Flame mode transitions in outer recirculation zone of lean premixed prevaporized n-decane/air swirling flames

Abstract

The outer recirculation zone (ORZ) flame modes and their transition were experimentally investigated in lean premixed prevaporized n-decane/air swirling flames. The flame structure and flow field were measured employing simultaneous CH${}_2$O/OH-PLIF and OH-PLIF/PIV laser diagnostics. The ORZ temperature was estimated with thermocouples. Three ORZ flame modes were identified: (1) mode II, characterized by extensive CH${}_2$O but nearly absent OH in the ORZ; (2) mode III, with flickering CH${}_2$O and OH in the ORZ; (3) mode IV, with flickering OH in the ORZ but nearly without CH${}_2$O. Besides, the mode I is defined as the inner shear layer (ISL) flame and mode V is the ISL/outer shear layer (OSL) flame. A theoretical diagram from mode I to V is achieved describing transition from the ISL to ISL/OSL flames. It is confirmed that the presence of CH${}_2$O in the ORZ results from the first-stage low-temperature autoignition, which is validated to be predictable with the ignition Damköhler number ($D{a}_{ig}$) after examining the fluid residence time and ignition delay time. It is found that the high-temperature ORZ flame indicated by the flickering OH radicals cannot be predicted merely using $D{a}_{ig}$. Instead, the increased flame speed and intensified ISL flame propagation into the ORZ due to the elevated equivalence ratio influence a lot, which can also be promoted by the shear layer vortices. These processes are expected to motivate the entrainment of hot burnt gas into the ORZ thus to form the high-temperature ORZ flame. The flickering ORZ flame is undesirable due to the potentially induced combustion instabilities, and this work can broaden its understanding and provide insights for its suppression.