Optimum placement of Helmholtz resonators for damping pressure oscillations.

Abstract

Cold flow and hot firing tests were conducted to find the optimum design and positioning of an acoustic resonator for damping high intensity pressure oscillations. The variables in the cold flow tests were the position of the resonator and its volume. In the hot firings more than one resonator was used and the gas properties were also varying. Pressure node or antinode locations of the resonator were emphasized in the tests and the resonator cavity volume was varied from fully closed to three and a half times the resonant volume calculated by the Helmholtz resonator theory. It was found that the resonator at the pressure node provided little or no damping while when positioned at the pressure antinode the sound intensity in the cold flow duct was reduced to less than 50% of the undamped amplitude. The range of damping was found to be narrow and centered around the volume calculated by the Helmholtz resonator theory. The results of the phase lag between the pressures in the cavity and duct and the velocity in the orifice, connecting the cavity and the tube closely corresponded to those predicted by Sirignano in Reference 14. An investigation of the effective length of the orifice in the Helmholtz resonator indicated that a single end correction was appropriate (rather than the double end correction for conventional acoustic amplitudes) - this also substantiated the jet flow model.