Lifting of Single and Coaxial Nonpremixed Jet Flames Under Transverse Acoustic Forcing

The dynamics of gaseous reacting turbulent nonpremixed single and coaxial jet flames in unforced and acoustically forced environments was investigated at atmospheric pressure. Two burner configurations were used: one having a single fuel jet and one having a coaxial jet of fuel and oxidizer streams. The flames were exposed to transverse acoustic waves tuned to produce pressure antinode acoustic forcing at various frequencies and amplitudes. The flame responses were recorded using high-speed Schlieren and OH* chemiluminescence imaging. The center jet was set to be the same between the single and the coaxial jets at two different Reynolds numbers. The annular flow of the coaxial jet was set to have the same bulk velocity as the ambient coflow, a configuration here called a coaxial single jet. The configuration was so named because it was expected that a coaxial jet having the same annular bulk velocity as the ambient coflow would produce results similar to that of the single jet in the same coflow. It was found instead that the flame lifting behavior was considerably different, even qualitatively. The flow visualizations revealed differences in large-scale structures that explain the different lifting regimes.