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Lasers and Holographic Imaging Team up for Combustion Flow Study

Photonics Spectra
Jan 1998
Ruth A. Mendonsa

Researchers at the Wright Patterson Air Force Base in Dayton, Ohio, are using holography and laser technology in an effort to develop a practical 3-D diagnostic technique for evaluating aircraft combustion flow. In a program jointly funded by the Air Force base's Aero Propulsion and Power Directorate and its Office of Scientific Research, the team is developing two techniques -- holographic flow visualization and holographic particle image velocimetry -- for combustion studies.

Flow visualization techniques currently are limited to a 2-D slice of a flow illuminated by a laser sheet or a 2-D field that results from the integration of a 3-D density field along the path of a laser beam. These techniques do not provide full visualization of 3-D information.

Holographic imaging, which provides 3-D representation of spatial objects instantaneously, holds great promise as a qualitative and quantitative 3-D diagnostic tool for spatially and temporally evolving complex flow structures.

Under this program, S. Gogineni, J. Estevadeordal and L. Goss of Innovative Scientific Solutions Inc. and Professor H. Meng of Kansas State University in Manhattan, Kan., have demonstrated a holographic recording and reconstruction system that can provide 3-D flow visualization and velocity measurements in simple combustion flows.

Matching wavelengths

To record the holograms, the researchers use two 532-nm Spectra-Physics Nd:YAG lasers that are injection-seeded to provide a coherence length greater than 1 m. They also use a single Spectra-Physics Millennia Nd:YVO4 continuous-wave laser at 532 nm to reconstruct holograms. They use the Millennia not only because of its convenience, but also because its wavelength matches that of the recording lasers.

The team has employed simple in-line recording and off-axis viewing techniques for single-exposure holographic flow visualization of a drop of fluid falling into water. The results demonstrate the effectiveness of holographic visualization in flows under unstable conditions (see figure). They have extended this technique to combustion flows and have demonstrated the limitations that result from distortion of the reference beam by temperature and density gradients.

Scientists have used simple in-line recording and off-axis viewing for single-exposure holographic flow visualization of a drop of fluid falling into water, and have extended the technique to combustion flow studies.

As an alternative to this technique, the group has developed an off-axis-based system that can perform holographic flow visualization and holographic particle image velocimetry. This system was applied to reacting and nonreacting propane jet flows in the presence and absence of a co-flowing air stream. The flow field seeded with TO2 particles provided images suitable for 3-D flow visualization, and the same flow seeded with Al2O3 provided images suitable for qualitative and quantitative 3-D velocity measurements.

With this research well under way, 3-D imaging should allow scientists to study complex flow phenomena.

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