Scalar mixing in a confined rectangular jet in crossflow

Abstract

An experimental investigation of a confined rectangular jet in crossflow was performed. The rectangular jet is highly confined in that it spans almost 80% of the crossflow duct, rather than issuing into a semi-infinite crossflow. Furthermore, the jet is confined in the cross-stream direction because it issues into a relatively narrow duct. In addition, the flow rate of the secondary jet is large (up to 50% of the crossflow flow rate) which also influences the jet–crossflow interaction. Configurations of this type are found in a variety of different industrial manufacturing processes used to mix product streams.

A systematic variation of three pertinent parameters, i.e. momentum ratio, injection angle and development length, was performed. A full factorial experiment was run using three velocity ratios ($Vr\,{=}\,0.5, 1.0, 1.5$), three downstream distances ($x/D_{h}\,{=}\,6, 10, 19$) and six injection angles ($\alpha\,{=}\,18^\circ, 24^\circ$, 30^\circ, 48^\circ, 60^\circ, 90^\circ$). A planar Mie scattering mixing diagnostic system was used to evaluate the relative mixing effectiveness at various conditions within the parameter space studied. Three regimes for the jet–crossflow interaction and the resulting scalar concentration field were revealed: ‘wall jet’, ‘fully lifted jet’ and ‘reattached jet’. To understand the flow physics in these regimes, a more detailed exploration of the secondary flow and coherent structures was required. This was accomplished by acquiring velocity field data at measurement locations and conditions that demarcate the different regimes ($\alpha\,{=}\,30^{\circ}$ and $48^{\circ}, Vr\,{=}\,1.0$ and 1.5, $x/D_{h}\,{=}\, 3, 6, 10$, 15 and 19) using a laser-Doppler velocimetry (LDV) system. The combined scalar concentration and velocity field data provided an understanding of the large-scale mixing and the role of coherent structures and their evolution. The investigation revealed that the flow does not necessarily develop symmetrically and also highlighted some of the effects of confinement.