An experimental investigation of the effect of the velocity ratio on the flow and wall heat transfer characteristics of a wall-bounded dual jet

A wall-bounded dual jet, consisting of a wall jet and a co-flowing offset jet, possesses a complex flow structure that has proved advantageous across many industrial applications. Despite this, dual jets remain relatively unexplored through experimentation and their flow phenomena are largely misunderstood. This investigation is the first to examine the effect of varying the velocity ratio (V_r) on the flow and heat transfer behaviour of a dual jet through experimental means. A velocity ratio range of 0.5≤V_r≤2 is achieved by varying each jet Reynolds number (Re_w/o) in the range 5500≤Re_w/o≤12,000. Infrared thermography is used to acquire the local Nusselt number (Nu_x) profile along the bounding wall when subject to a uniform wall heat flux of 1670 W/m² for the offset ratio range 1≤OR≤7. In tandem, a particle image velocimetry study collects the corresponding flow data for 1≤OR≤3. For V_r<1, the wall jet is pulled farther from the boundary, increasing the extent of the separation region and moving the locations of the local minimum and maximum values in the Nu_x profile either downstream (OR=1) or upstream (OR≥3), which in turn affects their respective magnitudes. For V_r>1, the separation of the wall jet becomes increasingly suppressed and the Nu_x profile approaches that of a single wall jet, where the ‘peaking’ effect previously observed for OR=1 is no longer apparent. The time-resolved data shows an increasing Strouhal number (St) associated with the vortex shedding frequency for OR=2 and OR=3 within a limiting V_r range, however a general decline is observed for OR=1.