Effects of self-pulsation on combustion instability in a liquid rocket engine

•Spray and flame transform from stable behavior to self-pulsated behavior as recess increases.•Flame and pressure of the liquid manifold oscillate at the same frequency during self-pulsation.•Self-pulsation can cause combustion instability only when frequencies are coupled.•The blocking actions of the conical liquid sheets are the basic reason of all oscillations. The flame oscillation characteristics as well as the relationship between self-pulsation and combustion instability in a combustor burning gaseous oxygen (GOX) and liquid ethanol (LET) through a liquid-centered swirl coaxial (LCSC) injector were investigated experimentally. Data were obtained at pressures from 0.53 to 0.6 MPa and mixture ratios from 0.16 to 0.3. The side-on images of spray and flame were captured synchronously through two high-speed cameras. With the increase of recess length, spray and flame transform from the stable behavior to the self-pulsated behavior with periodic intensity variations of flame emission. When self-pulsation occurs, flame together with the pressure of the LET manifold oscillates at the same frequency, and the peak-to-peak pressure amplitudes of the LET-manifold are greater than 26%. Due to the blocking actions of the conical liquid sheet, spray self-pulsation caused by the periodic fluctuation of pressure in recess chamber is found to be the essential reason of flame and LET manifold pressure oscillation. Furthermore, when the self-pulsation frequency is coupled with the first-longitudinal-mode (1L) natural acoustic frequency of combustor, combustor pressure will be induced to oscillate at the same frequency. This further indicates that self-pulsation may probably be a critical factor inducing combustion instability in liquid rocket engines.