Xuehu Ma

Influence of processing conditions of polymer film on dropwise condensation heat transfer

Abstract The effect of processing conditions of polymer film on dropwise condensation heat transfer of steam under atmospheric pressure is investigated to find an effective technique to prepare a viable polymer film sustaining long-term dropwise condensation pattern state. The polytetrafluoroethylene (PTFE) films were coated on the external surfaces of brass tubes, copper tube, stainless steel tube and carbon steel tube by means of the dynamic ion-beam mixed implantation technique, with a variety of surface processing conditions. The experimental results indicated that heat flux is increased by 0.3–4.6 times and condensation heat transfer coefficient by 1.6–28.6 times of film condensation values for the brass tubes treated with various conditions. The surface processing condition is crucial to the adhesion between polymer film and metal substrate, different substrate material requires different optimal processing condition, and leads to different condensation heat transfer characteristic.

Dropwise Condensation Heat Transfer Model: Effect of the Liquid-Solid Surface Free Energy Difference

Dropwise condensation heat transfer performance depends not only on the condensing conditions, but also on the interfacial interaction between condensate and condensing surface material. Based on the well-established Rose’s model, a modified model of dropwise condensation heat transfer is proposed by considering the interfacial interaction between liquid and solid, and established by rebuilding the space conformation of drop distributing into time conformation. The simulation results indicate that the heat transfer coefficient increases with the surface free energy difference increasing and the contact angle hysteresis decreasing. The larger contact angle and the smaller departure drop size result in the higher heat transfer coefficient. Different interfacial effect gives rise to the different heat transfer curves. For the identical solid-liquid-vapor system, the simulation results agree very well with the present experimental data and those reported in literature. The controversy among experimental results in literature might be well understood with the concept of the present paper.Copyright

Transport Phenomena for Falling Liquid Film Process: Coating Configurations Effect

A specific coating was sprayed on the external surface of brass tube with design surface division form. The characteristics of heated liquid films flowing over a vertical coated division tube were experimentally investigated with a highly sensitive infrared camera, THERMOVISION™ A40. For different flow conditions, the temperature fluctuation and the flow patterns of falling film were determined. It was found that the temperature fluctuation on the coated division tube was larger than that on the smooth tube. The disturbance of falling liquid film was strengthened from the existence of a surface tension gradient that was induced by the coated region, resulting in stronger mixing occurred and drove the liquid particles from internal of liquid film to surface. The interfacial effect could therefore strongly affect the flow characteristic and thus the transfer process of falling liquid films.

Effect of Surface-Free-Energy-Difference on Condensation Characteristic Curves of Binary Mixture of Steam and Ethanol Vapor

The condensation heat transfer process of binary mixture of steam and ethanol vapor was studied experimentally with various ethanol concentrations and surface subcooling at atmospheric pressure. The experiments demonstrate that the condensation modes alter from filmwise, to transition and finally to dropwise with respect to the operation conditions. The marangoni effect was always used to interpret this phenomenon in the published literature. In this paper, the mechanism of condensation heat transfer with the liquid-solid-surface-free-energy-difference is used to further understand this process and the conversion of condensation modes. From this viewpoint, the condensation characteristics of binary mixture of steam and ethanol vapor can be explained reasonably with the corresponding condensation modes. Base on the data of the present paper and those reported in literature, the conversion criterion of the condensation modes is proposed. The condensation state is filmwise for the surface-free-energy-difference less than 14±1 mJ·m−2 and dropwise for more than 21±1 mJ·m−2 and transition for between 14±1 mJ·m−2 and 21±1 mJ·m−2 .Copyright