表面分割方式对滴膜共存冷凝传热特性的实验研究

Study on Effects of Surface Division Patterns on Condensation Heat Transfer for dropwise and Film Condensation Coexisting Surfaces

实验研究了滴膜面积比为1:1且滴状区域采用较厚的有机涂层时,垂直管外滴膜共存表面的滴膜区域分割方式对冷凝传热特性的影响。结果表明,在表面分割方式一定时,滴膜共存表面的冷凝传热性能随着表面分割数的增加而增加, 但总仍是介于全部表面为滴状冷凝和全表面为膜状冷凝型态之间的传热特性。分析表面分割数分别为2和4的滴膜共存表面的传热结果,发现凝液环的缓冲作用对于滴膜共存表面的冷凝强化传热具有重要作用。滴膜共存表面上滴状冷凝区面积对其下游膜状冷凝传热的影响不仅与滴膜区域的面积比有关,而且实验操作条件也有一定的控制作用,且两者之间存在最佳匹配值,即此时的强化传热效果最好。

The Experiments were taken to examine the possibility of enhancing condensation heat transfer characteristics of steam by using a thick polymer film promoting dropwise condensation on dropwise and film condensation coexisting (DFC) surfaces. A polymer film with thickness of more than 1 μm was coated on the dropwise-condensation regions of the external surface of brass tubes with different surface divisions. The area ratio of dropwise parts and filmwise parts was fixed at 1:1 for the six surfaces, whilst the division numbers were different with each other. It was found that the condensation heat transfer characteristics were greatly influenced by the surface division number. Compared with the bare surface, the condensation heat transfer of dropwise and film condensation coexisting surface showed an enhancement ratio of 1.3 to 4. Analyzing the experimental results of this paper and the reported in literatures indicated that the heat transfer enhancement characteristics of dropwise and film condensation coexisting surface depend not only on the surface division pattern at a fixed area ratio of dropwise and film parts, but also on the operating condition. The heat transfer enhancement ratio showed that there exists a maximum value while increasing the surface subcooling degree. An optimal choice exists between surface division numbers and surface subcooling degree.