On the Role of Control Fuel in Active Suppression of Thermoacoustic Instabilities in Non-Premixed, Fuel-Blended Combustion Chamber

This work investigates the active control of thermoacoustic instabilities through fuel supply modulation in a turbulent, non-premixed combustion chamber featuring direct injection of methane and hydrogen directly into the combustion zone. The purpose of the research is to assess differences in the operational performance of the control system when selecting one fuel over another to modulate the flame heat release rate, with particular focus on the settling time of fluctuations and the effects of control action saturation. The thermoacoustic behavior of the system was simulated through a one-dimensional Reduced Order Model framework. The response of the turbulent, non-premixed flame to acoustic disturbances was borrowed from recent experimental findings in the literature, whose trends were used to fit a simple second-order Flame Transfer Function. Extension of this model to a non-linear perturbation regime was performed through a well-established saturation term allowing to construct a compact Flame Describing Function. Control was actuated by a Self-Tuning Regulator governing a servo-valve that modulated the amount of control fuel injected into the combustion zone. The control effort was constrained by limiting modulation of the heat release rate to a small fraction of its mean value, preventing large fuel fluctuations for reasons concerning combustion stability and environmental impact, thus incorporating saturation effects. Identical Actuator Transfer Function parameters and control action saturation limits were chosen for both fuels, ensuring that any observed difference between the two strategies is attributable solely to intrinsic fuel properties, chiefly the Lower Heating Value.