Acoustic decay coefficients of simulated rocket combustors

Abstract

Cold acoustic bench tests were made to obtain coefficients of decay of cylindrical acoustic modes in simulated rocket combustion chambers with no throughflow.The tests were performed to obtain decay coefficients for typical chambers with and without baffles and to indicate damping trends with chamber length, nozzle angle, and injector and baffle shapes.The tests were made by generating the desired mode with an acoustic driver in the simulated combustor at atmospheric pressure.When the power to the driver was abruptly shut off, the decay of the field was monitored with wall-mounted condenser microphones.All tests were run at acoustic pressure levels of 123 to 143 decibels (0.0041 to 0.041 psi, 28 to 280 N/m ); so the acoustic field was essentially sinusoidal in behavior.The nozzle was plugged at the throat.ing transverse mode increased with increasing internal area-to-volume ratio of the test chamber.The effective acoustic length of the chambers was determined from the resonant frequency of the longitudinal mode, and the decay per cycle of this mode increased with increasing effective length.The effective length of tapered chambers depended on injector shape and was a function of nozzle convergence angle.Pure radial modes usually could not be generated in noncylindrical chambers but exhibited low damping per cycle compared to the other two fundamental modes.by obstructing particle motion and usually produced much higher damping than unbaffled chambers.Data for baffled chambers a r e included, but no correlation for the baffles was found. 2The results for unbaffled chambers showed that the damping of the fundamental stand-Baffle shapes comprising radial vanes and circular rings distorted the acoustic fields INTRO DU CTlQNThe high-frequency pressure and particle-velocity oscillations which occur in liquid-