Characterizing the dynamic liftoff in a bluff-body-stabilized flame via the fringe effect of electrical capacitance tomography sensors

Flame liftoff poses a significant risk in industrial systems, necessitating reliable liftoff monitoring methods. Electrical capacitance tomography (ECT) is widely used in non-contact flame detection. However, direct detection of flame liftoff remains challenging for conventional ECT arrangements, because flame liftoff is mainly characterized by axial flame displacement rather than a significant change in combustion intensity. This study utilizes the fringe effect of ECT sensors to achieve real-time detection and characterization of flame liftoff. During liftoff, flame displacement away from the sensor weakens the fringe field, causing a detectable change in the capacitance value. Experiments were conducted using a bluff-body combustor with acoustic excitation to generate controllable liftoff. The Calderon method adapted for annular regions was employed to reconstruct flame combustion images. The comparison between the ECT reconstructed images and the high-speed camera images was completed to verify the effectiveness of the method. The results indicate that the reconstructed grayscale values decrease with the increase of flame liftoff intensity, confirming the method’s ability for real-time and accurate detection. To quantitatively characterize the liftoff intensity, a liftoff index (LI) was proposed, integrating the frequency, height, and duration of liftoff events. At a fixed acoustic frequency, LI increased monotonically with sound pressure level (SPL), whereas at a fixed SPL, LI exhibited nonlinear variation with excitation frequency. Effective liftoff monitoring and characterization are crucial for the safe operation of combustors.