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

Flame speeds based on the outer edge of the shadow cast by the
laminar Bunsen cone were determined as functions of composi-
tion for methane-air mirtu-res at initial mixture temperatures
ranging from —182° to 342° 0 and for prOpane-air and ethylene—
air mixtures at initial mixture temperatures ranging from —?’3°
to 844° 0. The data showed that maximum flame speed in-
creased with temperature at an increasing rate. The percentage
change inflame speed with change in initial temperature for the
three fuels followed the decreasing order, methane, propane, and
ethylene. Empirical equations were determined for maximum
flame speed as a function of initial temperature over the tempera—
ture range covered for each fuel. ‘

For each fuel it was found thdt, with a fixed parallel-beam
shadowgraph system, the ratio of flame speed based on the outer
edge of the shadow cast by the flame cone to flame speed based on
the inner edge of the shadow was a constant, independent of
temperature or composition. The flame speed of propane—air
flames was independent of tube diameter from 10 to 22 milli—
meters or stream-flow Reynolds number from 1500 to 2100.

The observed eject of temperature onflame speed for each of
the fuels was reasonably well predicted by either the thermal
theory as presented by Semenov or the square-root law of Tanford
and Pease. The importance of actire radicals in flame pro paga-
tion was indicated by a simple linear relation between maximum
flame speed and eguilibrum radical comentrations for all three

fuels. Equally good correlations resulted from using either
hydrogematom concentration alone or a. summation of ejectioe
relative concentrations of hydrogen atoms, hydroxyl radicals, and
oxygen atoms and from using either flame temperatures based on.
a sodium D—line measurement for a room-temperature mixture
or adiabatic flame temperatures.