Ethylene-fueled Scramjet Research Articles

Effect of injection distance on supersonic combustion under different combustion modes

The effect of injection distance on supersonic combustion under different modes in an ethylene-fueled scramjet combustor at Mach 2.52 is experimentally investigated. The study is based on three injectors at injection distances of 42, 21, and 7 mm. As the injection distance increases, a higher equivalence ratio is required to complete the transition from the Scram to Dual mode. The increase in injection distance will cause the reaction zone to move downstream in the Scram mode. However, the opposite is true in the Dual and Ram modes. In addition, the increase in injection distance will intensify the reaction zone oscillation under all modes. Moreover, in the Scram mode, shortening the injection distance will increase the heat release and the oscillation of heat release intensity. In the Dual mode, although shortening the injection distance cannot enhance the heat release, it will decrease the oscillation of heat release intensity. In the Ram mode, increasing the injection distance will enhance the heat release and will hardly increase the oscillation of heat release intensity. Considering the combustion heat release and oscillation, the injection distance should be shortened in the Scram and Dual modes and increased in the Ram mode.

On the scale effects of flame stabilization under different combustion modes in an ethylene-fueled scramjet combustor

The combustion characteristics in two geometrically similar ethylene-fueled scramjet combustors with mass flow rates of 0.69 and 1.41 kg s-1 are experimentally investigated to explore the scale effects of flame stabilization under different combustion modes with a Mach number 2.52 inflow. The static pressure distribution, schlieren images, and CH* chemiluminescence images are used to illustrate the combustion process. As the combustion heat release increases, the combustion in scramjet combustors of different scales generally undergoes the mode transition process of "Scram-Dual-Ram." The transition between the Scram and Dual modes is intermittent, with the "pressure step" phenomenon and significant combustion oscillations in the initial state of the Dual mode. The scale effects of flame stabilization include two aspects. On the one hand, the transition between the Scram and Dual modes occurs at different equivalence ratios-0.15 for the small-scale combustor and 0.19 for the large-scale combustor. On the other hand, the scale effects of flame stabilization are inconsistent under different combustion modes. The main reason is that the boundary layer that does not change proportionally with the combustor scale has different effects on the combustion in the three modes. In the Scram and Ram modes, the combustion scale has little impact on the combustion heat release and only affects the length of the pre-combustion shock train in Ram mode. However, in the Dual mode, slight differences in the relative thickness of the boundary layer are amplified. The increase in the combustion scale will lead to a significant decrease in the combustion heat release. In all combustion modes, the relatively thick boundary layer of the small-scale combustor makes it easier for the flame to propagate upstream, leading to stronger flame oscillations.

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.

Application of static integrated skeletal reduction and tabulation of dynamic adaptive chemistry in the combustion simulation of ethylene-fueled scramjet combustor.

The high-fidelity reduced mechanism is one of the key elements in the combustion simulation of scramjet combustors to reveal their combustion and flow phenomena. In the present work, the hierarchically constructed NUIGMech1.2 (2857 species and 11 814 reactions) is applied to the combustion simulation of an ethylene-fueled scramjet combustor using the method of static integrated skeletal reduction and tabulation of dynamic adaptive chemistry (TDAC). The integrated skeletal reduction strategy successively consists of species elimination using the revised directed relation graph with error propagation method of fixed species scheme and improved sensitivity analysis method, and reactions elimination based on computational singular perturbation importance index. A preferred ethylene skeletal mechanism (26 species and 117 reactions) is obtained through the integrated skeletal reduction strategy under target working conditions of temperature range of 900-1800 K, pressure range of 1-4 atm, and equivalence ratio range of 0.25-5.0. The compact skeletal mechanism is comprehensively validated against the experimental results of ignition delay times, laminar flame speeds, and key species concentration profiles. Meanwhile, it shows consistent results with the detailed mechanism on the adiabatic flame temperature profiles and "S"-curves. When applying this skeletal mechanism to combustion simulations of ethylene-fueled scramjet combustor with double parallel cavities, the path flux analysis method and in situ adaptive tabulation algorithm of TDAC is further utilized to speed up the chemical reaction solution process at run-time. Under the scramjet and ramjet modes, the corresponding simulation results in terms of flame luminosity images, schlieren images, and static pressure distributions, coincide well with those of experimental measurements. The combustion and flow characteristics of the two modes are investigated and analyzed comparatively based on above results and combustion performance parameters. Present work contributes to the application of fuel kinetic mechanisms in scramjet combustor combustion simulation.

ROLE OF DUAL CAVITY ON THE COMBUSTION PERFORMANCE OF A SPLITTER PLATE–ASSISTED SUPERSONIC COMBUSTOR

An unsteady-state simulation using Reynolds-averaged Navier-Stokes was conducted to explore the performance of an ethylene-fueled scramjet combustor employing a dual cavity floor injection strategy and a splitter plate. The validity of this approach was confirmed by comparing it to experimental data, which showed a reasonable match between the observed experimental results and the simulation. A thorough analysis of the injection system's performance is conducted using variables such as the scramjet combustor's total pressure loss, wall pressures, and combustion efficiency. The findings revealed that the leading edge cavity shock waves' interaction from both the walls created a highpressure area close to the cavities, which forces the fuel jets in the direction of the combustor walls. Fuel distribution into the combustor is thereby prompted by the intense interactions that arise between the cavity's aft walls and fuel jets. Moreover, the recirculation zones within the cavity are significantly expanded, improving both the performance of combustion and mixing within the cavity. The combustion efficiency in the current study has shown a notable increase of 13% compared to our previously investigated configuration involving a splitter plate combined with a single cavity combustor.

Effect of thermal boundary condition and turbulent models on the combustion simulation of ethylene-fueled scramjet combustor

Wall thermal boundary conditions and turbulent models can affect flow and combustion simulations but are seldom considered in the turbulent modeling of supersonic combustors. This work investigated the effect of thermal boundary conditions and four turbulent models on turbulent combustion in a cavity-stabilized scramjet combustor. Results showed that the thermal boundary condition had a noticeable influence on the temperature fields. Changing the thermal boundary condition from zero gradient to a fixed lower temperature considerably reduced the maximum temperature but did not affect the temperature distribution. The fixed temperature boundary condition generated a slightly larger reaction heat release near the upper region of the cavity. However, the mass fraction of carbon dioxide was low for a fixed low temperature. The pressure increased near the rear of the cavity but decreased elsewhere at a fixed temperature. Reynolds-averaged models (k-epsilon, k-omega, and realizable k-epsilon) tend to over-predict the temperature and turbulent kinetic energy but under-predict the mass fraction of carbon dioxide. The detached Eddy simulation also under-predicts carbon dioxide but predicts a more accurate temperature.

Visualization of supersonic combustion using high-speed camera/dual-component planar laser-induced fluorescence simultaneous diagnostic technique

High-spatiotemporal-resolution diagnostics are important for capturing fine physicochemical structures in supersonic combustion. In this study, a high-speed camera (HSC)/dual-component planar laser-induced fluorescence (PLIF) simultaneous diagnostic technique is developed and applied to an ethylene-fueled scramjet to determine the flame structures and heat release characteristics of cavity shear-layer stabilized combustion. CH2O-PLIF and OH-PLIF simultaneous imaging techniques are used to capture the transient structures in the preheat and product zones of the flame, while the heat release zone (HRZ) is identified by the product of these two signals. Synchronized HSC techniques are used to capture the full-band flame fluorescence. The flame base was found to stabilize in the cavity shear layer at a certain distance downstream of the leading edge. The HRZ was initiated near the flame base and gradually spread into the mainstream. Upstream of the HRZ, the partially premixed fuel jet undergoes preheating oxidation reactions, forming the preheat zone adjacent to the HRZ. Downstream of the HRZ, hot products were produced, forming the product zone distributed in both the cavity and mainstream. The central role of the cavity is to provide a favorable environment for stabilizing the flame base rather than participating in heat release reactions. Schlieren images and wall pressure distributions are also documented, constituting a dataset that could be used to validate computational models.

Gliding Arc Plasma-Controlled Behaviors of Jet-Wake Stabilized Combustion in a Scramjet Combustor

A multichannel gliding arc (MCGA) plasma is used to control the jet-wake stabilized combustion in a Mach 2.92 cavity-based and ethylene-fueled scramjet combustor. Optical diagnostic methods (including high-speed photography, schlieren, and planar laser-induced fluorescence with acetone tracer) combined with plasma kinetic simulations and Reynolds-averaged Navier–Stokes/large-eddy simulations were employed to investigate the combustion behaviors. The results show that the flame is mainly located near the cavity leading edge and operated at the jet-wake stabilized mode. When the MCGA plasma is added at the upstream wall of the cavity leading edge, the vigorous region of the flame spreads upstream 6.5 times longer than the original one without the plasma. Once the plasma is turned off, the flame returns back to the cavity leading edge. The species (Nitrogen in the excited state) and hydrogen atoms produced by the plasma are favorable for igniting the flame near the plasma, and the average penetration depth of the fuel above the plasma is increased by about 10%. Intense combustion near the plasma occurs with higher pressure, establishing the recirculation zone with the boundary-layer separation. The combined effects of the reactive species produced, the elevated temperature, and the modified flowfield induced by the plasma contribute to the MCGA-controlled behaviors of jet-wake stabilized combustion.

Effect of boundary layer thickness on supersonic combustion in a scramjet combustor

In this study, the effect of boundary layer thickness on supersonic combustion in an ethylene-fueled scramjet combustor under Mach 5.5 flight conditions is studied by employing experiments, numerical simulations, and theoretical modeling. The plate flow quantitative results show that the absolute thickness of the boundary layer is higher for a larger combustor. But the relative thickness is thin. Although boundary layer thickness changes greatly as the combustion chamber scale enlarges, the influence on the transverse jet and cavity flow is limited. In contrast, the combustion chamber scale change has a great impact on the combustion. More violent flames exist in large-scale combustion chambers. A theoretical model demonstrated that a large subsonic region led to a long fuel residence time, resulting in larger Damköhler numbers. This indicated that a large combustor could maintain large combustion. Based on the above analysis, although the relative thickness of the boundary layer is thinner, the absolute subsonic region of the boundary layer full of combustion is increased, which is conducive to intense combustion. In addition, a verification calculation of the 50 mm shortening isolation section is carried out. It is found from the flame cloud diagram and the quantitative wall pressure data that the combustion is significantly weakened.

Effects of Thermal/Chemical Nonequilibrium on a High-Mach Ethylene-Fueled Scramjet

An ethylene-fueled scramjet operating at Mach 10 was experimentally tested in the JF-24 shock tunnel and modeled using improved delayed detached eddy simulation based on up to 368.34 million cells. An in-depth analysis of the effects of thermal and chemical nonequilibrium on combustion characteristics and engine performance was conducted. The contrary effects of nonequilibrium heating and nonequilibrium cooling that occur in different sections of a scramjet were revealed. The underlying mechanism can be attributed to the delayed relaxation of thermal nonequilibrium under energy addition or deduction. The nonequilibrium case has better mixing, while the equilibrium case has higher combustion efficiency. The synchronous reductions in thrust and drag counteract each other and lead to a higher final net thrust under nonequilibrium. The net thrust increases with the global equivalence ratio, whereas the specific impulse decreases. The evolution of flamelets and reaction paths were analyzed to reveal the effect of chemical nonequilibrium, which produces an abundance of O, OH, and NO radicals through endothermic dissociation reactions and significantly alters the rate-limiting reaction paths.

Performance analysis of an ethylene-fueled scramjet with adjustable finite-rate chemistry

This study aimed to assess the effect of fuel reaction rate on the auto-ignition characteristics and propulsion performance of an ethylene-fueled scramjet engine operating under various working conditions. Based on ethylene combustion, a quasi-one-dimensional model was constructed and thermodynamic analysis was performed systemically with adjustable reaction rates. The reaction rate was controlled by modifying the pre-exponential factor of each elementary reaction in the finite rate chemistry module. By accelerating the reaction rate, flow parameters required for auto-ignition were significantly relaxed, which are shown as the reduction of flight Mach number for self-starting of a scramjet combustor and the improvement of ignition reliability. Each condition has an optimal pre-exponential factor corresponding to its maximum specific thrust. The optimal pre-exponential factor decreases as the flight Mach number increases due to elevated temperature and pressure in the combustor, which promotes combustion. Additionally, the effects of the combustor inlet Mach number, dynamic pressure, and wall friction on the specific thrust and optimal pre-exponential factor are investigated. The results of this study provide a new perspective on the fuel-structure co-design for an advanced scramjet engine.

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.

Experimental investigation of flameholding in a cavity-based scramjet combustor by a multi-channel gliding arc

A multi-channel gliding arc (MCGA) plasma with 4973 W of the average power was utilized to promote flameholding in an ethylene-fueled scramjet combustor with an inflow of Ma2.92. Schlieren, photography, and CH* chemiluminescence coordinated with electrical and pressure measurements were used to examine the effect of the MCGA plasma on flameholding. The results show that the flame can be basically held by the plasma in the presented experimental conditions, whereas the flame gradually spreads backward in the mainstream and is blown out eventually without the plasma. The MCGA plasma is able to re-ignite the fuel/air mixture with a short period, and the unstable combustion is more likely to be kept by the MCGA plasma for a longer period. The average flameholding time is increased by 51% when the plasma is added. A plausible flameholding mechanism for the flame promoted by the MCGA plasma is revealed that the temperature in the cavity is enhanced by the heat and kinetic effect of the MCGA plasma, decreasing the ignition delay time of the fresh fuel near the cavity ramp, which strengthens the flame stability.

Numerical investigation of the scale effects of the flame flashback phenomenon in scramjet combustors

The scale effect of flame flashbacks inside an ethylene-fueled scramjet combustor was investigated through an approach that combines numerical, experimental, and theoretical analyses. In the current study, a hybrid large eddy simulation (LES)/flamelet-progress variable (FPV) combustion model was compared with experimental observations and measurements, and a good level of agreement was shown. As the scale changed, the boundary-layer profiles showed differences to some extent. A larger-scale engine could obtain higher fuel mixing efficiency and thus had the potential to yield intense combustion, enhancing the downstream backpressure and boundary-layer separation. The gradually separated boundary layer formed a quick thermally choked flow, prompting the flame front to accelerate forward until it was transmitted back to the fuel injection position. The low-speed zone in the interior of the boundary layer was more conducive to combustion enhancement and the formation of positive feedback. In addition, the simplified flame flashback model combined with the flame stabilization model was used to illuminate the scale effects of flame flashback.

Effect of pilot hydrogen on the formation of dynamic flame in an ethylene-fueled scramjet with a cavity

Pilot hydrogen is used for the ignition of gaseous ethylene to improve combustion efficiency at low flight Mach numbers. The process of dynamic flame stabilization has been investigated experimentally in a scramjet equipped with a cavity by using a variety of fuel injection strategies involving the injection of pure hydrogen, pure ethylene, and combinations of fuels to understand the effects of pilot hydrogen on supersonic combustion. This study reports experiments conducted in a direct-connected facility that simulated flight conditions at a Mach number of 4, a total temperature of 953 K, and a total pressure of 0.82 MPa. The data were collected by using hydroxyl planar laser-induced fluorescence (OH-PLIF), methylene (CH), luminosity images, and 10-kHz static pressure transducers. The results show that the internal flow oscillated at a dominant frequency of approximately 450 Hz without fuel injection. Oscillation in internal flow was suppressed when pilot hydrogen was injected into the cavity, whereas the influence of other injection strategies was minor. The OH-PLIF images indicate that the pilot flame was unstable, with different locations and occupying different areas. The normalized pressure was apparently higher when pilot hydrogen was injected ahead of the cavity step. The pure ethylene flame was difficult to stabilize without the application of pilot hydrogen. Once the pilot hydrogen had been turned off, the ethylene flame moved toward the cavity ramp and its area expanded. Thus, pilot hydrogen is a precondition for the stabilization of the ethylene flame.

Pilot hydrogen enhanced combustion in an ethylene-fueled scramjet combustor at Mach 4

This paper describes a numerical and experimental investigation of the combustion process in an ethylene-fueled scramjet combustor. The combustion process could be divided into six parts. Part 1 consists of a nonreacting flow before the hydrogen was injected. In part 2, hydrogen injection led to the generation of a shock wave, resulting in an increase in the monitor pressure. Part 3 involved hydrogen combustion, including ignition and flame stabilization. The ignition time of the pilot hydrogen was about 26 ms, and the shock train generated by hydrogen combustion moved at about 20 m/s. In part 4, with the injection of ethylene, there was a hydrogen and ethylene combustion flow, the combustion became more intense, and the shock waves were pushed into the isolator and disappeared from the schlieren images. In part 5, with the cessation of hydrogen injection, the combustion involved ethylene alone, and the ethylene flame moved from the front of the cavity to the back. The combustion mode changed from subsonic to supersonic, and the flame stabilization mode changed from cavity recirculation to cavity shear layer combustion. In part 6, the flame was blown out and combustion ceased.

Lean blowoff behavior of cavity-stabilized flames in a supersonic combustor

Experiments are performed to investigate the near blowoff characteristics of cavity-stabilized flames in an ethylene-fueled scramjet combustor. Heated air enters the combustor with conditions of Ma = 2.52, stagnation pressure p=01.6 Mpa and stagnation temperature T0= 1400 K. The lean blowoff and ignition limits are obtained for different-sized cavity flameholders with upstream flush-wall fuel injection, and the flame structures and dynamics near blowoff are captured with high-speed chemiluminescence imaging. Though the flow residence time is believed to increase with increasing cavity size, it is observed that the lean ignition and blowoff limits do not vary monotonically with the cavity size. Under the same fuel injection conditions, the flame intensity in larger cavities is weaker than that in smaller cavities though the residual flame can survive for an obviously longer period in larger cavities. This may be attributed to the leaner mixture within larger cavities when the lean blowoff limits are approached. Thus, there exists an optimal cavity size which is the most suitable for the ignition and flame stabilization near lean blowoff conditions. This optimal cavity size is determined by the competition between the flow residence time and the local equivalence ratio that greatly depends on the interactions between the fuel jet and the cavity.

Ignition modes of a cavity-based scramjet combustor by a gliding arc plasma

A gliding arc discharge was employed to ignite an ethylene-fueled scramjet combustor with an inflow speed of Ma = 2.92. Flame chemiluminescence and Schlieren photography were recorded simultaneously with CH∗ emission images and discharge waveforms for showing the ignition characteristics in the cavity. The direct ignition mode and the re-ignition mode can be identified. In the direct ignition mode, the initial flame can be directly ignited. In the re-ignition mode, the flame kernel is unable to form an initial flame in the early stage of the ignition process, which can be found from the quenching CH∗ emission of the flame kernel. The CH∗ emission can be seen again although its quenching lasts for ∼700 μs and the re-ignition of the initial flame can be developed to further establish a cavity-stabilized flame. Local equivalence ratio, the instantaneous power of the gliding arc, the area of the flame kernel, and the moving trail of the flame play important roles in determining the ignition modes of the scramjet combustor. The re-ignition mode is more likely to be observed in the local fuel-lean environment and exhibits a longer flame propagation time (1.6 times) and a smaller flame kernel (15%) than those of the direct ignition mode.

Ignition and flame stabilization characteristics in an ethylene-fueled scramjet combustor

Successful ignition process is the prerequisite for achieving stable combustion in a scramjet combustor coupled with a cavity. A single pulse laser is adopted to ignite the fuel/air mixture that is generated by a sonic ethylene jet injection into heated Mach 2.92 crossflows. In order to reveal the complicated combustion regime, large eddy simulations are performed to reproduce the unsteady reactive process observed in the experiments, from an initial ignition kernel to the cavity-stabilized flame. From qualitative comparisons, the predicted results show reasonably good agreement with experimental observations. After ignition, the flow dynamics drive the flame kernel to the leading edge of the cavity. Herein, a flame base is established, and provides hot intermediate products and high temperature to continually ignite the fresh combustible mixture downstream. Finally, a partially-premixed flame is stabilized in the cavity. Due to a shorter flow residence time, the intensity of chemical reactions becomes weak in the downstream region of cavity. In comparison with no-reactive flows, the chemical heat released during the reactions induces high temperature, and the jet wakes penetrate a deep height into the mainstream. A thorough understanding of unsteady flame regime is propitious to instruct the design and optimization of combustor.

Dynamic characteristics of a gliding arc plasma-assisted ignition in a cavity-based scramjet combustor

A gliding arc discharge with an averaged power of 630 W at atmospheric pressure was used to ignite an ethylene-fueled scramjet combustor with an inflow speed of Mach number 2.92. The instantaneous power of the plasma and the high-speed CH* chemiluminescence imaging of the ignition process was simultaneously measured to show dynamic characteristics of the gliding arc plasma-assisted ignition. The results show that the power spikes of the spark-type discharges are the key factor to generate the flame kernel in the scramjet combustor and the instantaneous power can reach 3168 W during the power spikes. The flame kernel ignited by the gliding arc discharge can be developed to form an initial flame, which can become larger with increasing contained discharge energy of the gliding arc. Moreover, a comparison between the successful initial flame and the failed initial flame is performed to demonstrate that the larger area of the successful initial flame can propagate for achieving the mainstream flame, whereas the failed initial flame generated multiple times until the successful initial flame was observed. It is concluded that the successful initial flame contains more discharge energy during the initial flame formation, leading to form the resident flame in the recirculation zone.