A backward-facing step combustor: reduced-order modeling and control

Model-based combustion control enables one to analyze the behavior of a combustor, quantify its characteristics in terms of parameters such as geometry, equivalence ratio, flow rate, flame stabilization mechanism and heat loss, thus allowing an optimal and robust control design toward the desired objectives such as extending flammability limits, reducing emissions, and suppressing pressure oscillations. The most difficult aspect of deriving combustion models is the presence of multiple interacting mechanisms of acoustics, heat release dynamics, and hydrodynamics. While the first two and their interactions have been modeled and understood quite well, a control-oriented model of the hydrodynamics and its interactions with the first two components is only beginning to be formed. As a first step towards this objective, in this paper, we consider separating shear flow behind a backward facing step under non-reactive and reactive flow conditions. Models based on numerical simulation using POD expansion and system identification, and corresponding controllers are presented. Validation of the control strategies using both numerical simulation studies and experimental apparatus are presented. For the latter, a 80 kW backward facing step-stabilized combustion tunnel is used, where using a photodiode array and multiple air-modulating valves as actuators, a 7.5 dB pressure reduction was achieved.