Combustion Oscillations in Dump Combustors with a Constricted Exit

Combustion oscillations in axisymmetric dump combustors have been examined in terms of amplitude and frequency characteristics for two dump-plane area ratios and as a function of combustor length, exit constriction, diameter, flowrate, equivalence ratio and swirl. The flammability limits are similar to those previously determined in disc- and dump-stabilized flames without a constricted exit, but the stability limits are not. Rough combustion, characterized by radiated sound levels more than 12 dB above that in smooth combustion, was observed at equivalence ratios close to the flammability limits for values of swirl number less than 0.2 and was associated with the bulk-mode frequency. With swirl numbers in the range from 0.2 to 0.4, rough combustion was not encountered and, for higher values, existed in a range of equivalence ratios from around 0.8 to 1.4, provided the combustor length and flowrate led to half-wave frequencies less than around 800 Hz. In those ranges of equivalance ratio where the combustion was smooth, discrete frequencies corresponding to the bk-mode and half-wave were observed. The amplitude of the discrete frequency increased when it coincided with the shedding frequency of the shear layer.