Effects of hydrogen enrichment on flame stability, thermoacoustic oscillations, and combustion characteristics in premixed propane swirl flames: An experimental study

This paper depicts the results and analysis of experimental investigations on the effect of hydrogen enrichment on the combustion characteristics of premixed C₃H₈/air flames in a Dual Annular Counter Rotating Swirl (DACRS) combustor. The novelty of the burner, developed for this experimental study, lies in its ability to independently regulate swirl and jet flows in concentric annuli. This design enables operation in multiple modes: swirl, jet, partially swirling, or stratified, thus effectively mimicking practical gas turbine conditions. In this work, the burner was operated under fully premixed swirl conditions. Hydrogen enrichment (0–50 % by volume) and global equivalence ratios (from blowout limits up to 1.0) were systematically evaluated. The primary objectives were to quantify the effects of hydrogen addition on flame stability, thermoacoustic dynamics, temperature distributions, and emissions performance. Experimental results demonstrated that hydrogen enrichment significantly extends lean blowout limits up to 21 %. This improvement is attributed to increased flame speed, diffusivity, and OH radical production. Thermoacoustic analysis, utilizing Fast Fourier Transform (FFT) and Phase Space Reconstruction (PSR), revealed a clear transition from stable to unstable combustion regimes with increasing hydrogen content. This instability was characterized by enhanced pressure-heat release coupling and limit-cycle oscillations. Temperature profiles exhibited limited sensitivity to hydrogen fraction, showing stronger dependence on equivalence ratio. Emission analysis revealed substantial reductions in CO₂ (10.5 %) and CO (33.3 %). However, these benefits were accompanied by a moderate increase (21 %) in NOx emissions. This increase in NOx is attributed to elevated flame temperatures and radical formation. Overall, these findings highlight hydrogen's potential to improve flame stability and combustion efficiency. They provide critical experimental benchmarks for modeling and advancing low-emission, hydrogen-enriched combustion technologies.