Dynamics of acoustically excited coaxial laminar jet diffusion flames

This experimental study explored combustion dynamics associated with coaxial laminar jet diffusion flames in the presence of acoustic excitation. The methane–air jets burned inside a closed cylindrical waveguide at atmospheric conditions, where flame behavior was captured via direct high-speed visible imaging. As the acoustic forcing increased at a fixed frequency in the vicinity of a pressure node associated with a standing wave, the flame underwent a transition from sustained oscillatory combustion (SOC) to periodic lift-off and reattachment (PLOR) and eventual flame blow-off (BO). The nature of this transition and flame–acoustic coupling was explored by varying a wide range of experimental parameters for five different coaxial jet geometries, including the jet Reynolds number, outer-to-inner jet velocity ratio, coaxial jet wall thicknesses and diameters, and amplitude of acoustic excitation. Flame–acoustic coupling processes were observed to vary significantly based on the annular-to-jet area ratios and tube wall thicknesses under similar flow conditions. Analyzing the spatiotemporal flame dynamics via proper orthogonal decomposition (POD) of high-speed imaging data revealed different signatures of the transition process, including abrupt changes in the mode energy distribution and a significant increase in the complexity of the phase portraits when flame dynamics involved additional time scales. Results from this study suggested that increased annular-to-inner area ratio and velocity ratio can greatly enhance flame stability and resistance to blow-off, with wall thickness playing a lesser role.