Steam Condensation Heat Transfer Enhancement And Lattice Boltzmann Simulation For Droplet Dynamic Behaviors

Steam condensation was extensively applied in petro-chemical engineering, refrigeration, power generation plant, electronic systems, nuclear power station and the aero space engineering. Dropwise condensation heat transfer has drawn much attention due to its higher heat transfer coefficient than that of filmwise condensation. Solid-liquid and steam-liquid interfacial effects were significant in regulating the condensation mode and exploiting the novel condensation heat transfer enhancement technology. The departure radius or the maximum radius of droplet on the condensing surface was one of the most significant parameters during dropwise condensation for a given condensing surface. Therefore, reducing the maximum radius of droplet and accelerating the movement of the droplet were the effective protocal to enhance dropwise condensation heat transfer. It seemed that the droplet size adjustment and droplets rapid movement induced by surface configurations and wettability were significant to enhance the dropwise condensation heat transfer. In this dissertation, the hybrid surfaces were used to adjust the droplet dynamic properties and size distribution for the dropwise condensation heat transfer enhancement. Consequently, the adjustment of the droplet size, droplet size distribution and droplet movement was investigated by the experimental test, theoretical analysis and lattice-Boltzmann simulation.The movement features, maximum radius and size distribution of droplet on hydrophobic region of hydrophobic-hydrophilic hybrid surfaces were visually observed. The influence of hydrophobic region width on droplet motion, droplet size and droplet size distribution on hydrophobic region was eaxamed. Therefore, the adjustment mechanism of droplet dynamic properties by the hybrid mode was studied. The results indicated that the drainage mode of droplet on hydrophobic region could change accordingly with the increase of the hydrophobic region width. When the hydrophobic region width was smaller than the departure diameter of droplet for the complete dropwise condensation, droplet on hydrophobic region was drained through migrating into hydrophilic region due to the surface free energy difference between the two regions. However, when the hydrophobic region width was equal or even larger than departure diameter, droplet on hydrophobic region was drained with two modes. The droplet on hydrophobic region could spontaneously drain through hydrophilic region and depart from condensing surface. On the other hand, the departed droplet on hydrophobic region could also migrat into hydrophilic region when the droplet contacted with the liquid film on hydrophilic region during the departure process. The maximum radius of droplet on hydrophobic region decreased significantly due to the spontaneous motion of droplet. The maximum radius of droplet on hydrophobic region decreased with the decrease of hydrophobic region width, which was smaller than the value estimated by the geometrical relationship. The density of droplet with small radius increased with the decrease of hydrophobic region width. Therefore, droplet size distribution on hydrophobic region could be adjusted for the hybrid surface. Simailar work was conducted for the superhydrophobic-hydrophobic hybrid surfaces at atmospheric pressure. The results showed that the droplet on superhydrophobic region was at Wenzel wetting mode and the surface adhesion was much larger. The droplet on hydrophobic region migrated into the superhydrophobic region spontaneously. Droplet size on hydrophobic region could be adjusted by hydrophobic region width.Steam condensation heat transfer performances at atmospheric pressure on hydrophobic-hydrophilic and superhydrophobic-hydrophobic hybrid surfaces were measured experimentally. The results indicated that steam condensation heat transfer characteristics of hydrophobic-hydrophilic hybrid surface increased with the increase of hydrophobic region width firstly and then decreased with the further increase of hydrophobic region width. An optimum hydrophobic region width existed. However, steam condensation heat transfer performance decreased with the increase of hydrophilic region width. It was also found that steam condensation heat transfer performance was larger than that of complete hydrophobic surface when the width of hydrophobic and hydrophilic regions was designed appropriately. From the experimental renstls, it can be concluded that the condensation heat transfer could be enhanced with the hydrophobic-hydrophilic hybrid surfaces by adjusting the maximum departure radius of droplet and droplet size distribution. The heat transfer performance of hybrid surface increased with the increase of area fraction of hydrophobic region. The performance was higher than the weighted average value on the complete superhydrophobic and hydrophobic surfaces.Integrating the dropwise and filmwise condensation heat transfer models, the condensation heat transfer enhancement with dropwise-filmwise hybrid surfaces was analyzed. The influence of dropwise region width on heat transfer performance of dropwise region, the thickness of liquid film on filmwise region and the heat transfer characteristics of filmwise region was investigated. The influences of filmwise region width, surface subcooling, surface contact angle and contact angle hysteresis on heat transfer performance of dropwise-filmwise hybrid surface was also studied. Results indicated there was a good agreement with theoretical and experimental results. An optimum dropwise region width existed and the filmwise region was as narrow as possible. The maximum steam condensation heat transfer performance was1.18times of that on complete dropwise condensation. It also indicated the dropwise-filmwise hybrid surface could be effectively applied to enhance steam condensation heat transfer at low surface subcooling and for the condensing surface with smaller contact angle or larger contact angle hysteresis.The dynamic characteristics during coalescence of two droplets with the same volume on superhydrophobic surface were visualized with high speed camera system and the jumping height of the coalesced droplet was measured experimentally. The velocity distribution and energy conversation evolution were calculated with lattice Boltzmann method. Combining the energy analysis method, droplet jumping phenomenon induced by droplet coalescence on superhydrophobic surface was studied. The results indicated the present model based on the lattice Boltzmann simulation and the energy conservation model could satisfactorily predict the jumping height and the initial jumping velocity of the coalesced droplet. Therefore, the good agreement between the experimental and model predicted results indicated that this modified model could provide good guidelines for deep getting insight into the droplet jumping phenomena induced by droplet coalescence during dropwise condensation of steam with high concentrations of non-condensable gas on superhydrophobic surface.Droplet coalescence on solid surface was simulated by lattice Boltzman method based on free energy model. The perturbation in steam bulk induced by droplet coalescence was intensified with the wettability of solid surface decreasing. Droplet deformation and motion on solid surface driven by steam velocity and external force was simulated as well. The results indicated that droplet deformation and moving velocity increased with an increase of steam velocity, external force acceleration, droplet radius and contact angle. The simulation results could facilitate the interpretation of the temperature distribution at droplet surface induced by droplet coalescence, condensation heat transfer enhancement with the aids of vapor velocity and gravitational force qualitatively or semi-quantitatively.