Equivalence ratio and fuel type impacts on limit cycle and beating oscillations in combustion instability

With the growing emphasis on developing clean fuels to reduce emissions, hydrogen has become a critical focus due to its potential for lowering environmental impact. However, the operational challenges in lean fuel systems, particularly combustion instability, highlight the necessity of parametric studies to ensure stable performance with alternative fuels. This study numerically examines the thermoacoustic instability of pure hydrogen and methane in a Rijke tube combustor using the Unsteady Reynolds-Averaged Navier-Stokes method. The investigation focuses on two distinct instability behaviors: beating and limit cycle oscillations. Key findings reveal that for both fuels, increasing the equivalence ratio toward richer mixtures tends to damp thermoacoustic instabilities. It was observed that a 20% increase in fuel flow rate caused a transition from beating to limit cycle oscillations. A comparative analysis shows that lean methane flames produce stronger fluctuations, whereas hydrogen has a wider instability range under fuel-rich conditions. Advanced modal analysis using Dynamic Mode Decomposition identifies the dominant longitudinal acoustic modes and localized oscillations near the fuel nozzle that influence flame structure. These results provide critical insights into the unique thermoacoustic characteristics of hydrogen and methane, which can inform the design of stable, next-generation combustion systems.