Cavity Flameholder Research Articles

Laser ablation ignition modes in a cavity-based supersonic combustor

A numerical and experimental study was conducted to investigate the Laser Ablation (LA) ignition mode in an ethylene-fueled supersonic combustor with a cavity flameholder. The experiments were operated under a Mach number 2.92 supersonic inflow, with stagnation pressure of 2.4 MPa and stagnation temperature of 1600 K. Reynolds-averaged Navier-Stokes simulations were conducted to characterize the mixing process and flow field structure. This study identified four distinct LA ignition modes. Under the specified condition, laser ablation in zero and negative defocusing states manifested two distinct ignition modes termed Laser Ablation Direct Ignition (LADI) mode and Laser Ablation Re-Ignition (LARI) mode, correspondingly. LA ignition in a local small cavity, created by depressing the flow field regulator, could facilitate the ignition mode transforming from LARI mode to Laser Ablation Transition Ignition (LATI) mode. On the other hand, the elevation of the flow field regulator effectively inhibited the forward propagation of the initial flame kernel and reduced the dissipation of LA plasma, further enhancing the LADI mode. Based on these characteristics, the LADI mode was subdivided into strong (LADI-S) and weak (LADI-W) modes. Facilitating the transition of ignition modes through alterations in the local flow field could contribute to attaining a more effective and stable LA ignition.

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.

Role of different cavity flame holders on the performance characteristics of supersonic combustor

Abstract 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. 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. 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°. This advantage is critical, especially considering the compact length of the combustor, which is a desirable attribute in scramjet design.

Effect of flameholding cavity geometry on the flowfield of a solid fuel scramjet

An optically accessible solid fuel scramjet was used to investigate the supersonic combustion of varying polymethyl methacrylate solid fuel grain geometries. The rear wall angle of the flameholding cavity was varied (30, 60, 75, and 90°) and an alternative geometry (flush fuel grain) containing the addition of a backwards facing step to the front of a conventional fuel grain was explored. Optical diagnostics including high-speed shadowgraph, CH* chemiluminescence, and HD video were used to analyze flammability characteristics and determine quantitative regression rates for each geometry. The optical results were supplemented by static pressure traces along the length of the combustor for the duration of a test firing. All tests were conducted with Mach 2 inlet flow into the combustor at nominal inlet conditions of 12.9 atm and 900 °C. Shadowgraph imaging for both cold flow and combusting flow was used to characterize the varying flow fields associated with their respective geometries and their core flow oscillatory frequencies were preliminarily inspected. Chemiluminescence data reveals increased heat release in the downstream constant area and expansion sections of the flush fuel grains compared to conventional geometries. This heat release appears to correlate with increased time-averaged regression rates in these regions while the varying cavity angle fuel grains demonstrate differing magnitudes of maximum regression along the rear cavity wall and onset of the constant area section. In addition to increased overall regression rates, flush geometries increase the flameholding capabilities and demonstrate self-sustained burning at lower inlet temperatures than similar geometries designed without a flush section.

The fuel mixing of multi-annular extruded fuel jets in cavity flame holder at supersonic combustion chamber: Computational study

A cavity-flame holder is widely used for the injection of the fuel jet in a supersonic combustor due to high fuel residence time. This paper presents extensive results about the impacts of extruded annular multi-jets for improving fuel mixing inside the cavity flame holder. A three-dimensional model of the cavity with multiple extruded nozzles in different lengths and conditions has been examined in this study to investigate the flow and fuel mixing inside the cavity. In addition, the usage of internal air jets is investigated and extensive comparisons between the elongation of the extruded nozzle and the use of the inner air jets for better fuel distribution are done. The presented results show that the usage of both inner air jet and nozzle elongation significantly enhances the fuel distribution within the cavity flame-holder. The achieved finding confirms that the latter technique is more efficient for the fuel mixing in the combustor of a scramjet engine.

Characterization of a Cavity Flameholding Solid-Fuel Ramjet in an Optical Combustor

Three fuel grain geometries were investigated in a two-dimensional, optically accessible solid fuel ramjet combustor to improve the understanding of the internal ballistics as it pertains to flameholding and overall performance. Two-cavity flameholding-configured fuel grains demonstrated increased flameholding and similar performance to the baseline flat fuel grain. Optical diagnostics such as high-speed CH* chemiluminescence and high-speed three-color camera pyrometry were utilized to measure and analyze the emissive species to draw conclusions about the reacting flowfield. Additionally, optical access afforded the ability to determine spatially resolved regression rates within the flameholding region of each fuel grain. The measured fuel grain regression rates demonstrated similar performance between the three configurations; however, the spatial distribution of the regression rates resulting from the high local heat flux due to the cavity corner is postulated to be the driving factor in increasing the flameholding capability of the cavity fuel grains.

Impact of an extruded nozzle on flame stability and fuel mixing in a cavity flame holder

Impact of an extruded nozzle on flame stability and fuel mixing in a cavity flame holder

Numerical investigation of mixing performance in supersonic cold flow over dual-cavity in scramjet

Mixing performance in the combustion chamber is important in achieving the efficiency of scramjet for its compact structure and the short residence time caused by high velocity. This study focuses on predicting the mixing performance in a viscous supersonic flow past the cavity flame holder in the scramjet, using the discontinuous spectral element method with direct numerical simulation. The arrangement and distribution of a series of cavities are calculated at various inlet velocities. By comparing the contours of Mach number and static temperature, as well as analyzing the airflow residence time with a new calculating formula and the drag based on the numerical results, it is concluded that higher inlet velocities result in faster stabilization. It also leads to longer airflow residence time when the supersonic flow passes through the dual-cavity with a tandem connection rather than the parallel one or the single-cavity structures. As for the shortened rear wall of cavities, these structures can decrease the drag quickly, but they also decrease the airflow residence time seriously, which destroys the mixing performance. In addition, these conclusions are applied to a practical case of the cavity flame holder, verifying the effectiveness of tandem dual-cavity structures in enhancing the mixing performance by increasing the airflow residence time and maintaining or reducing the drag. This study can provide valuable suggestions in further design of cavity flame holders for different flow conditions.

Estimation of the fuel mixing of annular extruded fuel multi-jets in cavity flame holder at the supersonic combustion chamber via predictive surrogate model

Cavity flame holder is a well-organized method for fuel mixing inside the combustion chamber of supersonic vehicles. In this study, the combined machine learning technique of proper orthogonal decomposition (POD) combined with Long Short-Term Memory network (LSTM) is used for prediction of fuel jet penetration inside the cavity flame holder at free stream Mach=2.2 is investigated. Computational Fluid Dynamic is applied for the three-dimensional model of a cavity flame holder with extruded multi-nozzles. A comparison of single and extruded multi-nozzles for the fuel mixing at the combustor with cavity configuration is also done. The presented results show that the use of multi-jet efficiently increases the fuel mixing inside the cavity since the vortex structure is enhanced in this configuration. The usage of a shock creator pushes the shear layer into the cavity and limits the fuel penetration inside the cavity. Therefore, the shock creator is not recommended for the fuel mixing of extruded multi-jet injection systems.

Usage of multi-extruded nozzles for hydrogen fuel mixing in the double cavity flame holder in the existence of the shock generator

Improvement of the fuel mixing inside the cavity flame-holder is important for the development of this injection system in scramjet engines. This article presents significant flow results on the mixing role of multi-extruded nozzles inside dual cavity flame holders with a single shock generator. A three-dimensional model of the dual cavity with injectors located on the inclined rear wall is produced to simulate real flow physics associated with proposed jet configurations. The impression of air injection from the inclined rear wall is also examined by computational modeling of the compressible flow inside the dual cavity. Based on the achieved results, the injection of multiple extruded nozzles enhances fuel diffusion in the cavity when the shock generator is applied. Our findings disclose that the injection of the fuel from the inclined rear wall results in more efficient fuel mixing inside the dual cavity flame holder. Mixing of the annular jet is increased about 14% via backward air jet inside the cavity.

Supersonic Combustion of Solid Fuels

Supersonic combustion of solid fuels is investigated experimentally using a rectangular-cross-section combustor in the context of understanding ignition characteristics, studying fuel regression, and determining operating conditions. Fuel grains consisted of 3D printed polymethyl-methacrylate of varying geometry. Specifically, the cavity flameholder dimensions are systematically studied to develop a flammability map. Initial examinations reveal that a longer and larger cross-sectional area of the flameholding zone in the fuel grain creates the most optimal conditions for ignition and sustained flameholding. The longer and larger flameholding zone also creates conditions for faster time-averaged regression rates. The ignition characteristics and fuel regression are also examined by varying inlet temperatures of the air, which showed that a higher inlet temperature is conducive to sustained ignition, as well as higher fuel regression rates. This work extends the available literature on supersonic combustion of solid fuels to lower temperatures than has previously been reported.

Experimental and numerical investigation on the Ignition performance of the cavity flame holder in the integrated afterburner

Integrating components, as a new afterburner concept, is currently an important trend in afterburner development research. Spurred by this trend, this study proposes an inner cone cavity flame holder for an integrated afterburner. The experimental and numerical investigation evaluates the ignition performance under various pneumatic parameters, illustrating that stable flame is successfully achieved in the cavity of the integrated afterburner for a wide range of flow conditions. This is important, as an appropriate cone expansion angle benefits the ignition and flame stability. By comparing the experimental results, the optimum case is obtained, which reduces the ignition FAR. Furthermore, double fuel line can effectively broaden the ignition boundary. The successful completion of our ignition experiment demonstrates that the cavity flame holder of the integrated afterburner has a good structural design and ignition performance.

Combustion instability analysis in an ethylene-fueled scramjet combustor under various fuel penetration height conditions using an image-based nonlinear dimensionality reduction method

The present study investigates the effects of fuel penetration height on combustion instabilities in an ethylene-fueled scramjet model combustor with a cavity flameholder. Experiments were performed at the stagnation temperature of 1900 K, the stagnation pressure of 0.37 MPa, and the Mach number of 2. Three fuel injection orifice diameters:2.3, 2.4, and 2.5 mm, were tested to elucidate the effects of ethylene penetration height on combustion instabilities. Furthermore, high-speed measurements of CH* chemiluminescence and shadowgraphs were performed with high-speed video cameras. To examine the dynamics of the combustion instabilities, time-resolved CH* chemiluminescence images and shock parameters, extracted from snapshots of shadowgraphs, were analyzed using a non-linear dimensionality reduction algorithm: Gaussian Process Dynamical Model (GPDM). Thereafter, further analyses on the acquired latent variables were conducted with Recurrence Plot (RP). The experimental results showed that the fuel equivalence ratio (ϕ) range for cavity shear-layer combustion mode expanded as d decreased. Furthermore, for ϕ = 0.18, the instability behavior remarkably changed at around d = 2.4 mm. Therefore, the instability dynamics for ϕ = 0.18 were investigated using GPDM and RP including results from our previous study (d = 2, 3, and 4 mm), revealing differences in the instability behaviors. For d = 3 and 4 mm, jet-wake combustion and ram combustion modes were established alternately with a frequency of about 1600 Hz. In contrast, at d = 2.4 and 2.5 mm, although a similar instability in the d = 4 mm case was present at almost the same oscillation frequency, a different instability behavior was also confirmed. This additional instability exhibited an intermediate state between jet-wake combustion and ram combustion modes. These two instabilities emerged aperiodically. For an unstable combustion in shear layer observed for d = 2 and 2.3 mm, corresponding RPs exhibited black patches, indicating that the oscillation amplitude diminished substantially.

Regulating scramjet combustor mode transition using fuel distribution control

Dual-mode scramjets can operate efficiently over a range of flight speeds from moderate supersonic to hypersonic conditions. Depending on the fueling and flight conditions, the combustion mode operates in either a thermally-choked mode or a supersonic combustion mode. Direct-connect experiments were conducted using a laboratory-scale scramjet combustor with hydrogen as fuel, and its combustion mode transition behavior was characterized over various equivalence ratios. It was observed that the combustor became susceptible to combustion instability when mode transition was occurring naturally. To explore the possibility of actively triggering combustion mode transition while alleviating the combustion instability concerns, a new strategy of changing fuel injection distribution was formulated and a series of spatially distributed fuel injection experiments were conducted. The results showed that the critical amount of fueling for mode transition depends on the degree of fuel distribution. Subsequent experiments demonstrated that the combustion mode transition timing could be effectively controlled by scheduling spatial distribution of fuel injection. When fuel was injected at one location, most of heat release was concentrated near the cavity flame-holder, leading to thermal choking at a relatively low equivalence ratio. With distributed fuel injection, heat release became more evenly distributed across the expanding portion of the combustor, effectively delaying the mode transition to a higher equivalence ratio. Through the use of fast-acting solenoid valves, it was shown that changing fuel injection distribution could be used to trigger a timely combustor mode transition while holding the total fuel flow rate unchanged. When mode transition was actively triggered, the entire transition process occurred over a significantly shorter time scale compared to the natural mode transition process. The results indicate that combustion mode transition process could be controlled at the desired timing by actively scheduling fuel injection distribution, with reduced risks of encountering combustion instabilities while transitioning.

Strut-assisted injection of liquid fuel in a supersonic combustor

This paper investigates mixing and combustion of liquid fuel in a model scramjet combustor with strut-assisted injection adjacent to a cavity flame holder. Fuel/air mixing in a Mach 2.2 cross-flow and stagnation temperature of 1300 K was studied using filtered IR imaging. The insight gained by this method is discussed along with the method limitations. The impact of the strut assisted injection on the mixing of the liquid fuel was seen by the shorter vaporization distance, as the fuel vaporized upstream of the cavity, in contract to a liquid jet that expanded above most of the cavity when the strut was absent. Moreover, the fuel jet was seen to propagate laterally to the tip of the strut, which lead to lateral dispersion of the fuel up the strut height. The combustion experiments were conducted by piloting the cavity with ethylene and were imaged using CH* chemiluminescence. At stagnation temperature of 1300 K thermal choking and stable combustion occurred at equivalence ratio of 0.43, with the flame stabilizing at the strut wake, above the cavity. Intermittent thermal choking occurred at leaner condition. The flame stabilized on the cavity shear layer when the shock train moved downstream of the cavity and the flame stabilized at the strut wake when the shock train moved upstream. Thermal choking with the strut guided injection relative to a combustion only in the cavity without the strut is the result of the observed higher mixing efficiency, that enabled flame spreading into the supersonic flow and higher heat release. At a higher stagnation temperature of 1500 K the shock train moved downstream of the cavity, and the flame stabilized on the cavity shear layer. Finally, it is shown that the combustion efficiency could be further improved by injecting the fuel from the strut side walls.

Role of cavity in a Mach 8 axisymmetric scramjet combustor: Flame stabilization vs combustion enhancement

The cavity-assisted scramjet has been proven to be the most promising propulsion system for air-breathing hypersonic vehicles. In this paper, numerical simulations of a Mach 8 axisymmetric scramjet combustor are conducted and validated to investigate the effect of the cavity. The results indicate that the combustion state undergoes significant changes as the combustion heat release increases. Detailed analysis reveals that the role of the cavity in flame stabilization and combustion enhancement also changes with combustion heat release. Under weak heat release conditions, the high-speed environment results in reduced combustion efficiency, and the primary role of the cavity is to stabilize the flame. Increasing the cavity size does not yield significant gains but could bring redundant mass. As heat release intensifies, the combustion enhancement effect of the cavity becomes more prominent. The presence of the cavity dramatically improves fuel combustion efficiency. The distribution of supersonic and subsonic combustion modes, as well as that of premixed and diffusion combustion modes, is also affected by cavity size and combustion heat release. In the engineering development of scramjets, it is suggested that the design of the cavity flameholder should involve careful consideration of combustion heat release.

Color and multi-band imaging of a cavity-based flameholder in supersonic flow

Utilizing time-resolved CH*/C2* and multi-angle color flame imaging recorded at 1 kHz/22.5 kHz frequencies, this study investigates the dynamics and stabilization of supersonic flames in a cavity flameholder combustor at Mach 2, with ethylene fuel and air. A stagnation pressure of 289 kPa was used for the stable cavity burning experiments with fuel flow rates of 30, 60, and 90 slpm and 483 kPa for ignition transient experiments with fuel flow rates of 55 and 90 slpm, injecting fuel at the closeout ramp. The stagnation temperature was 597 K. Chemiluminescence analysis focused on the equivalence ratio (ER) and combustion intensity, while a fiber-based endoscope captured color flame image, informing on premixedness, flame structure, and flame surface density. Results from transient ignition showed that a progression from lean to stoichiometric, and ultimately to fuel-rich conditions was observed, with marked transitions occurring along the cavity floor. High-intensity CH* regions were consistently associated with fuel-rich zones. Digital flame coloration discrimination (DFCD) analysis provided insights into the mixing efficiency, affecting the flame color and structure. Despite reduced fuel flow rates significantly altering flame characteristics, such as thickness and the persistence of a 'W' flame structure, the shear layer remained a focal point for optimal combustion conditions. The study demonstrated that the shear layer's intense turbulent mixing is crucial for flame stability and structure, with chemiluminescence surface density (CSD) profiles suggesting balanced combustion at 60 and 90 slpm flow rates. However, an asymmetry of CSD at 30 slpm indicated a shift towards fuel-rich conditions at the burning surface, indicating potential instability and elevated blowout.

Large Eddy Simulation of cavity stabilized ramjet combustion

In this study we use finite rate chemistry Large Eddy Simulation (LES) to examine flow, injection, mixing, self-ignition and turbulent combustion in a dual-mode ramjet combustor with a cavity flameholder. The target case is the dual-mode ramjet combustor studied experimentally by Micka & Driscoll in a series of publications under different operating conditions characterized by varying air stagnation temperatures, T0, and fueled with hydrogen or hydrogen-ethylene blends. Here, only hydrogen fuel is considered. Three skeletal and comprehensive chemical reaction mechanisms are used to evaluate the influence of the reaction mechanism. Based on comparisons with ignition, laminar flame, and dual-mode ramjet combustor experiments the Z22 mechanism is found superior to the D7 and J20 mechanisms, which is then used throughout this study. For the ramjet experiments, two stabilization modes were reported: cavity-stabilized combustion at low T0 and jet-wake-stabilized combustion at high T0. For in-between values of T0, combustion oscillates between these modes. The LES results using the Z22 reaction mechanism are in good agreement with the experimental data: cavity-stabilized combustion is observed for low T0, and for this case combustion anchors at the leading-edge of the cavity. For high T0 jet-wake-stabilized combustion occurs in the near-wake of the fuel-plume. Between these two modes, combustion oscillates between modes that can be roughly identified as jet-wake and cavity stabilized. The LES predictions also reveal details about high-frequency oscillations (∼1 kHz) that are observed in particular for low and intermediate T0. The H2-air mixture appears to burn as an auto-ignition assisted premixed flame in the cavity-stabilized case, and as a diffusion-flame in the jet-wake stabilized case, whereas for the in-between cases, the burning modes changes accordingly.

Characteristics of the flame flashback in a dual-mode scramjet combustor by the gliding arc plasma

A multi-channel gliding arc (MCGA) plasma with an average power of 6000 W is utilized to control the flame flashback in a dual-mode scramjet combustor with a cavity flameholder. Optical measurements, including synchronous high-speed CH* emissions and high-speed schlieren, simultaneous high-speed camera with the oblique view and high-speed CH* emission, accompanied by the three-dimensional RANS simulation, were employed to characterize the flame flashback with or without the MCGA plasma. The results show that a typical flame flashback cycle can be observed with a period of ∼2 ms in the combustor. The back pressure reduced by the dynamic heat release of the reaction zone is not sufficient to form the steady thermal choking in the mainstream in the absence of the MCGA, which results in the oblique shocks and bow shocks appearing in dual-mode scramjet combustion. When the plasma is added, the flame is strengthened near the plasma region, and the combustion oscillation is suppressed significantly. The re-ignition and attachment characteristics of the MCGA plasma provide the new flame kernels to the mainstream flame, and the leading edge of the mainstream flame follows the MCGA plasma. Due to the relatively stable heat release near the fuel jet assisted by the plasma, the thermal choking is stable presented in the combustor with bow shocks by higher back pressure, indicating that the combustion oscillation is suppressed by the MCGA plasma in the dual-mode scramjet combustor.

Combustion characteristics of a supersonic combustor model for a JAXA flight experiment

JAXA is conducting a flight-ground test comparison program to clarify the “facility effect” on hypersonic aerodynamics and combustion phenomena and to develop a CFD tool for predicting flight data from ground test data. The aim is a flight experiment to obtain data on aerodynamic heating and supersonic combustion under actual flight conditions and to validate the CFD tool using flight data and the corresponding ground test data. This study aimed at determining a flow-path geometry and a fuel injector configuration for a supersonic combustor suitable for clarifying the influence of the different test flow compositions between flight and ground test conditions on combustion. The candidate configurations proposed by the CFD study were evaluated by direct-connect combustion tests using ethylene fuel. The results showed that a combustor flow was symmetric when the fuel equivalence ratio was low and asymmetric when the equivalence ratio and the pressure in the combustor were high. Because an asymmetric flow is unsuitable for validating CFD based on steady RANS, the total equivalence ratio was limited to 0.44. The combustor model uses two-stage fuel injectors and cavity flame holders to combust ethylene fuel. The depth of the cavity flame holder had little influence on combustion. However, the number of injection holes for the injector located downstream of the cavity affected the combustor pressure. The combustor flow-path design was finalized based on the combustion test results. In addition, an ethylene fuel ignition method using pilot hydrogen injection, adopted for the flight experiment, was also demonstrated successfully.