Combustor performance enhancement through direct shear layer excitation

Abstract

Previous studies of turbulent nonreacting shear flows have shown that flow excitation can provide enhance entrainment and mixing. In the present study, the effects of periodic flow excitation on the performance of a two-dimensional dump combustor were investigated for lean premixed conditions. The flow excitation was in the form of a sinusoidal cross-stream velocity perturbation applied just upstream of the flow separation. The forcing frequencies, chosen such that they corresponded to resonant and off-resonant vortex shedding frequencies indentified in unforced combustion, ranged from 35 to 400 Hz. The effect of forcing on both nonreacting and reacting flowfields was to modulate the formation of vortex structures just downstream of the flow separation. In the nonreacting flowfield, the shear layer spreading rate increased when forcing was applied. In the reacting flow, forcing caused a modulation of the flame structure. Forcing increased the mean CH emission intensity from the flame, which is related to mean volumetric energy release, up to 15%, reduced the rms pressure fluctuation level by up to 30%, and reduced the equivalence ratio at the lean blowoff limit up to 6%. NO x emissions were reduced by up to 20% with forcing. The forcing location and excitation frequency and amplitude are important parameters in gaining effective combustion control. Performance improvements generally increased with increasing excitation amplitude and increasing frequency within the operating constraints of the excitation system. The mean CH emission intensity was found to be proportional to the mean flame surface area, which increased with forcing, suggesting that the observed increase in volumetric energy release was due to an increase in flame area with forcing. The coupling of heat release and the pressure field was investigated using Rayleigh's criterion, and the analysis showed decreased flame driving of the dominant low-frequency modes with forcing applied, resulting in a reduction of the magnitude of rms pressure fluctuations.