Effect of overall equivalence ratio on minimum nitric oxide emission index in laminar and turbulent partially premixed flames

NO formation in laminar and turbulent partially premixed flames is studied as a function of the amount of air introduced into both the central fuel tube and the annular co-flow air passage of a concentric tube burner. The fuel tube equivalence ratio (phi(B)) is varied from 1.0 to 10, while the overall equivalence ratio (phi(o)) is varied from 0.3 to 0.9. Surprisingly, for both the laminar and turbulent flames, absolute EINOx levels tend to decrease with increasing phi(o). Moreover, for each phi(o), EINOx displays a minimum at an intermediate value of phi(B). However, for the laminar flames, the greatest relative EINOx minimium is observed for phi(B) = 2.5 at phi(o) approximate to 0.7. Similarly, for the turbulent flames, the greatest relative EINOx minimum is observed for phi(B) = 1.6 phi(o) approximate to 0.6. In both cases, the greatest reduction in EINOx as phi(o) rises occurs for phi(B) values greater than those at minimum EINOx.