Effect Of Hydrophobicity On Microchannel Flow And Heat Transfer

Within the scope of the micro/nano scale, high flow resistance is a choke problem which is difficult to solve due to the significant surface effect and the high area/volume in the application of micro/nano system. Therefore, searching for the reliable and effective method for drag reduction has an important engineering application value.Up to now, great attention has been concentrated on the exploration concerning micro pin-fins internal flow and super drag reduction of microchannels hydrophobic surface. However, Experimental studies available dealing with the drag reduction of micro tubes and micro pin-fins with the hydrophobic surface, especially the study of the flow drag reduction relationship with different contact angles, in the open literature are very limited.Therefore, in present work, the preparation method of hydrophobic coating and hydrophobic microchannels were firstly discussed systematically and the flow drag reduction characteristics of the de-ionized water flowing through the hydrophobic microchannels were experimentally investigated. Moreover, the flow and heat transfer characteristic of the dei-onized water flowing through the micro pin-fin arrays heat sinks were systematically analyzed. The result and conclusions are summarized as follows:(1) The studies on the preparation of super-hydrophobic micro-tubes and the law of drag reduction in microchannels were conducted. Firstly, the super-hydrophobic solution was obtained by adding 2% perfluorinated octyl triethoxy silane and other types of additive to modified-organosilicone dilute solution and then the solution was titrated into three micro-tubes with different diameters of 0.447mm, 0.728mm and 0.873mm, respectively, to form the super-hydrophobic micro-tubes. After the super-hydrophobic modification, the contact angles of water on inner surfaces of the micro-tubes are all larger than 150°. Furthermore, the experiments were carried out to explore the drag reduction characteristics in micro tubes with de-ionized water with working fluid at Re changing from 100-3000 and the frictional resistance coefficients (f) of de-ionized water flowing through micro-tubes with super-hydrophobic layer and without super-hydrophobic layer were comparatively measured. The result shows the value of f with super-hydrophobic layer is significantly lower than that without the layer at the same Re number, especially in micro tubes with larger diameter, and the maximum discrepancy reached 29.08%. Besides, the transition of flow from laminar to turbulence in super-hydrophobic micro-tube was apparently delayed by the super-hydrophobic modification and the transitional Re in micro tubes became large with the increase of the diameter.(2) The studies on the preparation of the hydrophobic micro pin fins arrays heat sinks and the effects of hydrophobic surface on flow characteristics were conducted. Hydrophobic solution was prepared by adding 2% perfluorinated octyl fluorine silane and micro-nano particles to modified silicone dilute solution and this hydrophobic solution could be solidified to obtain the hydrophobic surfaces. The contact angle of water on these surfaces could be adjusted by changing the content of micro-nano particles in hydrophobic solution. Based on the method above, different micro cylinders groups with contact angles of 99.5°,119.5°and 151.5° (de-ionized water) were prepared. Furthermore, using de-ionized water as working fluid, the flow resistance and pressure drops in micro cylinders groups with different contact angles were measured. The results show that the value of friction factor of test section with hydrophobic layer is apparently lower than that without the layer at the same Re, and this is because the surface tension on hydrophobic interaction interface leads to a reduction of the friction resistance. The larger the contact angles is, the smaller the distance between the micro/nano bumps on the double structure of hydrophobic layer becomes at the same Re, meanwhile the ratio of liquid-air contact area to the total flow area becomes higher, thus the flow resistance decreases. The results of the present study also illustrate that the drag reduction rates attributed to the three kinds of hydrophobic coatings on surfaces of micro pin fins decrease continuously with the increase of Re.(3) The effects of hydrophobic surface on heat transfer characteristics of the micro pin fins arrays heat sinks were studied. The heat transfer characteristics of the ellipse micro pin fins arrays heat sinks had been experimentally investigated when P=100w, micro pin fins with contact angles of 99.5°,119.5°and 151.5°, respectively. The results show that changing flow had big effect on floor surface temperature under low flow condition and changing the flow has little effect under large flow condition; Convective heat transfer coefficient decreases with the increase of contact Angle at the same Re; The convective heat transfer in ellipse micro pin-fins was enhanced with the increase of Re under the different contact Angle; The contact deteriorated with the increase of contact Angle, and the heat transfer performance of the heat sinks deteriorated. The results of the present study also illustrate that the drag reduction rates attributed to the three kinds of hydrophobic coatings on surfaces of micro pin fins is better than that of the heat transfer effect.In summary, the flow and heat transfer characteristics of the de-ionized water flowing through the micro pin-fin arrays heat sinks had been experimentally investigated. In addition, the de-ionized water flowing through the micro pin-fin arrays heat sinks were analyzed systematically. Moreover, the feasibilities of hydrophobic coating applied to the microchannels are explorely studied and a new method of drag reduction in microchannels is developed. The corresponding research results provid a theoretical and technical support for drag reduction in microchannels, which is an effective supplement and perfection for the flow and heat transfer in microchannels.