Investigation on auto-ignition and chemical energy release characteristics of pilot hydrogen in supersonic combustion flow

Abstract

This study investigates auto-ignition and heat release characteristics of pilot hydrogen chemical energy in a scramjet combustor equipped with a single cavity. Experiments are conducted in a direct-connected facility simulating Mach 6.0 flight conditions with a total temperature of 1350 K and total pressure of 1.75 MPa. Data are obtained from schlieren imaging, hydroxyl planar laser-induced fluorescence, flame emission, and 10-kHz static pressure transducers. The present investigation extends the pilot hydrogen ignition delay experimental dataset and clarifies the instabilities present in the ignition process. The results show that the supersonic internal flow of a confined cavity exhibits self-oscillating behavior with a dominant frequency of approximately 141.3 Hz. The primary chemical reaction occurs at mid-cavity, where the chemical energy of the pilot hydrogen begins to be converted into heat energy, then approaches the cavity ramp before finally being distributed across the whole cavity. The combustion mode is the cavity-stabilized scramjet mode. The distribution of hydroxyl radicals varies significantly because the combustion in the cavity is unsteady. The ignition delay time increases as the injection pressure rises. However, an injection pressure of 4.0 MPa produces an ignition delay of 24.7 ms, which is apparently shorter than the delay under an injection pressure of 3.5 MPa and similar to that under an injection pressure of 3.0 MPa. The injection of pilot hydrogen under high pressures induces greater heat release and more intense blockage effects, thus enhancing the probability of successful ignition and stable combustion.