This experimental study aimed to characterize the coupling of an acoustic field with turbulent methane nonpremixed flame dynamics under atmospheric pressure conditions. High-speed Schlieren and OH* chemiluminescence images recorded the near field behavior of the flame under both unforced and acoustically forced conditions. Acoustic forcing established a standing wave with either a pressure node or pressure anti-node located at the acoustic channel center, where the flame was located. High-speed imaging showed two different phenomena associated with these standing waves. When the flame was forced while situated at a pressure node, an asymmetric, sinuous response of the flame was observed, in addition to transverse oscillations of the center fuel jet, which shortened the intact fuel core length. The flame "flattened" into an ellipsoidal shape in the direction of the acoustic waves. Conversely, at a pressure anti-node, the coupling of the acoustics and flame gave rise to an axisymmetric response (puff-like oscillations), which prompted the flame to become unstable at the anchoring region. This could lead to periodic liftoff or permanent flame liftoff. A receptivity study at a jet Reynolds number of 5,300 and an ambient oxygen concentration of 40% showed that the reacting jet was able to respond at the frequency of the unsteady acoustic field even at higher frequencies, but with a diminishing response of the flame for both the pressure node and the pressure anti-node. At the pressure node at higher frequencies the center fuel jet was changed by the acoustic field more than the flame. In the case of the pressure anti-node the higher frequencies reduced the mean standoff distance and the amplitude of the flame standoff fluctuations. In addition, anti-node forcing showed that the flame standoff distance was tightly coupled to the acoustic field with nearly out-of-phase behavior. The study showed that both PN and PAN forcing of the jet disrupted the reacting jet.
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