Coupling of cryogenic oxygen-hydrogen flames to longitudinal and transverse acoustic instabilities

Abstract

A rectangular combustor with acoustic forcing was used to study flame–acoustic interaction under injection conditions that are representative of liquid propellant rocket engines. Hot-fire tests using liquid oxygen and gaseous hydrogen were conducted at pressures of 40 and 60 bar, which are sub- and supercritical conditions, respectively, for oxygen. Examined samples of hydroxyl-radical emission imaging, collected using a high-speed camera during periods of forced acoustic resonance, showed significant response in the multi-injection element flame. Fluctuating acoustic pressure causes in-phase fluctuation of the emission intensity, producing response factor values of around 0.8. Transverse acoustic velocity causes shortening of the flame, concentrating heat release near the injection plane. The flame is also convectively displaced with transverse acoustic velocity, a process believed by many to be responsible for driving transverse mode high-frequency combustion instabilities. The analysis in this work was extended to detect the response of internal flame processes to the transverse acoustic disturbance, namely, pressure response and enhanced transport and mixing of non-premixed propellants. A flame-tracking technique was implemented to isolate the response of these processes from that of collective convective displacement. The results show a significant contribution to overall flame response from the response of internal processes; however, further work is required to quantify the relative contributions of pressure or propellant transport.