Understanding the dynamics of molecular mixing in turbulent flows is fundamental to a wide variety of engineering problems. Flows in which chemical reactions occur can be particularly sensitive to the details of the underlying mixing processes. In airbreathing combustion problems, for example, the complex reaction kinetics involved in pollutant formation are very dependent upon mixing at fine scales. The same is true for any reacting flows in which the chemistry is far from equilibrium.
This project aims to obtain a large set of three-dimensional measurements of gas-phase mixing in a planar turbulent jet. The jet fluid in these experiments is propane (into which is seeded a small amount of acetone vapor) which issues into air. The diagnostic approach is simultaneous, two-plane Rayleigh scattering from the propane and planar laser-induced fluorescence (PLIF) from the seeded acetone. By applying these two different measurement techniques, laser power requirements are significantly lower than with, for example, two-plane Rayleigh scattering. As shown in the schematic of the optical arrangement above, a single Nd:YAG laser with 320 mJoule output at 532 nm serves as the laser light source for the experiments. Using a beam splitter, 25 percent of the output is diverted through an external frequency doubler to provide the ultraviolet, 266 nm light for the acetone fluorescence, while the remaining 532 nm light is used for the Rayleigh scattering. Independent beam steering and sheet forming optics for each wavelength allow for precise control of the relative sheet positions.
The figure above shows sample scalar field measurements taken in a jet with a local Reynolds number of 7800. The image on the left obtained was by acetone PLIF and that on the right by Rayleigh scattering. The jet flow is upward in the images, where the imaging area spans from 81 to 115 jet nozzle widths downstream; the nozzle width is 1 mm and the images are separated (in the out-of-plane direction) by 200 microns. False color assignments range from blue, representing low jet fluid concentrations, to red, representing high jet fluid concentrations.
Because the measurements are fully resolved spatially, the scalar fields can be differentiated numerically to obtain the scalar gradient vector fields. Of particular interest is the scalar energy dissipation rate, defined as the square of the scalar gradient magnitude, which is a measure of the efficiency of mixing in a flow. The planes above show two measures of the scalar dissipation. The plane on the left is two-dimensional result using only in-plane derivatives, while the plane on the left is the true three-dimensional result incorporating the out-of-plane differentiation. Results of this type will be applicable to comprehensive studies of the mixing process in gas-phase turbulent flows.
Publications:
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Planar Measurements of the Full Three-Dimensional Scalar Dissipation
Rate in Gas-Phase Turbulent Flow, Su, L.K. and Clemens, N.T.,
Experiments in Fluids, Vol. 27, pp. 507-521, 1999.
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Three-Dimensional Measurements of Fine-Scale Mixing in Gas-Phase Planar Turbulent Jets,
Su, L.K. and Clemens, N.T.,
FEDSM99-7766, ASME/JSME Fluids Engineering Division Summer Meeting, San Francisco, CA, July 1999.
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The Structure of the Three-Dimensional Scalar Gradient in Gas Phase
Planar Turbulent Jets, Su, L.K. and Clemens, N.T., AIAA 98-0429,
36th Aerospace Sciences Meeting, 1998.
- Measurements of the three-dimensional scalar dissipation rate field in gas phase planar turbulent jets, Su, L.K. and Clemens, N.T., AIAA 97-0074, 35th Aerospace Sciences Meeting, 1997.