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National Advisory Committee for Aeronautics, Technical Notes - Effect of Initial Mixture Temperature on Flame Speeds and Blow Off Limits of Propane - Air Flames

The effect of initial mixture temperature on flame speeds in the
laminar-flow region and on blow-off limits in both the laminar- and
turbulent-flow regions was investigated for Bunsen type flames of
propane and air. Flame speeds were determined at various mixture
compositions at each of several temperatures from 85° to 650° F
by measuring the area of the Bmsen cone from shadowgraphs.

Flame speed increased with temperature from 1.54 feet per second
at 85° F to 5.25 feet per second at 650° F. The observed relative
effect of temperature on flame speed was in good agreement with that
predicted by the thermal theory as presented by Semenov, whereas the
effect predicted by the square-root law of Tanford and Pease is as
much as 55 percent lower than experimental results. Flame speed
was independent of tube size from 5/8 to 7/8 inch or stream-flow
Reynolds number'from 1500 to 2100.

Blow-off limit data. were obtained for the tempbrature m
90° to 650° F. The relation between mixture composition and velocity
gradient at the tube wall at blow-off was dependent on the initial
temperature.

A knowledge of the effect of temperature on the normal burning
velocity, called flame speed herein, and on the stability limits of
fuel-air mixtures is necessary to the understanding of the com-
bustion process and to certain applications of it, for example,
in the future design of Jet-engine combustors.

Theories of flamepropagation predict that flame speed
will increase with temperature at an increasing rate. Flame
speed was found to be approximately proportional to the
square of the absolute temperature over the range from_52° . .
to 1292° F for city-gee - air flames (reference 1). The data of ,
Johnston (reference 2) on natural as - air flames show a slight
increase in the rate of change of flame speed. with temperature
from 157° to 902° F. Sachsse (reference 3) reports an increasing
rate of change for methane-oxygen flames from 68° to 18520 r
although he found a linear relation between flame speed and -
perature for propane-oxygen flames from 68° to 952° F. According
to Breeze (reference 4), a linear relation exists for propane-air
and. butane-air flames from 68° to 592° F.

Stability-limit data for mixtures at room temperature are
correlated. by boundary velocity gradient - composition plots, elim-
inating tube diameter as an independent parameter, in references 5
to 7. -The effect of initial mixture temperature on blow-off limits
is not included in these references.