Improving fluid-thermal performance of impinging jet arrays with small-scale engineered surface augmentations in the fountain regions

While much research has been completed regarding jet array design, there is significantly less effort into the effect of using augmentations on the impingement surface. Micro-fins have been observed to successfully increase the heat dissipation, but large-scale structures can inhibit preferred flow mechanics, requiring careful design to achieve desired performance. Furthermore, crossflow effects from upstream jets can be detrimental to the performance of other jets downstream; spent fluid management is a necessity to attain uniform performance across the entire surface. The present study examines the use of surface augmentations between impinging jets of water, in the fountain region, under either a flat or angled upper confining wall examined both using a numerical model and through particle image velocimetry flow visualization. The augmentations were engineered with the intention to promote desired flow mechanics, with increased surface area being a secondary benefit. The augmentations include ribs of triangular cross-section oriented either parallel or normal to the direction of outflow, otherwise referred to as streamwise and transverse ribs, respectively. Regardless of the confining wall angle, the streamwise-oriented ribs were highly effective at reducing the surface temperature, with average surface temperature reductions of 0.9 to
$3.7^{\circ}\mathrm{C}$
corresponding to surface temperature rise reductions of 13 to 26%, through replacement of weak fountain interactions with impingement of the wall jet upon the rib face. The transverse ribs were effective in the same way under the angled confining wall, but flow separation under the flat wall resulted in highly impacted downstream jets, suggesting that ribs in this orientation are best applied with another form of spent fluid management. Cones placed between jets in the streamwise direction were also examined. These were particularly successful at deflecting fluid from the downstream jets, mitigating crossflow for both confining wall angles. While these resulted in less temperature rise reduction, from 0.1 to
$0.6^{\circ}\mathrm{C}$
corresponding to surface temperature rise reductions of 1.9 to 4.8%, further analysis revealed that, depending on the flow rate, 43 to 71% of this reduction was caused by improvement of flow mechanics, not an increase in surface area.