Acoustical Resonances Produced by Combustion of a Fuel-Air Mixture in a Two-Dimensional Duct. I.

Abstract

To better understand the mechanisms which initiate and maintain high-frequency combustion pressure oscillations in the air-breathing engine (commonly referred to as “screech”), an experimental program was conducted with a 212-in. by 10-in. model ramjet combustor. Results show that pressure oscillations characteristic of transverse acoustic modes occur as fuel-air mixtures approach stoichiometric values. The particular mode excited is related to orientation of the flame fronts in the duct, which in turn are functions of the particular flame holder geometry. It is shown that heat release associated with the flame fronts occurs in high impedance zones for the modes excited. High-speed motion pictures of the combustion zone during screech show that the flame fronts are serrated, the number of serrations being proportional to screech frequency. The serrations are interpreted to be related to vortex shedding from the flame holder lips. High amplitude oscillations corresponding to the fundamental longitudinal mode occurred when conditions of intense screech were achieved. This mode transference is attributed to increased heat addition in the forward portion of the duct, a position favorable for driving longitudinal modes. Preliminary results show that high turbulence levels produce screech at lower values of fuel-air mixture. (This work was performed under Contract AF 33(616)-2951.)