Study of drag reduction in microstructured channel with patterned cavities and wettability control
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This work is done in part as a requirement towards the fulfillment of the master's degree. Four chapters are covered in this work. Chapter 1 gives an overview of the various works that have been done in the field, including some basic background and theory. Chapter 2 discusses the details of the numerical model. Grid independency study and validation of these models are accomplished by comparing with the analytical solution and quantitatively with the experiments. Chapter 3 explores the parametric study of different initial condition and geometries. Here, we are particularly interested in the effects of initial interface assumption. It is found that the effective slip length of preset interface assumption is in good agreement with the initially air-filled interface assumption, as well as the experimental results, at low gas fraction. However, the effective slip length of a preset interface has a dramatic deviation at high gas fraction with the initially air-filled channel. Large difference occurs when changing the surfaces from hydrophobic to hydrophilic. For a preset interface, air patterns are trapped in the structured cavities and compressed by the liquid flow. An air-water interface still exists even the surface of the cavity is hydrophilic, thus, drag reduction increases. However, for an initially air-filled microchannel, water expel the air out of the microchannel and completely wets the whole channel at the end. No air-water interface is found and drag reduction decreases. Studies also show that drag reduction increases when increasing the contact angle of the cavity surfaces. To extend our work further, parametric studies have been carried out to investigate the effect of geometry by changing the cavity width, fraction and numbers at low gas fraction. It is found that increasing the gas fraction and decreasing the cavity numbers can provide better performance on drag reduction, as the interfaces provide longer and continuous slip-boundary condition. Conclusions and future works are discussed in Chapter 4. More cases are carried out to build the relationship between friction reduction and different configurations in transient state to find a smaller range of depth-to-width ratio in which the air pockets can be successfully trapped in the cavity. Different shapes of posts can be studied to observe the position of the air-water interface because the mechanism for depinning is strongly dependent on the details of the post geometry and on the Young's contact angle, and depinning from both the top and the side edges of the posts is important in controlling the fluid behavior. The effect of air diffusion and how to overcome this effect will be taken into consideration.
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