Mean flow regulation of a high frequency combustion control valve based on pulse width modulation and system identification

Strong combustion instabilities within gas turbine engines adverse combustion efficiency, shorten engine life cycles and may even cause hardware or structure failures. Fuel modulations, which introduce externally unsteady heat release rate perturbations out of phase with pressure oscillations, is an effective and practical approach for combustion instability control. A high frequency fuel valve capable of large fuel modulations (above 30% of mean flow) up to 750 Hz is presented in this paper. Fuel modulations are achieved by pushing fuel out of the valve cavity using a Terfenol-D rod that extends or contracts with external magnetic fields, and mean flow is controlled by a step motor using pulse width modulation. However, this valve suffers from significant variations of mean flow when starting fuel modulation. To follow the flow command and effectively reject strong interferences of fuel modulations on mean flow, a LQG pulse width modulation controller based on closed-loop system identification is developed, which achieves faster response than a traditional proportional derivative controller. With effective mean flow regulation, this fuel valve damps out strong pressure pulsations in an unstable swirling atmospheric combustor up to 23 dB.