A Study of the Nonlinear Acoustic Response of Area-contractions and Sense-lines

Abstract

Rocket and gas turbine engine combustion instabilities, which lead to rapid engine failure through enhanced heat transfer rates and high-cycle fatigue, continue to be a serious concern for engineers. Experimental pressure measurements remain the best approach in determining the susceptibility of an engine to acoustically coupled combustion instabilities. However, the harsh, high-temperature environment requires that pressure transducers be remotely mounted to the engine's main chamber using “sense-lines”, creating an area-contraction at the connection point between the sense-line and combustor. Preliminary measurements have shown large discrepancies between sense-line measured and engine pressure amplitudes. To elucidate these discrepancies, this experimental study measured the nonlinear acoustic response of the area-contraction and sense-line with and without purge air flow. To do this, a multiple-microphone impedance tube was used to measure the acoustic impedance of the combined area-contraction and sense-line. Measurements were performed over a range of frequencies, area-contraction ratios, acoustic velocity amplitudes, and sense-line length-to-diameter ratios. These measurements revealed the acoustic response of the sense-line and area-contraction combination is highly nonlinear.