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【4/24 09:50~10:50 Keynote Speech】Flame propagation in confined geometries: what Darrieus and Landau didn`t tell you.

latest update:2022/04/20

FRONT propagation in narrow channels:

What Darrieus and Landau didn't tell you

 

Paul D. Ronney

Department of Aerospace and Mechanical Engineering

University of Southern California, Los Angeles, USA

 

    It is well known that steadily propagating planar premixed-gas flame fronts are unstable to several disturbances, notably that due to thermal expansion as first analyzed by Darrieus (1938) and Landau (1944).  However, in most laboratory experiments the effects of the Darrieus-Landau (DL) instability are masked by using open geometries such as Bunsen, counterflow or V-flames, where thermal expansion is relaxed in the transverse directions.  With this motivation, the front speeds and wrinkling spectra of premixed flames propagating in quasi-2D channels (Hele-Shaw cells) were studied to avoid suppression of DL instabilities.  H2, CH4 and C3H8 fuels with N2 and CO2 diluents were used to assess the effects of Lewis number (Le) and thus diffusive-thermal (DT) instabilities.  Upward, downward, and horizontal propagation configurations were tested to assess the effects of buoyancy and thus Rayleigh-Taylor (RT) instabilities. Varying mixture strengths and thus laminar burning velocities (SL) were employed to assess heat loss effects.

 

    Wrinkling and thus flame speed enhancement was observed even for downward propagating (RT stable) flames have high Le (DT stable) because of DL and the viscosity increase (Saffman-Taylor, ST) instability across the front.  The quasi-steady wrinkled flame speed (UT) is always higher than (SL), typically by a factor of 3.  For Le ≈ 1 mixtures, values of UT/SL correlated well with a scaled growth rate parameter (K) based on the Joulin-Sivashinsky model of flame instabilities in narrow channels due to DL, ST and buoyancy effects.  The observed correlation was UT/SL = 1 + K, thus K serves a role similar to u' in turbulent combustion in the laminar flamelet regime.  Wrinkling spectra exhibited a marked change as the cell thickness decreased due to a change in the dominant instability mechanism from DL to ST.  Flame wrinkling in the plane of the cell and front curvature in the transverse dimension are found to be of similar importance in affecting UT.  The combined effects of diffusive-thermal and hydrodynamic instabilities are striking for H2-air mixtures and present challenges for identification of scaling parameters.

 

    These results indicate that the behavior of practical flames in confined geometries such as internal combustion engines or gas turbines may be quite different from that inferred from laboratory experiments conducted in open geometries.

 

 

 

 

 

 

“Long” short biography

 

    Paul D. Ronneyis a Professor and Chair of the Department of Aerospace and Mechanical Engineering at the University of Southern California in Los Angeles, CA. Prof. Ronney received a Bachelor of Science degree in Mechanical Engineering from the University of California, Berkeley, a Master of Science degree in Aeronautics from the California Institute of Technology, and a Doctor of Science degree in Aeronautics and Astronautics from the Massachusetts Institute of Technology. He held postdoctoral appointments at the NASA Lewis (now Glenn) Research Center and the Laboratory for Computational Physics at the U. S. Naval Research Laboratory and a position as Assistant Professor in the Department of Mechanical and Aerospace Engineering at Princeton University before assuming his current position at USC.  Prof. Ronney was the Payload Specialist Astronaut (Alternate) for Space Shuttle mission MSL-1 (STS-83, April 4 - 8, 1997) and the reflight of this mission (STS-94, July 1 - 16, 1997).

 

    Professor Ronney has extensive research experience in small-scale combustion and power generation, turbulent combustion, flame ignition by transient plasma discharges, micro-scale combustion, bioengineering (robotic insect propulsion), edge flames, flame propagation in confined geometries (Hele-Shaw cells), internal combustion engines, premixed-gas combustion at microgravity and flame spread over solid fuel beds.  One of his experiments, a study of premixed-gas flames at low gravity, flew on three Space Shuttle missions.

 

    Prof. Ronney has published over 80 technical papers in peer-reviewed journals, made over 250 technical presentations (including over 35 invited presentations at international conferences), holds 7 U.S. patents, and has received over $12 million in funding for his research projects. In recognition of his achievements, he is a Fellow of the American Society of Mechanical Engineers and the Combustion Institute, an Associate Fellow of the American Institute of Aeronautics and Astronautics, and is a recipient of the National Science Foundation Presidential Young Investigator Award. He has received the Distinguished Paper Award from the Combustion Institute (for a work published in the Proceedings of the Combustion Institute, Vol. 37) and the Starley Premium Award of the Institution of Mechanical Engineers (for the best paper of the year published in the Journal of Automobile Engineering.)