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Blow-off Characteristics of Turbulent Premixed Flames Under Upstream Flow Excitation and Spatial Mixture Gradients

Sponsor: National Science Foundation and United Technologies Corporation

Graduate Students: S.Chaudhuri (Ph.D. Candidate), M. Andel (M.S. Candidate), Alper Ata (M.S. in ME, 2003), Andres Chaparro (M.S. in ME, 2004)

Project Summary

This project concerns characterization of flame blow-off for lean premixed flames under upstream time-periodic velocity oscillations and spatial mixture gradients.  Experiments are carried out in two geometries.  First involves an axisymmetric configuration where premixed flames are stabilized at the nozzle exit on a bluff-body flame holder attached to a center sting.  Second experimental configuration is that of a rectangular duct where a V-gutter flanme holder spans the duct width.  In each case,  flame blow-off conditions are experimentally characterized for a range of mixture approach velocities (5 to 20 m/s), upstream velocity oscillation amplitude and frequency as well as the radial gradient of mixture equivalence ratio.   The employed flow diagnostics  include Rayleigh scattering for mixture characterization, CH* flame emission, particle image velocimetry and high speed flame imaging.

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Experimental set-up for the blow-off studies

Variation of blow-off equivalence ratio with excitation frequency for different bluff body shapes

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Disk shaped bluff body

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Rod shaped bluff body

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Premixed conical flame with higher fuel concentration in the inner nozzle

Time variation of velocity and photomultiplier voltages at 100 Hz

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Coherence between velocity and photomultiplier signals

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PMT1: masked at 500 mm height above the flame holder; PMT2: unmasked viewing the whole flame.

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Instantaneous velocity field and vorticity maps for the rod-shaped bluff body before blowoff at flow velocities of 5 and 15 m/s and excitation frequencies of 0, 100, 200, and 400 Hz. Vorticity scale: top row, ±3500 1/s; bottom row, ±12,000 1/s.

Flame images obtained by reversing the Mie scattering images obtained during the PIV experiments for the 10-mm-diameter rod-shaped bluff-body flame holder (arrows show the length scale l=Um/f).