Flowfield Imaging Laboratory

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heat release effects in planar, non-premixed turbulent flames

Jason E. Rehm and Noel T. Clemens

Achieving an understanding of fundamental flame/turbulence interactions is critical for the modeling of turbulent non-premixed flames. Of particular importance is the role of heat release in the possible modification of the turbulence, as changes in vortical length scales and strain rates have an impact on the structure of the reaction zone. In this project planar turbulent non-premixed jet flames are investigated using, simultaneously, planar laser-induced fluorescence (PLIF) of the OH radical formed in the combustion, and planar laser Mie scattering (PLMS) of aluminum oxide particles seeded into the flow. The OH fluorescence signal is useful as a marker of the flame zone, while the PLMS is used to visualize the fuel stream or, alternatively, for particle imaging velocimetry (PIV) purposes.

optical arrangement

A schematic of the optical arrangement used in these experiments is given above. While the frequency-doubled 532 nm output of an Nd:YAG laser can be used for the particle imaging, the OH fluorescence is excited by the 285 nm, frequency-doubled output of an Nd:YAG-pumped dye laser, as shown. For simultaneous PLIF/PIV imaging, a second, double-pulsed Nd:YAG laser augments the arrangement shown.

non-reacting air jets

The images above show PLMS visualizations of a planar jet of air issuing into air at local Reynolds numbers (at the far downstream location) of 1300 and 5100. Also shown is the RMS intermittency of the local scattering intensity, where blue represents low, and red, high intermittency. Large-scale organized motions are very pronounced in the PLMS images, and are also found to be characteristic of planar jets in which the jet fluid is both more and less dense than the ambient fluid. However, the situation changes when combustion is considered, as seen in the following figure.

hydrogen/nitrogen diffusion flames

The above figure shows simultaneous PLMS images from the jet fluid (grayscale) and OH PLIF (yellow-red) in non-premixed planar turbulent flames. The fuel in this case is hydrogen, which is diluted with nitrogen such that the jet fluid density is .52 times that of the ambient air. From local Richardson number calculations, these flames are determined to be momentum-driven. The PLMS images show no evidence of the large-scale coherent motions seen previously in the non-reacting case. This can also be inferred from the much narrower regions of high intermittency in the reacting case.

hydrogen/helium diffusion flames

The sequence of images above shows simultaneous PLMS/PLIF images taken in non-premixed flames where the jet fluid is now a 40/60 hydrogen/helium mixture. The exit Reynolds number is 500 in the leftmost image pair, rising to 1000 for the middle image pair and to 2500 in the images on the right, where the flame is lifted from the nozzle. Richardson number calculations indicate that, in contrast to the nitrogen-diluted flames considered earlier, the helium-diluted flames are strongly influenced by buoyancy. Here, the large-scale structure of the jet flow more closely resembles that of the non-reacting jets than of the momentum-driven flames. This is true even at Reynolds numbers sufficiently high to lift the flame. Also of interest in these images is the position and structure of the OH (re: reaction) zones. In the low and moderate Reynolds number cases, the OH zones lie outside the particles in the initial laminar region owing to differential diffusion, while at increasing downstream distances, the lower strain rates allow the OH zones to become more diffuse. In the lifted flame case, meanwhile, the OH zones are distributed to a degree that renders identification of an instantaneous reaction surface difficult.

Related work involving simultaneous PIV/OH PLIF in this flowfield is also described on this site.

Publications:

  • The Association of Scalar Dissipation Rate Layers and OH Zones with Strain, Vorticity, and 2-D Dilatation Fields in Turbulent Nonpremixed Jets and Jet Flames , J. E. Rehm & N. T. Clemens, AIAA-99-0676, 37th Aerospace Sciences Meeting, 1999.
  • The relationship between vorticity/strain and reaction zone structure in turbulent nonpremixed jet flames, Rehm, J. E. and Clemens, N.T., To appear, 27th International Symposium on Combustion, 1998.
  • An improved method for enhancing resolution of conventional double-exposure single-frame particle image velocimetry, Rehm, J. E. and Clemens, N.T., Experiments in Fluids, Vol. 26, pp. 497-504, 1999.
  • The Large-Scale Turbulent Structure of Nonpremixed Planar Jet Flames, Rehm, J. E. and Clemens, N.T., Combustion and Flame, Vol 116, pp. 616-626, 1999.
  • Effects of heat release on the large-scale turbulent structure of planar jet diffusion flames, Rehm, J.E. and Clemens, N.T., Paper WS-97S-053, Western States Section of the Combustion Institute, 1997.
  • A PIV/PLIF investigation of turbulent planar non-premixed flames, Rehm, J.E. and Clemens, N.T., AIAA 97-0250, 35th Aerospace Sciences Meeting, 1997.
  • The flow structure of planar hydrogen diffusion flames, Rehm, J.E. and Clemens, N.T., AIAA 96-0704, 34th Aerospace Sciences Meeting, 1996.

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