Self-excited longitudinal combustion instabilities were investigated in a hypergolic liquid bipropellant combustor, which applied single dual-swirl coaxial injector. Hot-fire tests were conducted for four different injector geometries, while extensive tests on injection conditions were carried out for each injector geometry. The synchronous measurement of the pressure and heat release rate was applied, successfully capturing the process of the pressure and heat release rate enhanced coupling and developing into in-phase oscillation. By calculating Rayleigh index at the head and middle section of the chamber, it is shown that Rayleigh index of the middle section is even higher than that of the head, indicating a long heat release zone. When the combustion instability occurs, the pressure in propellant manifolds also oscillates with the same frequency and lags behind the oscillation in the combustor. Compared to the oscillation in the outer injector manifold, the oscillation in the inner injector manifold shows a higher correlation with that in the chamber in amplitude and phase. Based on numerical simulations of the multiphase cold flow inside the injector and combustion process in the chamber, it is found that injector geometries affect longitudinal combustion instability by changing spray cone angle. The spray with small cone angle is more sensitive to the modulation of longitudinal pressure wave in combustion simulations, which is more likely to excite the longitudinal combustion instability. Meanwhile, the combustion instability may be related to the pulsating coherent structure generated by the spray fluctuation, which is determined by injection conditions. Besides, a positive feedback closed-loop system associated with the active fluctuation and passive oscillation of the spray is believed to excite and sustain the longitudinal combustion instability.

To estimate the effects of oxygen-fuel (O/F) ratio on the combustion stability of a model rocket engine with hypergolic propellant, experiments were carried out at different O/F ratios in a rectangular model engine with dual-liquid swirl coaxial injectors. The pressure oscillations and CH*-characterized heat release pulsations in the combustion chamber were simultaneously recorded by high-frequency pressure sensors and photomultiplier tubes (PMT). The effects of O/F ratio on combustion stability were obtained. The results show that the low-frequency oscillation of 41 Hz occurs in fuel mainfold after combustion initiation, which induces synchronous low-frequency oscillations in the combustion chamber during shutdown. In the process of increasing O/F ratio from 0.933 to 1.789, the combustion chamber undergoes a combustion stability transition process of stability, mild instability, first-order transverse instability, and second-order transverse instability. The amplitude of pressure oscillations in the 2W mode is only 4.69% of the mean combustion chamber pressure. By incorporating the PMT signal, it is found that the coupling of pressure and heat release signals is more obvious when combustion instability is more intense. Rayleigh index analysis based on the experimental data shows that the driving source of combustion instability in the 1W mode is mainly located at both sides of the combustion chamber, while suppression is found in the middle of the combustion chamber. The analysis suggests that the generation of combustion instability may be related to the interaction between the propellant and the combustion chamber walls.