Tunable passive control of thermoacoustic instabilities based on a variable geometry combustor outlet nozzle
Audrey Blondé, Bruno Schuermans, Bayu Dharmaputra, Nicolas Noiray
Abstract
This paper presents a tunable combustor outlet geometry, which enables control of thermoacoustic instabilities. It is based on a water-cooled conical piston inserted in the outlet orifice. By varying the axial position of the piston, the outlet cross section area can be smoothly adjusted to control the acoustic boundary condition, from strongly reflecting to non-reflecting, over a broad range of frequencies. The classic model of Bechert for the reflection coefficient of an orifice with low Mach bias flow is adapted to account for the non-isothermal nature of the flow at this tunable combustor outlet, and experimentally validated for a wide range of conditions. It is then showed that this broadband tunable passive control enables suppression of high amplitude thermoacoustic limit cycles during operation. The results suggest that this device can be particularly useful for measuring the response of a burner and its flame to acoustic perturbations, because this measurement requires thermoacoustically stable states for a wide range of operating conditions. • A tunable combustor outlet for control of thermoacoustic instabilities is presented. • A movable piston in the combustor outlet varies the acoustic boundary condition. • The outlet reflection coefficient can be varied from anechoic to fully reflecting. • A model predicts the dependance of reflection coefficient on the outlet Mach number. • The tunable passive control of instabilities is demonstrated experimentally.