Dual-mode Scramjet Combustor Research Articles

Experimental Study on Closed-Loop Control in a Dual-mode Scramjet Combustor with Kerosene Fuel

Experimental Study on Closed-Loop Control in a Dual-mode Scramjet Combustor with Kerosene Fuel

Dual-mode scramjet control using optical emission sensors.

Closed-loop fueling control of a dual-mode scramjet was successfully demonstrated using optical emission spectroscopy as the sole sensor for controller feedback. The optical emission from species of interest (O H ∗, C H ∗, C2∗) was first characterized throughout the combustor. The relative emission intensity between species pairs was studied over a range of fueling conditions and imaging locations throughout a dual-mode scramjet combustor flow path. The pair of emissive species (C2∗/O H ∗) and imaging location that were the most sensitive to changes in fueling condition were selected for use in the control system. Changes in optical transmission of the observation windows and the impact on fuel control were explored. To our knowledge, this paper is the first demonstration of fueling control of a dual-mode scramjet using only optical emission spectroscopy as feedback.

On the residence enhancement mechanism of the cavity-based dual-mode scramjet combustor through the Eulerian and Lagrangian analysis

The lower total temperature and pressure in a dual-mode scramjet engine lead to slower rates of evaporation and chemical reactions, while the inflow velocity is higher than that in a ramjet engine. Consequently, enhancing fuel residence time becomes a more critical challenge. The cavity is a crucial device for enhancing the residence time. However, the quantitative residence capacity and the mechanism have not yet been revealed, especially in wide-range speed inflow conditions. This work employs the delayed detached eddy simulation method to investigate the mass transport and fluid residence characteristics of the wide-range subsonic flow (Ma = 0.3, 0.4, 0.5, 0.6, 0.7) over the cavity. The Lagrangian coherent structure is utilized to characterize the dynamic evolution of the large-scale vortex in the cavity shear layer. Particle tracking is employed to delicately determine the net mass exchange rate and the quantitative cavity residence time. Based on the entrainment process of the large-scale vortex and mass exchange between the mainstream and cavity, this paper proposes a novel theoretical entrainment-impinging model of the large-scale vortex for calculating the residence enhancement coefficient (τr). The theoretical model demonstrates that the residence enhancement coefficient is a function of the cavity geometry (L, D), the vortex radius (rv), the shedding Strouhal number of the vortex (St), and the vortex/trailing edge interaction coefficient (η). Furthermore, it has been proven that the model proposed in this paper is applicable to a wide range of inflow turbulent conditions and cavity geometric configurations.

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.

Characterization and predictive modeling of a trajectory-oriented dual-mode scramjet combustor

Linear and nonlinear characteristics of a dual-mode scramjet combustor are investigated in ground-simulated acceleration and deceleration trajectory experiments. The experiments were conducted in the direct-connected transient flight trajectory simulator 1 at the Institute of Mechanics, Chinese Academy of Sciences. High-frequency pressure measurements, the schlieren, and CH* chemiluminescence high-speed imaging were applied for the diagnostics. Based on the quantitative analysis, the physical processes of acceleration and deceleration in general represent similar nonlinear characteristics. The linear characteristics are limited to the low-frequency oscillation period in the presence of physical governing mechanisms. A nonlinear predictive model of the dual-mode scramjet combustor based on historical measurements is proposed due to its generality for the acceleration and deceleration trajectory.

Unsteady Numerical Analysis of a Dual-Mode Scramjet Combustor with a Cavity

Unsteady Numerical Analysis of a Dual-Mode Scramjet Combustor with a Cavity

Numerical study on detonation initiation process in the chamber with characteristic structures of the dual-mode scramjet combustor

In order to improve the performance of traditional ramjet, a ramjet based on pulse detonation (PD-Ramjet) instead of isobaric combustion was proposed. Several special-shaped detonation chambers which retained the characteristic structures of the dual-mode scramjet combustor were designed. Characteristic structures included the expansion channel and the cavity. The filling process and detonation initiation process of the stoichiometric hydrogen/air mixture under the incoming flow condition of the sub-combustion mode were studied in the detonation chambers with characteristic structures by two-dimensional numerical simulation method, and the influences of the characteristic structures on the filling process and detonation initiation process were analyzed. The simulation and analysis results indicated that the hydrogen concentration at both sides of the chamber near the outlet was low due to the structure of the expansion channel, and the pressure and velocity of the detonation wave decreased gradually after the expansion channel. The cavity had a significant influence on the filling process, which resulted in the uneven hydrogen concentration downstream the cavity, especially near the outlet of the chamber. The intensity of the detonation wave attenuated to a certain extent by reason of the cavity, where the peak pressure first decayed due to the sudden expansion of the flow path and then rose owing to the reflection at the cavity aft wall.

On the mass exchange mode controlled by high-speed compressive mainstream/cavity interaction in a laboratory dual-mode scramjet combustor

Stable and efficient mass exchange is crucial to the combustion stability of a dual-mode scramjet. In subsonic mode, the progression of mass exchange between the cavity fluid and the mainstream, which is controlled by the cavity shear layer, is different from the scramjet mode due to the weak compressibility effects. However, mass exchange mode studies in the subsonic mode are rare. In this study, experimental research has been conducted to obtain the flow structure of Mach 0.3 and 0.5 inflow over a rectangular cavity and reveal the properties of mass exchange using particle image velocimetry technology. The results reveal the weak compressibility effects and geometry effects on the mass exchange by analyzing the growth rate and turbulence characteristics of the cavity shear layer. The mass exchange between the cavity fluid and mainstream is determined based on the vertical velocity along the cavity lip line. When the flow Mach number increased from 0.3 to 0.5, the growth rate of the cavity shear layer decreases due to the weak compressibility effects. Also, the growth rate of the cavity shear layer is smaller than that of the compressible free shear layer due to the effects of recirculation within the cavity. For length-to-depth (L/D) ratios ranging from 0.8 to 1.2, the cavity shear-layer growth rate decreases with increasing L/D. However, at an L/D of 1.5, the shear-layer growth rate increases, which changes the mass exchange mode. Finally, the effects of increasing the Mach number (Mach = 0.3 versus 0.5) show that the L/D ratio at the “crossover” point of the mass exchange mode transition is reduced (L/D = 1.5 versus 1.2) due to the weak compressibility at a higher Mach number.

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.

Investigation on the mode transition of a typical three-dimensional scramjet combustor equipped with a strut

A three-dimensional compressible Reynolds-averaged Navier-Stokes (RANS) equations coupled with the two equation RNG k-ε turbulence model have been employed to capture the mode transition properties in a dual-mode scramjet combustor. Based on the code validation and the grid independency analysis, two transition processes from scram to ram mode and from ram to scram mode are analyzed in detail. The fuel equivalent ratio is controlled by linearly increasing or decreasing the injection pressure of each nozzle to achieve the mode transition. During the mode transition, the thermodynamic properties of the combustion chamber change dramatically and the flow field structure changes correspondingly. The variation of the flow field structure is mainly reflected in the upstream flow field, while the variation of the flow field temperature is mainly reflected in the downstream flow field. The obtained results show that there is an obvious temperature variation in the downstream flow field. Besides, the upstream wave structures will change when the temperature varies to a certain degree, then the mode-transition occurs completely. Since the ram mode is a more stable mode than the scram mode, and the transition to the scram mode is more difficult than the transition from the scram mode to the ram mode. The variation of the shock wave position in the isolator is accompanied by the variation of the mode transition. During the transition from scram to ram mode, the shock wave at the combustor entrance is gradually pushed out and moves forward at the position with the lowest Mach number. During the transition from ram to scram mode, the shock wave in the isolator is gradually pushed forward into the flow field, but the shock waves is still in the isolator when the flow field turns into the scram mode.

Characteristics of reaction zone in a dual-mode scramjet combustor during mode transitions

The mechanism of controllable combustion modes (scramjet and ramjet) is one of the most significant challenges for dual-mode ramjet/scramjet engines. The detailed characteristics of the reaction zone during the transitions of combustion modes within the Hyshot II combustor are numerically investigated with 7 species-7 steps hydrogen/air reaction mechanism and the thickened flame model (TFM). In the case of continuous increasing or decreasing equivalence ratios (ERs), the critical ERs for the mode transitions are 0.474 or 0.539, respectively. As the ER increases or decreases, the location of the leading shock of the combustor shock-train moves upstream or downstream, respectively. When the ER continuously increases, the pressure ratio at the leading shock to the outlet of the isolator is greater than when the ER continuously decreases. Inversely, the mass-weighted average Mach number inside the combustor for a continuously increasing ER is less than that for the continuously decreasing ER. In the scramjet mode, the primary combustion reaction zone is located at the rear of the combustor. In the ramjet mode, combustion occurs in the separation zone at the middle of the combustion chamber, while the reaction zone is closer to equilibrium at the critical ER of 0.474 as compared to the scramjet mode.

Flowing residence characteristics in a dual-mode scramjet combustor equipped with strut flame holder

In this paper, the numerical and experimental investigations have been conducted to study the flowing residence characteristics in the supersonic combustor. A thin strut equipped in the center of the combustor was adopted as the flame holder. The airflow residence time was calculated by a new method, and results proved that the airflow residence time was longest in the center of the recirculation zone. Then, the interaction between the strut geometrical parameters and the airflow residence time was investigated. The results revealed that the thickness of the strut played a decisive role on the residence time and with the strut thickness increasing, the residence time became longer. The sweep angle and the length of the strut had few influences on the residence time. A series of experiments have been conducted at the flight condition of Ma=6.5. Experimental results indicated that, with the help of the long residence time in the recirculation zone, an initial flame was formed in the recirculation zone at the tailing of the strut, and grew to form a steady flame in the center of the main flow. These results are valuable for the future optimization of the combustion stabilization in the supersonic combustor using strut as the flame holder.

Numerical investigation of mode transition and hysteresis in a cavity-based dual-mode scramjet combustor

Abstract The effect of ethylene fuel equivalence ratio (ER) variation directions on combustion states in a dual-mode scramjet combustor was numerically investigated. The combustor employed transverse wall fuel injectors and downstream cavity flameholders without pilot fuel, which are fundamental components in many practical combustors. The isolator inflow Mach number was 3.1, and static pressure, stagnation pressure and stagnation temperature were 53 kPa, 2622 kPa and 1656 K, respectively. The ER was regulated abruptly in a piecewise constant manner, from 0.10 to 1.02, and then back to 0.10. A 3-D URANS method with a recognized two-step kinetics model was adopted. Results exhibited two combustion hysteresis loops, which indicated that different types of combustion mode transitions could result in hysteresis. The first was a hysteretic phenomenon between separated and shock-free scramjet modes based on steady quasi-one-dimensional combustor flow assumptions, and the second was between two different patterns of separated scramjet modes. Hysteresis mechanisms are elucidated from the viewpoint of combustion flow structures. The first hysteresis was attributed to flame stabilization mode transitions between the cavity shear-layer stabilized mode and the jet-wake stabilized mode, along with the transition hysteresis of a pre-combustion shock train's establishment and vanishment. The flame stabilization locations were greatly influenced by the flow separation states ahead of the fuel injectors, and the flow separations were in return determined by the flame distributions. The second hysteresis was attributed to transitions between weak-oscillation mode and intensive-oscillation mode with the transition hysteresis of shock reflection amount increase and decrease of the pre-combustion shock train structure, which were both in the jet-wake stabilized location. Flame in the low-speed region beside the separation bubbles ahead of the fuel injectors provided heat and hot radicals for downstream flame stabilization, and the pre-injector flame intensity greatly influenced the combustion oscillation states.

Flame Structure and Dynamics in a Premixed Dual-Mode Scramjet Combustor from Fluorescence Imaging

Formaldehyde () planar laser-induced fluorescence (PLIF) imaging was performed in a premixed, ethylene-fueled dual-mode scramjet combustor to study the flame structure and dynamic flame behavior. PLIF is used as a marker to provide information on the structure and position of the lower-temperature portion of the reaction zone. Low-repetition-rate (10 Hz) measurements were acquired at several cross-flow planes along the axial flow direction. Statistical analysis of the topology indicates sparse concentrations of formaldehyde that are intermittently detected by the PLIF measurement. Comparisons between previous large-eddy simulations, laboratory-scale flames, and the current experimental measurements indicate discrepancies between the expected and measured formaldehyde concentrations and structure. High-speed formaldehyde PLIF also was conducted at 50 kHz to further investigate the intermittency of the formaldehyde layers in the flow.

Investigation of turbulence flow characteristics in a dual-mode scramjet combustor using hydroxyl tagging velocimetry

The results of an investigation of the turbulence characteristics of a kerosene-fueled, dual-mode scramjet combustor are reported in this paper. The combustor had a cavity flame holder and was operated at an inlet Mach number of 2. The combustor was made to transition across different modes by adjusting the fuel injection and inlet flow total temperature. The velocity and fluctuations thereof in the flow field upstream of the combustion zone were measured using the hydroxyl tagging velocimetry method. Given a signal-to-noise ratio of about 4 for the OH lines and considering the effect of dithering of the optical equipment owing to strong vibration, the uncertainty in the measured velocity was about 29 m/s. The computational fluid dynamics solution of the flow was compared with the measurements, and the computed positions of the shock wave trains were used to distinguish the combustion modes. The results show that the turbulence fluctuation velocity (u'rms) in the supersonic mode was the same as that in the case of the nonreacting flow, but it was considerably higher in the subsonic mode. During the ram-to-scram process, the turbulence intensity (u'rms/ū) declined exponentially as the local Mach number of the flow field upstream of the combustion zone increased.

Experimental Investigation of Reacting Flow Characteristics in a Dual-Mode Scramjet Combustor

Abstract In this work, a hydrogen fueled dual-mode scramjet combustor was investigated experimentally. Clean and dry air was supplied to the combustor through a Mach 2 Nozzle with a total temperature of 800 K and a total pressure of 800 kPa. The high enthalpy air was provided by an electricity resistance heater. Room temperature hydrogen was injected with sonic speed from injector orifices vertically, and downstream the injector a tandem cavity flame holder was mounted. Except wall pressure profiles, velocity and temperature profiles in and at exit of the combustor were also measured using hydroxyl tagging velocimetry (HTV) and tunable diode laser absorption spectroscopy (TDLAS) respectively. Results showed that combustion occurred mainly at the bottom side of the combustor. And there were also an extreme disparity of the velocity and temperature profiles along the Y direction, i.e. the transverse direction.

Experimental investigation of influence factors on flame holding in a supersonic combustor

In this paper, combustion and flame stability are studied experimentally in a kerosene fueled dual-mode scramjet combustor at various air and fuel conditions. The lean and rich blowout limits of liquid kerosene injected from the wall upstream of a cavity flameholder are examined in Mach 2.0 airflow. The effects of injector configurations on the blowout limit are investigated. It is found that injector configuration has significant effect on the flame stabilization. The lean blowout limit is much lower when using injector C, which has a configuration of 2×Φ0.35 mm. But for the rich blowout limit, injector B which has a configuration of 3×Φ0.35 mm performs better than others. Therefore, the diameter and distribution of fuel injection holes should be considered fully in combustor design. Furthermore, the influence on blowout limits of combustor entrance total temperature and total pressure are also investigated. As the total temperature increasing, the equivalence ratio range between rich and lean blowout limit become larger. While, compared with the total temperature, the effect of total pressure on the blowout limit is slighter.

Effects of auto-ignition on combustion characteristics in a hydrogen-fueled dual-mode scramjet combustor

Combustion characteristics were investigated in a dual-mode scramjet combustor with hydrogen injected 160 mm upstream of a cavity. Tests were conducted under Mach 2.52 inflow conditions with the stagnation temperature of 1400 K and 1600 K, respectively. Under the stagnation temperature of 1600 K, strong auto-ignition induces the ignition process. When the stagnation temperature is 1400 K, intermittent blowoff and re-stabilization are observed in the jet wake. It is noted that auto-ignition induces an individual new flame upstream of the cavity, after the original jet-wake flame is blown downstream and stabilizes in the cavity. It is concluded that auto-ignition contributes to the flame re-stabilization in the jet wake.

Control-oriented modeling and real-time simulation method for a dual-mode scramjet combustor

The control-oriented modeling and real-time simulation method for a dual-mode scramjet combustor has been conducted in this paper. The 1-D unsteady model coupled with isolator shock train model and oblique shock wave modification can treat variable area, fuel addition, combustion heat release, variable specific heat, inflow air vitiation component, wall friction and mixing efficiency. Combustion is simplified into mass and heat addition process. So this paper discusses heat release distribution obtaining method which can be applied in scramjet combustor 1-D flowfield simulation in detail. It is fit for certain scramjet combustor configuration and acceptable over a wide range of inflow conditions. By mixed programming of C and Matlab, the code of model solving is capsuled into S-Function which can be used in SIMULINK. The control-oriented model is built on SIMULINK platform. When the model runs in a single-core 3.6 GHz Intel Core i7-4790 processor, computation time-consuming is found to require 5–10 ms within a control period. The computing speed indicates very promising, because model is compact enough to run in real time, and it can be used as an embedded model in control system research. This model can also be used to analyze unsteady and steady flowfield characteristic. The safety boundary codetermined by unstart and over-temperature is conducted using this model.

Analysis on Mode Transition in a Dual-Mode Scramjet Combustor

ABSTRACTAn ignition combustion test of different combustor configurations and different fuel equivalence ratios was carried out in a condition of total temperature at 880 K, total pressure at 8 atm, and Mach number of 2.0 in regards to dual-mode combustor. There are three combustor configurations that are dual-mode combustors with double cavity flame holder. Hydrogen is injected in the first cavity while kerosene is injected in front of the first cavity. Hydrogen, being a pilot flame, was the first to be ignited by the spark, then kerosene was ignited by hydrogen. After that, injection of hydrogen was stopped and sole kerosene was kept burning steadily. The distribution of other airflow parameters in the combustor was obtained by calculating with a 1D analysis method; the influence of combustor configuration and fuel equivalence ratio to combustion mode transition was analyzed. The results indicated that the gradual transition of combustion mode in the combustor from supersonic combustion mode to subsonic combustion mode was accompanied by increasing the fuel equivalence ratio. The disturbance arising from wall pressure rise of the combustor was gradually moved upstream. The subsonic combustion area in Model 2 was moved by 60 mm upstream when compared with that in Model 1, which was basically in conformity with the distance that the initial position of the first cavity in Model 2 was moved by 58 mm forward. When the fuel equivalence ratio was equal, the length of subsonic combustion areas in the two models was basically unchanged. In the case of combined combustion of hydrogen and kerosene, the pressure disturbance had been spread upstream and arrived at the entrance to the combustor; thus, the inlet Mach number condition of the entrance to the combustor was damaged. The damage to the condition of the entrance to the combustor was more serious to some extent in Model 2 due to antedisplacement of the first cavity when compared with that in Model 1.