Effect of inlet turbulence and premixer length on fuel distribution in swirling gas-turbine premixer

The mixing characteristics of a production gas-turbine combustor premixer have been investigated using a planar Mie-scattering imaging technique. Specifically, the effects of introducing controlled inlet grid turbulence (intensity and scale) and varying premixer length on large-scale mixing were independently studied. Baseline data acquired for a particular swirler configuration indicated a strong fuel concentration gradient in the annulus, with a maximum at the centerbody and decreasing outward, The measured fuel concentration, or equivalence ratio, also varied circumferentially, by as much as 45% over the entire premixer exit plane. The circumferential fuel concentration profiles showed that improved mixing with increased turbulence intensity was realized for most angular locations. Premixer length extensions of 2.5 and 5.0 cm, corresponding to 0.53 and 1.06 hydraulic diameters, respectively resulted in a considerable improvement in mixing, accompanied by significant structural changes in the scalar field. The circumferential variations were reduced up to 30% for the 5.0-cm extension case. Radial fuel concentration gradients were also reduced by 50%. Overall, increased residence time was a significantly more effective means of mixing enhancement than introducing moderate levels of freestream turbulence.