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National Advisory Committee for Aeronautics, Report - Propagation of a Free Flame in a Turbulent Gas Stream

SUMMARY

Efiective flame speeds of free turbulent flames were measured
by photographic, ionization—gap, and photomultiplier-tube
methods, and were found to have a statistical distribution
attributed to the nature of the turbulent field. The efiective
turbulent flame speeds for the free flame were less than those
previously measured for flames stabilized on nozzle burners,
Bunsen burners, and blufi bodies. The statistical spread of
the efiective turbulent flame speeds was markedly wider in the
lean and rich fuel-air-ratio regions, which might be attributed
to the greater sensitivity of laminar flame speed to flame temper—
ature in those regions. Values calculated from the turbulent
free-flame-speed analysis proposed by Tucker apparently form
upper limits for the statistical spread of free-flame-speed data.
Hot-wire anemometer measurements of the longitudinal velocity
fluctuation intensity and longitudinal correlation coeflicient
were made and were employed in the comparison of data and in

he theoretical calculation of turbulent flame speed.

INTRODUCTION

The high volumetric heat—release rate required in present
and future jet-engine combustors has placed an ever-growing
emphasis on turbulent combustion research. The theory of
turbulent flame propagation is as yet in a formative stage,
partly because of the lack of reliable experimental methods
for determining the effect of turbulence on the rate of flame
propagation. The purpose of this investigation was to ex-
plore a new experimental method believed to fpprom‘mate
more nearly a theoretical, or ideal, flame model than previous
methods.

The major portion of experimental data on turbulent flame
speeds has been taken in open flames stabilized on Bunsen
burners (refs. 1 to 3) or on Macho type burners (refs. 4 and 5).
A substantial amount of experimental data has also been
obtained from flames confined in a duct and stabilized on
blufl’ bodies (refs. 6 to 8). Another experimental method
consists in measuring the speed of a flame advancing into a
turbulent fuel-air mixture flowing through a tube (ref. 9).
In general, the definition of turbulent flame speed used in
these methods is that given in reference 8: