Effects of swirl on flow and mixing of acoustically excited swirling double-concentric jets

• Swirling double-concentric jets were acoustically excited.
• Flow and mixing characteristics were investigated.
• A train of vortex rings appears at low swirl numbers.
• Vortex rings quickly broke up into turbulent eddies at high swirl numbers.
• At low swirl numbers mixing enhancement showed better improvement in near field.

The influences of a swirl on the flow and mixing characteristics of acoustically excited swirling double-concentric jets were studied experimentally. The central jet was acoustically excited by a loudspeaker. The flow evolution process was captured by a high-speed digital camera, and the jet spread width was obtained from long exposure images through a digital edge detection technique. A one-component hot-wire anemometer was used to record the velocity instabilities in the flow. Tracer-gas concentration measurements provided information on jet dispersion. The results showed that one vortex ring was induced during each excitation period. At low swirl numbers, a train of vortex rings appeared in the downstream area. At high swirl numbers, the vortex rings quickly broke up into turbulent eddies in the near field. Consequently, the train of vortex rings observed at low swirl was lost. The vortex rings evolved in the axial direction and broke up into turbulent eddies at a downstream area. The breakup point moved upstream closer to the central tube exit as the swirl number increased. The mixing was dominated by entrainment during vortex ring formation and spread of fluid, due to the breakup of the vortex rings. Increasing the swirl number dampened the entrainment, due to the vortex ring formation in the near field. Hence, the mixing showed better improvement in the near field when the swirl number was lower. The mixing at the higher swirl number was slightly better in the far field, due to the combined effects of centrifugal force and early jet breakup, which caused higher jet spread rate.