Noninvasive Detection and Quantitative Characterization of Flashback in a Bluff-Body-Stabilized Flame Under Acoustic Excitation by Electrical Capacitance Tomography

Flame flashback poses a significant threat to combustors, necessitating the development of reliable detection methods. However, detecting flashback is challenging due to its occurrence within the premixing tube. In this study, we employed an electrical capacitance tomography (ECT) system, capable of nonintrusively reconstructing the permittivity distribution in opaque tubes, to address this challenge. A bluff-body stabilized combustor under acoustic excitation was utilized to generate controllable flashback. To reconstruct the permittivity within the annular flashback region of the combustor, the conventional Calderon-based reconstruction method was modified by compensating the scattering transform in the central area. The revised Calderon method was compared with original methods to validate its applicability in reconstructing the permittivity distribution in annular zones with a circular sensor. Reconstructed images of flashback flames demonstrated that the effectiveness of the ECT system in detecting flashback occurrences, with a distinct increase in permittivity corresponding to the intensification of flame flashback. To quantitatively characterize the flashback intensity, a flame flashback index (FI) was proposed based on the above findings. This index's distribution across various frequencies ({f}_{s}{)} and sound pressure levels (SPLs) was analyzed to investigate the effects of sound parameters on flashback. The results revealed several significant insights: 1) the existence of both a flashback boundary and an extinction boundary; 2) the nonlinear dependence of SPL on {f}_{s} at the flashback boundary; and 3) a monotonous increase in the flame FI for {f}_{s} < 110 Hz, followed by an initial increase and subsequent decrease for {f}_{s} > 110 Hz as SPL increases. The effective detection and characterization of flashback hold great significance for the secure operation of combustors.