Flame and flow characteristics of an excited non-premixed swirling double-concentric flame

• Jet pulsation was formed in swirling double-concentric jets by acoustic excitation.
• The characteristic flame modes were dominated by jet pulsation intensity.
• Time-averaged velocity fields of combusting oscillating jets were investigated.
• Counter-rotating vortices on circular disc reduce with increasing jet pulsation.
• The jet oscillation induced large turbulence properties around jet exit.

The flame behaviors and velocity fields of unexcited and excited swirling double-concentric jet flames were experimentally studied. Acoustic excitation was applied to the central fuel jet. The central jet Reynolds and swirl numbers were 2386 and 0.426, respectively. Three characteristic flame behaviors, wrinkled base flame, converged base flame, and diverged base flame were observed by the traditional photographic technique. Jet pulsation intensity dominated the change in the characteristic flame modes. We used a high-speed particle image velocimeter to measure the time-averaged velocity field; results of the excited swirling double-concentric jet flames showed that the streamlines that separate from the central fuel jet exit were significantly deflected toward the central jet axis, while the size of the rotating-inward single-ring vortex decreased as the jet pulsation intensity decreased. Partial flow emitted from the annular air jet flowed over the outer contour of the rotating-inward single-ring vortex and was then entrained into the central fuel jet, with the result that entrainment between the fuel and air was enhanced. The central jet region was formed by two adjacent vorticity-concentrated areas of opposite signs. As acoustic excitation was applied to the central fuel jet, these two areas expanded with increasing jet pulsation intensity. The jet pulsation induced vortical structures periodically evolving from the jet exit with the result that oscillation waveforms of the instantaneous velocities were obtained. The vortical structures entrained fresh air into the central fuel jet in radial direction, resulting in the extreme radial and axial turbulence intensities and improved mixing between fuel and air.