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The Study of Drag Reduction of Microbubbles on Nonsmooth Surface with Hydrophobic Property

Author: LiZheng
Tutor: XuZhong
School: Dalian University of Technology
Course: Mechanical Manufacturing and Automation
Keywords: Drag Reduction Microbubbles Nonsmooth Surface Low Energy Surface
CLC: U661
Type: Master's thesis
Year: 2013
Downloads: 110
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Abstract

In the drag reduction field of navigation and pipeline transport, the frictional resistance account for a large proportion of the total resistance. How to improve energy efficiency and reduce the frictional resistance has been attracted a large number scholars at home and abroad for the indefatigable effort. The drag reduction technology of microbubbles can effectively reduce the frictional resistance between the solid wall and fluid medium. The basic principle is to lead air into bottom surface and form mixing two phase flow of air and water at the bottom surface, which may reduce the vicinity fluid density and change the flow structure within the boundary layer. The microbubbles can convert a part of the solid-liquid interface to the shear-free air-water interface, thereby reducing the frictional resistance of the solid wall and then conserving energy.This paper combine microbubble drag reduction technology and nonsmooth surface with hydrophobic property technology. The special surface is benefit of retaining microbubbles in the boundary layer and the spaces of microstructures. It will take full advantage of microstructure of nonsmooth surface and low energy property of hydrophobic surface, to improve the stability and efficiency of drag reduction property of microbubbles at the bottom surface.In this paper, the numerical simulation method and experimental test are applied to research and analyze the drag reduction performance of microbubbles with the special surface. The pipe flow and hull model are build to simulate the drag reduction efficiency by using the gas-liquid two-phase flow module of Fluent software. The simulation analyze the influences between the drag reduction performance and factors of microstructure of the special surface, the low energy property and mainstream velocity et.al. The effect of factors are characterized by the parameters of skin friction, void porosity of the boundary layer and drag coefficient. During the experimental testing, the equipment is improved based on a platform of pressure drop. We test the drag reduction performance of microbubbles with the special surface, on which the pit structure and hydrophobic effect are made based on the low energy hydrophobic coating.In the course of study the experimental results mutually coincide with the simulation analysis. It shows that the drag reduction technology of the microbubble can significantly reduce frictional resistance of the solid wall in the fluid. The mainstream velocity and entilation pressure have great influence on the drag reduction effect. In the same boundary conditions, the dimple structure and the low energy property of hydrophobic surface can increase the void fraction near the boundary layer and improve the effect of microbubble drag reduction. The drag reduction effect is enhanced by increasing contact angle of hydrophobic surface but the lager dimple cause stronger turbulence and loss more energy.This paper combine microbubble drag reduction technology and nonsmooth surface with hydrophobic property technology. It take full advantage of microstructure of nonsmooth surface and low energy property of hydrophobic surface to improve the stability and efficiency of drag reduction property of microbubbles and promote the growth of microbubble drag reduction technology.

The Study of Drag Reduction of Microbubbles on Nonsmooth Surface with Hydrophobic Property

Abstract

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