Experimental investigation of the effect of the offset ratio on the flow characteristics and heat transfer behaviour of a wall-bounded dual jet flow

Abstract

A wall-bounded dual jet is formed through combining a wall jet, flowing directly adjacent to a solid wall, with a parallel, co-flowing offset jet. The combination of a wall and offset jet in this manner is commonly encountered across many industrial applications, such as electronics cooling, wastewater evacuation, and noise suppression technologies, despite remaining relatively misunderstood across the published literature. This study aims to contribute to the available dual jet experimental data and further fundamental knowledge of dual jet flows and their accompanying heat transfer characteristics. The primary objective of this experimental investigation is to analyse the effect of varying the separation distance between the wall and offset jets, i.e., the offset ratio (OR), on the dual jet flow and heat transfer behaviour, therefore providing the very first experimental dataset relating to a dual jet for varying OR. For this investigation, a uniform heat flux of 1670 W/m² is applied to the bounding wall for 5500 ≤ Re ≤ 12,000 and 1 ≤ OR ≤ 7, while the velocity ratio is maintained at a constant value of 1. This is achieved through a two-part experimental investigation, which separately adopts infrared thermography and particle image velocimetry techniques to respectively capture the heat transfer and flow characteristics. The results reveal a strong dependence on OR near the jet exit, while the flow characteristics remain generally unaffected further downstream. Increasing OR leads to a growth in the recirculation region, therefore increasing the streamwise position of the merge point. This consequentially moves the respective locations of the local minimum and maximum values in the characteristic local Nusselt number (Nu_x) profiles further downstream, while also inducing a general decline in the surface-averaged Nusselt number. Distinct differences in the observed flow and heat transfer behaviour are noted for OR ≤ 2 and OR ≥ 3, where lower OR values produce a unique ‘peaking’ phenomenon in the Nu_x profile that becomes more exaggerated for higher Re values.