激光选区微织构增强不锈钢表面冷凝传热

Enhancement of Condensation Heat Transfer on Stainless Steel Surface by Laser Selective Micro-texture

目的针对特殊润湿性表面常用的化学法修饰具有高污染、热阻大和加工周期长的缺点,采用无化学的激光选区微织构技术增强不锈钢表面冷凝传热性能。方法用纳秒脉冲光纤激光正交扫描微织构不锈钢表面,获得正方形网格沟槽-凸起结构的超亲水表面,在电热恒温干燥箱中热处理降低其表面自由能,获得接触角156.7°与滚动角4.2°的不锈钢超疏水表面。然后对超疏水表面进行激光二次微织构,制备出楔形超疏水-超亲水选区微织构表面。通过SEM、XPS及三维光学显微镜分析表面形貌、化学成分及三维轮廓。对比单一超亲水、超疏水及原始表面,测得试样背面温度、冷凝水量和冷凝液滴平均脱落直径,根据相关冷凝传热理论公式计算出四种试样的表面冷凝传热系数。结果冷凝实验表明,楔形选区微织构表面冷凝传热系数可达到(159.7±1.8) W/(m^2·K),其表面冷凝液滴平均脱落直径达到1.2 mm,约为单一超疏水表面冷凝传热系数的2.0倍,冷凝液滴平均脱落直径为全超疏水的55%。结论选区微织构表面超亲水楔形通道对液流有自驱动作用,使冷凝液快速汇聚后脱离冷凝表面,这一选区微织构图案相比单一原始、超亲水及超疏水表面拥有更好的冷凝传热效果;同时表面冷凝液滴平均脱落直径减小,从而具有更高的冷凝传热系数。

The work aims to adopt the chemical-free laser selective micro-texture technology to enhance the condensing heat transfer performance of stainless steel for the disadvantages of high pollution, large thermal resistance and long processing cycle in the chemical modification method used for special wettability. The micro-textured stainless steel surface was scanned by nanosecond pulsed fiber laser to obtain a superhydrophilic surface of a square grid groove-convex structure. The surface free energy was baked in a constant temperature drying oven to obtain a stainless steel superhydrophilic surface with a contact angle of 156.7° and a rolling angle of 4.2°. The superhydrophobic surface was then processed by laser for secondary micro-texture to get a wedge-shaped superhydrophilic-superhydrophobic selected micro-textured surface. Surface morphology, chemical composition and three-dimensional profile were analyzed by SEM, XPS and three-dimensional optical microscope, respectively. By comparing the single superhydrophilic, superhydrophobic and original surfaces, the back surface temperature, the condensed water volume and the average detached diameter of the condensed droplets were measured, and the surface condensation heat transfer coefficient of the four samples was calculated according to the relevant condensing heat transfer theoretical formula. Condensation experiments showed that the condensation heat transfer coefficient of the micro-textured surface of the wedge-shaped selection could reach(159.7±1.8) W/(m^2·K), about twice of single superhydrophilic surface condensation heat transfer coefficient, and the average detachment diameter of the surface condensation droplets reached 1.2 mm, about 55% of that of full super-hydrophobic surface. The heat transfer coefficient and the average detached diameter of the condensed droplets were twice and 55% respectively. The superhydrophilic wedge-shaped channel on the micro-texture surface of the selected area has a self-driving effect on the liquid flow, so that the condensate can be quickly concentrated and separated from the condensed surface. This selected micro-texture pattern has better performance than the single original, superhydrophilic and superhydrophobic surfaces. At the same time, the average detachment diameter of the surface condensation droplets decreases, thus leading to a higher condensation heat transfer coefficient.