ACOUSTIC ADMITTANCE OF AN ALUMINIZED COMPOSITE SOLID PROPELLANT BY LASER DOPPLER VELOCIMETRY AT LOW PRESSURE

Abstract

The present study involves the use of laser Doppler velocimetry (LDV) to determine the acoustic admittance of an aluminized composite solid propellant at low mean chamber pressure. The aim of this study is to measure the fluctuating velocity component perpendicular to the burning surface of the particles in the gaseous phase above the burning surface of the aluminized composite solid propellant for the imposed acoustic pressure oscillations. To check the accuracy and precision of the mini-LDV used in this study the acoustic mode shape was generated for the imposed acoustic oscillations with amplitudes of 215 and 6 Pa. The results obtained are very satisfying; they show that the predicted acoustic velocity mode shape matches with the generated mode shape using the LDV. The response function of a solid propellant indicates the inclination of the propellant to cause acoustic combustion instability. Both velocity fluctuations and imposed pressure fluctuations are measured experimentally and used to determine the acoustic admittance of the solid propellant and hence deducing the pressure coupled response function from acoustic admittance. The probe volume of the laser is maintained at a constant distance of 800-900 μm from the burning surface by a servomechanism using a linear actuator. The incorporation of the linear actuator dramatically increases the number of particles passing through the laser probe volume. The linear actuator is controlled using a microcontroller and the servomechanism is operated under an open loop system. A high-pressure window bomb is designed and fabricated. Low-frequency ranges (140-280 Hz) of the longitudinal acoustic modes are investigated in this paper. Tests were conducted at a mean chamber pressure of 0.5 MPa. To generate the acoustic pressure oscillations a rotary valve is designed and fabricated. The rotary valve generates a standing wave of the characterizing frequency depending upon the length of the chamber. Tests are conducted for two aluminized composite solid propellants. The results obtained are in good agreement with the literature for the pressure coupled response function at these mean chamber pressures.