Study on the initiation mechanism and flow field evolution characteristics of self-pulsation for gas–liquid swirl coaxial injectors under continuous gas flow regulation

Gas–liquid swirl coaxial injectors are widely used in bipropellant liquid rocket engines due to their excellent atomization performance. However, under specific conditions, they may generate self-pulsation, which in turn can induce unstable combustion. Through cold-state atomization tests under continuous gas flow regulation, this study focuses on the initiation process of self-pulsation; systematically explores the evolution law of spray flow field characteristics with gas flow and reveals the initiation process and mechanism of self-pulsation. With the continuous increase in gas–liquid ratio, the spray of the non-recessed injector undergoes a continuous transition sequence of “steady state-transition state-self-pulsation.” However, for the injector with a large recess, an “intermittent” phenomenon occurs, characterized by a sequence of “self-pulsation-steady state-transition state-resumed self-pulsation.” The transition state mainly includes three stages: near-steady state, oscillation initiation, and oscillation intensification. In the transition stage, there exists low-intensity oscillation in the spray width, with alternating occurrences of steady spray and self-pulsation. Moreover, the conical liquid film inside the recess chamber squeezes the annular gas, leading to a reduction of the flow channel and an increase in the pressure outside the liquid film, which in turn pushes the liquid film toward the central axis. As the gas flow increases, the energy exchange between gas and liquid intensifies, eventually triggering self-pulsation. This study provides a theoretical reference for understanding the self-pulsation mechanism and optimizing the injectors of liquid rocket engines.