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National Advisory Committee for Aeronautics, Technical Notes - Further Experiments on the Stability of Laminar and Turbulent Hydrogen Air Flames at Reduced Pressures

Stability limits for laminar and turbulent hydrogen-air burner
flames were measured as a function of pressure, burner diameter, and
composition. The average pressure exponent of the critical boundary
velocity gradient for turbulent flashback was 1.51, which is not sig-
nificantly different from the laminar value. The use of a simple flame
model and measured turbulent flame speeds indicated that turbulent
flashback could involve a smaller effective penetration distance than
laminar flashback. Turbulent blowoff velocity was nearly independent
of pressure and varied about as the inverse square root of the burner
diameter. Of several recent theoretical treatments, none satisfactorily
predicts the observed dependence of blowoff on pressure and burner di—
ameter. Extrapolation of stability loops to the quenching point showed
that the quenching pressure was inversely proportional to burner diam—
eter. The actual pressures were higher than those obtained by other
quenching measurements.

Relatively little attention has been paid to the stability limits
of turbulent burner flames as a function of pressure. Reference 1
(p. 82) reports data on the flashback of unpiloted turbulent propane-
air flames at pressures above 1 atmosphere. It was observed that the
critical boundary velocity gradient was several times higher than that
for corresponding laminar flames at the same pressure and composition.
Reference 2 presents blowoff and flashback data for acetylene flames at
lOW'pressures; these data extend into the turbulent region. H0wever,
data in the higher Reynolds number region are not discussed in detail.

The present study is concerned with the stability of unpiloted
turbulent hydrogen-air flames at subatmospheric pressures and extends,
into the turbulent region, previous work done on properties of laminar
hydrogen-air flames at subatmospheric pressures (ref. 5). Turbulent
flashback was studied at various pressures, equivalence ratios, and
burner diameters. Results are compared with results in the laminar re-
gion. A possible explanation of the results based on the extension of
the laminar model to the case of turbulence is offered. Blowoff in the
turbulent region was studied at various pressures and burner diameters
at equivalence ratios of l. l and 1.5. The results are compared with
predictions of several recent theoretical treatments, none of which
give satisfactory predictions.