Active control of combustion instability using a fluidic actuator

Abstract

Active instability control was applied to an atmospheric bluff body combustor using an open loop control scheme. Actuation was realized by a fluidic actuator that modulated parts of the fuel flow. This actuator had one inlet and two outlets. While maintaining a constant inlet mass flow, the fluidics' outlet mass flow oscillated asymmetrically between the two outlets. The frequency of oscillation was controlled by varying the inlet mass flow of the actuator. The use of a fluidic actuator rendered the use of fast moving and hence non-durable components, as for example found in valves, unnecessary. The fluidic actuator's ability to generate an oscillating flow was verified in cold conditions for natural gas, nitrogen and mixtures of both. Hot wire measurements showed that the oscillation frequency at the outlets is a function of the volume flow rate through the actuator inlet. The actuator was then incorporated into a bluff body combustor, where it modulated parts of the fuel flow blended with nitrogen. Pressure and heat release fluctuations in the combustor as well as combustion emissions in the exhaust were recorded. The heat release response of the flame to fuel flow modulation was first studied during stable combustion. The spectrum of the heat release signal showed a clear peak corresponding to the fluidics' oscillation frequency, thus validating the ability of the actuator to influence the combustion process. As in cold conditions, the oscillation frequency could be controlled by controlling the mass flow of the gas-nitrogen mixture fed into the fluidics. As the next step, the combustor was operated at conditions that featured a strong low-frequency combustion instability when no fuel was modulated. Applying fuel modulation resulted in attenuation of the combustion instability for some oscillation frequencies. The attenuation was highest when modulating the fuel flow in between the fundamental instability frequency and its subharmonic. Modulating the fuel flow at the subharmonic, however, resulted in an amplification of the instable mode. The results obtained in this work show that the fluidic actuator in use allows for fuel modulation and hence instability control without the need for complex and fast moving parts, thus ensuring a long actuator lifetime. This makes the fluidic actuator highly appropriate for application in industrial gas turbines. Copyright © 2008 by the authors.