Nonlinear acoustic response of stand-off tubes used in acoustic pressure measurements—Part I: Experimental study

Rocket engine combustion instabilities, which lead to rapid engine failure through enhanced heat transfer rates and high-cycle fatigue, continue to be the most serious concern facing engine designers. Experimental testing and pressure measurements remains the best approach to determine the susceptibility of an engine design to acoustically coupled combustion instabilities. But, the harsh, high-temperature environment requires that pressure transducers be remotely mounted to the engine’s main chamber using “sense-tubes,” thereby creating an area-contraction at the connection point between the sense-tube and combustor. Preliminary measurements showed large discrepancies between sense-tube measured and engine acoustic pressure amplitudes. To elucidate these discrepancies, this experimental study measured the nonlinear response of the area-contraction/sense-tube geometry. Specifically, the sense-tube was attached to a two-microphone impedance tube, allowing the measurement of the acoustic impedance of the combined area-contraction and sense-tube; the acoustic pressure was also measured at the sense-tube termination, allowing the the direct measurement of the frequency response function. Measurements were performed over a range of frequencies, area-contraction ratios, acoustic velocity amplitudes, and sense-tube length-to-diameter ratios. These measurements reveal that the acoustic response of the sense-tube was highly nonlinear—even for low amplitude forcing.