| Summary: | 碩士 === 國立成功大學 === 航空太空工程學系碩博士班 === 98 === In the liquid rocket propulsion, the combustion instability may severely damage the system. This research focused on the design analysis of the suppression mechanism for combustion instabilities using acoustic simulations. The model combustors with variable inside diameters (5.0cm and 6.4cm) and lengths (6.5cm, 8cm and 11.5cm) were used to simulate the acoustic characteristic in liquid rocket engines. By utilizing a speaker to transmit sound waves into the model combustors, microphone was set to measure their acoustic properties.
The experimental results showed that the length of the combustor affected mainly the frequencies of the longitudinal resonance of sound waves, and, the diameter of combustor affected the frequencies of the tangential sound-wave resonance. In the experiments, the installation of a convergence nozzle complicated the resonant frequencies in the chamber, while the chamber pressure showed little effects on the resonating frequency up to 150psi.
In the acoustic suppression simulation, quarter-wave resonator, Helmholtz resonator, and baffle devices were tested. The results showed that baffles on injector plate suppressed tangential sound-wave resonance, and the height of the baffle’s significantly increase the effectiveness of suppression. Compare to quarter-wave resonator, Helmholtz resonator showed a better suppression effect. To avoid inducing new resonating frequencies, the inlet area of the resonator should be 0.2%~0.5% of the injection surface area, while the total inlet area of the installed resonators should be less than 3% of the injection surface area. By maximizing the cavity’s volume and keeping the cavity’s length-to-diameter ratio closed to unity, the Helmholtz resonator showed the best suppression effects on the resonating wave in the chamber. In practical, the speed of sound and the instability in the combustion chamber were too complicate to be predicted. The design of an effective Helmholtz resonator for instability suppression shall require the information of the measured frequency spectrum of instabilities as well as the measured temperatures near the position of the resonator.
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