The influence of swirl on oscillations in ducted premixed flames

Abstract

Combustion oscillations in premixed flames stabilized behind a disk, rearward-facing steps, and annular rings without and with bypass flow in round ducts have been examined as a function of swirl number, upstream and downstream duct lengths, exit constriction, and equivalence ratio. With disk stabilization, rough combustion was associated with a quarter-wave in the upstream duct and swirl extended the length over which heat release occurred, thereby decreasing the susceptibility to rough combustion. In contrast, swirl shortened the length over which heat release occurred downstream of the rearward-facing steps and increased the susceptibility to rough combustion associated with quarter-waves in long downstream ducts without an exit nozzle. Since swirl improved flame stabilization, bulk-mode oscillations associated with exit constriction were experienced only for exit area ratios less than 0.25. The amplitude of oscillations in flames stabilized behind multiple-step expansions was small for shorter ducts without an exit nozzle, due to the flame stabilizing behind the second step, but longer ducts without and with constriction and swirl led to higher amplitudes. Annular rings, like disks, were associated with quarter-wave oscillations in the upstream duct, but swirl appeared to have no influence on the amplitude and frequency of oscillations. When the clearance between the outer diameter of the ring and the pipe was small, the ring acted like short rearward-facing steps and gave rise to rough combustion associated with a three-quarter wave in the entire duct with swirl increasing susceptibility to rough combustion. Bypass flow arrangements led to lower flammability limits and smaller amplitudes of oscillation due to poor mixing between the main and bypass flows, and swirl improved mixing between the flows and led to results similar to those in premixed flows.