Flow field characteristics of swirling double concentric jets

Abstract

Abstract The flow structures and turbulence properties of double concentric jets with a large separation between the central jet and swirling annular flows are studied using the smoke-wire flow visualization technique and a two-component laser Doppler velocimeter. The smoke-streak flow patterns, characteristic lengths, velocity vectors, streamlines, velocity distributions, and turbulence properties are presented and discussed. The smoke-streak patterns show that a large spatial separation at the exit between the central and swirling annular jets can expedite the formation of a recirculation zone at low swirl and Reynolds numbers. Complex flow structures, single bubble , dual rings , vortex breakdown , and vortex shedding , are found in the recirculation zone. Large central jet velocity induces a large entrainment of the fluids in the recirculation zone and thus reduces the size of the recirculation bubble. The streamline patterns of the dual-ring mode show no stagnation point existing on the central axis, which is different from the non-swirling concentric jets. An off-axis saddle point which fits the topological rule is observed. The turbulence intensities and shear stress around the saddle point are particularly large. A carbon dioxide tracer gas experiment is conducted to examine the mixing capability. Effects of the swirl number and separation between the concentric jets are presented.