Interaction Between Fluid-Dynamic and Acoustic Instabilities in Combusting Flows Within Ducts

Abstract

Measurements of pressure fluctuations in sudden-step expansion flows of round cross-section are examined to determine the effect of matching the fluid-dynamic frequency associated with large scale coherent structures in the near field of the jet to the acoustic or bulk-mode frequency of the confining duct. In one case, the fuel and air were premixed with an equivalence ratio of 1.0 and in the other the fuel was injected radially through discrete holes a short distance upstream of the expansion with an overall equivalence ratio of 0.52. The results show substantial increases in the amplitude of the instability when the two frequencies are matched and the linear relationship between the velocity and frequency implied by a constant Strouhal number of around 0.3 leads to frequencies outside the range previously shown to be associated with the much higher amplitudes of oscillations of rough combustion.