Flame Stabilization Mechanism Research Articles

Characterization of Additive Layer Manufacturing Swirl Burner Surface Roughness and Its Effects on Flame Stability Using High-Speed Diagnostics

Abstract In this study, two Inconel 625 swirl nozzle inserts with identical bulk geometry were constructed via additive layer manufacturing (ALM) for use in a generic gas turbine swirl burner. Further postprocessing by grit blasting of one swirl nozzle insert results in a quantifiable change to the surface roughness characteristics when compared with the unprocessed ALM swirl nozzle insert or a third nozzle insert which has been manufactured using traditional machining methods. An evaluation of the influence of variable surface roughness effects from these swirl nozzle inserts is therefore performed under preheated isothermal and combustion conditions for premixed methane-air flames at thermal power of 25 kW. High-speed velocimetry at the swirler exit under isothermal conditions gives evidence of the change in near-wall boundary layer thickness and turbulent fluctuations resulting from the change in nozzle surface roughness. Under atmospheric combustion conditions, this influence is further quantified using a combination of dynamic pressure, high-speed OH* chemiluminescence, and exhaust gas emissions measurements to evaluate the flame stabilization mechanisms at the lean blowoff and rich stability limits. Notable differences in flame stabilization are evident as the surface roughness is varied, and changes in rich stability limit were investigated in relation to changes in the near-wall turbulence intensity. Results show that precise control of in-process or postprocess surface roughness of wetted surfaces can positively influence burner stability limits and NOx emissions and must, therefore, be carefully considered in the ALM burner design process as well as computational fluid dynamics (CFD) models.

Numerical Characterization of a Premixed Hydrogen Flame Under Conditions Close to Flashback

This work presents a numerical study of a technically premixed swirling combustor with central air injection at conditions close to flashback using large-eddy simulation with flamelet modelling. This burner has the characteristics of showing flashback at low equivalence ratios, so numerical simulations are set to identify the mechanisms behind the flashback formation. Experimental findings suggest the axial momentum ratio between fuel and air dominates the flame stabilization mechanism and flashback resistance over mixing and equivalence ratio fluctuations. This aspect is investigated here for two operating conditions with the same axial momentum ratio as in the experiment using a perfectly premixed assumption. The two test cases correspond to two stable operating points, far and close to the flashback point. The study shows the assumption of perfect premixing is valid during the stable operation of the burner up to flashback conditions. The experimental results are well predicted under inert and reacting conditions by using a perfectly premixed mixture. It is found that the non reacting flow field develops a self-excited oscillation in the form of a precessing vortex core. This oscillation is attenuated by the fuel injection due to the respective increase in axial momentum and it is ultimately suppressed in the reacting flow field. Both experiments and simulations confirm the same trends. The analysis of the flames have shown certain dynamics as the flashback point is approached. The flashback resistance of the burner is minimized due to an increase in the velocity deficit of the incoming mixture. The recirculation region is shifted upstream, the central recirculation is altered and the flame position is displaced towards the inlet of the reactants in the combustion chamber. The analysis of instabilities and flow dynamics suggest that the formation of flashback can be attributed to combustion induced vortex breakdown, which in turn is associated to the lower axial momentum introduced by the fuel jets in leaner conditions.

Investigation of the Flame Stabilization Mechanism in Phase-Inhomogeneous Air—Fuel Mixtures

The mechanism of flame stabilization in phase-inhomogeneous air—fuel mixtures is considered. A method is proposed for refining the thermal theory of flame stabilization by taking into account the heat consumed for heating and evaporation liquid fuel in the ignition zone. An analytical dependence is obtained for the calculation of flaming out curves during the steady combustion of two-phase mixtures. Analysis of the experimental data shows their compliance with the laws of the calculated dependence.

Organization of Effective Combustion of Kerosene in a Channel at High Flow Velocities

The boundaries of stable combustion in a flow of an aluminum particle–air mixture in a wide range of changes in an air-to-fuel ratio (0.1–2.0) are experimentally determined. A characteristic feature of the relationship of a flame blowoff velocity with the air-to-fuel ratio is revealed: it has two maxima. Taking into account the principle of the flame stabilization mechanism, based on the equality of the flow velocity at which the flame is blown off to the flame front propagation velocity, it is concluded that there are two maxima in the relationship of the front velocity with the air-to-fuel ratio, which correspond to the maximum values of heat release and combustion temperature of the aluminum particle–air mixture with an air-to-fuel ratio of 0.2 and 1.0. Previously, these maxima were obtained by other researchers in thermodynamic calculations.

Numerical studies of flame extinction and re-ignition behaviors in a novel, ultra-lean, non-premixed model GT burner using LES-ESF method

The flame dynamics in a novel, ultra-lean non-premixed model gas turbine (GT) burner flame was numerically studied using large eddy simulation (LES) coupled with probability density function (PDF) based on the Euler stochastic field (ESF) method. One non-reacting case and three reacting cases with global equivalence ratio ϕglob=1.0,0.6,0.3 were simulated. Comparison of mean flow fields and OH distributions between numerical and experimental results was conducted. Flame dynamics including flame stabilization, structures and transitions of combustion modes were investigated. The simulation results were in close agreements with the experimental measurements showing the capability of PDF-ESF LES model in predicting the flame behaviors. At higher ϕglob, the fuel jet velocity was higher, which yielded higher scalar dissipation rate, χ, near the burner exit, leading to local extinction and flame lifted-off. In the extinction region, a series of relatively low to medium temperature reactions were active, providing favorable conditions for re-ignition downstream where χ is lower than the critical scalar dissipation rate for flame extinction, χcrt. In addition, with ϕglob decreasing, the flame height decreased due to a smaller jet velocity and χ, and thus the mechanism of flame stabilization changed from the swirl-stabilized to the bluff-body stabilized.

Modelling of hydrogen flame in perfectly clean combustion regimes using LES-CMC

The LES coupled with the first-order CMC method and detailed chemical mechanism is used for the calculations of hydrogen jet burning in the atmosphere of water vapour and oxygen mixture. The flame is initiated by a spark, then it spreads and propagates through the domain and eventually stabilises as a lifted or attached one. Three dimensional calculations were proceeded with analysis of the flame in the mixture fraction space. The main focus of the paper is on the effect of oxidizer composition on the flame characteristics and stabilization mechanisms. The flame structure during the propagation phase is represented based on the so-called pre-ignition species. Time-averaged results show the flame lift-off distance, shape and radial size. It is found that these factors may be effectively controlled by the amount of water vapour in the co-flowing oxidizer.

High-fidelity numerical analysis of non-premixed hydrothermal flames: Flame structure and stabilization mechanism

Hydrothermal flame is a fast oxidation process that occurs in supercritical aqueous environment. Flame structure and stabilization mechanism of laminar non-premixed hydrothermal flames (under a pressure of 25.0 MPa) are studied via homogeneous auto-ignition calculations and two-dimensional high-fidelity numerical simulations, and the effects of oxidizer temperature are investigated. The simulations are conducted with a low-Mach number reacting flow solver where the numerical framework for real-fluid properties are implemented. The bi-branchial edge flame structures are investigated in detail via the distribution of key intermediate species. An isolated reactive region, characterized by the existence of moderate radical H, is observed at the upstream of the stabilization point in the fuel-rich side. Transport budget analysis is conducted on the simulation results. With a high oxidizer temperature, the hydrothermal flame is mainly stabilized by auto-ignition. And with the decreasing of oxidizer temperature, the contribution of flame propagation to flame stabilization increases. The isolated reactive region is further identified as a propagation-dominated reaction zone, which is facilitated by radical H2O2.

Analysis of flame stabilization mechanism in a hydrogen-fueled reacting wall-jet flame

In this study, the novel conservative representation of chemical explosive mode analysis is augmented to analyze the key flame features in the Burrows-Kurkov flames simulated by both Reynolds-Averaged Navier-Stokes (RANS) and large eddy simulation (LES). Subtle difference are revealed in flame stabilization mechanisms resulting from the difference in modeling and spatial resolution. RANS shows that, ahead of the flame onset location, the composition diffusion and shock wave compression play dominant roles in chemical explosion indicating that the flame is stabilized by the assisted-ignition combustion mode. In contrast, LES shows that the flame is stabilized by the auto-ignition mode since the nonchemical contribution counteracts chemical reaction during the development of ignited flame kernels. For RANS, the radical pool builds up through the unphysical back diffusion near the flame stabilization front, which reveals the limitation of RANS method in the resolution and characterization of the key flame features in Burrows-Kurkov flames.

Thermographic phosphor heat flux measurements of laminar methane/air flame impinging on a cylindrical surface

The stagnation point heat fluxes of methane/air flames impinging normal on a cylindrical surface were determined experimentally. Light induced phosphorescence from thermographic phosphors was used to investigate surface temperatures at the stagnation point from a nearly 1D laminar premixed flame burning against a water-cooled ceramic tube. The ceramic tube was coated with 1.1% chromium-doped alumina (ruby) at the impingement area and excited with a green light-emitting diode (LED) to measure the surface temperature. The flame temperature profiles were also measured with a thermocouple of type R (Pt/Pt + 13% Rh). Effects on variations in cold gas velocity (0.1 m s−1–0.5 m s−1) relative to the flame speed, equivalence ratio (Ф = 0.85–1.2), burner to impingement surface spacing (H/d = 0.5–2) and surface curvature are reported. The stagnation point heat fluxes are strongly influenced by the flame stabilization mechanism, which changes from burner to wall stabilization, which also is seen from the measured flame temperature profiles. Increasing the cold gas velocity of the reactants leads to higher stagnation point heat fluxes. In addition, decreasing the distance between the burner and impingement surface increases the heat flux, with higher heat fluxes recorded for a tube compared to a flat plate.

Understanding the diesel-like spray characteristics applying a flamelet-based combustion model and detailed large eddy simulations

This investigation analyses the structure of spray A from engine combustion network (ECN), which is representative of diesel-like sprays, by means of large eddy simulations and an unsteady flamelet progress variable combustion model. A very good agreement between modelled and experimental measurements is obtained for the inert spray that supports further analysis. A parametric variation in oxygen concentration is carried out in order to describe the structure of the flame and how it is modified when mixture reactivity is changed. The most relevant trends for the flame metrics, ignition delay and lift-off length are well-captured by the simulations corroborating the suitability of the model for this type of configuration. Results show that the morphology of the flame is strongly affected by the boundary conditions in terms of the reactive scalar spatial fields and Z–T maps. The filtered instantaneous fields provided by the simulations allow investigation of the structure of the flame at the lift-off length, whose positioning shows low fluctuations, and how it is affected by turbulence. It is evidenced that small ignition kernels appear upstream and detached from the flame that eventually merge with its base in agreement with experimental observations, leading to state that auto-ignition plays a key role as one of the flame stabilization mechanisms of the flame.

Flame stabilization mechanism in reacting jets in swirling vitiated crossflow

Flame stabilization in reacting jet in crossflow (RJICF) is often affected by interaction between different mechanisms, involving auto-ignition, partially-premixed flame behavior, and vortex-induced recirculation zones. These flame stabilization mechanisms depend on the local flow dynamics involving the interaction between the jet fluid and the crossflow. In this research a staged combustion system featuring a reacting jet in crossflow as means of secondary combustion zone is investigated. The fuel jet is injected into the crossflow through a contoured nozzle which is protruded into the crossflow to avoid flame contact with the combustor wall. with a top-hat velocity profile. The ignition dynamics of the reacting jet is discussed based on high-repetition-rate OH-PLIF measurements along the RJICF trajectory. The recirculation region in the leeward side of the jet promotes fuel–air mixing and provides conditions favorable for a sustained flame kernel which leads to complete ignition of the jet. There is a difference in the flame structure of a non-premixed H2/N2 flame as compared to a premixed natural gas flame indicating a difference in flame stabilization mechanism. The H2 flame is primarily stabilized at the stagnation plane between the crossflow and the windward shear layer of the jet. Due to the higher propensity of strain rate induced extinctions of premixed natural gas (NG)/air jet flame, a stable reaction zone is seen only in the jet recirculation region. Conditional statistics of the simultaneously computed velocity magnitude, vorticity magnitude and in-plane strain rate were extracted from the location of the flame front in the windward and leeward side of the jet. The conditional statistics show that the mean strain rate plays an important role in flame stabilization particularly for premixed NG/air jets which is found to be susceptible to strain rate induced extinction. Measurements at a plane half jet diameter below the injection location of the jet indicate that there is a continuous entrainment of PIV particles as well as reaction products and unburnt fuel-air mixture. In this study high-repetition-rate simultaneous PIV/OH-PLIF measurements along the jet cross-section are discussed.

Numerical Modeling for N-Heptane Spray Combustion Processes with Various Ambient Oxygen Concentrations

This work has numerically analyzed the structure of spray flames with various ambient oxygen concentrations. To realistically predict the spray evaporation characteristics in the high-pressure diesel-like environment, the present study has employed the high-pressure effective conductivity vaporization model. The detailed n-heptane/air chemistry is represented by 114 elementary steps and 44 chemical species. The Representative Interactive Flamelet (RIF) model is adopted to consider the interactions between turbulence and chemistry in the fast transient reacting flows. In order to account for the spatial inhomogeneity of the scalar dissipation rate, the multiple RIFs are introduced. Moreover, the present approach realistically models the vaporization effects on the mixture fraction fluctuations. For the various conditions of ambient oxygen concentration, numerical results are compared with experimental data including the ignition delay and the lift-off length. These results indicate that the present MRIF approach yields the reasonably good agreement with experimental data in terms of ignition delay time and lift-off length. To gain the physical insight into the two-stage ignition behavior of diesel fuels, the detailed discussions are also made for the temporal evolution of several key species (CH2O, OH, CO) mass fraction and maximum temperature of the transient flamelet as well as the flame stabilization mechanism of the n-heptane spray flames with the much lower ambient oxygen concentration.

Experimental study on the diffusive flame stabilization mechanism of plasma injector driven by AC dielectric barrier discharge

A plasma injector element was designed to experimentally study the mechanism of methane-air diffusive flame stabilized by a discharge plasma. The air plasma was generated within the annulus gap of the injector by alternating current dielectric barrier discharge. The discharge voltage, current and photographs were recorded first. Three internal effects of the plasma on combustion were later investigated separately through several diagnostic methods, including optical emission spectrometry (OES), infrared thermography, thermocouple, infrared thermometer, schlieren imaging, photos and CH* chemiluminescence. Finally, the return on investment (ROI) was calculated. The results showed that a large number of filamentary micro discharge paths occur within the discharge gap. These discharge paths rotate anticlockwise at high speed and act as a virtual ‘fan’ to induce the flow jet. The velocity of the induced jet increases with increasing discharge voltage. The original jet expansion angle is enlarged by the radial velocity component of the induced jet, resulting in the mixing enhancement of the air and methane. The plasma rotational temperature (the first negative system ) obtained from OES is close to the discharge gas temperature measured by infrared thermography, indicating that the discharge gas temperature can be approximately represented by the rotational temperature. According to the measured temperature of the injector and the jet, the impact of the thermal effect of the plasma on flame stabilization is negligible. Due to the plasma, the height of the flame center and its representative length are generally reduced as the voltage rises, and the methane-air mixture becomes ignitable, and a stable flame can be reached under the conditions in which direct ignition fails. The combustion is enhanced with increasing heat release rate of the flame by the plasma. This finding revealed that the ROI of plasma-assisted flame stabilization is lower under a higher flowrate and a larger equivalence ratio for unstable flame situations.

Flame stabilization of supersonic ethylene jet in fuel-rich hot coflow

The stability limit of a supersonic ethylene jet flame in a fuel-rich hot coflow was examined by investigating the influence of the injection pressure, which was varied from 2.0 atm to 4.5 atm, and of the equivalence ratio of the coflow, which was varied from 1.2 to 1.6. The flames were investigated with time-resolved chemiluminescence and schlieren images, as well as a large-eddy simulation of combustion. The results show that, with increasing injection pressure, the flame state changes from stable to unstable and blow-off, and the flame brush thickness, heat release, and height of coflow decrease. The flame stability limits decrease as the equivalence ratio of coflow increases. Lastly, a large-eddy simulation was performed to investigate the mechanism of flame stabilization, and the numerical simulation results are in good agreement with the experimental results. It was found that the stability of a supersonic flame is affected by the chemical time scale and flow time scale.

Flame Diagnostics with a Conservative Representation of Chemical Explosive Mode Analysis

A conservative representation of a chemical explosive mode analysis is formulated for flame diagnostics in compressible reactive flows. The flame characteristics are detected by the quantification of individual contributions of chemical reaction, diffusion, and compressibility to chemical explosive modes. The diagnostic method is first applied to analyze one-dimensional representative flames. The analysis shows that compressibility plays the dominant role among the nonchemical effects for chemical explosive modes in high-speed flows. Then, the diagnostic method is further applied to analyze the flame stabilization mechanisms in the Burrows–Kurkov supersonic flames, for which large-eddy simulations have been performed and validated against measurement data. Results show that chemical reaction controls the flame kernel formation, indicating that autoignition governs the flame stabilization. Furthermore, compressibility has the same order of magnitude contribution to chemical explosive modes as the chemical reaction in the surrounding area of the ignited flame kernels, which counteracts chemical reaction for the growth of the flame kernels.

An experimental study of turbulent lifted flames at elevated pressures

Studying the lift-off behavior of non-premixed jet flames is important to understanding the flame stabilization mechanisms in practical systems, including gas flares and combustors, and to improving the safety of pressurized fuel tanks in case of fuel leaks. This is typically done with the canonical configuration of an axisymmetric fuel jet issuing into a quiescent or co-flowing oxidizer and abundant data are available in the literature. However, most of these data were collected at normal or sub-atmospheric pressure and little data are available at elevated pressure and high Reynolds numbers, conditions relevant to practical configurations. The present study fills this gap by reporting lift-off height measurements of methane and ethane non-premixed jet flames for pressures up to 7 bar and Re = 57,500 in the presence of an air co-flow. Data are interpreted using Kalghatgi's model for the dimensionless lift-off height, which was previously proven successful for sub-atmospheric to normal pressures, as well as elevated pressure but only for low turbulent Reynolds numbers and propane. In this contribution, Kalghatgi's model has been shown to accurately predict the slope of the lift-off height vs. jet velocity curves at elevated pressure and large Reynolds number for methane and ethane. A new term, a function of the stoichiometric mixture fraction, the laminar burning velocity, and a turbulent Schmidt number, is also introduced to extend the predictive capabilities of Kalghatgi's model to configurations featuring a co-flow.

Observations on the Role of Auto-Ignition in Flame Stabilization in Turbulent Non-Premixed Jet Flames in Vitiated Coflow

Turbulent combustion of non-premixed jets issuing into a vitiated coflow is studied at coflow temperatures that do not significantly exceed the fuel auto-ignition temperatures, with the objective of observing the global features of lifted flames in this operating temperature regime and the role played by auto-ignition in flame stabilization. Three distinct modes of flame base motions are identified, which include a fluctuating lifted flame base (mode A), avalanche downstream motion of the flame base (mode B), and the formation and propagation of auto-ignition kernels (mode C). Reducing the confinement length of the hot coflow serves to highlight the role of auto-ignition in flame stabilization when the flame is subjected to destabilization by ambient air entrainment. The influence of auto-ignition is further assessed by computing ignition delay times for homogeneous CH4/air mixtures using chemical kinetic simulations and comparing them against the flow transit time corresponding to mean flame liftoff height of the bulk flame base. It is inferred from these studies that while auto-ignition is an active flame stabilization mechanism in this regime, the effect of turbulence may be crucial in determining the importance of auto-ignition toward stabilizing the flame at the conditions studied. An experimental investigation of auto-ignition characteristics at various jet Reynolds numbers reveals that turbulence appears to have a suppressing effect on the active role of auto-ignition in flame stabilization.

OH planar laser-induced fluorescence measurements with high spatio-temporal resolution for the study of auto-ignition.

For the detailed understanding of transient combustion processes, in particular, of auto-ignition, quantitative measurements with high spatio-temporal resolution are desirable. These can, for instance, serve as validation data for time-resolved numerical simulations and in particular for the combustion models used in those simulations. In the current study, a jet-in-hot-coflow (JHC) burner, developed at the German Aerospace Center (DLR), the DLR JHC, was used to inject a turbulent methane jet into the hot exhaust gas of a lean hydrogen/air flame, and a steady state jet flame was established. In addition, fuel could be injected in a transient manner. Here, an auto-igniting jet was observed. The flame stabilization of the steady state jet flame and the auto-ignition during transient fuel injection were studied using high-speed laser-based and optical measurements. A strategy for quantifying high-speed OH planar laser-induced fluorescence is presented, and the measurement uncertainties are evaluated. The flame stabilization mechanism in steady state jet flames was assessed using probability density functions of the OH concentration at different axial and radial locations. The formation of auto-ignition kernels during transient fuel injection is evaluated based on time series of the OH concentration. It is shown how the OH concentration levels and PDF shapes can be used to characterize the chemical state of the reacting flow and to distinguish between auto-ignition and flame propagation.

Autoignition-cascade in the windward mixing layer of a premixed jet in hot vitiated crossflow

This numerical study builds upon the experimental work of Wagner et al., e.g. [Combust. Flame 176 (2017)], and focuses on the flame stabilization mechanism of a reactive jet in crossflow (RJICF). A 300 K premixed ethylene-air jet (ϕj=1.2) is injected into a hot vitiated crossflow at 1500 K. A compressible three-dimensional (3-D) large eddy simulation (LES) was performed with an analytically reduced chemistry mechanism. Comparisons between LES and experiments are in good agreement in terms of jet trajectories and flow velocities. The LES captures the flame-flow interactions that were experimentally observed in the central plane of the RJICF by means of particle image velocimetry and OH planar laser-induced fluorescence. It gives access to the spatio-temporal evolution of the autoignition process along the windward mixing layer of the RJICF. Chemical explosive mode analysis shows that autoignition is the dominant flame stabilization mechanism on the windward side of the jet. “Low-heat-release” autoignition starts at the front root of the jet, at the “very lean” most reactive mixture fraction, which is the spot where the autoignition time of the mixture is the smallest. Immediately downstream, the released heat is advected and diffuses, contributing to a decrease of autoignition times in the neighboring richer inner side of the jet. Autoignition in this richer layer of the jet is characterized by higher heat release rate that again diffuses toward even richer regions until the stoichiometric layer ignites. This autoignition-cascade mechanism determines the position of the “visible” windward flame. Moreover, a 3-D flame analysis shows that the resulting “high-heat-release” autoignition patches expand within the stoichiometric layer and ultimately merge with the leeward propagating flame while being advected toward the flame tip. It is also shown that the latter layer, and therefore the growing autoignition patches, get wrinkled by the coherent structures that originate from the hydrodynamically unstable front root of the jet in crossflow.

Ozonolysis activated autoignition in non-premixed coflow

While the majority of fuel oxidation reactions require high-temperature conditions, those involving ozone (O3) and unsaturated hydrocarbons, e.g. ethylene (C2H4), occur very rapidly at low temperatures, even at room temperature. The subsequent radical production and heat release could induce further fuel oxidation and even autoignition in a fuel/oxidizer mixture. The autoignition events activated by ozonolysis reactions were investigated in a non-premixed coflow burner using C2H4 as the fuel. Both gas chromatography samples and chemiluminescence images indicated ongoing low-temperature reactions before autoignition occurred, which confirmed that ozonolysis reactions initiated autoignition. As expected, the autoignition time decreased with the increase of O3 concentration in the mixture. Moreover, the autoignition time decreased as the fuel jet Reynolds number (Re) increased. At lower Re, this trend was explained by the inverse relation between the mixing timescale and flow velocity. At higher Re, this trend was attributed to the mixing promoted by turbulence. Owing to the ozonolysis reactions, fuel can be ‘preprocessed’ and therefore the reactivity of the fuel-oxidizer mixture increased. As a result, the flame propagation speed increased significantly after autoignition in a stratified environment owing to the existence of ozonolysis reactions. This enhancement, owing to ‘in situ fuel preprocessing’, plays an important role in flame stabilization and propagation mechanisms.