Tunable passive control of thermoacoustic instabilities based on a variable geometry combustor outlet nozzle

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.