Experimental studies on combustion instabilities in a multi-element, rectangular rocket chamber

An experimental investigation of transverse mode combustion instabilities in a gaseous methane, liquid oxygen (LOX) rocket was conducted at The Pennsylvania State University. The combustion chamber was rectangular in design to facilitate study of the transverse modes and consisted of multiple swirl coaxial injector elements and nozzle elements. Dimensions chosen for the chamber were such that the 1W and 2W transverse modes were the lowest frequency acoustic modes of the chamber. The design of the injector allowed for variation in placement of individual injector elements.;An analysis of initial experiments in the chamber indicated that injector placement influences stability characteristics. Placement of injector elements at anti-node locations of the chamber 1W mode showed larger pressure oscillation amplitudes than injector placement at 1W node positions. Despite the influence of injector placement, no self-induced combustion instabilities of significant amplitude (> 10% P'p-p /Pc) were observed, although short duration bursts of pressure oscillations with significant amplitude were observed. This phenomenon showed large amplitude oscillations (∼ 10% P'p-p /Pc) occurring through the steady state combustion cycle. These bursts occurred at random times and for short durations.;The most important observations in the current study were the identification of two key parameters which influence stability levels. It was observed that as injector momentum ratio and the ratio of propellant mass flowrate to mean chamber pressure (m˙/Pc) increased, instability levels ( P'p-p /Pc) also increased. A parametric study undertaken to systematically change these parameters supported this observation.;Additional studies were conducted utilizing a pulse gun that was fabricated for the chamber to observe dynamic stability characteristics. Damping times for pressure oscillation amplitudes excited by the pulse gun were very short (< 2 ms), indicating largely dynamically stable behavior of the rocket engine. However, results indicate that for higher injector momentum ratio tests, decay times increase as m˙/Pc increases. This supports observations made during the parametric study on the stability characteristics of the chamber.;A review of potential mechanisms related to the observed trends in that data was also conducted. Despite a lack of data to support or refute these mechanisms, several bodies of work were reviewed in an effort to discuss potential mechanisms influencing stability characteristics.