The pinch-off phenomenon in a flame refers to flame separation after reaching the critical frequency and amplitude under acoustic excitation. The pinch-off flame consists of the main flame attached to the nozzle and the detached pocket flame, and it is reported that more pollutants are discharged as the residence time of the pocket gas (i.e., the hot products) increases. This study focuses on the mechanism of the pinch-off phenomenon. The flame structure and flow field were analyzed by the simultaneous measurement of OH planar laser-induced fluorescence (PLIF) and particle image velocimetry (PIV). Due to a lack of previous research on the necessary conditions for a pinch-off flame, this study mapped the conditions for pinch-off in terms of the forcing frequency and velocity perturbation intensity (u′/u¯) under acoustic excitation. The flame height in the pinch-off region was defined in two ways: the main flame height from the nozzle tip to the tip of the attached flame, and the total flame height including the detached pocket. Strouhal number was calculated based on these two definitions of flame height. In addition, the vortex flow and reverse flow under external forces were measured through analyzing the non-reacting and reacting flow fields. Flame deformation due to the entrainment of air in the vortex flow and the reverse flow was confirmed, and it was found to correlate with the pinch-off phenomenon. To analyze this observation quantitatively, time-dependent strain rate analysis was performed, and a high strain was confirmed to be present upon flame extinction. Therefore, these results demonstrate that pinch-off is correlated with the vortex flow and the reverse flow, and caused by a high strain rate acting on the flame surface.
|Original language||English (US)|
|Journal||Combustion and Flame|
|State||Published - May 11 2021|
Bibliographical noteKAUST Repository Item: Exported on 2021-07-28
Acknowledgements: This work was supported by the Advanced Research Center Program (NRF-2013R1A5A1073861), Ministry of Trade, Industry & Energy Industrial Technology Innovation Program (No.10067074), and Global Ph.D. Fellowship Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (NRF-2017H1A2A1043206) and has been supported by the Institute of Advanced Aerospace Technology, Seoul National University.