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Molecular Dynamic Simulation of Friction Reduction in Two-Phase Flows with Nanostructures and Study of the effect of Electric Field

Saleki, Omid | 2023

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  1. Type of Document: M.Sc. Thesis
  2. Language: Farsi
  3. Document No: 56076 (08)
  4. University: Sharif University of Technology
  5. Department: Mechanical Engineering
  6. Advisor(s): Kazemzadeh Hannani, Siamak; Moosavi, Ali
  7. Abstract:
  8. The use of superhydrophobic surfaces is one the most promising methods for reducing the friction and increasing the flow rate in fluid transfer systems. Since in such systems the surface structure plays a key role, in the first part of this study, the performance of the hierarchical nanostructures is explored. These nanostructures are inspired by the superhydrophobic surface of the lotus leaf. The flow is considered between two walls with hierarchical nanostructures and simulate the system via the molecular dynamics (MD) method. The size of the nanostructures and the distance between them have been studied to find whether a design with maximum flow rate exists. The nanostructures have two parts, a bigger part on the wall which is a half-sphere and a smaller part which is a cylinder on top of the bigger part. The effect of wall materials was also examined by considering four different materials, namely carbon, silicon and two other hypothetical materials. Also, a two-phase flow consisting of water and air have been simulated to study the effect of the trapped airs in the performance. The results show that in the design with minimum pressure drop, the slip length increases by 111\% and 101\% for the carbon-made walls and silicon-made, respecticely. In the second part, water flow between two carbon walls with nanostructures made of Poly(N-isopropylacrylamide) via the molecular dynamics method has been studied. The structure of this polymer can change based on the temperature of the environment, so that by increasing the temperature the structure becomes hydrophobic. This property has been studied and the effect of multiple factors on the slip length is presented. The effects of the number of monomers in the polymer, the distance between the polymers, and the temperature on the flow field are investigated. The results reveal that the slip length and the flow rate increase with the temperature but both have a maximum with respect to the distance between the polymers and the number of monomers. The results show that at the temperature of 340 K the slip length increases by 230\%. In the third part of the study, water pumping by spinning electric field is investigated. Water can be pumped in nanochannels by confining it between the surfaces with different hydrophobicities and exerting a spinning electric field. The asymmetrical hydrophobicity combined with the spinning electric field and the fact that the water molecules have a dipole moment create a situation in which the angular momentum of water molecules is transformed into a linear momentum and the water is pumped into the nanochannel. The hydrophobicity of the surfaces can be manipulated by using nanostructures to reduce friction. In this study, two types of nanostructures have been used which are hierarchical nanostructures and polymer nanostructures made of Poly(N-isopropylacrylamide). The walls of the nanochannel are grafted with nanostructures asymmetrically, i.e. the upper wall is flat while the bottom wall is grafted with nanostructures. This allows for the fluid in the nanochannel to be confined between the surfaces with different hydrophobicity and by applying the spinning electric field, a plane Poiseuille flow is created. The effects of the frequency and the amplitude of the spinning electric field on the behavior of the fluid have been investigated. Also, the effects of adding air to the flow for two-phase flow systems have been studied. The results reveal that the electric field can increase the flow rate by up to 475\%
  9. Keywords:
  10. Molecular Dynamic Simulation ; Flow Discharge ; Two Phase Flow ; Carbon ; Silicon ; Functionalized Graphene ; Superhydrophobic Surfaces ; Slip Length ; Hierarchical Nanostructures ; Friction Reduction ; Spinning Electric Field

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