Unstable Flame Research Articles

Large Eddy Simulation of large-scale hydrogen deflagrations using the Thickened Flame Model with stretch sensitivity adaptation and thermo-diffusive instability modeling

Due to the high costs associated to experimental testing of hydrogen (H2) explosions, numerical simulations play an essential role in the design of safe industrial systems. In this work, we investigate the use of the Thickened Flame Model (TFM) to perform simulations of H2 deflagrations. Specific issues arise when considering hydrogen: (i) due to differential diffusion, the flame is very sensitive to stretch; (ii) instabilities of the flame front are generated and lead to an increased flame propagation speed. Both of these phenomena are affected by flame thickening. First, the flame sensitivity to stretch is artificially increased by the thickening. The recently developed Ma-TFM model proposes a correction of diffusive fluxes to reach the correct Markstein length and is selected in this work. Secondly, thermo-diffusive and hydrodynamic flame instabilities are also altered by thickening. By computing the dispersion relationship for a hydrogen/air mixture, we show that the instability scales are proportional to the thickening factor F. A simple subgrid scale (SGS) model, based on the computations of laminar unstable 2-D and 3-D flames, is then considered to take into account the effect of instabilities. The Ma-TFM and SGS instability models are applied together for the first time on a semi-industrial vented lean hydrogen deflagration, leading to promising results. In particular, it is found that considering 3-D unstable flames is necessary to accurately predict the experimental pressure peaks.

Scaling transition of turbulent flame speed for thermodiffusively unstable flames

This work presents an experimental set of Bunsen flames characterized by a moderate Reynolds number and a variable turbulence intensity. Ten lean hydrogen-enriched methane–air mixtures at three turbulence levels are investigated, ranging from methane–air to hydrogen–air mixtures. Such mixtures are selected to have an almost constant laminar flame speed while inducing the onset of thermal-diffusive (TD) instability by gradually increasing the hydrogen content of the blend. The flames' global consumption speed, stretch factor, and flame surface area are investigated and discussed as functions of the effective Lewis number of the mixture. As the interplay between TD instability and turbulence enhances the overall flame propagation, below a transitional Lewis number, flames are observed to be particularly sensitive to external turbulent forcing. This synergistic interaction is discussed in terms of Karlovitz and Lewis numbers. A parameterization of the turbulent flame speed is thus proposed, based on a functional form depending, concurrently, on both Karlovitz and Lewis numbers. The proposed form is shown to fit the experimental results at different turbulence levels and to capture the flame speed enhancement across the transitional Lewis number.

Three-dimensional dynamics of unstable lean premixed hydrogen-air flames: Intrinsic instabilities and morphological characteristics

The 3D swinging laser sheet technique was employed to study the development and morphological characteristics of premixed hydrogen-air unstable flames in a spherical explosion vessel. Pressure dependencies for laminar flame propagation were sought to exploit the role of the Darrieus-Landau (DL) and Thermal-diffusive (TD) instabilities in the unstable self-accelerating flame regime. A sufficiently low Markstein number, as a consequence of the increased pressure, leads to more cracking and smaller cells over the flame surface. The degree of wrinkling on the flame surface is proportional to the increase in flame burning velocity, a relationship that holds true for low pressures but is not applicable under high pressures. External turbulence can significantly alter the extent of flame surface wrinkling even at low root mean square velocities, producing a more wrinkled flame surface compared to intrinsic cellularity, and distinctly affecting flame dynamics. The increased wrinkling and flame speed due to external turbulence can be attributed to the synergistic effects between thermo-diffusive instabilities and turbulence, resulting in higher fuel consumption rates per flame surface area and the formation of finger-like structures that enhance flame displacement speed in curved segments. The parameters, ϵ, deviation of the Lewis number from a critical value, and ω2, obtained through classical linear stability analysis, display a clear linear relationship with the ratio of the wrinkled surface area observed in planar flames. This study enhances the understanding of hydrogen flame instabilities, which is crucial for preventing explosions in hydrogen storage and utilization, and provides valuable insights into flame dynamics, supporting the design of safer and more efficient hydrogen-fueled engines and turbines.

Influence of flame stability on iron oxide nanoparticle growth during FSP

Flame stability during flame spray pyrolysis (FSP) remains an active topic of investigation due to its impact on synthesised particle attributes and purity. The unique feature of the burner investigated here is the ability to control flame stability over newly defined stability maps. The novelty of the current work lies in understanding the influence of these broad stability modes on nanoparticle growth during FSP and on the attributes of collected products. Several distinct flame configurations are selected for iron oxide nanoparticle synthesis, ranging from stable to highly unstable flames. The flame stability regimes are characterised by OH∗ chemiluminescence and broadband flame luminescence imaging. Stability is correlated with the coefficient of variation of flame luminescence (CV) and flame height with mean OH∗ chemiluminescence. Planar Mie scattering is then used to identify the effect of flame luminescence intermittency on spray atomisation and evaporation quality. For particle analysis, in-situ thermophoretic sampling is performed from 30 to 200 mm above the burner exit plane and analysed via transmission electron microscopy (TEM). Further ex-situ analysis is also performed on the bulk-collected product via high-resolution TEM, X-ray powder diffraction (XRD), and attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR). It is demonstrated that flames with higher instability (CVmin ≥ 0.35) maintain increased spray heights (>26 %) and reduced flame heights (>79 %) compared to stable flames with the same precursor volume flowrate. This reduces the high-temperature particle residence time for primary particle growth and impacts subsequent agglomeration. For example, the mean diameter of gyration and primary particle diameter are found to vary by 44 % and 29 % depending on the flame regime, respectively. Ex-situ analysis also demonstrates that the dominant iron oxide phase produced is maghemite regardless of the stability regime. However, higher concentrations of organic impurities including methyl, methylene and carboxylate functional groups are found via ATR-FTIR with increased flame instability (CV).

Ultra-lean hydrogen-air flames initiated by a hot surface

The benefits of using hydrogen as a fuel for rocket engines dictate the necessity of a deep understanding of possible scenarios of hydrogen ignition and subsequent combustion. Safety requirements on launch sites and hydrogen transportation and storage facilities should be elaborated based on a thorough investigation of hydrogen flame development. The paper aims to provide a detailed numerical analysis of the hot wall ignition of hydrogen-air mixtures with compositions near low flammability limits. It is shown that the development of ultra-lean flames initiated by a hot spot on the solid wall possesses several features compared to ultra-lean flames ignited by a point energy source. Three modes of flame development are observed: stable flame column with bow-shaped flame structure on top, individual flame kernel, and unstable flame column with multiple kernels generation. Effects of mixture composition, along with the impact of hot spot size, are analyzed. Column tip acceleration mechanism is determined. All of the combustion modes obtained are grouped in the diagram, providing information on the limits of various combustion modes depending on the mixture concentration and hot spot size.

Isolating effects of large and small scale turbulence on thermodiffusively unstable premixed hydrogen flames

Lean turbulent premixed hydrogen/air flames have substantially increased flame speeds, commonly attributed to differential diffusion effects. In this work, the effect of turbulence on lean hydrogen combustion is studied through Direct Numerical Simulation using detailed chemistry and detailed transport. Simulations are conducted at six Karlovitz numbers and four integral length scales. A general expression for the burning efficiency is proposed which depends on the conditional mean chemical source term and gradient of a progress variable, and the amount of superadiabatic burning. At a fixed Karlovitz number, the normalized turbulent flame speed and area both increase almost linearly with the integral length scale ratio. The effect on the mean source term profile is minimal, indicating that the increase in flame speed can solely be attributed to the increase in flame area. At a fixed integral length scale, both the flame speed and area first increase with Karlovitz number before decreasing. The qualitative observations and trends do not change when Soret effects are neglected. Specifically, neglecting Soret diffusion is shown to reduce the flame speed, area, and burning efficiency. At higher Karlovitz numbers, the diffusivity is enhanced due to penetration of turbulence into the reaction zone, significantly dampening differential diffusion effects.Novelty and SignificanceLean premixed hydrogen flames are subject to thermodiffusive instabilities, which can lead to system level instabilities such as flashback. A comprehensive study of the thermodiffusively unstable turbulent flames with detailed chemistry and detailed transport (including Soret diffusion) was conducted across a wide range of turbulent intensities and integral length scales. Varying these two parameters independently was necessary to isolate the effects of large- and small-scale turbulence. Using these results, we propose a general expression for the burning efficiency to explain the relationship between the turbulence intensity, flame speed, and flame area. This work is an important step in developing predictive models which can aid in the design of practical combustion devices.

Spatiotemporal Surface Temperature Measurements Resolving Flame-Wall Interactions of Lean H2-Air and CH4-Air Flames Using Phosphor Thermometry

AbstractSpatiotemporal wall temperature (Twall) distributions resulting from flame-wall interactions of lean H2-air and CH4-air flames are measured using phosphor thermometry. Such measurements are important to understand transient heat transfer and wall heat flux associated with various flame features. This is particularly true for hydrogen, which can exhibit a range of unique flame features associated with combustion instabilities. Experiments are performed within a two-wall passage, in an optically accessible chamber. The phosphor ScVO4:Bi3+ is used to measure Twall in a 22 × 22 mm2 region with 180 µm/pixel resolution and repetition rate of 1 kHz. Chemiluminescence imaging is combined with phosphor thermometry to correlate the spatiotemporal dynamics of the flame with the heat signatures imposed on the wall. Measurements are performed for lean H2-air flames with equivalence ratio Φ = 0.56 and compared to CH4-air flames with Φ = 1. Twall signatures for H2-air Φ = 0.56 exhibit alternating high and low-temperature vertical streaks associated with finger-like flame structures, while CH4-air flames exhibit larger scale wrinkling with identifiable crest/cusp regions that exhibit higher/lower wall temperatures, respectively. The underlying differences in flame morphology and Twall distributions observed between the CH4-air and lean H2-air mixtures are attributed to the differences in their Lewis number (CH4-air Φ = 1: Le = 0.94; H2-air Φ = 0.56: Le = 0.39). Findings are presented at two different passage spacings to study the increased wall heat loss with larger surface-area-to-volume ratios. Additional experiments are conducted for H2-air mixtures with Φ = 0.45, where flame propagation was slower and was more suitable to resolve the wall heat signatures associated with thermodiffusive instabilities. These unstable flame features impose similar wall heat fluxes as flames with 2–3 times greater flame power. In this study, these flame instabilities occur within a small space/time domain, but demonstrate the capability to impose appreciable heat fluxes on surfaces.

Thermoacoustic parametric instability of premixed ammonia flames propagating downwards in an open-closed tube

This work investigates the self-excited thermoacoustic parametric instability of downward propagating laminar premixed ammonia flames in an open-closed combustion tube. NH3/O2/N2 flames were propagated at different laminar burning velocities at equivalence ratios of ɸ = 0.8 and ɸ = 1.2. Different unstable flame regimes were observed through pressure fluctuation measurements and synchronized high-speed imaging capturing the lateral and longitudinal perspectives of flame propagation. The observed propagation speed of quasi-planar flames showed a strong correlation with the calculated laminar burning velocities of NH3/O2/N2 mixtures. For the first time, the cellular structures of laminar premixed ammonia flames at the onset of parametric instability were clearly observed from the present experimental approach. The experimental cellular flame wavenumbers and acoustic velocity fluctuation amplitudes at the onset of parametric instability are in qualitative agreement with the theory of stability of laminar flames under acoustic excitation. Experiments have shown that NH3/O2/N2 flames are more unstable than CH4/O2/N2 flames at the same laminar burning velocity independent of Lewis number within the range of tested conditions. The influence of activation energy on parametric instability is investigated through a comparative thermoacoustic analysis between two different fuels at varied equivalence ratios. Ammonia flames are characterized by large overall activation energy and Zeldovich numbers compared to hydrocarbon flames. This work elucidates how this unique property characterizes the acoustic instability of downward propagating premixed ammonia flames in a tube based on classical theories on acoustic instability of laminar premixed flames.

Thermodiffusively unstable laminar hydrogen flame in a sufficiently large 3D computational domain – Part II: NOx formation mechanism and flamelet modeling

In this work, the NOx formation mechanism in three-dimensional (3D) thermodiffusively unstable premixed hydrogen flames is investigated through direct numerical simulations (DNS) and a reaction path analysis based on the DNS dataset. In part I of this study (Wen et al., 2024), the characteristic patterns of the instabilities were studied using the same dataset. Here, first, the effects of the computational setup (2D vs. 3D) and curvature on the flux ratio of nitrogen atom are quantified. In addition, the composition space model (CSM) proposed in previous works is extended to account for the NOx chemical reaction mechanism to predict NOx formation in thermodiffusively unstable premixed hydrogen flames. The flamelet solutions are obtained from the premixed flamelet equations in composition space so that the wide range of curvatures associated with the strongly corrugated flame front can be considered. The performance of the CSM in predicting the NOx species and the important radicals involved in the NOx formation pathways is evaluated through an a priori analysis. To investigate the effects of curvature on the NOx formation pathways and the performance of the composition space model, the positively- and negatively-curved regions in the DNS are studied separately. The results show that different from the 2D simulation, the NNH reaction pathway becomes dominant in the 3D simulation due to the increased range of curvature, which promotes the accumulation of the highly diffusive H radical in the positively-curved regions. The NNH reaction pathway is dominant in the positively-curved regions, while the N2O reaction pathway is more important in the negatively-curved regions. The flamelet model based on the composition space solutions that consider the effects of curvature yields accurate predictions for the radicals that are sensitive to the effects of curvature.Novelty and significance statementThe novelty of the present work includes the following aspects,(i) Analysis of NOx formation mechanism based on the large-scale 3D DNS dataset of a laminar fuel-lean premixed hydrogen flame on a sufficiently large domain;(ii) Quantification of the effects of computational setup (2D and 3D) and curvature on the NOx formation pathways;(iii) Extension of the previous composition space model (CSM) by incorporating the NOx chemical reaction mechanism to consider the effects of curvature on the NOx formation.

An experimental study on the flame instability of NH3/DME blends with H2 addition

It is of significance to investigate the effect of H2 on the combustion of NH3/DME blends since NH3 can partially decompose to produce hydrogen (H2) in the combustion process. However, few studies were conducted on NH3/DME/H2 mixtures, especially for flame instability. This work presents an experimental study of the flame instability of NH3/DME blends with H2 addition. Experiments were conducted using the spherical constant-volume combustion method at φ = 0.7 – 1.4, Tu = 298 K. The influences of NH3/DME blend ratio (BNH3/DME = 80/60, 60/40, 40/60, and 20/80), initial pressure (Pu = 0.1 – 0.5 MPa), and H2 fraction (XH2 = 0%, 20%, and 40%) on the flame instability were examined in detail. The results show that increasing either DME fraction or initial pressure can promote the hydrodynamic instability of NH3/DME/air flames because of the decreased flame thickness. For both XH2 = 0% and 20%, the cellular flame instability with BNH3/DME = 40/60 and 20/80 increases with increasing equivalence ratio. For XH2 = 40%, the flame instability shows a similar trend with BNH3/DME = 20/80, but the most unstable flame appears around stoichiometric combustion with BNH3/DME = 40/60. This is due to the compound effects of the diffusive-thermal imbalance and hydrodynamic instability for lean or rich fuel, while the diffusive-thermal instability can be neglected for stoichiometric combustion. Therefore, an effective control of H2 or DME addition can help enhance the combustion performance of the fuel while reducing the occurrence of dangerous events caused by potential flame instabilities. In addition, critical parameters, i.e., critical flame radius rcr, critical Peclet number Pecr, and critical Karlovitz stretch factor Kacr were analyzed to quantify the onset of flame instability. This work provides insights into the flame instabilities of NH3/DME with H2 addition.

Parametric learning of time-advancement operators for unstable flame evolution

This study investigates the application of machine learning, specifically Fourier neural operator (FNO) and convolutional neural network (CNN), to learn time-advancement operators for parametric partial differential equations (PDEs). Our focus is on extending existing operator learning methods to handle additional inputs representing PDE parameters. The goal is to create a unified learning approach that accurately predicts short-term solutions and provides robust long-term statistics under diverse parameter conditions, facilitating computational cost savings and accelerating development in engineering simulations. We develop and compare parametric learning methods based on FNO and CNN, evaluating their effectiveness in learning parametric-dependent solution time-advancement operators for one-dimensional PDEs and realistic flame front evolution data obtained from direct numerical simulations of the Navier–Stokes equations.

Investigation on the intrinsic instabilities of ethyl methyl carbonate flames

The intrinsic instabilities of ethyl methyl carbonate flames are investigated at equivalence ratios (0.6–1.8) and initial pressures (70–160 kPa) by using a 36-liter constant volume combustion chamber. The unstable laminar premixed flame is evaluated by the laminar burning velocity, explosion characteristics, flame cellularity, and flame self-acceleration. The experimental laminar burning velocity is well reproduced by the chemical kinetic mechanism. The extent of cellularization characterized by the crack length and average cell area non-monotonically varies with increasing equivalence ratio but monotonically increases with increasing initial pressure. Meanwhile, the critical flame radius for the onset of cellularity exhibits non-monotonic variation with the equivalence ratio, while it decreases monotonically with increasing initial pressure. The experimental critical Peclet number and critical flame radius are well captured by flame stability theory. The linear correlation between the critical Peclet number and the Markstein number is Pecl = 26.46 Ma + 122.22. In addition, the dependence of the critical Karlovitz number on the Markstein number is predicted as Kacl = 0.0226e-0.0880Ma. The flame self-acceleration increases as the flame instabilities increase. The acceleration exponent and fractal dimension are smaller than 1.5 and 2.33, respectively, which are the suggested values for flame self-turbulization.

Flame visualization and spectral analysis of combustion instability in a premixed methane/air-fueled micro-combustor

The present study experimentally explores the combustion instability characteristics of premixed methane/air flames in a planar micro-combustor by utilizing the high-speed camera and the noise data acquisition system. It aims to identify the sound pressure fluctuation of unstable flames and shed light on the relationship between flame structure and thermoacoustic instability under micro-scale conditions. Four types of flame structures, i.e., flame with repetitive extinction and ignition (FREI), weak flame, steady flame, and oscillating flame, are experimentally observed by varying operating parameters. Among them, the weak flame at low flow velocities and the oscillating flame at high flow velocities form thermoacoustic oscillation. For weak flames, the collision with the wall and the local deformation of the flame result in high-frequency harmonics. Varying the inlet velocity from 0.14 m/s to 0.15 m/s leads to the pressure fluctuation of the weak flame being reduced by 10 Pa, and the dominant frequency being changed from 125 Hz to 167 Hz. As for oscillating flame, the variation of the inlet velocity plays a decisive role in the sound pressure fluctuation. At the equivalence ratio of 0.7, an increase of 0.01 m/s in the inlet velocity reduces the pressure fluctuation by 4 Pa.

Can flamelet manifolds capture the interactions of thermo-diffusive instabilities and turbulence in lean hydrogen flames?—An a-priori analysis

Flamelet-based methods are extensively used in modeling turbulent hydrocarbon flames. However, these models have yet to be established for (lean) premixed hydrogen flames. While flamelet models exist for laminar thermo-diffusively unstable hydrogen flames, for which consideration of curvature effects has resulted in improved model predictions [1], it is still unclear whether these models are directly applicable to turbulent hydrogen flames. Therefore, a detailed assessment of stretch effects on thermochemical states in a turbulent lean premixed hydrogen-air slot flame through finite-rate chemistry simulations is conducted. Strain and curvature are examined individually using a composition space model, revealing their distinct influences on thermochemical states. An a-priori analysis confirms that the previously developed tabulated manifolds fall short of capturing all turbulent flame phenomena, necessitating a novel manifold incorporating both strain and curvature variations. These results underscore the significance of these variations in developing manifold-based combustion models for turbulent lean hydrogen flames.

Three-dimensional diffusive-thermal instability of flames propagating in a plane Poiseuille flow

The three-dimensional diffusive-thermal stability of a two-dimensional flame propagating in a Poiseuille flow is examined. The study explores the effect of three non-dimensional parameters, namely the Lewis number Le, the Damköhler number Da, and the flow Peclet number Pe. Wide ranges of the Lewis number and the flow amplitude are covered, as well as conditions corresponding to small-scale narrow (Da≪1) to large-scale wide (Da≫1) channels. The instability experienced by the flame appears as a combination of the traditional diffusive-thermal instability of planar flames and the recently identified instability corresponding to a transition from symmetric to asymmetric flame. The instability regions are identified in the Le-Pe plane for selected values of Da by computing the eigenvalues of a linear stability problem. These are complemented by two- and three-dimensional time-dependent simulations describing the full evolution of unstable flames into the non-linear regime. In narrow channels, flames are found to be always symmetric about the mid-plane of the channel. Additionally, in these situations, shear flow-induced Taylor dispersion enhances the cellular instability in Le<1 mixtures and suppresses the oscillatory instability in Le>1 mixtures. In large-scale channels, however, both the cellular and the oscillatory instabilities are expected to persist. Here, the flame has a stronger propensity to become asymmetric when the mean flow opposes its propagation and when Le<1; if the mean flow facilitates the flame propagation, then the flame is likely to remain symmetric about the channel mid-plane. For Le>1, both symmetric and asymmetric flames are encountered and are accompanied by temporal oscillations.

Thermodiffusively unstable laminar hydrogen flame in a sufficiently large 3D computational domain – Part I: Characteristic patterns

Thermodiffusive instabilities can have a leading order effect on flame propagation for lean premixed hydrogen flames. Many simulation studies have been performed to study this effect, but almost exclusively in two-dimensional (2D) or domain sizes too small to support the characteristic large-scale features of the instability. The main purpose of this study is to quantify the differences of 3D and 2D flames using simulations on sufficiently large domains. To this end, direct numerical simulations (DNS) of thermodiffusively unstable laminar premixed hydrogen flames stabilized in 3D domains were performed. The effects of confinement on the flame dynamics are rigorously investigated by varying the domain size. The flame burning velocity shows a strong dependence on the domain size when the lateral domain width is less than 50 laminar flame thicknesses. The characteristic patterns of the thermodiffusively unstable flame are analyzed in detail, including the instantaneous flame structure, global burning velocity, flame surface area, stretch factor, curvature distribution, and the cell size. The effects of the computational setup (2D vs. 3D) on the local flame front curvature and the distributions of the thermo-chemical quantities are quantified through a conditional analysis. The formation and destruction mechanisms of the distinct cellular structures observed in the 3D domain are analyzed focusing on the interactions between the flame dynamics and the flow field, and the contributions of the production of flame surface density, kinematic restoration, and curvature dissipation are quantified. Compared with the corresponding 2D simulation, the flame surface area in the cut plane of the 3D configuration is similar, yet the burning velocity and the stretch factor are increased. The reason for this is the difference in curvature statistics in 2D and 3D. In 3D, larger extreme curvature values are obtained, leading to quantitatively different distributions of the thermo-chemical variables. For example, the peak H radical concentration in the 3D simulation is about twice higher than in the 2D simulation.Novelty and significance statementThe novelty of the present work includes the following aspects:(i) First large-scale 3D DNS of laminar lean premixed hydrogen flame on domain large enough to obtain domain-independent results.(ii) First quantitative assessment of characteristic patterns of 3D thermodiffusively unstable premixed hydrogen flames.(iii) First quantitative assessment of differences between 2D and 3D thermodiffusively unstable premixed hydrogen flames for domain-independent results.

Flame structure of GCH4/GOX pintle model combustor with wall confinement

The pintle injector is a representative option for designing a throttleable rocket engine that can be applied for various applications, including the vertical soft landing of reusable launch vehicles. Although the most direct research method to investigate the characteristics of pintle injectors experimentally is the high-pressure combustion test, it is often limited due to its cost and operational hazards. Consequently, atmospheric combustion or cold flow tests are frequently applied in the rocket injector field to simulate and study the fundamental combustion or spray characteristics. However, previous works, including such simulated experiments under atmospheric conditions, involved an unrealistically wide combustor or open condition. Since only a single pintle injector is typically used per engine, the combustor wall may significantly influence the fundamental characteristics. Thus, in this study, a pintle injector combustor with adequate wall confinement is designed, and its flame structure is investigated across a wide range of total volume flow rates and mixture ratio conditions, incorporating several geometries with varying gaseous oxygen injection areas. This research primarily focuses on the flame structure and its angle. Under specific experimental conditions, the unreported unstable flame was newly observed. The overall experimental results, supported by numerical methods, indicate that wall confinement has substantial importance in simulating a realistic flame structure and internal flow field within the combustor.

Flame stability characteristics of a flame spray pyrolysis burner

The stability characteristics of an established flame spray pyrolysis (FSP) burner are quantified in order to determine viable operating conditions and map the structure of FSP flames. The primary novelty of this work lies in exploring a large parameter space which is necessary to explain the dependence of visible flame emissions on input variables. Using high-speed flame luminescence imaging (1–120 kHz), we demonstrate that the input pilot heat release and the flowrates of liquid and dispersion gases are primary control variables that dictate combustion quality. Indeed, variation in these input conditions allows the qualitative identification of four flame types: (i) stable, (ii) lifted, (iii) unstable and (iv) low intensity. Quantification of these flame types is derived via statistical analysis of the temporal luminosity fluctuations. Key metrics such as the mean and standard deviation effectively describe the behaviour of the aforementioned flame types and act as classifying parameters for FSP flames. High normalised mean intensities and coefficients of variation are characteristic of stable flames. However, flame extinction events in the ignition region and throughout the entire flame are characteristic of unstable flame configurations. In this case, poor combustion efficiency and significant intensity fluctuations are quantified by significantly lower mean intensities and higher coefficients of variation. Further insight is given via the implementation of phase-Doppler anemometry (PDA). It is demonstrated how variation in the input flowrates - and therefore variation in gas and droplet-phase velocities - correlate with the identified flame structures. This analysis demonstrates how input parameters can be varied in order to configure the combustion mode and control combustion stability during FSP. Both of which are relevant for understanding the boundary conditions for nanoparticle growth using FSP.

Control device for improving swirl flame stabilization.

Aiming to improve the stabilization of unstable swirling turbulent premixed flames, an actively controlled swirler and electrical hardware and control software are developed, implemented, and tested in the present study. Stereoscopic particle image velocimetry is performed to calculate the swirl number and study the flame stabilization. A mixture of methane and air with a mean bulk flow velocity of 5.0m/s and a fuel-air equivalence ratio of 0.6 is examined. This test condition, along with an original swirler vane angle of 30°, led to the initial anchoring of an unstable premixed flame at the burner exhaust. The developed actuation mechanism allows for changing the swirler vane angle from 30° to 60° and back to 30°. Increasing the vane angle from 30° to 60° increases the swirl number to relatively large values, which leads to the formation of a recirculation zone, a downward velocity along the burner centerline, and, as a result, the stabilization of an M-shaped flame. After the vane angle is reduced to 30°, the swirl number decreases but remains relatively large compared to its original value. As a result, the recirculation zone is present at the end of the actuation process and a V-shaped flame is stabilized. Improving the stabilization of the swirl flames is made possible because of the control apparatus and the method developed in the present study. The apparatus and technique designed and tested in this study facilitate the development of robust tools for improving the stabilization of swirling premixed flames in gas turbine combustors.

Experimental analysis and multi-objective optimization of flame dynamics and combustion performance in methane-fueled slit-type combustors

Achieving a high wall temperature and wide flame stability limit is of extreme significance for the maximization of power generation in hydrocarbon fuel-driven micro-thermophotovoltaic and thermoelectric devices. For this, detailed experimental investigations are conducted on variable channel height combustors, with the emphasis on analyzing the flame dynamics and thermal performance. A preliminary understanding of the multiple flame morphologies in the combustors is developed by varying the input working conditions and wall materials. The basic types of unstable flames are clarified. The pinch-off phenomenon of a weak flame in a slit combustor is observed for the first time, and the cell tends to decrease and emerge with an increase in the flow rate. The combustible range is shown to be extended with increasing the channel height, while it exhibits a non-monotonic changing trend with the combustor thermal conductivity which highlights the importance of the balance between heat transfer due to the flame-wall coupling and heat losses. Furthermore, a Kriging-NSGA-II optimization model is also developed to obtain the optimal characteristics in terms of radiation efficiency, standard deviation and volume power density. A Pareto-optimal front solution is determined to identify three distinct regions of thermal conductivities, which is crucially useful in guiding the design of practical micro-power systems based on different working requirements.