بررسي عددي ناپايداري‌هاي ترموآكوستيك خودتحريك احتراق در مخلوط‌هاي متان و هيدروژن با استفاده از LES-FGM

The modeling framework aims to capture the delicate coupling between turbulent flow structures, flame dynamics, an‎d acoustic feedback mechanisms under ultra-lean conditions representative of modern gas turbine operation. The study addresses two interconnected research objectives: (1) to elaborate on the fundamental instability mechanisms in pure methane flames at ultra-lean equivalence ratios, an‎d (2) to quantify the impact of hydrogen enrichment on flame topology, mode structure, an‎d instability growth behavior in partially premixed CH₄/H₂ mixtures. The first phase focuses on pure methane flames at global equivalence ratios of Φ = 0.7 an‎d Φ = 0.55, replicating the experimental configurations reported in the experimental results. These cases represent the ultra-lean operational envelope in which self-excited oscillations spontaneously emerge from the inherent coupling between hydrodynamic an‎d acoustic modes. The simulations employ a high-resolution computational grid of approximately 70 million cells an‎d utilize the compressible LES solver with a second-order implicit time integration an‎d non-reflective boundary conditions at the inlet an‎d outlet to prevent artificial reflections. The FGM approach, based on detailed GRI-3.0 an‎d UCSD 2016 chemistry an‎d a β-distributed mixture fraction manifold, allows accurate reconstruction of species source terms an‎d heat-release rate with manageable computational overhead. Detailed validation is performed against experimental pressure spectra, OH* chemiluminescence, an‎d flow field measurements. The simulations accurately reproduce the dominant instability frequencies, mode amplitudes, an‎d phase relationships between pressure an‎d heat-release fluctuations, confirming the predictive fidelity of the employed numerical framework. Detailed modal analysis reveals two distinct self-excited instability modes, whose competition depends strongly on the equivalence ratio an‎d flow configuration. At Φ = 0.7, a dominant high-frequency low-amplitude mode (~117 Hz / 4.7 Pa from LES-FGM, 131 Hz / 6.6 Pa from Experiment) associated with longitudinal acoustic excitation is observed, whereas at Φ = 0.55, a completely new oscillation trend with high-frequency low-amplitude mode characteristics (~87 Hz / 51 Pa from LES-FGM, 91 Hz / 60.8 Pa from Experiment) emerge due to intensified coupling between the weakened precessing vortex core (PVC) an‎d the flame front. The results show that local equivalence ratio fluctuations, induced by vortex–flame interaction in the mixing zone, act as a critical driver for instability growth, modulating the unsteady heat-release rate in phase with pressure oscillations in accordance with the Rayleigh criterion. The second phase extends the analysis to hydrogen-enriched methane flames at a fixed global equivalence ratio of Φ = 0.7, with hydrogen volume fractions studied for 10% an‎d 25%. Hydrogen addition is introduced as a practical pathway toward carbon-neutral combustion, offering enhanced reactivity, faster flame propagation, an‎d broader lean stability limits. However, these same properties can worsen thermoacoustic instabilities if not carefully managed. The simulations reveal that hydrogen enrichment significantly alters the flame topology an‎d anchoring mechanism, promoting more compact, wrinkled flame structures an‎d shifting the primary reaction zone closer to the swirler exit.

احمدرضا پيشه وراصفهاني , محسن دوازده امامي

محسن ثقفيان , حسين بلاغي اينالو