Influence of Surface Physical and Chemical Properties on Heat Transfer Characteristics of Steam Condensation
|School||East China University of Science and Technology|
|Course||Chemical Process Equipment|
|Keywords||Dropwise condensation Wettability Surface roughness Chemical inhomogeneity Nanocrystallined surface|
Dropwise condensation is the most efficient condensation mode in the steam condensation heat transfer, which is widely applied in the industrial fields including chemical engineering, energy, health care and so on. The effects of surface roughness, chemical inhomogeneities and nanocrystalline on condensation heat transfer characteristics have been studied to provide the basis for the development of condensation enhancement.The effect of surface roughness on wettability and steam condensation heat transfer characteristics of copper surfaces with various surface roughness, which are gained by grinding with abrasive paper. The results show that the steam presents dropwise and filmwise co-existing condensation on the surfaces, and the area of dropwise condensation and the condensation heat transfer coefficient reach a maximum when Ra=0.15μm. It is found that chemical inhomogeneities induced by oxidation are benefit to enhance condensation heat transfer during steam condensation. The condensation heat transfer coefficient on the copper surfaces with chemical inhomogeneities is 1.4 times greater than the pure copper surfaces, and 3.5 times greater than the surface with complete filmwise condensation. The wettability and condensation heat transfer characteristics on nanocrystallined surface, which is achieved by cryogenic normal pulse spinning technology, have been studied. The results demonstrate that the surface free energy of nanocrystallined surface is lower than annealed surface. The contact angle of nanocrystallined surface with chemical inhomogeneities increases to 114°and presents perfect dropwise condensation. While the hysteresis contact angle increase to 35°, which induces larger resistance of departure droplets and results in only 2.5 times of condensation heat transfer coefficient greater than complete filmwise condensation. The condensation heat transfer characteristics are determined by the wettability of the surface such as contact angle and hysteresis contact angle.