A Point Diffraction Interferometry (PDI) system was developed to measure two-dimensional and axisynimetric flame temperatures. PDI is a non-invasive optical technique that can be used to measure changes in the index of refraction. Flame temperatures can be determined using physical relations between changes in optical path length and gas phase properties. The system uses a single laser beam that is collimated, sent through a test section, decollimated, and aimed at a diffraction plate. The PDI plate is a small disk that consists of a diffraction hole centered in a semi- absorbing thin film. A portion of the beam passes through the film with some attenuation, but otherwise unchanged. This technique is unusual as it creates the reference beam after the test section; the light passing through the center hole creates a spherical diffraction wave that acts as the reference beam for the interferometer. An interference (or fringe) pattern is generated when the reference beam interacts with the light passed through the attenuating film. Temperature measurements from the PDI system were compared to thermocouple measurements and theoretical solutions for two-dimensional and axisymmetric configurations with constant gas composition and constant pressure. The PDI and thermocouple temperature data differed by 2% in tests with a heated vertical flat plate. In axisymmetric tests with a heated vertical cylinder, the PDI fringe data was used with the Abel transform to compute temperatures. The PDI temperatures were within 3% of the thermocouple data. The PDI system has also been used to measure temperatures in a steady two-dimensional methanol flame and an axisymmetric dilute methane laminar gas jet dif- fusion flame. This system has also been used for qualitative evaluation of flames. Interferograms of candle flames as well as single and interacting two-dimensional spreading flames at reduced pressure have been taken.
Goldmeer, J.S., Urban, D.L., Yuan, Z.G., Development of a Point Diffraction Interferometer for Measuring Flame Temperatures, Spring Meeting of the Western States Section of the Combustion Institute, Combustion Institute, Pittsburgh, PA, pp. 1-19, March 23, 1998.