Effect of excitation Strouhal number on velocity field and mixing capability of acoustically excited dual jets

Abstract

The present study experimentally investigated the effect of the excitation Strouhal number on the velocity field and mixing capability of excited dual jets. The dual jets were acoustically excited by a loudspeaker. The velocity pulsation at the jet exit was examined using a hotwire anemometer. The evolution process of the excited dual jets was captured using the smoke flow visualization technique. The jet spread width was measured from a long-exposure image by using the edge-detection method. The time-averaged velocity fields were measured using the particle image velocimetry (PIV) technique. The mixing capability was examined by tracer-gas concentration detection. Two characteristic flow modes, namely the bifurcated and parallel vortices, were classified in the domain of pulsation intensity and excitation Strouhal number. The excitation Strouhal number dominated the change in the characteristic flow modes. The leading vortices induced jet flow bifurcation in the bifurcated vortices mode, while the vortices broke up early in the parallel vortices mode. The recirculation region, merging length, combined length, jet spread, and later distribution of the dual jets excited in the bifurcated mode were larger than those in the parallel vortices mode. The tracer-gas value in the bifurcated vortices mode was smaller than that in the parallel vortices mode; therefore, the mixing capability in the bifurcated vortices mode was better than that in the parallel vortices mode. The unexcited and excited dual jets were compared to evaluate the effect of acoustic excitation on the flow and mixing characteristics.