Cool flame

Cool flames were accidentally discovered in the 1810s by Sir Humphry Davy, who inserted a hot platinum wire into a mixture of air and diethyl ether vapor. "When the experiment on the slow combustion of ether is made in the dark, a pale phosphorescent light is perceived above the wire, which of course is most distinct when the wire ceases to be ignited. This appearance is connected with the formation of a peculiar acrid volatile substance possessed of acid properties." Harry Julius Emeléus was the first to record their emission spectra, and in 1929 he coined the term "cold flame". ==Parameters==

Cool flames can occur in hydrocarbons, alcohols, aldehydes, oils, acids, waxes, and even methane. The lowest temperature of a cool flame is poorly defined and is conventionally set as a temperature at which the flame can be detected by eye in a dark room (cool flames are hardly visible in daylight). This temperature slightly depends on the fuel to oxygen ratio and strongly depends on gas pressure – there is a threshold below which cool flame is not formed. A specific example is 50% n-butane–50% oxygen (by volume) which has a cool flame temperature (CFT) of about at . One of the lowest CFTs () was reported for a CHOCH + O + N mixture at . The CFT is significantly lower than the auto-ignition temperature (AIT) of conventional flame (see table ==Mechanism==

Whereas in a hot flame molecules break down to small fragments and combine with oxygen producing carbon dioxide (i.e., burn), in a cool flame, the fragments are relatively large and easily recombine with each other. Therefore, much less heat, light and carbon dioxide is released; the premixed combustion process is oscillatory and can sustain for a long time. A typical temperature increase upon ignition of a cool flame is a few tens of degrees Celsius whereas it is on the order of for a hot flame. Most experimental data can be explained by the model which considers cool flame just as a slow chemical reaction where the rate of heat generation is higher than the heat loss. This model also explains the oscillatory character of the cool premixed flame: the reaction accelerates as it produces more heat until the heat loss becomes appreciable and temporarily quenches the process. ==Cool diffusion flames==

Unlike a cool premixed flame, a cool diffusion flame (CDF) burns in the presence of an equivalence ratio gradient. CDFs were first observed in 2012 in droplet experiments aboard the International Space Station. Since then CDFs have been observed in microgravity spherical flames burning droplets and gases and in normal gravity counterflow ==Applications==

Cool flames may contribute to engine knock – the undesirable, erratic, and noisy combustion of low-octane fuels in internal combustion engines. ==See also==