Effects of cavity length on operating characteristics of a ramjet rotating detonation engine fueled by liquid kerosene

Abstract

Our very recent work has experimentally demonstrated the successful operation of a liquid-kerosene-fueled ramjet/air-breathing rotating detonation engine with a cavity-based annular combustor. As a follow-up study, the present work aims to further reveal the effects of cavities on the initiation and propagation characteristics of detonation, and various cavity lengths ranging from 30 mm to 90 mm were employed in the combustor. An air-heater was designed to provide an incoming flow at a total temperature of 860 K, which resembles the specific flight Mach number of 4 at an altitude of 20 km. The total mass flow rate of the incoming air flow was kept at 1.0 kg/s. The results indicate that a sufficient cavity length, which is found to be around 45 mm in minimum, is necessary to achieve the successful initiation of the rotating detonation waves. By increasing the cavity length to 75 mm and 90 mm, the fuel-lean detonation limits in terms of equivalence ratios (ER) can be extended from 0.8 to 0.7. The maximum propagation stability and minimum velocity deficit of 39.1 % (with respect to the ideal Chapman-Jouguet velocity) were achieved when the cavity length was 75 mm with the ER of 0.95. The optical observation indicates that detonation mainly occurs downstream of the cavity. The detonability of the kerosene/air mixture can be enhanced by the preheating of the permanent flame, and better enhancement is provided by larger cavity lengths. However, when the cavity length reaches 90 mm, part of the mixture is pre-ignited and consumed by deflagration. Therefore, the propagation velocity and stability of the rotating detonation wave increase as the cavity is lengthened from 45 mm to 75 mm, while showing a decrease when the cavity length reaches 90 mm. It is suggested that better detonation performance can be achieved by placing the permanent flame in a proper axial position through changing the cavity length. In addition, the increase of the cavity length is beneficial to alleviate the disturbance of the upstream oblique shock wave on the incoming flow in the isolator.