چكيده به لاتين
Human observations and curiously explorations in the nature, alongside with the advancement of science and detection technologies development, leads to discovery an interesting phenomena, means the fluid slip on the wall. This phenomena can be used to improve the efficiency of the industry equipment’s and Nano-based applications. In recent years, many researchers could make hydrophobic surfaces by changing the chemical and mechanical structures of the surface, in which drag reduction is one of the most important applications of theses surfaces. In this regard, the researchers of Applied Hydrodynamic Laboratory of Iran University of Science and Technology have prospered to fabricate surfaces with static contact angle larger than 165 degree and dynamic slip angle less than 5 degree, so that these surfaces has very good hydrophobicity properties and are classified in “superhydrophobic surfaces”. Although, evaluation of the hydrophobicity efficiency is a challenge point in this field and so far no one report any universal and accurate method for quantifying the response of the surface to the slip condition and measurement of the slip length in the internal and external flow. So this research, by paying attention to this vague matter, has tried to resolve this important problem.
In this regard, measurement of the pressure drop in the internal flow has been used after literature review in order to reach the research goals. Then Pressure drop measurement system in the turbulent channel flow with hydrophobic walls was designed and built up. Other researchers data and numerical simulation results has been used in order to verify the accuracy of the measurement system. Also for estimating the slip length and making needless of measurement system from PIV, an algorithm was developed and programmed as an UDF code for using besides the momentum equations of fluid in the Ansys FLUENT software package.
In the experimental part of this study, hydrophobic surfaces were evaluated in the shear stress less than 25 pascal. In the case of turbulent flow in the channel with one hydrophobic wall, the measured drag reductions were began from a few percent in the low shear stress to asymptotic value of 17.3 percent in the higher shear stress. Also the predicted value of drag reduction in the case of turbulent flow in the channel with both hydrophobic walls was measured about three times more than the case of one side hydrophobic wall and the amount of this drag reduction was about 50 percent. Estimated slip length was between 30 to 90 microns for this hydrophobic surfaces and the maximum of drag reduction occurred in the corresponding shear stress of about 1.3 Pascal. Also the asymptotic state of slip velocity was about 20 percent of the center velocity in channel.
Keywords: Hydrophobic Surfaces, Slip Length, Slip Boundary Condition, Turbulent Channel Flow, Turbulent Flow Simulation.
