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National Advisory Committee for Aeronautics, Technical Notes - Effect of Initial Mixture Temperature on Flame Speed of Methane Air, Propane Air and Ethylene Air Mixtures

The flame speed, or normal burning velocity, of a fuel-air mixture
is a fundamental property governing flame propagation, which is one of
the several major processes occurring in combustion equipment for flight
propulsion. Inasmuch as plots of the performance data of aircraft com-
bustors show that the combustion efficiency is related to the combustor-
inlet temperaimre (for example, references 1 and 2) , knowledge of how
temperature affects the fundamental combustion properties of the fuel-
air mixture is desirable. Consequently, as part of the combustion
research program at the NACA Lewis laboratory, an investigation was
initiated to study the effect of initial mixture temperature on flame
propagation. The effect of temperature on flame speeds and stability
limits of propane-air flames is reported in reference 5. An extension
of the flame-speed phase of this investigation to include the effect of
temperature on methane-air and ethylene-air flames is reported herein.

Previous work (reference 5) showed that the flame speed of propane-
air mixtures increased with temperature at an increasing rate. It was
shown that the thermal theory of flame propagation, as presented by
Semenov in reference 4 for bimolecular reactions, predicted the effect
of initial mixture temperature on the maximum flame speed of propane-
air mixtures within approximtely 5 percent. It was further shown that
the square-root law of Tanford and Pease (references 5 and 6’) , which is
based on the diffusion of active particles ,‘ predicted relative increases
in maximum flame speed that were as much as 35 percent lower than the
emerimental results. Subsequent reevaluation of the temperature depend-
ence predicted by the square-root law has shown, however, that the rela-
tive increases predicted by this theory for propane are within 14 per-
cent of the experimental results.

Inasmuch as the activation enerar of the oxidation process is an
important factor in the thermal-theory equations, it was decided to
obtain flame-speed - temperature datafor a gaseous fuel having an
activation energy appreciably different from that of propane. Methane
was selected because it has an activation energy of 52 kilocalories per
gram-mole (from unpublished data obtained by A. D. Walsh, Leeds Univer-
sity, England), compared with 58 kilocalories per yam—mole for propane
(reference 7, p. 437).