Combined effect of equivalence ratio and velocity gradients on flame stability and emission formation

Premixed flames have shown to behave differently while operating under equivalence ratio or velocity gradients. This distinct behaviour can be used to improve the performance of premixed burners. The present work analysed the effect of both equivalence ratio and velocity gradients on the stability of methane premixed laminar flames. Additionally, the influence of these gradients on CO and NOx emissions was quantified. Results show that using equivalence ratio and velocity gradients can extend the stability limits of the burner. The lifting phenomenon commonly found on peripheral flames was cancelled by using higher equivalence ratios and lower flow velocities in the primary regime. Consequently, stable flames were achieved in the secondary regime for substantially leaner mixtures. However, flame extinction was eventually reached in the inner region of the burner, due to the tip opening of secondary flames. The minimum global equivalence ratio, phi(g), was extended from 0.80 to 0.65, solely by using equivalence ratio gradients. When combined with a velocity gradient, the latter effect was further extended up to phi(g) = 0.52. Regarding emissions, results show that different combinations of equivalence ratio and velocity gradients result in distinct NOx and CO emissions, even for the same phi(g). Under this stratified arrangement, a reduction of 43% was achieved in NOx formation, when compared with the homogeneous case. Overall, results suggest that introducing richer peripheral flames allows a higher extension of the lower stability limits, whereas leaner peripheral flames lead to lower NOx emissions.