EFFECT OF ACOUSTIC OSCILLATIONS ON FLAME DYNAMICS IN SWIRL BURNERS

摘要

The likelihood of self-sustained oscillations in modern combustion systems has increased since lean premixed or partially premixed combustion conditions are currently being employed as a way to reduce NO_x emissions and gain higher efficiencies. These undesirable large pressure variations, commonly found over a wide range of operating conditions, arise due to coupling between the periodic heat release and the natural acoustic modes of some geometric elements of the system. This paper presents an experimental study of a 100 kW scale model of a 2MW swirl burner/furnace system. Analysis of the interaction between pressure and velocity fields throughout the system is aided by a detailed study of phase locked laser anemometry and pressure measurements in both cold and reactive components of the system, i.e. inlet geometry and burner/furnace. The feedback mechanism, heat release-pressure wave coupling, is explained by the fluctuation of fuel/air flow rates. Two main instabilities are encountered. One type adapts to some of the natural acoustic modes of the supply inlet geometry and has a significant effect on the inlet flow, inducing reverse flow for approximately one sixth of the cycle. The oscillation driving mechanism is related to the fluctuation in reactants supply to the flame holder. Flame destabilization phenomena such as disappearance of the central reverse flow zone are a consequence of the oscillation and do not play a major role in driving the instability. The second instability encountered presents smaller oscillation amplitudes and a higher frequency matching the longitudinal acoustic mode of the furnace. The fluctuations in inlet flow rates, for this second case, are significantly smaller but still provide the necessary variation in mixture rate to generate a concentrated heat release at a certain time of the limit cycle.