Numerical simulation of passive control of self-excited thermoacoustic instabilities by using a secondary heater

Thermoacoustic instabilities can be triggered with small-amplitude flow disturbances in a Rijke type combustor. To suppress the instabilities, a simple but robust control approach is needed. For this, a passive control means is designed and numerically tested in a Rijke tube with a heat source confined in the bottom half. A secondary heater is placed in the top half of the Rijke tube. In order to gain insight on the interaction of the two heat sources and to evaluate the secondary heater damping effect and to shed light on its damping mechanism, 2D numerical investigations are conducted in time domain. The simulation is modelled by using finite volume method to solve the unsteady Navier-Stokes equations. It is found that implementing the secondary heater can dramatically mitigate the heat-driven oscillations by reducing the sound pressure level by approximately 20dB, depending on its surface temperature, i.e. heat flux. In addition, the frequency of heat-driven oscillation is shown to vary. This indicates that the initial thermoacoustic instability undergos transition to a new one of which the dominant mode is at different frequency and amplitude. The successful application of a secondary heater in a Rijke tube opens new possibilities for stabilizing unstable combustion system via passive control means.