Visibility Study Of Convective Heat Transfer And Micro-PIV Flow In Hydrophobic Microrib Arrays

With the increase of the integration of microelectronic devices, the heat flux in the per unit space increases dramatically. As a kind of highly efficient micro heat dissipation structure, micro fin array is widely concerned by researchers in the field of micro electronic heat dissipation due to its high aspect ratio, compact structure and high heat transfer efficiency. A large number of studies had shown that the micro fin array structure has excellent heat transfer ability, but the flow resistance of the micro fin is relatively high. The results showed that the heat transfer enhancement effect of the micro fin array was difficult to be fully developed under the condition of no power,which greatly hindered the application and development of the micro fin array cooling structure.In this paper, the flow and heat transfer channel of the micro fin array was designed, and the micro fin array flow and heat transfer experimental platform was built. Hydrophobic micro pin fin heat sinks with contact angles as 99.5°, 119.5° and 151.5° were obtained by solidifying hydrophobic layers involved nano-particles on the flowing surfaces, in which the pressure drop,the friction factor f and Nusselt number were experimentally measured under the different Reynolds number. Besides, the impact of contact angle on the power conservation and comprehensive heat transfer efficiency was analyzed. The results demonstrated that the flow resistance was reduced apparently due to the hydrophobic surfaces, and the pressure drop and flow resistance coefficient decreased with the increase of the contact angle. However, the Nu in hydrophobic micro pin fins was also reduced, and the deviation of the Nu among three kinds of hydrophobic micro pin fins increased with the increase of the heating power. Although the Nu in micro pin fins was reduced by the hydrophobic surfaces, the super-hydrophobic micro pin fins with elliptical cross-sectional shape had good comprehensive heat transfer enhancement performance, compared with the plain micro pin fins without hydrophobic surfaces. At the same time, considering the important influence of fin shape on flow and heat transfer,super-hydrophobic micro pin fins with circular, diamond and elliptical cross-sectional shapes were prepared by solidifying hydrophobic layers involved nano-particles on the flowing surfaces,and the flow resistance coefficient as well as Nusselt number in super-hydrophobic micro pin fins was measured. The integrated effect of super-hydrophobic surface on resistance reduction and heat transfer enhancement was explored by analyzing the integrate heat transfer enhancement factor ?. The results indicated that the friction resistance coefficient was 72%, 66% and 70%reduced in three kinds of micro fin fins with super hydrophobic walls and different cross-sectional shapes. Besides, although the Nu in all three kinds of micro pin fins was reduced,the value of the decrease was less than 44%, 17% and 47% under the high heating power in present research. In addition, the super-hydrophobic micro pin fins with diamond cross-sectional shape had good comprehensive heat transfer enhancement performance under the high heating power, and the value of ? is higher than 1.17 in the range of Re<1200.In this paper, the velocity field at the entrance of the micro fin array channel was measured by Micro-PIV measurement technology. The results showed that the Micro-PIV could accurately reflect the flow characteristics in the micro fin array. Then, the influence of hydrophobicity and cross section shapes on the flow separation characteristics in micro fin array was investigated. The results showed that for the circular fin array without hydrophobic treatment, when Re as 250, the middle cylindrical tail began to appear two symmetrical vortex, which had obvious lag compared with the macro scale. The values of the separation angle and the Lv of the different rows of cylinder tail increased with the increase of Re. Under the same Re, the Lv value of the upstream cylinder was smaller than that of the downstream cylinder. The boundary layer separation is delayed due to the hydrophobic treatment of the fin array and with the increase of the contact angle,the delay was more significant, which greatly reduced the pressure resistance. Boundary layer separation of the elliptical and diamond micro fin arrays being of good vortex flow line was obviously delayed, compared with non streamlined cylinder.