Examination of a helmholtz resonator for use in enhancing hybrid fuel grain regression.

The use of a Helmholtz resonator, placed in the oxidizer line upstream of the injector, to supply disturbances in the precombustion chamber of a hybrid rocket motor was examined. The intent was to disturb the combustion turbulent mixing with a resonant frequency of the Helmholtz resonator, hence to increase the hybrid fuel regression rate.

For these studies a hybrid rocket motor, test stand, oxidizer feed line, and Helmholtz resonator were constructed. Only cold flow tests were performed, using high pressure air as the test media.

The characteristics of the Helmholtz resonator were determined by placing a condenser microphone within the resonator and on the oxidizer feed line opposite to the resonator. Frequency spectra were measured for various resonator geometries as a function of the oxidizer line mean velocity.

The measured sound power level due to the Helmholtz resonator was in the same range as the measured noise inherent in the entire system. Lack of a distinct resonant frequency, stronger than the system noise, indicates that the Helmholtz resonator configuration used herein will not produce a resonant frequency strong enough to increase the fuel burn rate of a hybrid rocket engine. Thus, other passive means must be found to disturb the oxidizer flow strongly enough to have an influence on the fuel regression rate. One such design, namely a protuberance into the oxidizer line displayed a definite peak in the frequency in the precombustion chamber. Such designs, which may be more successful, need to be pursued.