Research On Droplet Spreading And Flow Boiling Characteristics Of Micro-nano Structured Surface On Copper Substrate

Thermal functional surface with micro-nano structures is a special functional surface for enhancement heat and mass transfer. Using various surface processing technologies not only can obtain the specific micro-nano structures, but also have important influence on the wettability of the surface. In this paper, we adopted the method of laser ablation combining redox to fabricate superhydrophilic surface with micro-nano structures. The droplet shape change processes and internal flow field changes between smooth surface and micro-nano superhydrophilic surface were researched by the approach which associated with the numerical simulation and test. The effects of micro-nano structures and surface wettability on boiling heat transfer performance were researched by sub-cooled flow boiling test bed.The copper was selected as the base material. Through polishing its surface with sandpaper and Metallographic Sample Polishing Machine, we get a smooth surface whose contact angle was 65°. Then using laser ablation to fabricate microstructure on smooth surface, the contact angle of the microstructure surface reached 19°. On this basis, the superhydrophilic surface with micro-nano structures was obtained by redox reaction. The analysis of surface wettability was on the basis of Wenzel model.The shape change process of spreading was recorded by two high-speed cameras from top-view and side-view respectively as the droplet impacting on smooth surface and micro-nano structure surface at different speeds. And the effect of different micronano structures on surface spreading properties was analyzed by using the L9(21×33) mixed orthogonal test. The results showed that droplets had different shapes and spreading properties impacting on smooth surface and micro-nano structured surface. After impacting on smooth surface, the droplet quickly spread, and its three-phase contact line began to retract after reaching the maximum diameter. In retracting process, the droplet appeared obvious oscillation phenomenon. Impacting on superhydrophilic surface with micro-nano structures, the droplet’s three-phase contact line continually spread outward, and only the center position occured rarely retraction and oscillation under the control of surface tension of liquid. The result of the range analysis for spreading performance of nine surfaces indicated that the primary and secondary factors influencing the surface spreading performance successively were the micro-structure shape, the redox reaction, the micro-structure size and the micro-structure space.Based on the VOF method, the numerical simulations of the deformation caused by the droplet impacting on the smooth surface and the micro-nano structured surface, of the velocity distribution of gas-liquid interface, of the contour deformation process and the velocity distribution of different sections were carried out. The maximum velocity of the gas-liquid interface was at the joint of the liquid and the liquid film, and the velocity gradually decreases along the radial direction. The spreading rate of the droplet on the micro-nano structured surface is greater than that on the smooth surface. On micro-nano structured surface, due to rectangular array of the micro square columns, the droplet presents different shape change compared to that on the smooth surface in the course of spreading. The velocity distribution of the droplet impacting micro-nano structure surface in different cross-section showed no obvious difference, which was due to the height of the micro-structure is too small, so in the vertical direction had no obvious effect on the velocity field.The enhancement heat transfer performance of nine surfaces were studied by a sub-cooled flow boiling test bed. The nine surfaces were designed by L9(21 × 33) mixed orthogonal test design, and we obtained the primary and secondary factors and optimal combination by range analysis. The results showed that the heat transfer performance of the nine micro-nano surfaces was better than that of the smooth surface. Among the four factors, the shape of micro-structure influenced the enhancement boiling heat transfer most, and followed by the micro-structure size, the micro-structure spacing and the redox reaction time. The reasons for micro-nano structured surface enhancement boiling heat transfer can be expressed as: The micro-nano structures increased the heat transfer area of the surface to a certain extent. Compared to the smooth surface, the nucleation density of the micro-nano structure surface was decreased, and the detachment diameter was reduced, but the bubble departure frequency was higher. The nine micro-nano surfaces showed a more hydrophilic property compared to the smooth surface, which indicated that the increase of surface wettability on the surface had a strengthening effect on the boiling heat transfer performance in the used operating condition.