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Ignition and flame stability in supersonic flow have always been the key problems of research in scramjet. In addition, ignition is difficult and the cavity flameholder is susceptible to support the flame stability under extreme conditions such as low equivalent ratio. In recent years, gliding arc plasma is recognized to expand ignition and extinction limit with lower energy consumption in the field of plasma assisted ignition due to its heating and chemical effects. In this paper, a gliding arc igniter has been designed and compared with the traditional spark plug in order to quantify the ignition ability. The igniter has the same size with the spark plug, using low-power AC gliding arc to carry out ethylene ignition test in Ma = 2.52 Ma supersonic flow. The average power of gliding arc discharge is 1199 W. A high-speed camera and CH ⁎ chemiluminescence were used to make combustion diagnosis. Founded in the same discharge period, the lean ignition limit of the gliding arc is lower than the ignition limit of the spark. The average expansion of ethylene ignition limit is 17%. The ignition process is that gliding arc continues to generate the initial flame kernels during the discharge period, but it is extinguished continuously due to the strong convection. Until generating an initial flame kernel which can successfully propagate the flame. The ignition process can be divided into four stages. It continues to generate new flame kernels in flame propagation process. Gliding arc reignites the fuel and generates the new flame kernels after forming a stable flame, appearing intermittent ignition in the cavity. The high equivalent ratio can make ignition delay time shorter, generating initial flame kernels more frequently. The heating effect of the gliding arc and reignition character make the thermal product and ethylene occur intermittent combustion more often in the cavity, increasing the area of the combustion reaction. Gliding arc plasma can achieve combustion enhancement during the flame stabilization process. The shear layer of flame thickness increased by the average of 2 mm on S-B-1 and G-B-1 conditions. Compared with the traditional spark ignition, gliding arc broaden the lean blow-off limit in different stages may be the significant reason for broadening lean ignition limit. It concludes that gliding arc makes the flame's ignition limit closer to the flame's blow-off limit. [ABSTRACT FROM AUTHOR]

Ignition enhancement characteristics of partially covered cavity in an ethylene-fueled scramjet combustor were investigated experimentally under the inflow conditions of Ma = 2.1, stagnation pressure P0 = 0.7 MPa, and stagnation temperature T0 = 846 K. It is concluded that under the influence of the baffle, the covered region will become a region with sufficient fuel during the stable combustion process and wall-pressure distribution will be increased due to this restricted region. Comparing with the uncovered cavity under the same equivalence ratio, partially covered cavity is more suitable for the formation and propagation of flame kernel, and ignition in the scramjet combustor could be intensified due to this cavity configuration. [ABSTRACT FROM AUTHOR]

The present study attempts to fix the location of thermal throat by employing transverse fuel injection downstream and adding a short constant-area section in the divergent part in a realistic scramjet combustor configuration in order to depress the pressure rise effects on the isolator. Ground tests using a direct-connect model along with pressure measurements and three-dimensional numerical simulation are also performed for the present investigations. The results indicate that the thermal throat is fixed in the constant-area section for the configuration with a short constant-area section in the divergent part. Numerical simulations also indicate that the constant-area section is helpful for ignition and flame holding. The configuration with a short constant-area section increase heat release in the combustor and decrease entropy production. Numerical simulation results imply that there could exist more than one thermal throat in dual-mode scramjet combustor, and the constant-area section in a divergent part could reduce the number of thermal throat. [ABSTRACT FROM AUTHOR]

The supersonic combustion ramjet (scramjet) engine is expected to become the most efficient air breathing propulsion system in the hypersonic flight regime. The behavior of wall pressures propagation was examined using a direct-connect model scramjet experiment along with high-frequency pressure measurements. High-frequency pressure transducers are employed to record the history of pressure response during ignition process. A first order plus dead time (FOPDT) model is employed to model the S-shape step response curve. The time scale distributions of pressure propagation are reported in this paper. The characteristics of pressure propagation are closely related to the combustion mode of the scramjet. [ABSTRACT FROM AUTHOR]

Abstract: By using a plasma jet (PJ) torch with 1.5 kW input power as an igniter, successful ignition for liquid-kerosene fueled combustion experiment was conducted in a direct-connected supersonic test facility. The incoming flow has total temperature of 950 K and local Mach number of 1.8, corresponding to Mach 4 flight condition. In this study, several optical techniques, including high speed photography, high speed schlieren photography, and planar laser scattering (PLS) technique, were combined to study the ignition process, flame propagation, and mixing features of liquid kerosene fuel with air around the cavity. The effect of fuel injection position, injection pressure, and feedstock gas on ignition performance has been analyzed. The results indicate that local mixing is a critical factor for ignition. It is also shown that the PJ torch with N2 +H2 feedstock is superior to the PJ torch with N2 feedstock for the ignition of liquid-kerosene fuel. These results are valuable for the future optimization of kerosene-fueled scramjet engine when using a PJ torch as an igniter. [Copyright &y& Elsevier]

The performance of a scramjet engine relies heavily on the combustion characteristics of fuel. This work aims to propose a concept of controlling the self-starting characteristics and propulsion performance of a scramjet via fuel reactivity modification. The fuel reactivity was modified by adjusting the activation energy of each elementary reaction. The thermodynamic analysis was then systemically performed to evaluate the effects of fuel reactivity on the lowest flight Mach number required for self-starting and the specific thrust. The results indicate that the lowest flight Mach number for self-starting of a hydrogen-fueled scramjet reduces from 6.2 to 5.1 when the activation energy is decreased by 50%. Under a given flight condition, fuel reactivity has a remarkable influence on the characteristic ignition length, which affects the specific thrust, especially at low flight Mach numbers. It is demonstrated that there exists an optimal reaction rate to achieve the maximum specific thrust, rather than increase the reaction rate infinitely. The optimal reaction rate relates to the supersonic flow condition, the exothermic heat of fuel, and the wall friction. The results obtained in this investigation provide a theoretical basis for the design of high-reactivity supersonic combustion fuel and further research of advanced scramjet engine operation. • Control self-starting characteristics via fuel reactivity modification. • Establish a relationship between activation energy and self-starting Mach number. • The specific thrust is improved by adjusting activation energy. • Exist an optimal reactivity to achieve the maximum specific thrust. • The optimal reactivity relates to flow condition, exothermic heat, and wall friction. [ABSTRACT FROM AUTHOR]

The ignition characteristics of a flush wall, liquid-kerosene-fueled scramjet combustor are tested in a directly connected ground test rig. The total temperature and Mach number of the vitiated airflow at the exit of the nozzle are 1,505 K and 3.0, respectively. The ignition process is accomplished by air throttling and a micro hot gas generator (MHGG). Without any cavity or backward steps, the flame holding is also accomplished by the MHGG. By controlling the air-throttling pressure and overlap time of the air throttling and the MHGG, different ignition results can be achieved. This paper divides these different results into four ignition modes and provides a detailed description of the characteristics of each mode. In addition, this paper summarizes the experiment data and proves that the ignition laws have some relationship to the fuel-air equivalence ratio. [ABSTRACT FROM AUTHOR]