Numerical Simulation Of Transfer Characteristics In Super-Hydrophobic Micro-Tubes

Water transfer characteristics in super-hydrophobic micro-tubes is studied. Emphasizes are put on heat transfer characteristics, numerical simulation of heat transfer and water flow in super-hydrophobic micro-tubes.With super-hydrophobic surface successfully fabricated in aluminous micro-tubes, heat transfer characteristics in it is researched. It is found that the apparent heat transfer coefficient of super-hydrophobic micro-channels is a little lower than that of super-hydrophilic micro-tubes. The analysis result is that the air-layer existing in the micro-nanostructures of the super-hydrophobic surface decreases flow resistance evidently and blocks heat transfer. Considering difference between heat resistance of super-hydrophobic and super-hydrophilic micro-tubes, the equivalent thickness of air layer is calculated in this paper. The result is that it is less than 600nm, however, the scale of cave on the super-hydrophobic surface investigated in SEM is micron-sized. Therefore, the enhancement of heat transfer mechanism exists in super-hydrophobic micro-tubes.With heat transfer in the super-hydrophobic micro-tubes investigated, heat transfer for caves on the super-hydrophobic surface is modeled. With the inner flow field and temperature field analyzed, heat transfer coefficient distribution in the cave is found to be explained in field synergy principle. Heat transfer coefficient of the cave with 2μm,3μm and 4μm-semidiameters is simulated and it is found that heat transfer coefficient is increasing with the decrease of cave scale. According to experimental heat transfer coefficient of super-hydrophilic micro-tube with 0.68mm diameters, heat transfer coefficient of super-hydrophobic micro-tube is computed, and result is that heat transfer coefficient of super-hydrophobic is higher than that of super-hydrophilic micro-tube considering heat conductance of air layer of which thickness equals to the cave scale. The simulate heat transfer coefficient of the cave with 2μm agree with experiment results well, and the maximum relative error is less than 2%. The result of super-hydrophilic micro-tube with 0.72mm diameters have the same trend with that of 0.68mm diameters.Water flow in super-hydrophobic micro-tubes is numerical simulated. Flow model is established on the basis of slip velocity existing on the wall and inner velocity field in tubes is simulated on the slip wall and no-slip wall and flow characteristics is analyzed. The dimensionless pressure drop is decreasing with Reynolds number (Re) increasing, and the trend is the same with experimental results. The slip effect is decreasing with Re number increasing. With the deduced expression for slip length in circular tubes analyzed, the slip effect weakens with Re number increasing, and slip length can reflect the slip effect. Finally friction factor and Poiseuille number (Po) are computed by simulate results, which prove the resistance reduction on slip wall.