Abstract:

The study uses PTFE nanoparticle coatings to construct a super-hydrophobic surface and a striped pattern mixed wettable surface on a 50 mm×100 mm copper-based surface. In order to explore the influence of the inclination angle of the stripes on the condensation heat transfer, a mixed wettable surface with stripes and the width direction of the copper plate at 90° and 60° was used in the experiment. Besides, the width of the hydrophilic region and superhydrophobic region were both set to 1 mm, and the proportion of hydrophilic area were 48.8% and 48.7% respectively corresponding to the above-mentioned hybrid wettable surfaces. The condensation phenomena were recorded by a high-speed camera (Phantom, Miro Ex4). Dropwise condensation (DWC) was achieved on the super hydrophobic surface and superhydrophobic region of the hybrid wettable surfaces, while filmwise condensation (FWC) was observed on the hydrophilic region of the hybrid wettable surfaces. The result of the experiment indicated that the condensation heat transfer coefficient (HTC) of the super hydrophobic surface and the hybrid wettable surfaces has a strong correlation with the droplet departure rate and surface renewal. The faster the droplet departure rate is, the higher the condensation heat transfer coefficient will be. Moreover, due to the coarse textures of the superhydrophobic surface, the wetting model of droplets formed on the superhydrophobic surface tended to represent the Wenzel wetting model. As a result, the droplet departure rate and HTC of the superhydrophobic surface was lower than FWC when the condensation heat transfer temperature difference \mathrm{\Delta }{T}_{\mathrm{s}\mathrm{u}\mathrm{b}} was in the range of 0—20 K. However, the hybrid wettable surfaces were able to enhance the HTC of the superhydrophobic surface by increasing condensation surface renewal owing to the liquid bridge departure. The highest HTC of 16.64 \mathrm{k}\mathrm{W}/({\mathrm{m}}^{\mathrm{2}}?\mathrm{K}) and condensation heat flux of 188 \mathrm{k}\mathrm{W}/{\mathrm{m}}^{\mathrm{2}} when \mathrm{\Delta }{T}_{\mathrm{s}\mathrm{u}\mathrm{b}} was 11.3 K were observed on the hybrid wettable surface (60°-parallel-stripes pattern), whose HTC was 2.14-fold compared with that of the superhydrophobic surface. While an HTC of 13.63 \mathrm{k}\mathrm{W}/({\mathrm{m}}^{\mathrm{2}}?\mathrm{K}) which was 1.68-fold compared with that of the superhydrophobic surface was observed on the hybrid wettable surface (90°-parallel-stripes pattern) when \mathrm{\Delta }{T}_{\mathrm{s}\mathrm{u}\mathrm{b}} was 13.8 K.

Key words: condensation, heat transfer, PTFE, nanoparticle, fabrication, superhydrophobic, hybrid wettable surface

摘要：

采用聚四氟乙烯纳米颗粒涂料在50 mm×100 mm铜基表面构造超疏水表面以及条纹图案混合润湿性表面。为探究条纹倾角对冷凝换热的影响，条纹与铜板宽度方向分别成90°与60°的混合润湿性表面被应用于实验中。实验结果表明，超疏水表面传热系数h_{\mathrm{d}\mathrm{w}\mathrm{c}}与混合润湿性表面冷凝传热系数h_{\mathrm{h}\mathrm{y}\mathrm{b}\mathrm{r}\mathrm{i}\mathrm{d}}随换热温差\mathrm{\Delta }{T}_{\mathrm{s}\mathrm{u}\mathrm{b}}增大而增大。同时，h_{\mathrm{d}\mathrm{w}\mathrm{c}}和h_{\mathrm{h}\mathrm{y}\mathrm{b}\mathrm{r}\mathrm{i}\mathrm{d}}与冷凝液的脱落频率存在强相关关系，脱落频率越高，h_{\mathrm{d}\mathrm{w}\mathrm{c}}、h_{\mathrm{h}\mathrm{y}\mathrm{b}\mathrm{r}\mathrm{i}\mathrm{d}}越大。超疏水表面冷凝液脱落频率低，h_{\mathrm{d}\mathrm{w}\mathrm{c}}在0~20 K的换热温差范围内始终低于膜状凝结。混合润湿性表面能有效强化超疏水表面的冷凝换热，条纹倾角为60°的表面在\mathrm{\Delta }{T}_{\mathrm{s}\mathrm{u}\mathrm{b}}为11.3 K时测得最高传热系数16.64 \text{?}\mathrm{k}\mathrm{W}/({\mathrm{m}}^{\mathrm{2}}?\mathrm{K})，是超疏水表面传热系数的2.14倍；而条纹倾角为90°的表面在\mathrm{\Delta }{T}_{\mathrm{s}\mathrm{u}\mathrm{b}}为13.8 K时测得最高冷凝传热系数13.63 \mathrm{k}\mathrm{W}/({\mathrm{m}}^{\mathrm{2}}?\mathrm{K})，是超疏水表面传热系数的1.68倍。

关键词: 凝结, 传热, 聚四氟乙烯, 纳米粒子, 织构, 超疏水, 混合润湿性表面