Cavity-based Scramjet Combustor Research Articles

Investigation of combustion mode conversion driven by fuel flow variation in a cavity-based scramjet

This work aimed to investigate combustion mode conversions by rapid variation of the fuel flow rate in a cavity-based scramjet combustor. The experiments were carried out on a direct-connected facility with an inflow condition of Mach number 2.52, a total pressure of 1.35 MPa, and a total temperature of 1650 K. The fuel injector consisted of two injection ports: fuel was continuously injected from one port while the other controlled the fuel flow for mode conversions by switching it on or off. Simultaneous schlieren and CH* imaging techniques were used to characterize the dynamics of combustion mode conversions. It was recognized that the combustion modes characterized by different flow field structures and heat release distributions can be classified into three types: the shear-layer mode, the transition mode, and the jet-wake mode. During the combustion mode conversion, the mixing region of the transverse jet and air became thicker with the increase in fuel flow rate, and the gradient of the flow field density and the flame area increased, making the flame more likely to propagate upstream. The combustion suppression induced by rapid fuel addition was observed at low equivalence ratios. It was speculated that the weak heat supply was insufficient to provide adequate heat for the rapid ignition of the added fuel. Furthermore, it was found that the flame-flow matching process with frequent flame propagation upstream occurred during the combustion mode conversion. This process was attributed to the mismatch between the increasing heat release and the original flow field structure.

Effect of variation of temperature on the internal flow characteristics of a strut and cavity-based SCRAMJET combustor

Effect of variation of temperature on the internal flow characteristics of a strut and cavity-based SCRAMJET combustor

Implication of injection angle and jet-to-cross flow ratio on the combustion characteristics of a scramjet combustor

The present work deals with the implication of injection angle and jet-to-cross flow ratio on the combustor performance of a cavity-based scramjet combustor. Unsteady, two-dimensional compressible flow through the scramjet combustor is considered to comprehend the effects of Mach number (Ma), jet-to-cross flow ratio ([Formula: see text]) and injection angle (α) on the combustion characteristics. Results are presented for the following range of parameters: 2.0 ≤ Ma ≤ 2.7; 15° ≤ α ≤ 60° and 0.5 ≤ [Formula: see text] ≤ 1.2. Intense recirculation zone is observed at θ = 45° irrespective of inlet Mach number which are quintessential for proper mixing. Further, the size of the recirculation zone is the largest when jet-to-cross flow ratio is equal to 0.8 acting as a precursor for enhanced mixing performance. Moreover, static pressure reaches an optimum value at injection angle of 45° beyond which further increase would result in thermal choking. Our study reveals that mixing is improved with an increase in Mach number (Ma). It is observed that the size of the recirculation zone is strongly dependent on the jet-to-cross flow pressure ratio along with the injection angle. To put it in a nutshell, the injection angle of 45° along with a jet-to-cross flow pressure ratio of 0.8 is conducive the optimal performance in scramjet combustor.

Experimental Study on Combustion Modes and Oscillations in a Cavity-Based Scramjet Combustor

This study conducted experiments to investigate flame behaviors in a cavity-based scramjet combustor operating under Mach 2.92 supersonic inflow. Pressure measurements and flame chemiluminescence observations were combined to study the initial flame formation and combustion mode transitions. In addition, flame luminosity standard deviation and fast Fourier transform power spectral density (PSD) were post-processed to assess the impact of equivalence ratio and aft wall height on combustion modes as well as oscillations. It is indicated that increasing the equivalence ratio leads to a transition from lifted shear-layer mode to ramjet mode. Elevating the cavity aft wall height improves the combustor’s self-ignition capability and promotes the transition from lifted shear-layer mode to ramjet mode as long as flame stabilization is achieved. It is found that increasing the equivalence ratio and aft wall height also results in more intense combustion oscillations; besides, raising the cavity aft wall height could accentuate the predominant frequency of low-frequency oscillations in the combustor. Finally, it is demonstrated that the application of spark plasma effectively suppresses these low-frequency combustion oscillations, which is a promising active control method in the scramjet engine.

Experimental Study on the Ignition Characteristics of Scramjet Combustor with Tandem Cavities Using Micro-Pulse Detonation Engine

This experimental investigation focused on the ignition and combustion characteristics of a tandem cavity-based scramjet combustor with side-by-side identical cavities. This study utilized the Pusan National University-direct connect scramjet combustor (PNU-DCSC), which was capable of simulating flight conditions at Mach number 4.0–5.0 and altitudes of 20–25 km using the vitiated air heater (VAH). The combustion tests were conducted under off-design point conditions corresponding to low inlet enthalpy. It is a condition in which self-ignition does not occur, and a micro pulse detonation engine (μPDE) ignitor is used. The results revealed that as the injection pressure of the gaseous hydrogen fuel (GH2) and the corresponding equivalence ratio increased, the combustion mode transitioned from the cavity-shear layer flame to the jet-wake flame. Furthermore, the measured wall static pressure profiles along the isolator and scramjet combustor indicated that the region of elevated pressure distribution caused by the shock train expanded upstream with higher equivalence ratios. When ignited from the secondary cavity, the combustion area did not extend to the primary cavity at lower equivalence ratios, while it expanded upstream faster with higher equivalence ratios. Therefore, the combustion characteristics of the tandem cavity were found to vary based on the overall equivalence ratio of the main fuel (GH2) and ignition position.

Numerical Simulation of Combustion in Kerosene Fueled Ramp Cavity Based Scramjet Combustor

The flow field of a ramp cavity based scramjet combustor with kerosene fuel is explored numerically using commercial CFD software. Three-dimensional Navier-Stokes equations are solved alongwith K - e turbulence model and fast rate chemical kinetics. Liquid kerosene is considered as disperse phase fluid and is modeled through Lagrangian tracking method. Simulation captures all essential features of the flow field. Good agreement between computational and experimental values forms the basis of further analysis. The flow phenomena in the combustor are presented through the distribution of important thermochemical parameters at different cross sections. Normal shock is seen to occur in the combustor and significant upstream interaction was observed due to heat release. The computed combustion efficiency is near unity as the fuel equivalence ratio is small.

Dynamic response characteristics of flame during condition transition in a cavity-based scramjet combustor

Experimental study is performed to investigate the dynamic response characteristics of the flame when global equivalence ratio increases suddenly during the condition transition in a cavity-based scramjet combustor. High-enthalpy supersonic flow enters the combustor on conditions that Mach number, stagnation temperature, and total pressure are 2.52, 1486 K, and 1.6 MPa, respectively. In this paper, more attention is paid to the characteristics of flame evolution after sudden increase of fuel injection pressure and combustion characteristic when the injection pressure gets stabilized again. Three flame modes after condition transition have been observed and analyzed according to the transient chemiluminescence image captured by high-speed photography. There are two kinds of stable flame modes after transition mission at a low injection pressure difference, which are mainly reflected in the distribution of flame and intensity of visible light. Whereas, once the injection pressure difference exceeds a critical value, intense periodic flame flashback phenomenon begins to appear in the combustor. It can clearly capture the back and forth motion of the flame in the combustor, along with the local blowout and reignition phenomenon. Furthermore, the dominant frequency of flame flashback decreases and contrast of combustion intensity becomes more obvious as the injection pressure difference increases.

Rocket-augmented flame stabilization and combustion in a cavity-based scramjet

The scramjet engine has become an important propulsion system in hypersonic vehicles for its high specific impulse, but it also has obvious shortcomings, such as narrow reliable operating and thrust adjustment range, and consequently, insufficient maneuverability. To solve these problems, a rocket is introduced into the scramjet to improve the thrust performance while stabilizing flame and enhancing combustion, referring to the operating principle of the RBCC engine. The function mechanism of the high mass flow rate rocket jet on the flame stability and combustion characteristics in a cavity-based scramjet combustor is investigated through the ground direct-connect test. The experimental results show that: 1) Effective secondary fuel combustion enhancement and thrust enhancement can be obtained through the rocket augmentation. The embedded rocket leads to a 69% increase in the peak combustion pressure. 2) In the rocket-augmented combustion mode, the core combustion zone concentrates in the central region of the flow passage and simultaneously expands upstream, resulting from the increased mixing efficiency of kerosene/air and a more stable flame, which effectively augments the combustion and the engine thrust. 3) As the kerosene injection positions move upstream to the exit of the rocket nozzle, the secondary combustion efficiency can be further improved by inhibiting the negative effects of oxygen consumption by the fuel-rich rocket jet. The length of the high combustion pressure region is increased by 150%, and the scramjet performance has been enhanced significantly. 4) During the process of the kerosene injection position transition, the flame in the combustor keeps sustained under the function of the embedded rocket jet, and the rocket-augmented combustion exhibits favorable robustness.

Numerical study of periodic flame flashback in a cavity-based scramjet combustor

The periodic flame flashback phenomenon in an ethylene-fueled cavity-based scramjet combustor was numerically investigated by a three-dimensional unsteady Reynolds-averaged Navier–Stokes solver with two-step kinetics. The air inflow stagnation temperature is 1225 K, and its Mach number is 2.6. Spectral analyses revealed the combustion oscillations with flame flashbacks maintained in the separated scramjet mode with the establishment/vanishment of flow separation near the fuel injector, differing from previous studies of flame flashbacks connected to the ramjet/scramjet mode transitions. A mechanism with four evolution stages was proposed to elucidate the flow-flame interaction. In stage I, a rapid flame flashback upstream and shock-train extension were caused by the high-temperature induced auto-ignition tendency of well-mixed unburned gas in the near-sidewall low-speed region. In stage II, the combustion-induced back pressure and shock train gradually achieved an aerodynamic balance. The combustion flow barely changed in stage III. Meanwhile, a simplified model suggested that the gradual temperature rises occurring upstream of the cavity and away from the sidewall were caused by spanwise heat conduction. The higher temperatures would cause upstream flame propagation with enhanced heat release due to auto-ignition. However, the enhanced heat release occurred mostly in the subsonic flow, resulting in pressure decreases according to one-dimensional flow equations. A smaller near-sidewall separation was produced by the lower back-pressures, which prompted the rapid flame recession downstream in stage IV. Moreover, a simplified flame-spreading model was proposed to illuminate the flame propagation nature. The comparison of flame speeds with theoretical estimations indicated that the current flame was in the regime of turbulent flame propagation, rather than the C–J detonation or deflagration speculated in previous studies.

Numerical investigation on the performance and combustion characteristics of a cavity based scramjet combustor with novel strut injectors

A three-dimensional numerical investigation has been carried out on the performance and combustion characteristics of a scramjet combustor with and without cavity for various strut injector configurations. The geometrical shape of the injectors considered for the present study are circle, square and rectangle. The numerical model of scramjet combustor has been discretized hexahedrally through finite volume approach using ANSYS ICEMCFD 18.0 and the necessary boundary conditions are specified in ANSYS Fluent 2021 CFD code. The three-dimensional fluid flow variations through a scramjet engine with strut-based injectors and cavity have been analyzed by solving the corresponding governing equations namely continuity, momentum (Navier-Stokes) equation and energy transport equation for the non-reacting flows. In addition to the above equations species transport equation is also solved for the reacting flows. The turbulence induced in the fluid is modelled using standard K-ε turbulence model. The effect of injector shapes, mixing length, combustion efficiency and total pressure loss have been numerically simulated and plotted with and without cavity of a scramjet combustor. The results from the study predicted that circular injector with cavity showed better mixing characteristics and combustion characteristics compared to the rectangle and square model. The total pressure loss of circular injector with cavity is 5.4% higher than the other models investigated, still it is preferred because of high combustion efficiency attained with shorter span of combustor length.

Impact of passive fuel injections techniques in the flow field of the scramjet combustor

• The introduction of passive injection techniques in a parallel cavities scramjet combustor influences the internal flow filed charateristics. • The effect of fuel injection location on the internal flow field charaterictics is also investigated. • Out of all the configuration , the scrmajet combustor showef optimum performance when the fuel is injected through the wall only. The development of scramjet technology has paved the path of possibilities for researchers to develop light-weighted space missions in the years to come. Many countries around the globe have already tested successful scramjet engines. The researchers are making continuous efforts to improve the combustor design by improvising turbulence inserts and flame holders in the combustor. One such improvisation is the introduction of strut into the parallel-cavities combustor. The research paper uses computational fluid dynamics to investigate the effect of internal flow field variables on a strut and cavity-based scramjet combustor. The current work presents a comparative study by analyzing two different scramjet combustor configurations namely case A and Case B to understand the effect of presence of strut in the cavity combustor. The velocity and static pressure profiles are analyzed at different streamwise locations. At x = 70 mm, the velocity values are reduced to lower magnitude for case B as compared to case A and the static pressure values towards downstream detoriates with reduction in intensity of shock waves. It is seen that the magnitude of velocity values and pressure values increases with increase is Mach number. The high-pressure values are observed for M-2.75. A performance assessment of the combustor is carried out for both the configurations, maximum combustion and mixing performance is obtained for M-2.52 for case B as compared to Case A. The paper further analyses three different fuel injection locations namely fuel injection strut only, fuel injection through the wall only and fuel injection through both strut+wall recognized as the case I, II, and III respectively. From various flow-field parameters analysis, it is seen that the production of eddies and vortices in the combustor region indicates proper mixing which is observed for case I. On studying the mass fraction concentration of H 2 0 species for all the cases it is observed that the H 2 0 species is increasing in the lateral direction which indicates that the combustion efficiency is incrementing in the same direction. The maximum combustion and mixing efficiency is observed for case I than the other two cases.

Effect of Fuel-Injection Distance and Cavity Rear-Wall Height on the Flameholding Characteristics in a Mach 2.52 Supersonic Flow

The ethylene-fueled flameholding characteristics of a cavity-based scramjet combustor are experimentally and numerically investigated. The test facility used the air heater, which heats air from room temperature to total temperature 1477 K. A nozzle is installed behind the heater outlet to increase the air speed to Mach 2.52. Two cavity geometries with different rear-wall heights of 8 mm and 10 mm and two injection distances upstream of the cavities of 10 mm and 40 mm are compared to show the effect of these parameters. The CH* spontaneous emission images obtained by dual-camera synchronous shooting and the wall-pressure distribution obtained by a pressure-scan system are used to capture the flame dynamics. The global equivalence ratio range for different combination schemes is controlled from 0.14 to 0.27 in this paper. The results show that the conventional cavity (the rear-wall height is 10 mm) and the shorter injection distance can effectively decrease the lean blowoff limit of the combustor, while the rear-wall-expansion cavity (the rear-wall height is 8 mm) and the longer injection distance can effectively increase the rich blowoff limit. Compared with the injection distance, the rear-wall height of the cavity has little effect on the oscillation distribution of the shear layer-stabilized flame. However, the fuel-injection distance and cavity rear-wall height both have great influence on the spatial distribution of the flame.

Comparison of combustion characteristics of four strut injectors with that of three strut injectors by numerically investigating doubly-dual cavity-based scramjet combustor

A numerical investigation has been conducted using 2D RANS (Reynolds Averaged Numerical Solution) equation and SST k- ω turbulence model in a cavity-based scramjet combustor with a doubly-dual cavity and hydrogen-fuelled, to study the performance and combustion characteristics of three struts and four struts injection. Fluid flow characteristics of four-strut injectors were compared with that of three-strut injectors. The reason for better performance output from four strut injection is understood and discussed briefly. Compared with the three-strut injector configuration, the formation of recirculation regions and vortices are enhanced in the four-strut injector system affecting shock wave interactions (shock-shear, shock-shock layer). Due to increased levels of shock layer interaction, and lower presence of oxygen, there was a better mixing of the fuel which resulted in improvement in combustion when four strut injectors were introduced. The intensity of combustion also increased which is evident from static temperature plots.

Enhancement of blowout limit in a Mach 2.92 cavity-based scramjet combustor by a gliding arc discharge

Plasma-assisted combustion (PAC) has shown excellent performance in the ignition and combustion enhancement of scramjet combustors. A multi-channel gliding arc (MCGA) plasma placed on the sidewall surface of a scramjet combustor was employed to enhance combustion near the flame blowout limit (BL) in a C2H4-fueled and cavity-based model scramjet combustor. Wall static pressure measurements, simultaneous 10 kHz CH* chemiluminescence imaging from the top and side views, optical emission spectroscopy, and discharge waveform measurements were used to elucidate the combustion physics throughout the PAC events. For the current cavity configuration and fueling condition, a general estimate of the flame BL in a rich-fuel environment was provided, in which a fuel flow rate (FFR) was 7.5 g/s. Once the FFR was exceeded, the flame was unstable and blown out, whereas the MCGA plasma demonstrated an excellent flame stabilization ability. The flame BL of the scramjet combustor in the presence of the MCGA plasma has an increase of ∼29%. Two PAC processes, including enhancement mode and re-ignition mode, were distinguished to play a vital role in extending the flame BL. The two PAC processes showed that the spark-type discharge generated by the MCGA could ignite the local fuel-rich mixture and increase the flame BL. The local ignition is dominated by the continuous accumulation of the massive flame kernels with the local fuel-rich mixture, as well as more heat and species exchanges.

Experimental investigation on the flame stabilization modes of liquid kerosene in a cavity-based scramjet combustor

The flame stabilization modes of liquid kerosene is investigated in a model scramjet combustor with flight condition of Ma 6. The effects of equivalence ratio and cavity configuration schemes on the flame stabilization modes are studied. Flame propagations and the evolution of the flow field from the ignition to the establishment of stable flame in the supersonic flow are acquired through high-speed photography and schlieren photography. Only cavity-recirculation-region stabilized combustion can be obtained in the single-cavity scramjet combustor. The intensity of combustion is weak and is termed as local ignition with low combustion efficiency. In the tandem-cavity scramjet combustor, the flame stabilization mode is largely determined by the total equivalence ratio. The flame could spread into the mainstream and even propagate against the supersonic flow as the equivalence ratio increases. The positive feedback mechanism between combustion and precombustion shock train is responsible for the evolutions of the flame stabilization modes, where the interactions between shock waves and turbulent boundary layer play a key role in the flame propagation process. The large-scale eddies in the shear layer of the upstream cavity shed at the cavity aft wall and could enhance the turbulent levels of the incoming flow of the downstream cavity. Consequently, an intense kerosene flame can be ignited in the tandem-cavity combustor. The flame could not spread to the mainstream in the single-cavity combustor due to the lower turbulence level weakening the interaction of combustion products in the recirculation zone with unburned reactants in the supersonic flow.

Numerical investigations on the implication of hydrogen fuel jet injection pressure on the performance characteristics in a multi-strut in a double-dual cavity-based scramjet combustor

To analyze the influence of flow physics and operating parameters (i.e., injection pressure (2, 4, 6, and 8 bars), a simulation is performed on a scramjet combustor (modified) using the shear stress transport (SST) k- ω turbulence model and Reynolds Averaged Navier Stokes (RANS) equations. Wall pressures, the pattern of shock-wave, and static temperature of the combustor are three major parameters that have been analyzed in this study. An acceptable range of results indicates that the simulation approach adopted can be used for additional studies. According to the results of this study, the shock-wave position is destabilized when the hydrogen jet pressure is enhanced by varying the flame characteristics. Moreover, it was also observed that the shock wave travels upstream due to the pressure exerted by the main flow. A normal shock wave moves ahead of the strut as soon as the pressure in the hydrogen jet increases to 4 bar. According to this study, it is beneficial to have lower hydrogen jet pressures in supersonic flows for the geometry considered. The penetration depth of fuel in the axial direction increases, which deteriorates the combustion efficiency with an increase in the total pressure losses.

Flame characteristics of a cavity-based scramjet combustor using OH-PLIF and feature extraction

It is vital to analyze the flame characteristics and identify the flame states efficiently in scramjet. Flame combustion and oscillation behaviors in a hydrogen-fueled cavity-stabilized scramjet combustor have been investigated in this study. A 500 Hz high-speed PLIF (planar laser-induced fluorescence) technique with a 20-cm-wide view field is introduced to characterize the combustion flow. The spatial distribution of the flame structure under different fuel injection conditions is studied. The results indicate that the position of the flame peak shifted downstream of the cavity when increasing the injection pressure to a high level. The dynamic characteristics of flames under different flow conditions are analyzed, and the correlation between flame features and hydrogen injection conditions is obtained. These flame features include the vertical range, the flow direction position of the peak flame, the flow direction position of the centroid, the vertical position of the centroid, the flame area, and the flame circumference. For the present cases, the flame of 4.0 Mpa is more unstable than that of 1.5 Mpa under any flow conditions. Moreover, an experiment based on feature extraction results shows that the KNN (K-nearest neighbor) classifier could achieve high accuracy for flame state recognition in this scramjet combustor.

Combustion Characteristics in Scramjet Combustor Operating at Different Inflow Stagnation Pressures

In this study, combustion processes in a cavity-based scramjet combustor are experimentally studied based on the inflow conditions of different stagnation pressures (, 1.3, and 1.6 MPa), which correspond to a Mach 6 flight at trajectories of different dynamic pressures of 0.25, 0.34, 0.43 atm, respectively. Three combustion modes are identified according to flame distribution, flowfield structure, and wall pressure. Cases at the same equivalence ratio and combustion mode with different inflow stagnation pressures are selected to compare flame characteristics. For each inflow condition, flame structure, combustion intensity, and coherence between upstream and downstream flames are analyzed based on CH* chemiluminescence and schlieren imaging measurements. Combustion instability is examined according to statistical results, frequency domain and spatial distribution. It is found that the linearly increased inflow stagnation pressure causes irregularity variance in the combustion oscillation spectra, indicating the quasi-linearly enhanced combustion induces complicated aerodynamic and thermodynamic variances. Chaos of the combustion oscillation is quantitatively evaluated by a classical nonlinear analysis. The dynamic mode decomposition analysis presents that the high-frequency oscillation induced by the self-sustaining shear-layer instability exist mainly in the combustor upper region while the low-frequency oscillation that could be caused by the injector–flame feedback is predominant in the cavity region. In addition, thermo-acoustic couplings, which are in connection with these two kinds of oscillations, have been analyzed. It is further revealed that the variance of the shock waves caused by the backpressure destroys the existing thermo-acoustic feedback related to the high-frequency oscillation, which is the main reason for the chaotic phenomena shown in the transitional state between the scramjet mode and the ramjet mode.

Numerical Investigation on Flame Stabilization of Cavity-Based Scramjet Combustor Using Compressible Modified FGM Model

The technology of flame stabilization on cavity-based scramjet combustor has great significance in the field of future spacecraft. In this paper, a compressible modified FGM model was established based on the idea of the flamelet model, which was adopted to simulate the unsteady combustion process of the hydrogen transverse jet in the upper cavity of the scramjet. The results show that the compressible modified FGM model can accurately reflect the flow field and the propagation process of the flame in the supersonic cavity, and can capture the fine shock structure in the flow field. The coupling effect of shock waves and shear layer cause the shear layer to quickly destabilize, resulting in the turbulence effect, which promotes the mixing of air and fuel. The boundary layer separation at the upper wall of the combustion chamber will reduce the stability of the shear layer.

Numerical investigation on implication of innovative hydrogen strut in comparison with multi strut injector on performance and combustion characteristics in a scramjet combustor

Detailed numerical analysis on a cavity-based scramjet combustor is carried out by introducing an innovatively shaped strut and multi strut with backward-facing step to generate intense vorticity, which helps in efficient mixing of fuel and oxidizer. In this study, the flow dynamics with finite volume approach on commercial software Ansys-Fluent 20.0 to solve the compressible two-dimensional fluid flow with RANS equation by considering the density-based solver with SST k- ω turbulent model. The species transport model with volumetric reaction and finite rate/eddy dissipation turbulence chemistry interaction is adopted to study the combustion phenomena. Numerically calculated results are validated with its corresponding experimental results by comparing pressure distribution along the length of the combustor, distribution of H2 mole fraction for different axial locations of combustors, and it is found that the interaction of the shear shock layer enhances the mixing rate by intensifying turbulence. It is found that the multi strut improves the mixing and combustion efficiency compared with that of the single strut owing to the formation of a significant separation layer, resulting in multiple shocks, vortices, and a larger recirculation zone.