PREMIXED TURBULENT FLAME INSTABILITY AND NO FORMATION IN A LEAN-BURN SWIRL BURNER

Computed results are presented from a Reynolds stress, stretched lamin ar flamelet model, for premixed swirling combustion in a rotating matr ix burner. These are in good agreement with flame photography and cohe rent anti-Stokes Raman spectroscopy (CARS) temperature measurements. T he principal variable is the equivalence ratio, phi, while the swirl n umber and mean axial entry velocity remain constant. At higher equival ence ratios, the flame is stabilized by hot gas in both the inner and outer recirculation zones. As phi is reduced below 0.6 another steady- state solution appears, in which the flame is stabilized only by the h ot gas in the inner zone. In this regime, the experiments show unstabl e combustion with low-frequency oscillations between the two states. T his instability is shown to originate in flame quench in the regime be tween the two recirculations. At the values of phi below 0.6, most of the NO produced originates as prompt NO in the reaction zone. When phi is reduced to 0.56, there is only one steady-state solution. While th e investigations were of a laboratory scale burner, some of the genera l findings are relevant to practical combustors. (C) 1998 by The Combu stion Institute.