An experimental investigation of the nonlinear acoustic response of acoustic sense-ports

Abstract

Combustion instabilities continue to hinder the development of rocket engines and high-efficiency, low NOx combustion technology used in gas turbine engines. Experimental pressure measurements remain the best method to assess combustion instabilities. However, the harsh, high-temperature environment requires remotely mounting pressure sensors using sense-ports, which cause large discrepancies in measured thrust chamber acoustic pressure amplitudes. For this study, a multi-microphone impedance tube was used to investigate the nonlinear response of an acoustic sense-port. Measurements were performed for frequencies and driving amplitudes ranging from 100 Hz to 1500 Hz and 120 dB to 175 dB, respectively. Measurements were made using four different sense-port area-contraction ratios and for different extension tube lengths. The sense-port and extension tube acoustic responses were measured separately to enable the determination of the abrupt area contraction acoustic response. Measured sense-port area contraction length corrections were found to be in close agreement with the literature. The rigidly terminated sense-port extension tube exhibited linear acoustic damping. Measurements of the abrupt area contraction acoustic response revealed highly nonlinear damping even at low acoustic pressure amplitudes due to flow separation at the abrupt area contraction caused by the local acoustic velocity.