Theoretical and experimental investigation of thermoacoustics transfer function

The coupling between unsteady heat release and acoustic perturbations can lead to self-sustained thermoacoustic oscillations, also known as combustion instability. When such combustion instability occurs, the pressure oscillations may become so intense that they can cause engine structural damage and costly mission failure. Thus there is a need to understand the coupling physics between acoustic waves and unsteady combustion and to identify a measure to quantify the interaction between the flame and acoustics. The present work studies linear and nonlinear response of a conical premixed laminar flame to oncoming acoustic disturbances. Unsteady heat release from the flame is assumed to be caused by its surface area variations, which results from the fluctuations of the oncoming flow velocity. The classical G-equation is used to track the flame front variation in real-time. Second-order finite difference method is then used to expand the flame model. Time evolution of the flame surface distortions is successfully captured. To quantify the dynamic response of the flame to the acoustic disturbances, system identification is then conducted to estimate the linear and nonlinear flame transfer function. Good agreement is obtained. Finally, transfer function of an actuated open–open thermoacoustic system is experimentally measured by injecting a broad-band white noise. The present work opens up new applicable way to measure heat-driven acoustics transfer function in a thermoacoustic system by simply implementing white noise.