The stability of jets and flow structure has been a subject of research in various applications. The low-density gas jet injected in a heavy-density ambient or vice versa is a study of interest with relevance to engine exhaust, fuel leaks, diffusion flames, and natural phenomena such as volcanic eruption and fires. The flow structure in the far-field is influenced by the near field process such as buoyancy, entrainment, diffusion, and jet instability. The present work focused on the flow structure of the helium jet in the near field, whereby buoyant effects are significant. 
A numerical investigation is carried out on a helium jet of Reynolds number 150. The effect of air co-flow on a self-excited helium jet is studied in the near field using commercial software ANSYS Fluent V18.1. The co-flow velocity ratio varied in the range of 0.17-0.87. The contours of the helium mole fraction along with the streamlines show the interaction of the toroidal vortex with the jet. The suppression of toroidal vortex is observed as the air co-flow velocity induced to the jet flow. Due to the suppression of vortices, radial spread/diffusion is limited, resulting in large gradients at the shear layer. The flickering frequency linearly increases with the air co-flow. The amplitude of the oscillation increases up to a certain co-flow velocity and then drops significantly at high co-flow velocity ratios. However, at upstream (near jet exit plane), oscillation amplitude decrease with increase in air co-flow. The velocity difference in the shear layer to the ambient elucidates the stabilization mechanism of the self-excited helium jet.

Numerical Investigation Into the Effects of CoFlow Air on a Self-Excited Helium Jet

Paper Proceedings: https://asmedigitalcollection.asme.org/GTINDIA/GTINDIA2021/volume/85536

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