Effect of mixture flow stratification on premixed flame structure and emissions under counter-rotating swirl burner configuration

An investigation of the flame structure and emission performance of stratified swirl methane/air flames was performed by using a double-annulus counter-rotating premixed swirl burner. Stratification of the flow and mixtures were established by varying the bulk air flow rates and mixture equivalence ratios between the inner and outer annuli. Two distinct flame fronts were stabilised at the burner outlet, separated by a shear layer due to velocity differences. Higher swirl flow in the inner annulus generates an elongated and enlarged area of flame reaction zone due to increased flame intensity, as the flame shape is strongly dependent on the velocity magnitude exiting the annulus. Mixture and flow stratification affect local emissions. A richer mixture stratification within the inner channel at 70:30 flow split results in 91% and 49% higher emission rates of NO and CO respectively compared to premixed arrangement, in spite generally aiding flame stability. Enrichment of the outer annulus at 70:30 split flow shows only slightly higher levels of NO and CO emissions by 3% and 9% respectively compared to a homogenous mixture.