The macrostructure and stability of lean stratified methane-air flames

• Stratified dual-annular methane-air flames studied for stability and emissions. • Rich-core flames (SR > 1) improve lean blowoff limit by 19–55% over SR = 1. • Stability gain from radical diffusion outward enhances flame propagation. • SR > 1 flames are compact but produce higher NOx due to local hot spots. • Lean-core flames (SR < 1) show 2–10% lower LBO and up to 37% higher NOx. Stratified flames have received increased attention due to their enhanced flame stability and resilience to high turbulence. In the present study, the flame macrostructure, stability, and emissions of stratified diffusion (jet) flames produced using a dual annular burner are investigated. Stratified flames with a rich mixture in the inner annulus (stratification ratio, SR > 1) were found to exhibit improved stability, resulting in delayed blowoff (higher lean blowoff limit (LBO)) compared to flames with the same mixture composition in both annuli (conventional premixed flames). These flames showed an improvement in the lean blowoff limit by 19–55% relative to non-stratified flames, allowing for the combustion of leaner mixtures and driving all the benefits that entail. This enhancement in stability is attributed to the diffusion of heat and radicals from the rich mixture at the centre of the flame outward toward the extra lean mixture from the outer annulus. Structurally more compact, shorter flames produced by the favoured stratified flames were observed from the Abel deconvoluted images captured using CH* chemiluminescence. However, stratified jet flames with a rich inner annulus also exhibited higher thermal NOx emissions due to the elevated temperatures in the rich inner mixture forming localized hot spots. One way to fully reap the benefits of the stratified flames is to enhance the mixing of the streams by the use of a swirler. Conversely, jet flames with a leaner inner annulus (SR < 1) demonstrated poor stability with lifted flames but lower NOx emissions and poorer combustion efficiency. For cases where SR < 1, there is an unrecovered loss of heat and radicals away from the flame from the outer richer stream, which explains the poorer stability and efficiency. In fact, the lean blowoff limit in these cases decreased only by 2–10% compared to non-stratified flames. Additionally, higher NOx emissions were observed at higher equivalence ratios, owing to the elevated temperatures in rich flames. However, as a consequence of better mixing and reduced hot spots, thermal NOx emissions were found to decrease with increasing Reynolds number.