铜表面物理化学有序度对蒸汽冷凝特性的影响

Influence of surface physical and chemical inhomogeneities on heat transfer characteristics of steam condensation

研究了经不同粒度砂纸研磨后不同粗糙度纯铜表面的润湿性和蒸汽冷凝性能,分析了表面物理粗糙度和氧化引起的表面化学成分异质性对润湿性和冷凝传热性能的影响规律。结果表明,当铜表面粗糙度小于0.15μm,增大粗糙度有利于增大接触角,而当粗糙度增大到0.15μm后,继续增大粗糙度反而会减小其表面接触角。铜表面氧化后造成的化学成分异质性有利于增大接触角。表面微观结构和化学成分异质性的共同作用决定了铜表面的润湿性与蒸汽在其表面的冷凝模式。当粗糙度为0.15μm左右时,氧化后铜表面表现出最佳的滴状冷凝状态,其冷凝传热系数达到Nusselt理论模型的3.5倍左右。但是表面粗糙结构同时也会增大液滴在金属表面的运动阻力,对冷凝传热性能产生不利影响。

Visual experiments were employed to investigate wettability and heat transfer characteristics of steam condensation on vertical copper plates with different surface roughness by grinding.Effects of surface roughness and oxidation-induced chemical inhomogeneity on wettability and heat transfer characteristics of steam condensation were analyzed.The results indicate that the contact angle increases to a maximum value when the surface roughness reaches 0.15 μm and then decreases with increasing surface roughness.Moreover,chemical inhomogeneity induced by oxidation during steam condensation on the modified surface always contributes to enlarge contact angle and thus increase hydrophobic property.The results suggest that the wettability and steam condensation mode on the copper surface are determined by micro-scale surface roughness and chemical inhomogeneity.The experimental results show that the oxidized surface with surface roughness of about 0.15 μm exhibits complete dropwise condensation,increasing condensation heat transfer coefficient by a factor of 3.5 compared to complete filmwise condensation,but a surface with larger roughness will inhibit the drops to depart from the condensing surface and hence reduce the condensation heat transfer coefficient,so that the condensation heat transfer of the surface with roughness of 0.09 μm is larger than that with surface roughness of 0.15 μm.