet flames, and tested by three-dimensional large-eddy simulation of an experiment, for which very detailed data are available. Auto-ignition-delay times of different mixtures of fresh gas and products have been introduced as a novel optimization criterion for the mechanism development. The new mechanism has been developed by a genetic algorithm-based reduction and optimization, and consists of 15 species and 18 reactions. The large-eddy simulations are performed using a finite rate chemistry (FRC) approach and the dynamic thickened flame (DTF) model to investigate a lifted jet flame at high pressure in a gas turbine model combustor. In the simulations, the novel mechanism is compared to a similar mechanism that was generated without this criterion, and the well-established Lu19 mechanism. With the new mechanism, the LES predicts the flame as accurately as with Lu19, at a significantly lower cost. Further post processing with Lagrangian tracer particles confirmed that ignition events occur in the region corresponding to the liftoff height estimated in the experiment, which is corroborated by a chemical explosive mode analysis (CEMA). Overall, the newly developed mechanism with the novel optimization criterion was found to provide a better agreement with the experiments than previous mechanisms of similar cost, or a comparable agreement to a mechanism of significantly higher cost.

Large eddy simulation of a supersonic lifted jet flame in the high-enthalpy coflows

The stabilization mechanism of a supersonic lifted jet flame in the heated coflows should be attributed to the competition between autoignition and the propagation of active kernels at the flame base. As a result, the characteristics of the lifted hydrogen jet combustion gradually vary along the axial direction, which have not yet been deeply addressed in the previous studies. Highly-resolved large eddy simulation of a hydrogen jet injection into the air flow is carried out in this paper, the particular emphasis of which is placed on the evolution process of the turbulent structures as well as the flameholding mechanism. To analyze the effect of turbulence on the chemistry, an assumed probability density function method is adopted to close the source term of the governing equations. And the temporal and spatial resolution of the numerical schemes is high enough to capture the unsteady motion inside the supersonic reactive flowfeild. Predicted concentration of some important species agrees well with that measured by the experimental methods, demonstrating the responsibility of numerical results. From the instantaneous images, large eddy simulation reproduces the three-dimensional flowfield generated by the interaction of the inner jet and heated air flow. According to the time-averaged analysis, the combustion flowfield can be divided into three stages, namely the initial mixing, autoignition and subsequent turbulent flame. Evidently, the supersonic reactive flow passes through the transition from autoignition to the partially premixed flame, and the statistical features along the axial direction change correspondingly. Further, the effect of small-scale turbulent fluctuations on the reactive front within the shear layer is analyzed in detail, and the significant founding is propitious to understand the turbulent combustion regime in high-enthalpy conditions.

Stationary ethylene–air edge flames in a wedge-shaped region at low and high strain rates

ABSTRACT Edge flames are a canonical two-dimensional flame structure appearing in lifted jet flames and in the growth and repair of flame holes in nonpremixed turbulent combustion. Computational studies of edge flames with hydrodynamic-coupling at high strains are difficult owing to difficulties defining a stationary state. A wedge-shaped counterflow configuration is here used to provide control over the position of the edge flame, and allowing access to stationary, hydrodynamically-coupled retreating flames (at low and high strains). ethylene–air edge flames are established in the resulting non-uniformly strained counterflow, with combustion modeled using a skeletal reduction of the USC Mech II. The details of the ethylene–air edge flame are discussed, and comparisons are made between stoichiometric, fuel-lean, and fuel-rich compositions. Mixture-fraction-based coordinates local to the flame front are developed and analysed for a range of edge-flame speeds and compositions. A strain-rate parameter is described that admits a speed-strain relationship that is insensitive to the mixture stoichiometry, and a model for retreating flame speeds at high strain rates is discussed.

The Reattachment Process of a Lifted Jet Diffusion Flame by Repetitive DC Pulse Discharges

On research of plasma assisted combustion, effects of electric and plasma discharges in DC, AC and pulse forms on reattachment of a lifted flame have attracted extensive attention. However, the detailed plasma assisted reattachment process and mechanism and roles of induced corona discharge and corona-induced ozone on the reattachment process are still unclear and undocumented. The forced reattachment process of a lifted diffusion jet flame by repetitive DC electric pulse discharges was experimentally investigated in this study using high-speed flame imaging, conditioned particle image velocimetry (PIV), and planar ozone concentration imaging. The forced reattachment process can be divided into three stages in sequence: ionic wind prior to corona initiation, corona initiation, and corona enhancement propagation. The conditioned PIV results showed that the instantaneous flame base propagation velocity is sufficiently enhanced beyond the laminar burning velocity for high pulse-repetition-frequency (PRF) cases at the instant of pulse discharge (on pulse) due to the enhanced oxidation of the corona induced ozone. By observing the dynamic flame-base behavior and evolution characteristics of the short-lived corona induced ozone for various PRFs, the novel forced reattachment process and mechanism of a lifted jet flame induced by repetitive DC electric pulse discharges is proposed.

On the flame structure and stabilization characteristics of autoignited laminar lifted n-heptane jet flames in heated coflow air

The characteristics of the flame structure and stabilization of autoignited laminar lifted n-heptane jet flames in heated coflow air are investigated by performing 2-D numerical simulations with a 68-species skeletal chemical mechanism of n-heptane oxidation. The present simulations can reproduce a distinct transition of a lifted jet flame from a tribrachial edge flame mode to a moderate or intense low-oxygen dilution (MILD) combustion mode observed from a previous experimental study, featuring a significant variation in the liftoff height with the fuel jet velocity, U0. It is found that a lifted flame with the MILD combustion mode can exist further downstream of the stoichiometric mixture fraction isoline due to autoignition occurring upstream of the flamebase. The displacement speed and chemical explosive mode analyses reveal that the autoignition of lean mixtures plays a critical role in stabilizing lifted flames with the MILD combustion mode. It is further elucidated from additional numerical simulations that an autoignited laminar lifted n-heptane jet flames can be stabilized as one of the following forms depending on the inlet temperature, T0, and U0: a MILD combustion, a partially-premixed edge flame, a tribrachial edge flame, and a tetrabrachial edge flame. Based on the flame structures and stabilization mechanisms of the lifted flames, a flame regime diagram is constructed in the normalized U0 and Damköhler number space.

Effects of Schmidt number on non-monotonic liftoff height behavior in laminar coflow-jet flames with diluted methane and ethylene

Stabilization characteristics of laminar lifted jet flames in a coflow were investigated experimentally to elucidate the effect of Schmidt number in methane and ethylene fuels diluted with N2, He, and Ar. A non-monotonic (decreasing and then increasing) liftoff height (HL) behavior with jet velocity (U0) was observed previously for methane fuel diluted with N2. To further elucidate the fuel Schmidt number (ScF) effect in exhibiting such a non-monotonic (U-shaped) behavior, various diluents (N2, He, and Ar) were added to the fuel streams and methane and/or ethylene fuels were used. The result showed three flame types in terms of ScF and fuel density; nozzle-attached flame, stationary lifted flame, and oscillating flame. Among stationary lifted flames, two distinct HL behaviors with U0 were observed; monotonic and non-monotonic HL behaviors. A critical Schmidt number (ScF,cr1) existed over which monotonically increasing behavior was observed. A second critical Schmidt number (ScF,cr2) also existed such that U-shaped behavior was observed for ScF,cr2<ScF<ScF,cr1. An oscillating lifted flame was observed for ScF<ScF,cr2. The oscillating and stationary lifted flames can be categorized in terms of the density differences among the fuel, air, and burnt gas. For the increasing HL cases (including the increasing regime in U-shaped behavior), HL behavior can be characterized in terms of ScF, the density difference between fuel and air, ScF,cr1, and U0. While the decreasing HL regime in the U-shaped behavior can be characterized with ScF and/or the Richardson number (defined based on the density difference between fuel and air). Oscillating flames were observed with the frequency range of 2.1–2.7 Hz by the repetitive action of positive (by burnt gas) and negative (when the fuel heavier than air) buoyancies.

Hydrogen generation, and its venting from nuclear reactors

Reactor fires and explosions are centred around the use of graphite as a neutron moderator, and the high temperature generation of hydrogen in reactions of steam and zirconium. An alternative to uncontrolled, excessive, build-up of pressure within the reactor, is the provision of a buffer vessel, within which there is permeable membrane separation of hydrogen from radioactive products. Possible rates of production of hydrogen are compared with the rates at which it might be separated and then flared in lifted jet flames, giving high burn rates. There are few data on the behaviour of H2 flares in air cross flows, and this is synthesised from available data for other flammable gases. Destruction of hydrogen lifted jet flames by the cross flow of atmospheric air would seem to be less likely than for hydrocarbon jet flames. The H2 relationships are different from those of the hydrocarbons, due to the higher chemical reactivity of H2, its small laminar flame thickness, reduced air requirement, higher acoustic velocity, and minimal flame lift-off distance. Flaring with micro-tubes might be advantageous for integrating flaring with membrane hydrogen separation, whilst high mass flow rates can be achieved with large diameter flares in the lifted flame, supersonic regime.

Assessment of the stabilization mechanisms of turbulent lifted jet flames at elevated pressure using combined 2-D diagnostics

The stabilization mechanisms of turbulent lifted jet flames in a co-flow have been investigated at a pressure of 7 bar. The structure of the flame base was measured with combined OH and CH2O planar laser induced fluorescence (PLIF) and the spatial distribution of equivalence ratio was imaged, simultaneously, with CH4 Raman scattering. The velocity field was also measured with particle imaging velocimetry (PIV). Different bulk jet velocities Uj and co-flow velocities Uc were examined. Data show that flames with Uc = 0.6 m/s stabilize much further away from the nozzle than those with Uc = 0.3 m/s and that their structure does not resemble that of the edge-flames found closer to the nozzle. In addition, for Uc = 0.6 m/s, the measured lift-off height decreases with increasing bulk jet velocity, which is opposite to what is typically observed for lifted flames. Statistical examination of CH4 Raman images shows that the flames with Uc = 0.6 m/s propagate through regions of the flow where the equivalence ratio is not always stoichiometric but, instead, spans the whole flammability range. This is not consistent with edge-flames and is, instead, indicative of premixed burning. This is corroborated by PIV results which show that the flame base velocity exceeds that typically reported for edge-flames. Measurements of relevant flow properties were also conducted in non-reacting jets to predict the turbulent burning velocity of these lifted flames burning in a premixed mode. For Uc = 0.6 m/s and relatively large bulk jet velocities (Uj = 10 and 15 m/s), the predicted turbulent burning velocities are sufficiently high to counter the incoming flow of reactants and, in turn, allow flame stabilization. However, for a lower bulk jet velocity of Uj = 5 m/s, the predicted turbulent burning velocity is much less, leading to blow-out. This explains why the lift-off height decreases with increasing jet velocity for methane at 7 bar and Uc = 0.6 m/s. Data also shows that increasing pressure promotes transition from edge-flames to premixed flames due to reduced laminar burning velocity and enhanced mixing.

Large-eddy simulation of turbulent autoigniting hydrogen lifted jet flame with a multi-regime flamelet approach

In this work, the combustion model is focused on to describe a multitude of reaction regimes that are deemed to affect the flame stabilization. For this purpose, an efficient flame indicator is formulated to differentiate the differing flame structures and make use of flamelet chemistry that accounts for autoignition and multi-regime reactions. The large eddy simulation with this methodology is carried out to compute a turbulent lifted hydrogen-nitrogen flame in vitiated coflow. The canonical flame models of a laminar premixed flame and an unsteady counterflowing flame have been used to simulate the flamelet structure at different regimes. Present model improves the prediction of mean and rms profiles for temperature and species mass fraction in the comparison with experiments and a reference simulation, adopting the single-regime flamelet. The computed results also demarcate the formation of a triple flame structure at the flame base, where combustion develops into the premixed reaction that extends to the fuel-lean and rich branches. The counterflow mixing mode with autoignition is identified as the major mechanism for stabilization and is responsible for the propagating premixed zone above the liftoff height. The developed multi-regime flamelet approach properly accounts for the interactive different modes of burning.

Modelling of non-premixed turbulent combustion with Conditional Moment Closure (CMC)⋆.

Conditional Moment Closure (CMC), an advanced turbulent reacting flow method, has been applied to the challenging cases with a varying degree of turbulence-chemistry interactions. The CMC approach may be used either in the RAMS or LES context, this is reviewed in the first part of this paper, while the second part is dedicated to applications on Sandia piloted jet flames D and F and lifted hydrogen jet flame. In case of the Sandia piloted jet flame D, the RANS-CMC simulation results are in agreement with the experimental data. On the other hand, when one comes to the results for the Sandia flame F, extinction is not captured. These discrepancies are attributed to the use of RANS in combination with the boundary conditions set in CMC. However, in case of turbulent lifted jet flame in vitiated co-flow, the LES-CMC model is able to capture the axial and radial profiles of mixture fraction, temperature and major species. The lift-off height is found to be very sensitive to the co-flow temperature as well as the co-flow velocity. The LES-CMC results highlight the potential of the technique to simulate the problems which involve complex turbulence-chemistry interactions.

Influence of Boundary Conditions on the Flame Stabilization Mechanism and on Transient Auto-ignition in the DLR Jet-in-Hot-Coflow Burner

Transient auto-ignition is a key factor for flame stabilization and flame initialization in several technical combustion systems such as internal combustion engines or gas turbine combustors. Reliable numerical simulations of auto-ignition stabilized flames are important for the development of new combustor systems. For detailed model validation, knowledge of the sensitivity of different system response quantities (SRQs) of interest to the boundary conditions in combination with the accuracy of boundary conditions is essential, especially with respect to uncertainty quantification of numerical simulations. In the current study, the flame stabilization and auto-ignition in the DLR Jet-in-Hot-Coflow burner was examined experimentally using high-speed OH* chemiluminescence. Here, methane was either injected continuously to study the flame stabilization mechanism of steady state lifted jet flames, or in a pulsed manner to study the formation of auto-ignition kernels, into the hot exhaust gas of a lean, premixed hydrogen/air flame. The flame stabilization height, and the location and time of initial auto-ignition kernels for a case with transient auto-ignition were evaluated with respect to several boundary conditions, such as coflow temperature as well as coflow- and jet-velocity. A relative sensitivity of the measured SRQs on the boundary conditions was introduced in order to quantitatively compare steady state flame to transient auto-ignition characteristics and to assess the quantitative influence of different boundary conditions. Comparison of the auto-ignition dynamics in the steady state and during transient fuel injection allowed assessing the role of auto-ignition in the flame stabilization mechanism for different boundary conditions; accompanying chemical kinetic calculations were used to quantify the influence of strain on auto-ignition and flame propagation for the current conditions, allowing further insight into the flame stabilization mechanism in Jet-in-Hot-Coflow flames.

LES/CMC modelling of ignition and flame propagation in a non-premixed methane jet

The Large Eddy Simulation (LES) / Conditional Moment Closure (CMC) model with detailed chemistry is used for modelling spark ignition and flame propagation in a turbulent methane jet in ambient air. Two centerline and one off-axis ignition locations are simulated. We focus on predicting the flame kernel formation, flame edge propagation and stabilization. The current LES/CMC computations capture the three stages reasonably well compared to available experimental data. Regarding the formation of flame kernel, it is found that the convection dominates the propagation of its downstream edge. The simulated initial downstream and radial flame propagation compare well with OH-PLIF images from the experiment. Additionally, when the spark is deposited at off-centerline locations, the flame first propagates downstream and then back upstream from the other side of the stoichiometric iso-surface. At the leading edge location, the chemical source term is larger than others in magnitude, indicating its role in the flame propagation. The time evolution of flame edge position and the final lift-off height are compared with measurements and generally good agreement is observed. The conditional quantities at the stabilization point reflect a balance between chemistry and micro-mixing. This investigation, which focused on model validation for various stages of spark ignition of a turbulent lifted jet flame through comparison with measurements, demonstrates that turbulent edge flame propagation in non-premixed systems can be reasonably well captured by LES/CMC.

Differential diffusion effect on the stabilization characteristics of autoignited laminar lifted methane/hydrogen jet flames in heated coflow air

The characteristics of autoignited laminar lifted methane/hydrogen jet flames in heated coflow air are numerically investigated using laminarSMOKE code with a 57-species detailed methane/air chemical kinetic mechanism. Detailed numerical simulations are performed for various fuel jet velocities, U0, with different hydrogen ratio of the fuel jet, RH, and the inlet temperature, T0. Based on the flame characteristics, the autoignited laminar lifted jet flames can be categorized into three regimes of combustion mode: the tribrachial edge flame regime, the Moderate or Intense Low-oxygen Dilution (MILD) combustion regime, and the transition regime in between. Under relatively low temperature and high hydrogen ratio (LTHH) conditions, an unusual decreasing liftoff height, HL, behavior with increasing U0 is observed, qualitatively similar to those of previous experimental observations. From additional simulations with modified hydrogen mass diffusivity, it is substantiated that the unusual decreasing HL behavior is primarily attributed to the high diffusive nature of hydrogen molecules. The species transport budget, autoignition index, and displacement speed analyses verify that the autoignited lifted jet flames are stabilized by autoignition-assisted flame propagation or autoignition depending on the combustion regime. Chemical explosive mode analysis (CEMA) identifies important variables and reaction steps for the MILD combustion and tribrachial edge flame regimes.

Lifted and reattached behaviour of laminar premixed flame under external acoustic excitation

The flame chemiluminescent emission fluctuations and the vortex structure of the lifted jet flame under acoustic excitation were studied in this investigation. By employing high-speed visualization and DFCD (Digital Flame Colour Discrimination) image processing method, the fluctuation of the instantaneous mixture fraction has been found highly correlated with the lifted height variations. It has been observed that during the flame drifting to downstream, there is no obvious shifting on the mixture fraction. However, when the flame travels back to upstream, the fuel mixture has been evidently diluted. In addition, the stabilisation mechanism can be further explained by analysing the velocity fluctuation of the vortices in the shear layer via PIV. Measurements show that, the turbulent stretching at the shear layer generated by the excitation leading to the flame lift-off. On the other hand, the Kelvin-Helmholtz vortices in the unburn part play a key role in preventing flame lift-off. But, the excessive external acoustic excitation leads to blow-off due to over dilution and increased lifted height.