Cavity Flameholder Research Articles

Regression and Flame Structure in Cavity Flameholding Solid-Fuel Ramjet Fuel Grains

Introducing cavity flameholders into a solid-fuel ramjet fuel grain demonstrated increased fuel loading with sustained combustion in previously unfavorable geometries. Volumetric fuel loading improvements of up to 26% were demonstrated to sustain combustion. Regression patterns of cavity fuel grains are presented and show that the effect of implementing a cavity flameholder is to change the location of maximum regression and the reattachment point. The addition of a cavity flameholder does not appear to have a significant effect on combustion efficiency. However, it is noteworthy that longer cavities increased the chamber pressure above what was observed for a center-perforated fuel grains as a result of the increased mass addition and higher equivalence ratio associated with the higher regression rate. Large-eddy simulation computations were performed using a fourth-order discontinuous Galerkin finite element solver with a novel flamelet and progress variable formulation. The predictions agree well with the experiments and point to the increased heat transfer for longer cavities as the main flameholder mechanism. The larger heat feedback is supported by the formation of a stronger recirculation region, which leads to increased coherent fluctuations due to the transition between local and global instabilities.

Asymmetric heating effect on heat transfer and carbon depositions of pyrolytic hydrocarbon fuel in curved cooling channels

Numerical study was carried out to investigate the effect of asymmetric heating on fuel pyrolysis, heat transfer and carbon depositions in curved cooling channels around the cavity flame-holder. The results reveal that the secondary flow velocity determines the flow structure, heat transfer and carbon depositions in asymmetrically heated curved cooling channels. In the concave heated case, the drastic variations of the radial pressure gradient with fuel pyrolysis induce higher secondary flow velocity, which is of benefit to the utilization of fuel heat absorption capacity and the inhibition of carbon deposition. Under the asymmetric heating and secondary flow effects, compared with the convex heated case, the heated wall HTC rises, while the heated wall temperature and the total carbon depositions drop in the concave heated case. In addition, the increasing radius of curvature reduces the asymmetric heating effect on heat transfer and carbon depositions in the curved cooling channel.

Experimental investigation on the ignition limits of a kerosene-fueled cavity in model scramjet combustor

The ignition limits of liquid kerosene injected upstream of a cavity flameholder using a spark plug are investigated in a model scramjet combustor with the isolator entrance Mach number of 2.52, the total pressure of 1.6 MPa, and stagnation temperature of 1486 K. The effects of injection pressure and mixing distance on ignition are studied. Flame dynamics and the evolution of the flow field during the ignition process are acquired through high-speed photography and Schlieren photography. For near-field injection, successful ignition of kerosene can only be achieved when the injection pressure is extremely low or in a higher injection pressure range. The ignition is failed due to the fuel-rich local equivalence ratio in the cavity recirculation zone when the injection pressure is in the middle of injection pressure range. For far-field injection, the ignition trend is totally opposite compared with near-field injection. The ratio of the fuel that the cavity can react completely to that consumed by the mainstream is extremely low due to the minimal mass exchange rate between the cavity and the mainstream. The local equivalence ratio in cavity is the controlling factor determining whether successful ignition could be achieved. Only local ignition with low combustion efficiency is achieved in the present single-cavity combustor. The flame could not spread to the mainstream due to the lower turbulence level weakening the interaction of combustion products in the recirculation zone with unburned reactants in the supersonic flow.

超音速流における気流境界層吸込みが流れ場および保炎性能へ与える影響

The changes in the flow field and combustion behavior of a scramjet combustor with a cavity flameholder under boundary layer bleeding condition were investigated by direct imaging, PIV measurement and numerical calculations, at the mainstream conditions of Mach number of 2.5 with total temperature of 580--760K and total pressure of 0.6MPa. Hydrogen gas was used as fuel, and the ignition was forced by precombustion gas injection. From PIV measurement and numerical calculations, it was found that the boundary layer bleeding decreased the static pressure, the density and the static temperature of the mainstream and increased the maximum velocity and the velocity of the boundary layer. Direct imaging of the combustion experiments and numerical calculations showed that the boundary layer bleeding increased the momentum flux of the mainstream and changed the interaction between the precombustion gas jet and the mainstreams, which in turn changed the static temperature and the local equivalence ratio distribution inside the cavity flameholder, resulting in a change in the combustion behavior more favorable to flameholding. It was concluded that the increase in momentum of the mainstream could improve the flameholding performance.

Influence of strut on trapped vortex cavity flameholder: Ignition and flame propagation performances

In order to improve the performance of pilot flame propagation to the core flow, a novel strut design was proposed for the trapped vortex cavity flameholder. Experiments were conducted under high subsonic flow conditions to investigate the influences of strut on the ignition performance and flame propagation performance of trapped vortex cavity combustor. Furthermore, the flame development was acquired using a high-speed camera and the flame information and the flow field were combined to explicate the flame stabilization mechanisms of two flameholder layouts. The results showed that, both flameholder layouts achieved reliable ignition and flame stability under inlet flow velocity lower than Ma = 0.42. Cavity combined with strut weakened the ignition performance of the trapped vortex cavity-only flameholder based combustor, but improved the blowout performance and enhanced the flame stability. Compared with the cavity-only flameholder layout, the lean ignition equivalence ratio of the cavity-strut flameholder layout was increased of 14.6%, and the lean blowout equivalence ratio was decreased by 56.2%, under the inlet flow at 600 K. Strut changed the original flame propagation path of the cavity-only layout, and caused the flame stabilization mechanism change from cavity flame stabilization to strut flame stabilization. Although the change of flame stabilization mechanism prolongs the ignition delay time, it enhances the flame stability of the combustor. Moreover, the flame propagation ability to the radial direction behind the cavity combined with strut is enhanced, which is more suitable to the engineering combustor with higher requirements on circumferential flame propagation.

Numerical investigation of turbulent flame-vortex interaction in premixed cavity stabilized flames

The combustion dynamics of a mixture of ethylene and air in a cavity flameholder are examined using high-resolution numerical simulations. The simulations are performed using both laminar and turbulent inflow boundary conditions to characterize the effects freestream turbulence on the flame-vortex interactions within the combustor. Experimental test results are utilized to validate the simulation's ability to capture the relevant flow and flame behavior. The turbulence intensity is shown to influence the structure of the flow field by creating a more dominant primary vortex within the combustor and decreasing the shear layer reattachment length. The integral length scales and energy spectra are calculated throughout the domain and show that the flow is driven by smaller turbulent eddies within the ramp region of the combustor. The flame-vortex interaction is further examined and revealed to cause an instability forming in the combustor under both laminar and turbulent conditions. The instability is confirmed by examining the pressure fluctuations within the combustor and it is demonstrated that the time scale of the pressure fluctuations corresponds to the frequency of the vortex shedding from the ramp wall.

Mixing and Evolution of Laterally Injected Liquid Fuel in Mach 2 Air Channel with Cavity

ABSTRACT The present study deals with the flow structure and mixing characteristics of liquid fuel injected laterally into a supersonic crossflow. The geometry, with a cavity downstream of the injector, mimics a supersonic combustor with a cavity flame holder. A variety of injection schemes and cavity configurations were investigated by means of high-speed schlieren photography. A reflected shock tunnel was employed to generate a Mach 2 air flow and to simulate the high-temperature environment in the combustor. JP-4 liquid fuel was injected at different jet-to-crossflow momentum flux ratios upstream of and from the front wall of the cavity. Results showed that in direct injection into the cavity adjusting the injection angle has significant effect on the local fuel-air ratio. Upstream injection far from the leading edge of the cavity yielded more effective fuel mixing and shorter dissipation length (Ld) than closer injection, due to the development of a vortex on the lee side of the jet core. These results suggest that a combination of upstream injection far from the leading edge of the cavity and inclined direct injection into the cavity may be beneficial for effective mixing in a supersonic combustor cavity flame holder.

Design and testing of an additively manufactured flameholder for scramjet flows

A small-scale cavity flameholder for high-speed, premixed ethylene combustion has been designed, additively manufactured, and tested at Mach 5 enthalpy. The work has enabled a comprehensive study on the effects of flow compressibility and heat release on turbulent flame structures and stabilization mechanisms. The cavity is sized to produce a practical computational domain for Direct Numerical Simulations. Likewise, the flameholder is supported away from the flow path walls to minimize boundary layer growth upstream and simplify the numerical calculations. Optical access is provided for laser diagnostics. Challenges related to transforming a direct-connect combustor flow path into a semi-free jet flow path for immersed models were encountered, primarily flow blockage and heat removal. The final design can sustain an ethylene flame for repeated cycles lasting more than an hour. The flameholder was fabricated from Inconel 718 with internal cooling passages incorporated throughout. This paper presents a methodology for design and fabrication of additively manufactured scramjet combustor components and documents the first published conversion of a direct-connect, high-speed combustion wind tunnel to a semi-free jet configuration. While additive manufacturing enables novel designs with complex geometries, residual stresses and geometric distortion are notable concerns that must be considered in the design stages.

Flow and Combustion in a Supersonic Cavity Flameholder

Simulations of both inert mixing and sustained combustion are analyzed for a cavity flameholder based on corresponding experiments. An round ethylene jet fuels a cavity with length-to-depth ratio and a 45° inclined downstream wall. Oxidizer mixes into the cavity from the core flow. The simulations reproduce shock angles and wall pressures of the corresponding experiment. The effects that cavity combustion has on the core-flow gas dynamics and cavity entrainment are analyzed in detail. Relative to the inert case, heat release leads to a complex core flow, with upstream boundary layers transiently separating and highly unsteady shocks over the cavity. Their collective effect is the formation of a virtual throat, which decreases the core flow to above the cavity. Lagrangian trajectories assess the roles that turbulence, combustion, and three-dimensional side-wall boundary layers have on oxidizer entrainment into the cavity. Overall, sustained cavity combustion suppresses entrainment by a factor of about 2.

Ignition Characteristics of Scramjet Combustor with Laser Ablation and Laser-Induced Breakdown

The ignition characteristics of an ethylene-fueled scramjet combustor with a cavity flame holder were experimentally investigated in a Mach 2.92 inflow using laser ablation (LA) and laser-induced breakdown (LIB). For the LA method, a 532 nm Nd:YAG laser was focused directly on the cavity floor composed of stainless steel. Meanwhile, for the LIB method, the laser was focused at 2 mm above the cavity floor to ignite the combustor. Optical techniques, including optical emission spectroscopy, x-ray diffraction, and schlieren imaging, were utilized to analyze the characteristics of the two different laser-induced plasmas. High-speed chemiluminescence imaging was employed to visualize the ignition process by the LA and LIB methods. The LA plasma could successfully ignite the flame at more positions than the LIB plasma with the same laser energy, showing an improved ignition capability due to the higher energy utilization, longer lifetime, and more components with a longer existence time than that of the LIB plasma. The initial flame kernel ignited by the LIB plasma could propagate faster than the LA plasma once the flame is successfully ignited, possibly due to the direct heat convection in the airflow.

Appraisal Burning Characteristic and Analysis Effect of Cavity in Scramjet Combustion Chamber

The combustion chamber clearly plays an critical role in generating thrust force so the aircraft can move forward. A scramjet (supersonic combustion ramjet) is a variant of a ramjet airbreathing jet engine in which combustion takes place in supersonic airflow. Researchers are constantly working to improve the efficiency of ultrasonic combustion furnaces by various methods such as: optimize fuel injectors, optimize combustion chamber geometry design, create hole cavity. In this research, the characteristic of supersonic airflow were investigated, and a comparison between the standard chamber and advanced chamber was made to determine the effects of a circular hole (cavity) on pressure and velocity of the fuel mixture through the scramjet. Two dimensional Reynolds-Averaged Navier-Stokes governing(RANS) equations with k− turbulence model and finite rate/eddy dissipation chemistry model have been considered for modelling chemical reacting flows. From the comparison of numerical results, it is found that the development of recirculation regions and additional shock waves from the edge of cavity flame holder is increased and achieved stabilized combustion. From this research analysis, the performance of the scramjet engine with cavity is significantly improved as compared to the design without cavity

Numerical Study on Flow and Heat Transfer of Supercritical Hydrocarbon Fuel in Curved Cooling Channel

The fluid flow and heat transfer of hydrocarbon fuel play a significant role in developing regenerative cooling technology for advanced aeroengines. Numerical simulations have been conducted to investigate the flow and heat transfer characteristics of China RP-3 aviation kerosene with pyrolysis in a 3D, 90° bend, square cooling channel around the cavity flame-holder of a scramjet. A chemical kinetic model, composed of 18 species and 24 reactions, was adopted to simulate the fuel pyrolysis process. Results indicate that the secondary flow enhances the mixing of the fluid, thus, the transports of heat and components are improved between the near-wall region and main flow field in the curved channel. Compared with a straight cooling channel, fuel conversion and heat-absorbing capacity are higher, and the heat transfer is effectively enhanced in a curved cooling channel. In addition, with the increasing inlet mass flow rate and the decreasing radius of curvature, the velocity of the secondary flow increases. The heat and components are easily transferred from the near-wall region to the main flow. The non-uniformities of fuel temperature and conversion at the cross section decreases, which is helpful for improving the utilization of the level of fuel heat-absorbing capacity, and beneficial for enhancing the heat transfer.

NUMERICAL INVESTIGATION ON FLOW ACROSS STRUT-BASED FLAME HOLDER WITH CONVERGING GROOVES

The flow across a modified strut-based flame holder is numerically analyzed using the two-equation Menter SST k-ω turbulence model. The modified strut geometry contains alternative converging cavities and ramps on the top side. This design superimposes the advantages of cavity flame holders into strut-based flame holders. The effect of Mach number, the variation in geometrical parameters such as the length of the ramp, depth of the cavity and convergence angle of the cavities are extensively studied. The numerical simulations were conducted using a commercial software ANSYS FLUENT. The validation of the code is accomplished using an available experimental data for a simple strut configuration with fuel injection. The numerical predictions bear a close resemblance with the experimental data. As the Mach number of the flow is varied, the flow structures obtained is consistent with the flow physics. Different ramp lengths and cavity depths were analyzed to determine the behaviour of the flow. The shorter ramp length seems to carry a higher turbulence rate downstream of the flame-holder. Moreover, by increasing the depth of the cavity larger rate of production of turbulent kinetic energy is achieved in the cavity. The variation in convergence angle of the cavity also helps to impart velocity reduction downstream of the strut. The velocity reduction without shock losses is desirable for a supersonic combustor. The present study reveals that the same can be achieved by increasing the convergence angle of the cavity

Visualizations of direct fuel injection effects in a supersonic cavity flameholder

This paper is associated with a video winner of a 2020 American Physical Society's Division of Fluid Dynamics (DFD) Gallery of Fluid Motion Award for work presented at the DFD Gallery of Fluid Motion. The original video is available online at the Gallery of Fluid Motion, https://doi.org/10.1103/APS.DFD.2020.GFM.V0026.

Influence of the Multicavity Shape on the Solid Scramjet

In this paper, a modular solid scramjet combustor with multicavity was proposed. The influence of multicavity shape on the performance of solid scramjet was investigated by the direct-connected tests. The experiments simulated a flight Mach 5.5 at 25 km. The boron-based fuel-rich propellant was used. The microstructure of combustion products was analyzed by SEM. The experimental results show that the fuel-rich mixture produced by the gas generator would ignite rapidly in the solid scramjet combustor. The combustion process showed a typical characteristic of establishment-development-maintenance-attenuation. Compared to the flame-holding cavity, the other shapes of cavities, e.g., narrow and lobe, can improve mixing and combustion. In our experiment, the combustion efficiency increased from 0.41 to 0.48, and the total pressure recovery was 0.36. In summary, the proposed solid scramjet combustor can effectively solve the ignition delay problem of the fuel-rich mixture, and the narrow/lobe cavity shows the ability to improve the mixing and combustion of the fuel-rich mixture.

Numerical Investigation of an Axisymmetric Model Scramjet Assisted with Cavity of Different Aft Wall Angles

An axisymmetric model scramjet assisted with cavity flameholder is numerically investigated. Three-dimensional Reynolds-averaged Navier-Stokes simulation is carried out to reveal the fuel mixing and combustion characteristics. The simulation results show reasonable agreements with experimental data. The analysis indicates that the axisymmetric and rectangular scramjet has some similarities to the cavity shear layer in the nonreacting flow field. The configuration of the cavity shear layer changes hugely due to the significant chemical reaction and heat release in the reacting flow field. Typically, two more configurations with different cavity aft wall angles are compared with the experimental configuration to optimize the configuration of the cavity. When the cavity aft wall angle is small, the cavity shear layer bends to the cavity floor and more fuel enters into and stays in the cavity, which results in poor fuel mixing performance. With the increase of the aft wall angle, the fuel distributes more uniformly and the fuel mixing efficiency improves. In the reacting flow field, the volume of the cavity full of hot products and free radicals increases while the interaction between the cavity and main flow decreases with the increase of the aft wall angle. The improved combustion efficiency shows that larger cavity volume weighs more than reduced interaction between the cavity and main flow. The combustion is more violent in the case with a larger aft wall angle. Therefore, a proper increase of the aft wall angle is beneficial to the performance of cavity-assisted axisymmetric scramjet when designing the cavity flameholder.

Adaptive mesh based combustion simulations of direct fuel injection effects in a supersonic cavity flame-holder

We present high-fidelity reacting simulations of a supersonic cavity flame-holder configuration. The focus of this work is on flame stabilization brought about by varying the location of fuel injection in a cavity stabilized supersonic flow of air. Central to our approach is a compressible multi-species reacting flow solver that uses adaptive-mesh-refinement (AMR), enabling the resolution of flame, shock-waves, boundary-layers, and small-scale structures in the computational domain. Our analysis indicates that fuel injection closer to the ramp at the aft end of the cavity allows for greater mixing and lower peak temperatures compared to fuel injection upstream that is closer to the backward facing step of the cavity. This difference is mainly due to greater turbulent fluctuations generated from the shear-layer towards the cavity ramp, thereby enhancing the mixing of fuel and air. A low frequency oscillatory behaviour in heat-release and pressure was also observed for the upstream injection case while a much higher-frequency phenomena was observed in the near-ramp injection case. By identifying the important physical determinants of the combustion processes, this study illustrates a promising pathway to design and optimize direct fuel injection strategies in supersonic cavity flame-holders that can improve flame stability, combustion efficiency, and reduce emissions.

Hybrid Large Eddy Simulation/Reynolds-Averaged Navier–Stokes Analysis of a Premixed Ethylene-Fueled Dual-Mode Scramjet Combustor

Hydrocarbon fuels offer optimal high energy per volume for scramjet applications for sustained hypersonic flight but require additional residence time due to slower ignition delays (compared with hydrogen fuel). The injection of ethylene at the start of the isolator of a dual-mode scramjet combustor, operating in ramjet mode, allows sufficient mixing to achieve efficient premixed turbulent combustion. A cavity flameholder anchors the flame, supplying sufficient radicals to sustain a stable flame in the high-speed environment. This work investigates flame structure and stabilization limits of a new configuration with a scaled-down cavity embedded in the flow path of the combustor with a strut and insert. The cavity is reduced in size by one-third to enable eventual direct numerical simulations of the flame stabilization process. This work, however, focuses on modeling the full isolator/combustor geometry using a hybrid large eddy simulation/Reynolds-averaged Navier–Stokes simulation strategy. Particle image velocimetry, planar laser-induced fluorescence, coherent anti-Stokes Raman spectroscopy, and pressure measurements are compared with numerical predictions to analyze and characterize the conditions within the combustor, including flame structure, flow velocities, species composition, and wall pressure. An adjustable air throttle in the extender is able to control the placement of the shock train in the isolator and maintain a stable flame at various equivalence ratios. The simulations show reasonably good agreement with the experimental scalar and velocity data and predict a flame angle consistent with premixed turbulent flame-speed correlations.

Effect of fuel injection distance and cavity depth on the mixing and combustion characteristics of a scramjet combustor with a rear-wall-expansion cavity

The mixing and combustion characteristics of a model scramjet combustor with a rear-wall-expansion cavity under a Mach 2.0 inflow are experimentally and numerically investigated. Two different cavity depths and two fuel injection distances are compared to show the effect of these parameters. High-speed imaging system is used to capture the oscillation of flamebase and the flame oscillation of cavity shear layer in the transversal direction. Numerical simulation is carried out to reveal the jet mixing process around the cavity. The experimental results show that the combustor with longer fuel injection distance and smaller cavity depth has weaker flame and lower wall pressure due to the significant oscillation. The oscillation of flamebase is found to be stronger in the shallower cavity due to its smaller volume. Cavity with short fuel injection distance and large depth could suppress the oscillation, and the combustor can achieve a robust and strong combustion. The simulation results show that under the shorter injection distance, the center of cavity shear layer is rolled up to the main stream; thus the shear layer can interact with the fuel jets more intensively, which improves the mixing efficiency and promotes strong combustion. Compared to the cavity with larger depth, the shear layer in the smaller cavity has a larger thickness and displays more significant shear layer oscillation in the transverse direction. The oscillation of the cavity shear layer can promote the mixing efficiency but is detrimental to combustion. Therefore, a proper cavity depth should be carefully considered during the design of a cavity flameholder.

Lean fuel detection with nanosecond-gated laser-induced breakdown spectroscopy

Nanosecond-gated laser-induced breakdown spectroscopy (n-LIBS) has been used to quantify fuel mole fraction (ΧC2H4) in the cavity flameholder of a model high-speed combustor. The measurement locations selected, in the vicinity of the cavity shear layer, have low fuel concentrations. Previous n-LIBS measurements showed unexpectedly high values at these locations with expected low average fuel mole fraction (ΧC2H4) and intermittent fuel presence; thus, an effort was undertaken to understand potential sources of error with n-LIBS at low ΧC2H4. An improved direct spectrum matching (DSM) calibration matrix was thus constructed, focusing on low ΧC2H4 for both reacting and non-reacting conditions. Comparisons were made between n-LIBS measurements using two different laser systems for plasma generation, and the insensitivity of the signal to the specific laser was demonstrated for the first time. Uncertainty of the n-LIBS measurement technique at two different gas densities was analyzed through the processing of spectra acquired at known values of XC2H4. Measurements in the windtunnel, around the shear layer of the cavity flameholder, were conducted with a Mach-2 or Mach-3 freestream, and comparisons were made between n-LIBS results—using two DSM calibration matrices with different increments in XC2H4—and numerical results employing dynamic hybrid Reynolds-averaged Navier-Stokes and large-eddy simulation (DHRL). The high-resolution DSM matrix produced slightly lower XC2H4 than those from the coarse matrix, and comparisons with the simulation showed good agreement overall. For Mach-3 conditions, comparisons were made for both a non-reacting and a reacting cavity flameholder. Here, results showed a systematic difference between measured and simulated values of the mean XC2H4 within the shear layer but good (even excellent) agreement within the upper portion of the recirculation zone. For reacting condition, four reduced chemical kinetic models were implemented with the DHRL technique and compared to n-LIBS results. Overall, the four kinetic models produced a substantial range of results (e.g., temperatures and reaction progress), showing the importance of a validated diagnostic technique like n-LIBS for model testing.