Heat Transfer From Novel Target Surface Structures to a $3×3$ Array of Normally Impinging Water Jets

Impinging jet arrays provide a means to achieve high heat transfer coefficients and are used in a wide variety of engineering applications such as electronics cooling. The objective of this paper is to characterize the heat transfer from an array of $3×3$ submerged and confined impinging water jets to a range of target surface structures. The target surfaces consisted of a flat surface, nine 90 deg swirl generators, a $6×6$ pin fin array, and nine pedestals with turn-down dishes that turned the flow to create an additional annular impingement. In order to make comparisons with a previous single jet study by the authors, each impinging jet within the array was geometrically constrained to a round, 8.5 mm diameter, square-edged nozzle at a jet exit-to-target surface spacing, of $H/D=0.5$⁠. A custom measurement facility was designed and commissioned in order to measure the heat transfer coefficient and the pressure loss coefficient of each of the target surface augmentations. The heat transfer results are presented in terms of $Nu/Pr0.4$⁠, and the pressure results are presented in terms of pressure loss coefficient. Comparing the array of jets to a single jet showed a decrease in heat transfer. Full field velocity magnitude images showed that this decrease in heat transfer was caused by neighboring jet interference cross-flow coupled with a greater back pressure effect. The analysis of the different target surface augmentations showed that the performance of the pedestal with the turn-down dish was the least compromised by the addition of the surrounding jets. It showed both the highest fin efficiency of 95.1% and fin effectiveness of 2.27. However, it showed the highest overall pressure loss coefficient compared with the other target surfaces, and therefore the nine 90 deg swirl generators performed the best in terms of both pressure loss coefficient and thermal performance. The findings of this paper are of practical relevance to the design of primary heat exchangers for high-flux thermal management applications, where the boundaries of cooling requirements continue to be tested.