Effects of flow–flame interactions on the stabilization of ultra-lean swirling CH[formula omitted]/H[formula omitted]/air flames

Abstract

Hydrogen (H2) is regarded as a promising fuel to achieve decarbonization of power and propulsion systems. In this context, hydrogen enriched methane (CH4) combustion has attracted considerable attention in the development of low-emission gas turbines. To achieve low NOx emissions and avoid the dangers of flashback, combustion of CH4/H2/air mixtures under lean and/or ultra-lean operating conditions is of critical importance, while ultra-lean flames are prone to combustion instabilities and difficult to stabilize even in a bluff-body swirl burner. In this work, a series of confined lean premixed CH4/H2/air swirling flames with hydrogen enrichment (αH2) ranging from 0 to 80% is investigated under stable and ultra-lean conditions using simultaneous OH-PLIF and PIV measurements. The results suggest that decarbonization of combustion devices requires large volume fractions of H2 in the fuel mixture, e.g.,80% H2 to achieve half CO2 emission per heat of combustion. It is found that there is a flame topology transition when changing equivalence ratio and/or hydrogen enrichment. At a given αH2, the flames with 0 and 40% H2 always show “V” shapes, whereas an evolution from “M” to “V” shape can be observed for the 80% H2 flame when increasing the equivalence ratio. Moreover, at a given ϕ, the flame shape will shift towards “M” shape at αH2 = 80% from “V” shape at αH2 = 0,40%. Furthermore, H2-enriched flames would move to the inner recirculation zone (IRZ) and stabilize there when decreasing ϕ to ultra-lean conditions. Given that hydrogen enrichment can significantly enhance the resistance to flame strain and that under ultra-lean conditions, there is a strong diffusion of hydrogen from the swirling jet to the IRZ where the sufficient residence time and the increase in the local equivalence ratio contribute to the presence of flame pockets and flame stabilization in the IRZ.