Self-excited oscillation in homogeneous near-extinction combustion of H₂/NH₃/CH₄ mixtures

The control of combustion instability is crucial for maintaining safety and operational stability in practical burners. This study analyzes the self-excited oscillations resulting from coupled fuel chemistry, mixing and heat loss in the homogeneous combustion of ammonia (NH₃), hydrogen (H₂), methane (CH₄), and their mixtures for conditions extracted from industrial burners. The oscillation regimes near extinction have been identified with the non-adiabatic perfectly stirred reactor for various fuel mixtures, including the single fuels of CH₄, H₂, and NH₃, as well as the fuel mixtures of NH₃/CH₄, NH₃/H₂, and NH₃/CH₄/H₂. The complex coupling of fuel chemistry, mixing and heat loss on self-excited oscillation are identified by analyzing the oscillation mode and frequency using eigen analysis employing a reduced Li mechanism. It is shown that for a single fuel, it is practically infeasible for systems to exhibit sustained self-excited oscillation near extinction by heat loss. Three fundamental patterns are identified, namely: (1) unimodal-burning and (2) bimodal-burning oscillations in NH₃/CH₄ mixtures, where the fuel is nearly completely consumed, with temperature oscillations within a range of several tens of degrees and oscillation frequencies within a range of several tens of hertz; and (3) ignition-burnout-re-ignition oscillations found in NH₃/H₂ mixtures, characterized by temperature oscillations within a range of several hundreds of degrees and oscillation frequency below 1 Hz, corresponding to the residence time. The elementary reaction H + O₂ (+M) = HO₂ (+M) is emphasized for its dominant role in terminating the radical chain reactions at low temperatures in relation to the observed oscillation patterns under representative conditions for both NH₃/CH₄ and NH₃/H₂. The findings are of practical significance for the design and reliable operation of industrial burners utilizing various fuels containing NH₃, which underscore the importance of fuel coupling and operating conditions on the combustion dynamics.