CHAMPAIGN, Ill. -- Precise measurements of film thickness at liquid-vapor interfaces are important in commercial applications such as power plants, oil refineries and refrigeration systems, but are often expensive and difficult to make. Now researchers at the University of Illinois have developed an automated optical film-thickness measurement technique that is both inexpensive and nonintrusive.
The technique works by shining light from a light-emitting diode on the outside of a transparent tubular test section. Some of the light is reflected by total internal reflection at the liquid-vapor interface and forms a sharp ring on the outside of the tube. By measuring the diameter of the ring, the researchers can determine the film's thickness to an accuracy of one-hundredth of a millimeter.
The noninvasive technology is based on a measurement technique patented in 1997 by UI mechanical engineering professor Ty Newell and graduate student Evan Hurlburt (now a postdoctoral research associate in the UI department of chemical engineering). Recent improvements by graduate student Tim Shedd have turned the technique into a sophisticated film-thickness measurement system that also provides a unique research capability.
"In this application, light is reflected from the surface of a liquid film flowing over a transparent wall," Shedd said. "The reflected light generates an image on the outside of the wall which is captured by a CCD [charge-coupled device] camera and digitized in a computer. The resulting image is processed using custom software to produce an accurate film-thickness measurement."
Unlike other measurement methods that are based on electrical conductivity, dye injection or light absorption, the new technique does not disturb the flow itself, Shedd said. "So you can insert it nearly anywhere in a system and obtain reliable results. And because the technique is automated, several hundred film thickness readings can be taken in just a few minutes."
In addition to many commercial applications -- such as monitoring quality control in chemical-processing plants or sensing potential cooling failures in nuclear power plants -- the measurement technique can serve as a powerful, analytic research tool. In one project, for example, Shedd is exploring how effectively liquid refrigerants "wet" the walls in various square- and triangular-shaped tubes for potential use in highly efficient heat exchangers.
"We need to understand how the fluid is spread around the inside of the tube and measure its thickness profile in order to understand the effective heat transfer," he said. "Knowing the distribution of the liquid film is also very important in determining how much energy must be expended in pushing the refrigerant through the pipe."
Shedd's work was supported by the UI Air Conditioning and Refrigeration Center, a National Science Foundation industry/university cooperative research center.
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