REDUCTION OF COMBUSTION PRESSURE FLUCTUATIONS IN ROCKET ENGINE

While many studies to date on combustion instability in rocket engines have focused on high-frequency pressure fluctuations, there have been few studies on low-frequency pressure fluctuations in combustion chambers. Lowfrequency pressure fluctuations are thought to occur near natural frequencies of launch vehicles and can trigger vehicle vibrations. To determine the conditions where low-frequency pressure fluctuations increase, we reanalyzed combustion test data previously obtained at NAL and found that pressure fluctuations tend to increase at low LOX injection velocities. Based on this finding, we constructed two kinds of modified injector elements, one with a smaller flow area cross section and the other with a tapered LOX post. We then performed sub-scale model experiments with these elements. Experimental results showed these modifications to be effective in reducing low-frequency pressure fluctuations in combustion chambers. In addition, using a one-dimensional model for flow in the recess of the injector element, the influence of the tapered LOX post on atomization of the LOX jet was investigated from the point of view of an increase of momentum ratio of propellants. From calculations, it was found that LOX jet expansion in the recess has a major effect on the increase of the momentum ratio.