Degenerate Branching or Cool Flames

The flames are usually blue in colour with peroxides and aldehydes (alkanals) present, and with the aldehydes showing a peak in concentration during the cool flame episode. The main emitter in the cool flame is electronically excited HCHO. 

An observable induction period is often found, during which only traces of aldehydes are present. Addition of aldehydes can reduce or eliminate this induction period. 

If a cool flame mild explosion episode is followed by a second, then not all the reactant has been consumed in the first episode. What remains will be used up in the second, third and so forth episodes. This is in contrast to a genuine explosion, where all the reactant is rapidly removed. 

Sometimes, a cool flame may be followed by a genuine explosion. Here the cool flame behaviour is lost after the first increase in rate, which does not fall off, but moves straight into the genuine explosion, believed to be thermal in nature. 

A series of mild explosions is more general at low pressures and low temperatures, in contrast to the high temperatures and pressures at which a mild explosion leads into a genuine explosion. Typical temperature limits for hydrocarbons are approximately 280-410 °C, with pressure limits around 100-150 mmHg. 

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