The experimental analysis on the influence of Non-closed droplet shape and location on Thermal/Flow performance of pin fin heat sinks

With the rapid development of electronic technology, high-performance chips and integrated circuits have been used more and more widely. Due to the extremely high heat transfer coefficient, the boiling heat transfer of working medium in micro pin fin channel have attracted more and more attention, and how to effectively meet the requirements of efficient heat dissipation in small space is an important issue. This paper focuses on the irregular structure of pin fin array with a non-closed pin shape, who retains the streamline structure as droplet shape and enhances the heat transfer performance. In this paper, a visualized experimental analysis on the influence of non-closed droplet pin shape and location on thermal/flow performance of pin fin heat sinks have been conducted. Through macroscopic performance and microscopic bubble dynamic behavior, the inner influence law of pin shape, stream orientation, location on thermal/flow performance have been studied. Results show that from macroscopic view, the non-closed pin shape of the micro pin fin array has a great influence on the flow and boiling characteristics. With the increase of Reynolds number, the pressure drop undergoes three stages: rapid rise, rapid decline and then rise again. Similarly, the variation tendency of convective heat transfer coefficient also get through such three stages. All of which corresponding to the flow pattern changes, indicating four heat transfer stages: film boiling, transition period, nucleate boiling, and single-phase flow heat transfer. Besides, for the scale as 40 < Re < 200 as well as 400 < Re < 800, the heat transfer appearance of non-closed droplet pin fin is better than that of droplet one. While for different stream orientations, the general performance of stream orientation with the opening part opposite the upstream direction is better. For the different locations of the front, middle and rear region, the bubble growth diameter in the front region is much larger than that in the middle region of the channel, and more growth time is required, with the bubble disappear - appear- reappearance interval as 0.536 s and 6.110 s respectively. While the growth time and waiting time of the front region with stream orientation B is 4.85 s as well as 0.86 s, which is much shorter than that of the front region with stream orientation A. Through the visual observation of the bubble dynamic behavior in the front section of the micro pin fin array, it could be concluded that with the increase of heat flux,the bubble diameter increases with time. Furthermore, the bubble growth/migration diameter increases first and then decreases with the increase of Re. Moreover, the bubble diameter in zone II is larger than that in zone III. Generally, the comprehensive performance of the non-closed droplet with stream orientation B is the best over the droplet as well as stream orientation A pin fin array, when it is at the range as 300 < Re < 800. Under comprehensive consideration, non-closed droplet with stream orientation B design is recommended, under operating conditions with Re > 300.