Combustion characteristics of a dual-mode ramjet engine with a supersonic aerodynamic ramp injector

Abstract

The hypersonic air-breathing propulsion system is attracting attention as an engine that can be applied to long-range high-speed target strikes and high-speed commercial aircraft. Despite over 60 years of research, its practical implementation has been challenging due to the hypersonic flow inside the engine. The efficiency of a dual-mode ram/scramjet engine relies on the effective mixing of fuel and air, which requires deep penetration of fuel into the main flow. To achieve efficient injection of liquid hydrocarbon fuel, the Air-assisted Supersonic Aerodynamic Ramp Injection (ASARI) injector was proposed. Direct-connect experiments were conducted using a model combustor under conditions corresponding to a flight Mach number of 4 and an altitude of 22 km (enthalpy conditions). The newly proposed injector utilizes the combination of various injection methods, such as the aerodynamic ramp, premixing chamber, assisted air, and supersonic nozzle. The aerodynamic ramp injection method consists of four injectors with inclination angles of 40° and 80° and toe-in angles of 30° and 60°. This configuration was adopted to increase fuel penetration height while minimizing total pressure loss. The validity of this design was confirmed based on various fluid analyses including fuel quantity, injection angle, nozzle shape, and ramp angle. The premixing chamber with the assisted air injection device was employed to enhance atomization and fuel-air mixing. Additionally, a contoured Mach 3 supersonic nozzle was used to achieve flow acceleration. For performance comparison, combustion tests were also performed using the Air-assisted Oblique Injection (AOI) method, which applies a cylindrical nozzle to the premixing chamber with the same assisted air. Ignition was initiated using a hydrogen pilot fuel and a torch igniter. Once combustion stabilized, only liquid fuel was supplied. The Mach number was calculated using quasi-one-dimensional analysis based on the measured wall pressures and inlet conditions. The model combustor with the two types of injectors operated in the ramjet mode, with the area near the flame holder being subsonic and the combustor exit being supersonic. When the equivalence ratio varied from 0.25 to 0.56, the results obtained from pitot pressure measurements at the combustor exit showed that the newly proposed injector exhibited higher pressure levels and a more pronounced increase in pressure with increasing fuel mass compared to the baseline injector. Furthermore, in the accuracy analysis of the quasi-one-dimensional analysis method using the pitot lake, it showed an error of 38% under non-combustion conditions and an average error of 13% under combustion conditions.