Effect of stratification on thermoacoustic instability, emissions, and flame macrostructure in a swirl-stabilized dual annular burner: An experimental study

Stratified flames have attracted significant attention due to their superior resilience to turbulence and enhanced flame stability, enabling reduced NO_xand CO emissions. In this study, an innovative dual annular stratified burner was designed and experimentally investigated to characterize thermoacoustic instability, emissions and flame macrostructure in swirling and non-swirling (jet) methane-air flames. Experiments were systematically conducted across stratification ratios (SR = 1–3) and global equivalence ratios (Φ_g) ranging from lean blowoff to rich conditions (Φ_g = 1.2). Swirling flames exhibited consistently acceptable emissions (NO_xand CO < 20 ppm) under stable lean operating conditions (Φ_g = 0.55–0.8) for all SRs tested. Jet flames showed no thermoacoustic instabilities irrespective of SR or Φ_gvariations. Similarly, swirling flames remained stable for Φ_g < 0.8; however, at Φ_g = 0.8, thermoacoustic instability initiated, characterized by coupled oscillations of acoustic pressure and heat release fluctuations. These oscillations were sustained until Φ_g = 1.1, beyond which decoupling occurred. Limit cycle oscillations with heightened sound pressure amplitudes were observed at lower stratification ratios (SR = 1–1.5), whereas no limit cycles were detected at higher SR values (>1.5). Increasing SR significantly suppressed instability amplitudes, notably resulting in a 70 % reduction of oscillation amplitudes at Φ_g = 0.9 when SR increased from 1 to 3. Flame macrostructure analysis confirmed improved anchoring and mixing characteristics of swirling flames compared to jet flames, particularly at higher SR conditions. This work highlights that controlled stratification effectively enhances operational stability and produces more compact flames in swirling combustors, offering valuable insights for developing low-emission and high-efficiency combustion systems.