Abstract
Thispaperdescribesanexperimental investigation of thedynamicsof anear-blowoff, bluff-body stabilized flame. This work is motivated by a number of prior observations showing that near-blowoff flames exhibit enhanced unsteadiness. Laser sheet imaging studies and particle image velocimetry velocity field measurements show that the transient dynamics of these flames occur in two distinct stages before blowoff. The first stage is manifested by the presence of localized “holes” in the flame sheet, at locations where the instantaneous stretch rate exceeds the extinction stretch rate. During this stage, the overall flame and wake dynamics appear essentially unaltered and, moreover,the flamecanpersistindefinitely,althoughwithenhancedunsteadiness.Astheequivalenceratioisfurther decreased, the size of the flame region affected and the duration of these events increases monotonically. As the blowoff point is approached further, this leads to the second stage, large-scale alterations of the wake dynamics, violent flapping of the flame front, and even larger straining of the flame. In some cases, the flow in this second stage bears striking resemblance to the asymmetric von Karman type flowfield.