Flame Stability Research Articles

Experimental Design Validation of a Swirl-Stabilized Burner With Fluidically Variable Swirl Number

Abstract Swirling flows are commonly used for flame stabilization in gas turbine combustors, which are hence equipped with suitable swirler units. In these units the air rotation, quantified by the swirl number, is fixed through the geometry and represents a parameter significantly affecting flame stability and dynamics. The possibility of a continuous swirl variation, on the other hand, would be advantageous for the assessment and control of thermoacoustic instabilities. This is especially true for fuel-flexible applications, for which different swirl numbers are needed to stabilize flames arising from the combustion of different fuels. Most swirl-varying systems rely on mechanical adjustments. In this work, instead, a novel swirl-stabilized burner is investigated experimentally, which is based exclusively on fluidic actuation. For the experimental assessment of the resulting flow field, the axial and azimuthal velocity components are determined through laser Doppler anemometry (LDA) measurements. The measurements are performed in a volume downstream of the burner's mixing tube. The data are processed and computed into swirl-numbers in order to quantify the degree of swirl as a function of the fluidic actuation. The characteristics of two different burner geometries are investigated, with and without a central cone within the swirler, respectively. The configuration with the cone is found to generate higher swirl over the investigated operational range. For this configuration, the technically relevant operating range is determined in which the swirl number can be continuously set from zero to around 0.9. Our experimental results show that fluidic actuation is a viable way to continuously change the swirl number, and that the achievable swirl range is quantitatively comparable to that of state-of-the-art swirl-stabilized burners.

Complete Dynamics From Ignition to Stabilization of a Lean Hydrogen Flame With Thickened Flame Model

Abstract In recent years, attention has been paid to hydrogen thanks to its carbon-free nature and its interesting characteristics as an energy vector. Despite the large number of numerical analyses regarding hydrocarbon combustion in all the steady and unsteady processes, few papers that cover all those aspects are available in the literature for hydrogen flames. Therefore, a numerical methodology to explore the complete ignition sequence from the spark release to the flame stabilization is validated on a single-sector hydrogen burner. In this context, a preliminary direct numerical simulation (DNS) investigation of laminar spherical expanding flames is performed using different diffusive transport models to isolate their impact. The present work, carried out within the European project HESTIA, investigates the atmospheric test rig installed at the Norwegian University of Science and Technology operating with a lean, perfectly premixed, hydrogen–air flame stabilized on a conical bluff body. Four simulations are performed adopting the thickened flame model with an energy deposition (ED) strategy to assess the impact of preferential and thermal diffusion, as well as grid resolution, on flame dynamics. Three-dimensional (3D) flame structure visualization coupled with detailed particle image velocimetry (PIV)/OH-planar laser induced fluorescence (OH-PLIF) measurements allows the investigation of the key mechanisms involved during the ignition. The dynamic response of the flame through axial fluctuations once the ignition transient is concluded is reconstructed by the numerical strategy employed. Although the overall behavior is almost unchanged by including or not thermal diffusion effects, their local impact on the flame is evident leading to a better agreement with experimental data.

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.

Study on solid powder fuel transport characteristics and mixing degree in scramjet combustor

Abstract The powder scramjet engine is considered the best engine for hypersonic weapons because of its advantages of easy adjustment, flame stability, and high efficiency. However, due to the extremely short residence time of particle fuel in the combustion chamber under high Mach conditions, there is a risk of fuel mixing. Based on the existing combustion chamber models of scramjet engines, this paper proposes a new solid fuel blending enhancement structure by combining shock generator structure and transverse jet and compares the performance and mass transfer characteristics of different shock generators. The CFD method is used to solve the RANS formula to simulate the particle transfer process, and the SST k-ω model is used to calculate the complex turbulence. It was found that when the reverse shock generator was used, the mixing degree of solid particles was higher.

Experimental Study on Scale Effects of Kerosene Combustion in Scramjet Combustors

The combustion characteristics in two geometrically similar kerosene-fueled scramjet combustors with mass flow rates of 0.69 and 1.41 kg/s are experimentally investigated to explore the scale effects of flame stabilization at Mach 2.52 condition. As the equivalence ratio increases, the combustion usually changes from weak to intensive to blow-out mode. The weak combustion has little effect on the flow field, whereas the intensive combustion has the opposite effect. The transition combustion tends to occur between different modes. When the single injector is used, compared with the small-scale combustor, intensive combustion cannot occur in the large-scale combustor, and the flame stability range is also narrower. One probable reason is that as the combustor scale increases, the boundary layer becomes relatively thinner, resulting in a smaller low-velocity zone and a faster mainstream velocity at the downstream wall of the cavity, which is not conducive to the flame propagation upstream to form the intensive combustion. After shortening the isolator, all cases with intensive combustion in the small-scale combustor are transformed into weak combustion, further confirming the speculation. Compared to the single injector, the dual injector is required in the large-scale combustor to achieve intensive combustion and a wider flame stability range.

On the effects of oxygen fraction on stability and combustion characteristics of dual-swirl oxy-methane flames: An experimental and numerical study

On the effects of oxygen fraction on stability and combustion characteristics of dual-swirl oxy-methane flames: An experimental and numerical study

Study on heat release characteristics of the dual composite flame stabilizers for the dual-mode scramjet

The paper studies the heat release (HR) characteristics of a dual-mode scramjet equipped with the dual composite flame stabilizers under simulated free-stream conditions of Mach 6 and dynamic pressure of 30 kPa, through ground direct-connected tests and OpenFOAM numerical simulation. This study focuses on the three-dimensional distribution of subsonic/supersonic HR, local combustion modes, and the variations in HR during the ram-scram mode transition. The results indicate that the high-enthalpy jet transports the high-temperature and fuel-rich gas in the cavity to the mainstream through the ejection effect of the supersonic flow, causing the heat release zone to move to the mainstream. So that the mainstream subsonic-diffusion combustion zone is expanded and is evenly distributed, and then the HR trailing zone downstream of the thermal throat is shortened. The study highlights the different matching characteristics between subsonic flow and HR formed by the dual composite flame stabilizers under high and low equivalence ratio (ER). Optimized injection schemes need to be selected to obtain higher combustion efficiency at different equivalence ratios. In the region of the first-stage composite flame stabilizer, a distinct phenomenon of subsonic/supersonic mixed HR is observed. Subsonic HR predominates under the ramjet mode, while the subsonic HR tends to equal supersonic HR under the scramjet mode. Upstream of the thermal throat, subsonic HR exceeds 80%, and supersonic heat release has a tendency to gradually decrease. Furthermore, when the ER of two-stage dispersed injection is less than 0.5, the heating amount per kilogram of air is less than 1.06 MJ, and the thermal throat cannot be formed, causing the ram-scram mode transition to occur. Conversely, for single-stage concentrated injection, the boundary of ER and heating amount decreases. The study provides support for the design of injection schemes and the regulation of mode transition for dual-mode scramjet based on the HR performance.

Combustion performance of an evaporative flameholder under subsonic-supersonic mixing inflow

The combustion process in rocket-assisted subsonic ramjet engines represents a key advancement in integrated aerospace propulsion, particularly for embedded rocket-based systems. These engines offer the potential to improve combustion performance at altitudes of 25–35 km. However, the significant temperature and velocity differentials between the rocket jet and the subsonic ramjet flow restrict heat and mass transfer. Investigating the relationship between combustion performance and inlet parameters under subsonic-supersonic mixing conditions offers a promising approach to enhancing thrust performance. This study introduces subsonic and supersonic airflow mixing via a flat-plate shear layer in a rectangular channel, with an evaporative flameholder placed centrally to assess combustion. Results reveal that combustion efficiency decreases as the equivalence ratio exceeds 0.2, while the static temperature ratio has minimal impact on efficiency but strongly influences the maximum flame stabilization limit. As the temperature ratio increases from 1.30 to 1.80, the flame limit narrows from 1.656 to 0.237. Higher pressure ratios initially enhance combustion efficiency and flame coverage but eventually cause a decrease. The flame limit broadens from 0.900 to 1.626 as the pressure ratio increases from 1.12 to 1.50. While Mach number changes have little effect on efficiency, the flame limit exhibits an initial rise followed by a drop. Novel findings include an asymmetrical flame pattern and a “Z” shaped outlet temperature distribution, contributing to optimized combustion strategies for combined-cycle engines.

Design and performance analysis of a coupled burner for the Solid Oxide Fuel Cell system

The Solid Oxide Fuel Cell (SOFC) system is characterized by high energy conversion efficiency and low emissions. The energy supply and recovery of the SOFC system are facilitated through the ignition burner and the off-gas burner, respectively. This study proposes a coupled design of the ignition burner and the off-gas burner to reduce the burner volume, thereby enhancing the power density of the SOFC system. The ignition burner employs radially opposed jet combustion with fully premixed natural gas, while the off-gas burner uses swirl diffusion combustion with the anode off gas. When the premixed gas and lean anode off gas are unable to sustain the flame, catalytic combustion is initiated. Simulation and experiment methods are utilized to analyze the performance of the coupled burner in this paper. The results indicate that uniform mixing is achieved with a high swirl number (s1 = 2.6) for the premixed gas, and the maximum excess air coefficient for stable flame combustion at 400 °C is 2.47. As the temperature of the cathode off gas increases, the flameout boundary for the premixed gas gradually increases; conversely, with increasing burner power, the flameout boundary gradually decreases. The intersection cooling effect of mixed air and circumferential high-temperature flue gas is effective, and the flame does not exceed the tolerance temperature of the catalytic carrier. The diffusion combustion of anode off gas with a swirl number (s2 = 0.5) exhibits great gas mixing uniformity without a high-temperature core region. When transitioning from flame to catalytic combustion with premixed natural gas and anode off gas, there is a temperature lag of approximately 60 s. The isotropic cylindrical cordierite catalytic carrier with a 400 mesh and a coating of 60 g/ft3 Pt0.1Pd0.5 achieves stable catalytic combustion in an environment with preheated air temperatures ranging from 400 to 600 °C and H2 concentrations of 0.25–2.91 %.

Emissions and Flame Stability Assessment of Hydrogen Addition and Air Dilution in a Micro Gas Turbine Combustor Using 0D/1D Modeling by Chemical Reactor Network

Abstract Hydrogen emerges as a promising fuel for clean and sustain- able electricity production when utilized in micro gas turbines (mGTs). However, some challenges linked to hydrogen usage must be overcome as its high reactivity can lead to increased NOx emissions and flame instabilities. Literature suggests that combustion air humidification and/or exhaust gas recirculation (EGR) may be methods to reduce this reactivity. However, these measures are limited due to the small operating range of the mGT combustor. In this context, a 20 kWth mGT combustor is investigated un- der various inlet conditions to assess the effect of EGR towards increased flame stability and emission control when operating under hydrogen-enriched methane firing, using a Chemical Re- action Network (CRN) model. The CRN modeling is a fast and low computational complexity tool to model combustion perfor- mance and emissions by representing complex reactive flow fields through idealized reactor models, leading to reduced computa- tional costs. This study examines partial load conditions, fuel compositions, and the effect of combustion air dilution through EGR. The CRN model of the combustion process is designed based on combustion zones extracted from the main flow fields with similar thermo-chemical states from CFD, while emission predictions are experimentally validated for different operating points. Predicted NOx and CO emissions by the proposed CRN model show an acceptable agreement with the experimental data at high power loads. However, its accuracy diminishes for loads below 70% due to the variability in model tuning parameters across different loads, whereas the

The preparation and construction of biomimetic mineralization compatible interface of wood fiber/foamed magnesium oxychloride lightweight composites

The purpose of this study is to improve the application performance of foamed magnesium oxychloride cement (FMOC) and solve the problem of poor interfacial compatibility between the organic-inorganic interface of traditional inorganic composite materials. Inspired by the adhesion mechanism of barnacles to inorganic substrates through amyloid nanofibers and phosphoprotein, a simple green method was proposed to improve the strength and water resistance of the system. The FMOC composites (30×30×30 mm) were prepared by adding wood flour (WF) and phytic acid (PA) to the mixture of magnesium oxide and magnesium chloride. The influences of WF and PA on the compressive strength, composition, microstructure, pore structure, water resistance and flame retardant properties of FMOC were investigated. WF was used as a skeleton structure to induce cement particle aggregation, the PA molecule was used as a connecting bridge to activate the fiber skeleton to increase the active sites, and promote crystal nucleation and aggregation through hydrogen and ionic bond interactions to construct the biomimetic mineralized structure. The results show that the introduction of PA improves the compressive strength and water resistance of the FMOC/WF/PA composite system. When the PA addition amount is 0.8 %, the compressive strength of FMOC/WF/PA-0.8 % composites reached 6.64 MPa, which was 3 times compared to the FMOC composites. The softening coefficient and water absorption of the obtained FMOC/WF/PA-0.8 % composites were 0.81 and 13.84 %, respectively, which were superior to FMOC composites and presented better behavior in water resistance. Meanwhile, the modified composites have more stable pore structure and flame retardant properties, thus these green FMOC/WF/PA composites displayed potential application value in building energy conservation, aerospace and other fields.

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.

Modeling and experiments on plasma ignition of Ekibastuz coal in the form of dust

Due to the low cost of coal and the fact that it is readily available in most parts of the world, coal is a convenient fuel source. Despite the inefficiency of the systems when it comes to converting heat energy into electricity, new technologies are necessary in order to improve their efficiency. In contrast to traditional methods of starting-up boilers and stabilizing combustion, plasma ignition and combustion stabilization (PICS) of pulverized coal flames offers an effective and sustainable alternative to the use of fuel oil or gas. This technology involves heating the air-coal mixture with electric arc plasma until the coal devolatilizes and the coke residue partially gasifies. Consequently, low-rank coal is converted into a highly reactive two-component fuel (HRTF) consisting of combustible gas and coke residue. For these processes in a plasma-coal burner (PCB) using Ekibastuz coal in the form of dust, a kinetic analysis was conducted using the PlasmaKinTherm program. Modeling the kinetics of PICS of pulverized fuel allowed changes in temperature, velocity, and concentration to be determined along the length of a PCB. The composition, degree of carbon gasification, and temperature of a stable coal-dust flame were determined using plasma ignition of solid fuel. Based on the comparison between experimental and calculated data, it was found that the results were satisfactory.

LES investigation of a H2-fueled supersonic combustor: A two-strut injection approach

Efficient combustion schemes are increasingly vital as scramjet expansion into higher Mach numbers emerges as a leading trend. Thus, in the current work, an LES investigation on mixing improvement caused by a two-strut injector has been carried out for a Mach 2.5 model H2 fueled scramjet combustor. The research focuses on understanding the flow field, flame lift-off characteristics and combustion stabilization in the two-strut combustor. The simulation results indicate that the two-strut configuration exhibits a broader pattern of temperature fluctuation, implying a more efficient spread of combustion downstream compared to the single-strut configuration. Analysis of Fast Fourier Transform (FFT) pressure data reveals inherent instability in the combustion processes for two-strut under supersonic air inflow conditions. The findings also demonstrate the significant improvement in mixing performance achieved by the two-strut design compared to conventional struts. The primary mechanism is a two-strut's ability to create significant streamwise vorticity, which can improve the mixing of H2 fuel with supersonic air. According to the mixing and total pressure loss plots, a two-strut produces a rapid complete mixing at 0.182 m at the expense of a minute rise in total pressure loss. Finally, the entropy change plot reveals that the two-strut configuration exhibits a reduced entropy change compared to the single-strut configuration. This is mainly due to enhanced air-fuel mixing, efficient shock wave management, and enhanced flame stabilization in the two-strut setup. These factors collectively contribute to reduced irreversibilities and disorder in the flow, leading to lower entropy within the system.

Investigation of Ammonia/Hydrogen Mixtures and Pilot-Split Strategies in a Laboratory-Scale Radial Swirl Combustor

Abstract The transition to a decarbonized energy future relies on identifying the most suitable alternative fuels that can meet the needs of various energy sectors. While both ammonia and hydrogen are zero-carbon energy vectors, their physical properties and burning characteristics sit on either side of that of natural gas. Hence, mixtures of ammonia and hydrogen are being increasingly looked at as having the potential to fuel current energy systems without requiring significant combustor redesign. However, the combustion characteristics and operation limits for different ammonia/hydrogen mixtures still need to be evaluated. For gas turbine applications in particular, the effect of ammonia/hydrogen mixture composition and operating condition on flame behavior and stability is not well understood. The current work was carried out in a laboratory scale, radial swirl-stabilized turbulent combustor. A systematic study of two ammonia/hydrogen blend ratios (70:30 and 80:20 by volume) and a range of equivalence ratios were tested for different pilot-split ratios, to understand the effect on flame shape, stability and dynamics. Time-resolved pressure and integrated heat release fluctuations were measured to evaluate combustor dynamics, and NH2* chemiluminescence flame images were captured to understand spatial differences in flame structure. When comparing blend ratios, differences were observed in flame macrostructures and combustor dynamics, which could be largely attributed to the considerable difference in the laminar flame speeds of the blends. The addition of pilot generally improved the stability and lean operation for both blend ratios.

Prediction of the Flame Characteristics of an Industrial Gas Turbine Operated With CO2-Diluted Air Through a High-Fidelity Numerical Analysis: A Comparison Between Flamelet Generated Manifolds and Artificially Thickened Flame

Abstract In light of the global commitment to decarbonize industrial processes, carbon capture and storage (CCS) plays a pivotal role in mitigating greenhouse gas emissions from gas turbine (GT) power generation processes. Achieving an efficient GT–CCS coupling requires the employment of high percentages of exhaust gas recirculation (EGR) to maximize the CO2 content at the CCS inlet. Nevertheless, such operating conditions pose critical challenges for conventional combustion systems due to reduced oxygen levels associated with higher EGR, limiting engine operability. To address this challenge, the development of innovative technical solutions is essential to extend the combustor operational capabilities at high EGR rates. For this goal, a significant number of computational fluid dynamics (CFD) simulations are required to identify the flame stability limits across various EGR levels and burner designs. It is imperative, in this context, to minimize computational costs while maintaining high accuracy. In this work, a comprehensive comparative study of an extended version of the flamelet generated manifolds (FGM) and the artificially thickened flame (ATF) model is performed through a large eddy simulation (LES)-based CFD analysis. The investigation is performed within the context of an industrial lean-premixed burner manufactured by Baker Hughes, operating with natural gas and CO2-diluted air at atmospheric pressure. While the extended-FGM has been previously presented by the authors in a study on the same test rig under standard air conditions, the current work aims to extend its application to critical oxygen-depleted conditions, where near-blowout phenomena such as flame liftoff and length elongation may become significantly pronounced. Numerical validation is carried out through a direct comparison of the computed averaged heat release, representing the flame topology, with detailed OH* chemiluminescence images from a test campaign conducted by the technology for high temperature (THT) Lab of the University of Florence. The experimental data will serve as the primary benchmark for assessing the models’ effectiveness in capturing the main dynamics of such critical operating conditions. Furthermore, potential disparities in both thermal and flow fields at the burner exit region between the two models will be discussed.

Thickened Flame Model Extension for Dual Gas Turbine Combustion: Validation Against Single Cup Atmospheric Test

Abstract The development of predictive combustion models is more and more strategic in the design definition of gas turbine (GT) combustor. The thickened flame model (TFM), despite its high computational cost, is one of the most accurate approach available in literature since it can naturally take into account the nonequilibrium effects into the flame brush (i.e., strain and heat losses) as well as preferential diffusion when hydrogen is employed. Conversely, the original formulation of this combustion model needs several adjustments to accommodate the properties of the mixture when different streams of fuels and/or oxidizers are present in the system. The present work represents a first step in the extension of this combustion model to handle multiple streams of fuels and oxidizers. More specifically, an industrial burner fed with two different fuel streams and air as oxidizer is considered. The pilot fuel line is fed with microhydrogen injections with the aim to enhance the lean blow-out margin, while the main one is with pure methane. Dedicated tests are performed at the Technology for High Temperature laboratory (University of Florence) to retrieve the main information characterizing the burner (emissions, temperature, and pressure pulsations) as well as OH* chemiluminescence for the flame shape and position at the same operating conditions. The comparison between the numerical results and the experimental data will provide highlights about the ability of the extended-TFM to capture the main features of the flame stabilization mechanisms.

The analytical model of flame characteristics of hydrogen–air through wall and gas interaction analysis

AbstractIn this article, the effect of different boundary conditions and different thermal and physical properties of walls and gas on flame characteristics and stability of hydrogen–air mixture are investigated using an analytical method. This method solves the gas–wall energy equation, and the hydrogen mass conservation equations. The jump conditions are obtained by integrating the energy and mass equation into a small control volume around the flame. For validation of this model, the temperature distribution on the outer surface of the wall is compared with experimental data that show the maximum relative error of 3.5% for Q = 400 mL/min and 4.9% for Q = 200 mL/min. The maximum variation of gas temperature is nearly 6.5 times of wall temperature variation. The wall can be considered one‐dimensional for conventional wall materials with K > 10. For the existence of combustion inside the chamber, when the value of K is greater than 10, the Péclet number should also be considered greater than 10. In a constant equivalence ratio, increasing the medium temperature increases flame stability.

Mathematical thermo‐mechanical analysis on flame‐solid interaction: Steady laminar stagnation flow flame stabilized at a plane wall coupled with thermo‐elasticity model

AbstractA laminar stagnation flow flame stabilized at a plane wall is theoretically analyzed, coupled with a thermo‐elasticity model in the wall. Mathematical models for both the flame and the wall will be proposed, and corresponding analytical solutions based on their dimensionless forms will be obtained. The mathematical analysis of this flame‐solid interaction primarily focuses on the effect of the flame on combustion‐induced thermo‐mechanical stresses and the influence of wall material on flame properties such as flame temperature and stability against extinction. A sensitivity analysis is further performed to examine how thermo‐mechanical stress inside the wall is affected by changes in other combustion conditions (e.g., mixture composition, flame stretch rate). This study offers valuable insights into understanding the interaction between flames and solids, a relationship crucial in engineering applications such as the interaction between combustion processes and blades within gas turbine machinery.

Numerical investigation on influence of different geometrical cavity flame holders and flame stabilization mechanism in hydrogen fueled multi-strut-based scramjet combustor

A detailed computational study was carried out to explore how the presence of a cavity and cavity angles influence the performance and combustion mechanisms within a scramjet combustor. This research utilized ANSYS Fluent 2021 to create hexahedral meshes and simulate the complex fluid dynamics in scramjet engines equipped with dual strut injectors. The simulations addressed various critical aspects of fluid mechanics, including the continuity, momentum (via the Navier-Stokes equations), and energy equations. These equations were used to model non-reacting flows, while the species transport equations were applied to analyse reacting flows. The turbulence within the combustor was modelled using the standard K-ε model, a common choice for such high-speed aerodynamic evaluations. The study's primary goal was to assess the effects of different geometrical configurations of cavity flame holders on the performance characteristics of an H2-based scramjet combustor overall efficiency, focusing on mixing efficiency, combustion performance, and total pressure loss. Both configurations with and without a cavity were examined. Key findings indicate that incorporating a cavity into the combustor design leads to developing a robust recirculation zone within the cavity area. This recirculation zone is pivotal in enhancing fuel-air mixing and combustion efficiency, with cavity-based combustors showing an earlier onset of combustion and achieving peak combustion efficiencies around 90–95%. The extent of the recirculation region is notably influenced by the proximity of the strut injector to the cavity's length. Additionally, the presence of a shear layer within the cavity generates a weak shock at the cavity's trailing edge. This shock interaction can adversely affect scramjet combustor performance, especially at higher cavity angles (α) and specific geometric configurations, such as an L/D ratio of 4 and α=30°. The L/D ratio of 4 and α=30° combustor model results in a 13.8% increase in turbulence intensity, which raises air-fuel mixing and there by improves mixing and combustion efficiency by 14.23% and 13.34%, compared to the other depth of the cavities. This advantage is critical, especially considering the compact length of the combustor, which is a desirable attribute in scramjet design.