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National Advisory Committee for Aeronautics, Technical Notes - Stability Limits and Burning Velocities of Laminar Hydrogen Air Flames at Reduced Pressure

Laminar burning velocity was measured at pressures of 1 atmosphere
and below and critical boundary velocity gradient for flashback at pres-
sures below 1 atmosphere over a range of compositions for hydrogen-air
burner flames. Pressure exponents of 0.25 for burning velocity and_l.55
for flashback velocity gradient were found. In both cases the pressure
dependence was independent of composition between equivalence ratios of
about 1 and 2. A more general correlation relating flashback velocity
gradient, burning velocity, and quenching distance conformed to the
simple quenching model of Lewis and von Elbe.‘ From this correlatiOn and
recent thermal equations for flame propagation, a global reaction order
of 2.2 to 2.5 was calculated.

A complete laminar stability loop was obtained for one particular
burner diameter and equivalence ratio. Its shape is discussed in terms
Of quenching regions, normal laminar regions, and possible regions of
laminar-turbulent transition.

In the past, various relations have been reported among measured
properties of burner flames. In some cases, it has been possible to re—
late these empirical correlations te theories of flame propagation and
thus obtain an insight into the mechanism of combustion. This report
describes a study of certain of these relations for hydrogen—air burner
flames over a range of pressures and compositions.

Two combustion parameters, laminar burning velocity and critical
laminar boundary velocity gradient for flashback, were measured as func-
tions of composition and pressure.