Combustion oscillations close to the lean flammability limit

The nature of premixed methane-air flames stabilised on a symmetric, plane sudden-expansion has been examined in terms of wall pressures and the chemi luminescence of the CH radical. Large-amplitude acoustic oscillations at th e half-wave frequency of the entire duct were observed at high velocities a nd with near-stoichiometric mixtures, and instabilities with much lower-fre quencies close to the lean and rich extinction limits. The emphasis is on t hese near-limit instabilities and, due to the asymmetry of the plane flow, the experiments were extended to round ducts in which the flammability and stability limits were similar to those in the plane duct. The branches of f lame behind the steps of the plane expansion extinguished non-simultaneousl y, and gave rise to low-frequency flapping oscillations immediately prior t o extinction of the first of these, and the remaining branch gave rise to l ateral oscillations prior to its extinction. The oscillations were associat ed with axial movement of extinction along the shear layer due to the high strain rates close to the step, until the strain rate was sufficiently low and allowed upstream propagation of the flame through the recirculation reg ion. The flames in the round duct gave rise to oscillations of the same nat ure. The frequency of the oscillations increased with flow velocity and fla me speed, and the amplitude with heat release and much more with constricti on of the duct exit due to coupling with a bulk-mode of the combustor cavit y. These increased amplitudes caused the flammability limits to narrow due to the consequently higher strain rates and have implications for gas turbi ne operation.