Interaction of acoustic waves with a cryogenic nitrogen jet at sub- and supercritical pressures

Abstract

Abstract : To better understand the nature of the interaction between acoustic waves and liquid fuel jets in rocket engines, cryogenic liquid nitrogen is injected into a room temperature high-pressure chamber having optical access on its sides. A piezo-siren capable of generating sound waves with an SPL of up to 180 dB is used under three chamber pressures of 1.46, 2.48, and 4.86 MPa. The reduced pressures for these pressures are 0.43 (subcritical), 0.73 (near-critical), and 1.43 (supercritical), respectively. The assembly consisting of the acoustic driver and the high- pressure chamber form a cavity that resonates at several frequencies, the strongest being at 2700 and 4800 Hz. Three different flow rates are considered and the nature of the aforementioned interaction has been documented via a high-speed imaging system using a CCD camera. It is found that the impact of the acoustic waves on the jet structure is strongest from low to near-critical chamber pressures and at low injectant flow rates. No significant effects of the acoustic waves are detected at the supercritical chamber pressure examined. It suggests that engine operation either near the critical point or in transition passing through the critical point could be troublesome and may lead to or feed combustion instabilities in liquid rocket engines. Further work is needed to directly relate these effects to the observed instabilities.