The frequency and strength of combustion-induced oscillations have been measured for premixed flames stabilized on baffles located on the axis of a pipe. With the baffle located between 5 and 20 diameters upstream of the pipe exit, the frequency corresponds to longitudinal (standing quarter-wave) acoustic waves in the cold gas column upstream of the baffle and the strength is only weakly dependent on axial location, area blockage ratio (0.25 and 0.5), forebody shape (disk or cone), and Reynolds number (<70,000); with a 0.9 blockage ratio, a half-wave mode was observed. The instability is present in the range of equivalence ratios (defined as the fuel-toair ratio divided by the stoichiometric fuel-to-air ratio) from 0.8 to 1.2. The pressure coefficient based on the difference in mean pressure between the center of the baffle and pipe wall is a sensitive measure of the onset of oscillations.
Combustion-induced oscillations, with a predominant frequency and large amplitude, were not observed when the baffle was located between approximately 1.5 and 5 pipe diameters from the pipe exit. As the baffle was moved from the 1.5 diameter location toward the pipe exit, increasingly strong oscillations were observed with equivalence ratios between 1.1 and 1.5. The frequency of these oscillations increased with decreasing equivalence ratio and increasing Reynolds number. Instability could be obtained with the baffle between 1.5 and 5 diameters of the pipe exit by constricting the diameter of the exit plane.
The difference in behavior between confined and unconfined flames is ascribed to the existence of a selfinduced favorable pressure gradient in the confining duct. It is proposed that the instantaneous value of this gradient influences the rate of heat released by the flame through changes in the rate of turbulent mixing due to the preferential acceleration of products of combustion over reactants.