Investigation of the combustion process in an ethylene-fueled scramjet combustor

An experimental investigation was conducted in a direct-connect supersonic combustion facility simulating the inflow condition of Ma=2.0 to investigate the combustion process in an ethylene-fueled scramjet combustor with cavity and pilot hydrogen. The structure of the flow field and the flame development were visualized using the schlieren imaging, the flame luminosity imaging, the CH luminosity imaging and the planar laser-induced fluorescence (PLIF) of the OH radical. The equivalence ratios of pilot hydrogen and ethylene were about 0.33 and 0.10, respectively. The whole combustion process could be divided into six stages. In the first stage, there was a non-reaction cold flow before the hydrogen injection. And the frequency of oscillation was measured to be around 400 Hz, experimentally. In the second stage, the flow characteristics before the pilot hydrogen ignition were revealed, due to the hydrogen injection an oblique shock wave was generated, reflected on the bottom wall, and then interacted with the shock waves below the cavity, thus leading to the increase of the monitor pressure. The third stage was the hydrogen combustion, including ignition and flame stabilization. The process from the ignition of pilot hydrogen to the combustion stabilization lasted around 26.0 ms. In the first 0.1 ms, the ignition of pilot hydrogen had great effect on the flow field structures. The moving speed of the shock train caused by the combustion of pilot hydrogen was around 20 m/s. The stabilization mode of the pilot hydrogen flame was cavity recirculation stabilized combustion. The fourth stage was the intense combustion process of hydrogen and ethylene. The shock waves were pushed into the isolator, and thus exceeded the observation range. The ethylene flame stabilization mode was shear layer stabilized combustion. The combustion characteristics of ethylene were revealed in the fifth stage. When the pilot hydrogen was ceased, the ethylene flame moved from the cavity step to the cavity ramp. The last stage involved the combustion and flame blowout of pure ethylene. Preliminary analysis indicates that the CH luminosity images of ethylene combustion can be used to investigate combustion efficiency qualitatively.