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Flame Speeds of Methane Air, Propane Air, and Ethylene Air Mixtures at Low Initial Temperatures

Relative flame speeds at low initial temperatures were predicted
within approximately 20 percent by either the thermal theory as pre-
sented by Semenov or by the diffusion theory of Tanford and Pease. The
same order was found previously for high initial temperatures. The
low-temperature data were also found to extend the linear correlations
between maximum flame speed and calculated equilibrium active-radical
concentrations, which were established by the previously reported high-
temperature data.

Flame speed is an important combustion property of a fuel-air mix-
ture; as such, it is of interest in both fundamental and applied studies
of flame propagation. Data on the effect of initial mixture temperature
on flame speed are needed (1) to test the predictions of various theories
of flame propagation as to the effect of initial temperature on flame
speed in order to Obtain a better insight into the process of flame prop-
agation and (2) to add to the literature fundamental data, which should
be of value for correlating aircraft—combustor performance with one of
the important physical variables, the inlet temperature.

For these reasons, an investigation of the effect of initial tem-
perature on flame speed was undertaken at the NACA Lewis laboratory.
Data for methane-air, propane-air, and ethylene—air flames over the tem-
perature range from room.temperature to 3440 C are presented in refer-
ence 1. For these three fuels it is reported that:

(1) Flame speed increased with initial temperature at an increasing
rate.

(2) Changes of flame speed, relative to flame speed at room temper-
ature, with change in initial temperature followed the decreasing order:
methane, propane, and ethylene;

(5) Relative values of flame speed could be predicted within approx—
imately 20 percent by either a thermal theory or a diffusion theory of
flame propagation.

(4) Linear correlations existed between maximum flame speed and
calculated equilibrium.active-radical concentrations.

In order to test the validity of these results further, and partic-
ularly to determine whether any discontinuity would occur in either the
flame-speed - temperature curves or the flame-speed — radical-
concentration correlations at low temperatures, flame—speed data at low
initial temperatures are needed. Low-temperature data are also desira-
ble in order to extend the range of temperatures covered to the low
temperatures which might be encountered in flight.