Experimental investigation of a wall-bounded dual jet flow for varying Reynolds number: Flow visualisation, hydrodynamic characteristics, and associated heat transfer

Abstract

A wall bounded dual jet is the combination of a wall jet, flowing adjacent to a solid boundary, and a second parallel flowing jet offset from the boundary by some distance. The dual jet flow is distinctly different to that of either wall or offset jet, particularly in the region near the jet exit plane. This study represents just the 2^nd experimental investigation of the flow characteristics of a dual jet flow past a solid surface. The primary aim of the present investigation is to capture flow data to accompany the dual jet thermal data previously published by the authors and to provide further context to the reported findings. A bespoke experimental apparatus is constructed to observe the flow behavior using particle image velocimetry (PIV). The experimental setup is first validated by comparison of results for a single wall jet and a single offset jet with those available in literature. Then, a dual jet flow field is investigated for a Reynolds number range from $5,500\le Re\le 12,000$ for a jet width of $w=7mm$, where both the offset ratio and a velocity ratio are maintained at a constant value of $1$. Time-averaged PIV analysis reveals that the jets immediately deflect toward one another, with a slow-moving recirculation zone between them presenting as a pair of counter rotating vortices. The data suggests that the merge point moves marginally upstream with increasing $Re$, while moving further from the solid wall, whereas the streamwise positions of the vortex centres appear relatively unaffected by $Re$. Increasing $Re$ leads to a slight reduction in the size of the recirculation zone, where the defection of both jets is noticeably increased. The findings of the present study suggest that the shape of the characteristic local Nusselt number ($N{u}_x$) profiles previously reported in the literature can readily be attributed to the unique features observed inside the dual jet flow field. In particular, the observed deflection of the wall jet away from the solid wall is found to be the direct cause of the local $N{u}_x$ minimum, and the subsequent re-impingement of the jet flow on the wall boundary induces the succeeding local $N{u}_x$ maximum, where the occurrence of a wall jet deflection and re-impingement has not yet been reported on by any prior dual jet studies in the published literature. Time-resolved PIV analysis shows the occurrence of periodic von Kármán-like vortex shedding inside the dual jet flow field at a constant Strouhal number ($St)$ value for all $Re$ examined.