A small orange emission region, separated by a sharp boundary from a yellow one, is reported for hydrocarbon-air diffusion flames. The phenomenon, which has not been previously reported, occurs only in very small flames, only over a small range of fuel flow rates, and it has been demonstrated on a cylindrical as well as a slot burner. It has been shown to occur for the ten different aliphatic fuels tested and appears to be a general phenomenon. Thermocouple measurements showed that the gas temperature of the flame interior increases monotonically along the axis; the yellow color is in a lower temperature gas than the orange. Since color temperatures can be deceptive, temperatures in each zone were surveyed by a Kurlbaum method, as well as by a spectral measurement in the two regions. Both methods gave a higher soot particle temperature in the yellow zone than in the orange. Furthermore, measured composition profiles showed that the olefinic concentration approaches zero at the interface between the yellow and orange regions. On the basis of these experimental results and of theoretical estimates of soot-particle temperatures, it is suggested that the orange corresponds to radiant emission from soot particles whose temperatures are equilibrated with those of the gas, while in the yellow region the soot particles are hotter because of the energy release associated with their rapid growth by reaction with gas-phase unsaturates, primarily acetylene
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