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College of Science & Engineering > Department of Mechanical Engineering

Compressed Air Approach for Wind Energy Storage

     
 

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Research: Surface Texturing    surface
                  texturing


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Motivation:

In order to increase the heat transfer capability within the air-compressor/expander, surface areas for heat transfer are added by the use of heat exchanger structures and the injection of tiny water droplets. Proper surface treatment of the walls of the compressor/expander and of the heat exchanger can
  1. improve heat transfer effectiveness
  2. reduce mechanical losses due to liquid and air flow drag
The research team led by Professor Eric Loth and Dr. Adam Steele at the University of Virginia is investigating durable and cost-effective nano-texturing techniques for achieving these goals.

Introduction and Achievements

The story of superhydrophobic materials starts in nature and has been appropriately named the ‘lotus effect’ after the lotus leaf (Nelumbo nucifera) as shown in the figure below. Its characteristic micro- and nano-surface morphology combined with low surface energy chemical functionality allows for water droplets to roll and bounce freely on the surface.
Lotus leaves
              are anti-wetting because of their surface textures

This addition of surface texture can greatly reduce the solid-liquid surface contact area and leads to an apparent contact angle, θ*, for a droplet on the surface, which can be explained by two independently developed models: the Wenzel model and the Cassie-Baxter model.

How surface
              texture works.

Surfaces with an apparent contact angle between 150° and 180° are termed superhydrophobic. In addition to high contact angle, superhydrophobic surfaces also have very low water contact angle hysteresis below 10°, which is the difference between the advancing and receding contact angles. Artificial superhydrophobic surfaces as shown in the figures below have utilized this composite interface and low hysteresis to generate extreme non-wettability and low flow resistance. 

Superhydrophobic Nanocomposites



These superhydrophobic nanocomposite coatings can be applied to internal heat transferring components in this project's liquid piston system to prevent liquid films from degrading performance. Although key milestones such as coating mechanical durability have already been achieved, resistance to saturation remains the most important hurdle to address as demonstrated in the figure below.


A laser illuminance experiment (shown below) has been built to quantify this saturation effect in detail in order to understand and design more resistant coatings.





Finally, active methods for prolonging superhydrophobic performance and even reversing saturation are being investigated. For example, conductive superhydrophobic coatings have been created where an electrolytic reaction can be induced to regenerate the air layer in a submerged sample as shown below. Initial experiments have demonstrated a 2x improvement in prolonging the superhydrophobic effect on submerged samples.

To learn more - go to:  http://nanotexture.tumblr.com



  Contact: Prof. Perry Li
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