超疏水表面界面润湿行为与减阻特性研究

发布时间：2018-01-14 22:28

本文关键词：超疏水表面界面润湿行为与减阻特性研究　出处：《西北工业大学》2016年博士论文　论文类型：学位论文

【摘要】：近十多年,超疏水表面减阻受到许多学者关注,是当前减阻研究热点。该技术被认为经济、简便、可行性强,且兼具防污功能,能广泛应用于海洋工程和其它高技术领域。由于独特的润湿性,超疏水表面在水下能封存一层气膜,使得其近壁面流场呈现出多相流、跨尺度流动等特点,在进行理论分析和试验测试过程中,必须充分考虑壁面润湿性和微形貌等众多因素的影响,因此对超疏水表面减阻机理的研究提出了新的要求。论文采用试验和数值模拟方法,对超疏水表面界面润湿行为和减阻特性进行了深入地研究,通过对超疏水模型在不同流态下的阻力、近壁面流场、气液界面滑移等的试验测试与理论分析,揭示了不同微结构对超疏水表面减阻和滑移的影响规律。所取得的主要研究成果和创新有:(1)基于光刻和模板转印原理制备出具有规则微结构的PDMS超疏水薄膜,同时提出两种润湿性和粗糙度可调的超疏水表面制备方法:Teflon表面粗糙化,低表面能涂层添加微颗粒。随后,测试给出典型微结构超疏水表面上液滴接触角变化规律,并揭示了微结构对静态液滴各向异性的影响机制。通过设计、控制超疏水表面各向润湿异性,创新性地实现了水滴收缩过程的操控。(2)通过数值模拟,系统研究了顺流向沟槽、垂直流向沟槽、微凸柱和微凹坑等四种典型微结构超疏水表面在不同流态下的流场特性,从速度分布、剪应力特性、滑移速度、近壁面涡量等方面分析了其流场结构,给出了微结构类型、尺寸以及自由剪切面积比等参数对超疏水表面滑移长度、滑移速度、减阻量的影响规律。(3)通过理论分析和试验验证系统分析了层流状态下超疏水表面在平板间和外部扰流两种流动状态下的减阻规律。设计搭建起Teflon超疏水表面微通道装置和小尺度模型落体试验装置,测试给出不同流速下微结构尺寸、形状及模型尺度对滑移长度和减阻量的影响规律,并解释了层流状态下超疏水表面减阻效果随模型尺度增大而迅速衰减的原因。(4)搭建起基于连续激光和高速摄像机的专用PIV系统,通过荧光粒子和滤光片的组合使用,有效解决了常规PIV近壁区测试中存在的反光干扰难题。通过对超疏水平板边界层流场测试,发现滑移速度沿流向位置的后移而减小,但滑移长度变化并不明显,这与理论分析和落体试验结果相一致。(5)基于连续激光器、大景深显微镜头和高速摄像机改进了现有μ-PIV系统,实现了毫米尺度气液界面上滑移速度的直接测量。通过对不同沟槽上气液界面处滑移速度的直接测量,则发现顺流向沟槽上滑移速度远大于垂直流向沟槽,同时还意外观测到在矩形气液界面上存在局部负滑移现象。
[Abstract]:In recent ten years, the drag reduction of superhydrophobic surface has been concerned by many scholars, which is a hot spot in drag reduction. This technology is considered to be economical, simple, feasible and has anti-fouling function. It can be widely used in marine engineering and other high-tech fields. Because of its unique wettability, the superhydrophobic surface can seal a layer of gas film under water, which makes the flow field near the wall appear multi-phase flow, cross-scale flow and so on. In the course of theoretical analysis and test, the influence of many factors such as wall wettability and micro-morphology must be fully considered. Therefore, new requirements are put forward for the study of drag reduction mechanism of superhydrophobic surface. The wetting behavior and drag reduction characteristics of superhydrophobic surface interface are studied in this paper by means of experiments and numerical simulation. The resistance of the superhydrophobic model under different flow states, the flow field near the wall, and the slippage of the gas-liquid interface are tested and analyzed theoretically. The effects of different microstructures on drag reduction and slip on superhydrophobic surface are revealed. The main research results and innovations obtained are as follows: 1). Based on photolithography and template transfer principle, PDMS ultrahydrophobic films with regular microstructures were prepared. At the same time, two kinds of super-hydrophobic surface preparation methods with adjustable wettability and roughness were proposed. The surface of the superhydrophobic surface was roughened with low surface energy coating and microparticles were added. The change of contact angle of droplet on superhydrophobic surface of typical microstructures is given, and the mechanism of influence of microstructures on the anisotropy of static droplets is revealed. Through the design, the wettability of superhydrophobic surfaces is controlled. Innovatively realized the control of droplet contraction process. (2) through numerical simulation, we systematically studied the trench in the direction of flow and the trench in vertical direction. The flow field characteristics of superhydrophobic surfaces of four typical microstructures, such as microconvex columns and micropits, under different flow states, are analyzed in terms of velocity distribution, shear stress characteristics, slip velocity and vorticity near the wall. The parameters such as microstructure type, size and free shear area ratio on the slip length and velocity of superhydrophobic surface are given. Effect of drag reduction. The drag reduction law of superhydrophobic surface under laminar flow and external disturbance was analyzed by theoretical analysis and experimental verification. The Teflon superhydrophobic surface microchannel device was designed and constructed. Small-scale model falling test device. The effects of the size, shape and model size of the microstructures on the slip length and drag reduction are given. The reason why the drag reduction effect of superhydrophobic surface decreases rapidly with the increase of model scale in laminar flow is explained. The special PIV system based on continuous laser and high speed camera is built. Through the combination of fluorescent particles and filters, the problem of reflective interference in the conventional PIV near wall measurement is solved effectively, and the boundary laminar flow field of superhydrophobic plate is measured. It is found that the slip velocity decreases after moving backward along the flow direction, but the slip length does not change obviously, which is consistent with the theoretical analysis and the falling body test results. The large depth of field microscope head and high speed camera have improved the existing 渭 -PIV system. The direct measurement of slippage velocity on gas-liquid interface with millimeter scale is realized. Through the direct measurement of slippage velocity at gas-liquid interface on different grooves, it is found that the slip velocity on the trench is much higher than that on the vertical trench. At the same time, the phenomenon of local negative slip on the rectangular gas-liquid interface was observed accidentally.
【学位授予单位】：西北工业大学
【学位级别】：博士
【学位授予年份】：2016
【分类号】：U661.311;P75

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