Effects of cross-stream radial injections on a round jet

Abstract

An experimental study has been carried out to investigate cross-stream radial injections into a round jet using laser-induced fluorescence technique and a hot-wire anemometry. The aim is to study the fundamental interaction behavior and resultant flow structures caused by the presence of the radial injections. Flow visualization results show that radial injections lead to enhanced shear layer perturbations and instabilities, as well as improved transition to turbulence. For each radial injection, a counter-rotating vortex pair (CVP) is formed which aligns in the streamwise direction shortly after its inception and penetrates the round jet core progressively downstream. Their presence along the round jet peripheral region affects the jet cross-section shape and influences the jet width, depending on their exact locations. The quasi-steady nature of the CVP was investigated and deduced to be an effect of the transverse orientation of the injection. The formation of recursive flow structures imparted flow oscillations which accounted for the quasi-steadiness as well as other observed secondary flow structures. Hot-wire measurements showed that radial injections enhance centerline velocity decay and turbulence levels, effective up to fifty jet diameters downstream. Also, half-jet-width comparisons between measurements taken along injection planes and non-injection planes confirmed that jet-width alterations are possible across an appreciable range of Reynolds number with an appropriate positioning of the radial injection ports.

Keywords:

• Jet-mixing
• Laser-induced fluorescence
• Hot-wire anemometry

Acknowledgements

The authors would like to acknowledge K. S. Lim for his experimental assistance and the loan of experimental instrumentations from the Fluid Mechanics Laboratory of the Department of Mechanical Engineering, National University of Singapore.