Ph.D. Thesis Proposal

by

Shane V. Lympany

(Advisor: Dr. Krish K. Ahuja)

Acoustic Damping Mechanisms of Propellant Injectors

10:00 AM, Friday, June 23, 2017

Montgomery Knight

Room 317

Abstract:

Combustion instabilities in liquid rocket engines are caused by coupling between the combustion process and pressure oscillations in a combustor, and they are characterized by the frequencies and shapes of the acoustic modes of the combustion chamber. Acoustic resonators are commonly installed in combustors to provide passive acoustic damping and prevent combustion instabilities. Previously, it has been proposed that the propellant injectors in a combustor can be tuned to act as half-wave resonators and provide acoustic damping, which requires a thorough understanding of the acoustic damping mechanisms of injectors.

In this work, the acoustic damping of propellant injectors is measured experimentally. A new experimental facility is developed to measure the sound power reflection, transmission, and dissipation under the conditions of mean flow, high amplitude, high temperature, and higher-order modes. The effects of common design parameters – namely, the typical features of an injector, the number of injectors, the ratio between the cross-sectional area of the injectors and the combustion chamber, and the position of the injectors – on the absorption coefficient are investigated experimentally using the new facility. The effects of mean flow, high amplitude, high temperature, and higher-order modes are also investigated. Measurements of the fraction of sound power dissipated by the injectors and the velocity flow field at the ends of the injectors are used to elucidate the physical mechanisms responsible for the acoustic damping. Attempts are made to quantify the separate contributions of viscous dissipation and the conversion of sound to vorticity. An analytical model incorporating these acoustic damping mechanisms is developed to predict the absorption coefficient of the injectors for each of the measured geometric parameters and operating conditions.