Reflections on films help with process development
22 Aug 2000
Measuring the thickness of films at liquid-vapour interfaces is an important operation for many processes - for example, it can give valuable warning of a potential coolant failure in nuclear power plants and is useful for measuring quality in chemical processes. However, it's often an expensive and difficult measurement to make, as it generally uses equipment which interferes with flow rates.
Researchers from the University of Illinois at Urbana-Champaign have now developed a test which uses simple optical methods that are both cheap and quick. The test works by shining light from a light-emitting diode onto the outside of a transparent section of a tube. Some of the light undergoes total internal reflection at the liquid-vapour interface, and this reflected light forms a sharp ring on the outside of the tube. The diameter of this ring is related directly to the thickness of the liquid film.
Graduate student Tim Shedd has now refined the technique so that it is completely automated. `The reflected light generates an image on the outside of the wall which is captured by a charge-coupled device camera and digitised in a computer,' he explains. `The resulting image is processed using custom software to produce an accurate film-thickness measurement.'
The other methods generally used for film measurement include electrical conductivity tests and dye injection, but these all interfere with the flow. `You can insert it nearly anywhere within a system and obtain reliable results,' Shedd says.
Shedd's current research centres on measuring the thickness of liquid films inside square and triangular tubes, which could be used in efficient heat exchangers. `We need to understand how the fluid is spread around inside the tube and measure its thickness profile to understand the effective heat transfer,' he explains.
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