Unfortunately, the full-scale tests are expensive and time-consuming, and thus impractical. The most firsthand and effective methods are the full-scale tests. Therefore, HFCI should be paid sufficient attention for its theoretical and engineering significance. Almost every LRE suffered from HFCI during its development process, and the deep reason that results in HFCI has not been uncovered until now. Rayleigh thought that the time the pressure was at its peak was when heat was added to a system in an oscillatory cycle, as the oscillation would be encouraged and the amplitude would increase but if the heat was extracted from the system when the pressure was at its peak value, the oscillation would be discouraged, and the amplitude would decrease. Rayleigh Criterion ( Rayleigh, 1878 Rayleigh and Nachtrieb, 1957) is a widely accepted and significant consensus used for explaining the maintaining mechanism of thermo-acoustic instability. This was the thermoacoustic coupling process that maintained the HFCI. The heat release from combustion was fed into the resonant acoustic mode. Once the HFCI arose, no matter what standing mode or spinning mode, the pressure and heat release oscillated totally in phase temporally and coupled spatially. As the O/F was set 4.4 and the propellant distribution was completely uniform, the numerical result yielded a 1T wave node featured a spinning behavior, which was a traveling 1T wave mode. The pressure wave node was located at a fixed diameter, showing a 1T standing wave mode. When the mass flow of fuel for two injectors was modified, severe HFCI occurred. The chamber pressure obtained from the numerical simulation and experiment showed a good agreement, which validated the numerical model. Propellant was evenly distributed in all injectors and no HFCI occurred.

In the nominal operation condition, the ratio of oxygen-to-fuel (O/F) was 2.5. Two different kinds of 1T patterns, standing wave mode and traveling wave mode, were captured in the present work. Self-excited high frequency combustion instability (HFCI) of first-order tangential (1T) mode was observed in a staged-combustion LOX/Kerosene liquid rocket engine numerically. Department of Aerospace Science and Technology, Space Engineering University, Beijing, China.Kangkang Guo Boqi Xu Yongjie Ren Yiheng Tong* Wansheng Nie*

Apollo 4 combustion instability