Combustion of premixed methane and air in ducts

Extinction limits are reported for flames stabilized on sudden expansions in plane and round ducts, with acoustically open and closed upstream ends, with and without downstream nozzles and, in the case of the round duct, with added swirl. Two main regimes of combustion oscillations were observed, one close to stoichiometry and the other close to the limits, and details of the corresponding flame structures were obtained from simultaneous measurements of pressure and imaging of the chemiluminescence of the CH radical. Similar results are presented for disc-stabilized flames. The pressure measurements in the unconstricted sudden-expansion flows show that combustion oscillations with equivalence ratios close to unity were associated with longitudinal acoustic frequencies, and the chemiluminescence images show the roll-up of vortices close to the pressure maximum followed by their growth and convection. With an open upstream end, the dominant acoustic mode was a half wave in the entire duct, and it changed to a quarter or three-quarter wave when the upstream end was acoustically closed. The addition of a downstream nozzle introduced a bulk-mode oscillation that dominated in the round duct but the large-amplitude oscillations were still associated with a longitudinal mode in the plane geometry: the quarter wave in the upstream duct this time. The pressure signals showed low-frequency modulations that increased with flow rate and, therefore, with heat release, and these were associated with flame movements caused by high strain rates. As the equivalence ratio was reduced toward the lean-extinction limit, the pressure signals fluctuated with periods of the order of 100 ms and the chemiluminescence showed that these were associated with cyclic longitudinal movements of the flame. These movements were caused by local extinction due to the high strain rates in the shear layer close to the stabilizer, with subsequent downstream translation of the flame until the strain rate reduced sufficiently to allow the flame to re-establish and return through the region of recirculation. The disc-stabilized flames gave rise to similar oscillations prior to extinction. Constricting the ducts with an exit nozzle narrowed the flammability range by ca. 30%, increased the amplitudes of the near-limit oscillations by two orders of magnitude, and led to higher emissions of NOx. Attempts to suppress the near-limit oscillations in the sudden-expansion flows by imposing oscillations on the pressure field are described and reduced the amplitudes in the constricted ducts by up to 70% through suppression of the bulk-mode oscillation. They were less successful with near-stoichiometric oscillations and high flow rates, where the modulations presented problems that were not solved by active or open-loop control. Possible implications of the results for the operation of gas turbines are considered.