Active Control of Instabilities in High-Pressure Combustor by Non-Coherent Oscillatory Fuel Injection

Abstract

This paper describes the application of active, open loop, control resulted in effective damping of severe (i.e., peak to peak pressure amplitudes of up to 60 psi relative to a mean combustor pressure of 485 psi) combustion instabilities in a high pressure combustor. Active control was applied by harmonic modulation of the fuel injection rate into the combustor using a fast response actuating valve, which modulated the entire fuel flow rate. To determine the dependence of the performance of the active control system upon the amplitude of fuel flow modulations generated by the actuating valve the latter was varied between zero and about 16% (±8%) of the mean value of the fuel flow rate. Successful suppression of the 352Hz fundamental acoustic mode in the combustor was attained at about 12% fuel flow rate modulation at the frequency fCS=293Hz. These tests showed that the amplitude of the combustor pressure oscillations varied six fold over the range of investigated oscillatory outputs of the control valve, indicating that applying the investigated open loop control approach at the appropriate frequency could effectively damp detrimental combustion instabilities. It was shown that fundamental acoustic mode of the combustor collapsed at a certain level of the control output when it was gradually increased during control application. Modulation frequency at about 290Hz was determined to be optimal in our earlier study at relatively low power of the combustor (mean combustor pressure of 160 psi). In the current study this value has also proved to be optimal in the control of 352Hz instability at full power operating conditions by conducting frequency sweep controllability test in the range 285-330Hz. In an effort to gain better understanding of the control system operation its characteristics were investigated in the cold flow simulation tests using extensive instrumentation. These included dynamic pressure sensors in the actuating valve and in the fuel line near the injection orifice as well as hot film anemometer (which measured mass flow rate oscillations at the point of injection). This additional instrumentation provided data for the monitoring of the control input propagation through the system in the combustor.