Abstract

The object of this work is to explore the influence superhydrophobic (SH) surfaces exert in laminar jet impingement and when they are combined with riblets in turbulent channel flow. A SH surface generates an apparent slip due to the combination of micropatterning and chemical hydrophobicity. Because of surface tension, water does not enter the cavities between the features, increasing the contact angle of a water droplet on the surface and reducing the liquid-solid contact area. An analysis based on the integral momentum approach of Karman and Pohlhausen is presented that predicts jet impingement behavior on SH surfaces. The model is first applied to the scenario where the slip at the surface is isotropic and a downstream depth is imposed such that a circular hydraulic jump occurs. The model predicts the thin film parameters downstream of the jet and the radial location of the hydraulic jump. An increase in the hydraulic jump radius occurs as slip increases, momentum of the jet increases, or the downstream depth decreases. Modifications to the model are made for the scenario where the slip at the surface varies azimuthally, as would be the case for a surface patterned with microribs. The average behavior is similar, although now an elliptically shaped jump forms with the major axis aligned parallel to the rib/cavity structures. The ellipse eccentricity increases as the slip increases, the jet momentum increases, or the downstream depth decreases. Where there is no downstream depth imposed on SH surfaces, the thin film breaks up into droplets instead of forming a hydraulic jump. Further changes are made to the model to incorporate this behavior for isotropic and anisotropic surfaces resulting in circular and elliptically shaped breakups respectively. This work also explores SH surfaces with riblets in turbulent channel flow. Pressure drop measurements across surfaces exhibiting superhydrophobicity, riblets, and surfaces with both drag reducing mechanisms are presented. The SH surface reduces drag because the effective surface area is reduced and riblets are able to reduce drag by dampening the spanwise turbulence. Photolithography was used to fabricate all surface types. An aluminum channel with a control and a test section was used for testing. Pressure transducers recorded the pressure drop across smooth silicon wafers and patterned test surfaces simultaneously allowing for computation of the friction factors.

Degree

PhD

College and Department

Ira A. Fulton College of Engineering and Technology; Mechanical Engineering

Rights

http://lib.byu.edu/about/copyright/

Date Submitted

2013-08-28

Document Type

Dissertation

Handle

http://hdl.lib.byu.edu/1877/etd6509

Keywords

superhydrophobic, riblets, turbulent, jet impingement, laminar, hydraulic jump, breakup, slip, Joseph Prince

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