The Effectiveness of Passive Combustion Control Methods

Flame stabilization in a swirl-stabilized combustor occurs in an aerodynamically generated recirculation region which is a result of vortex breakdown. The characteristics of the recirculating flow are dependent on the swirl number and on axial pressure gradients. Coupling to downstream pressure pulsations is also possible. Control methods of unstable thermoacoustic modes and reduction of NO_x and CO emissions were investigated in a low-emission swirl-stabilized industrial combustor. Several axisymmetric and helical unstable modes were identified for fully premixed and diffusion type combustion. In addition to mode variation, the instabilities spanned a wide range of frequencies. The unstable modes that were associated with flow instabilities of the wake-like region on the combustor axis due to vortex breakdown (VBD), shear layer instabilities at the sudden expansion (dump plane) and equivalence ratio fluctuations were in a range of normalized frequency St = 0.5–1.1. Other unstable modes at higher frequencies of St = 7.77, were excited by the Kelvin-Helmholtz vortices shed at the burner’s exit. The combustion structures associated with the different unstable modes were visualized using phase locked images of OH chemiluminescence and analyzed using cross-correlations between OH detecting fiberoptics. After identifying the structure of the instabilities and determining their source, different geometrical changes were applied to disrupt their formation or vary their characteristics. These modifications reduced the periodic heat release and enabled decoupling of the heat from acoustic modes that led to thermoacoustic instabilities. The passive control techniques that will be described in this paper were effective in suppressing the thermoacoustic pressure oscillations and also reduced NO_x and CO emissions.