Spectra of cool flames and pre-ignition glows

Abstract

Cool flames and pre-ignition glows of rich and weak mixtures of propane with air and oxygen have been studied in a simple flow system at atmospheric pressure. In the cool-flame region the spectrum shows the usual formaldehyde emission bands, but in the pre-ignition glow near the high-temperature ignition limit the spectrum is quite different, consisting mainly of hydrocarbon flame bands, due to HCO. With weak mixtures the HCO bands are accompanied by strong OH and some CH and CO-flame bands and the spectrum is rather similar to that of the hot flame. With rich mixtures, the HCO bands are accompanied by some CH and formaldehyde bands and weak OH but no C 2 , so that the spectrum is quite different from that of the rich hot flame, which is dominated by C 2 and CH emission and shows little HCO. The unusual observation of strong HCO with rich mixtures is discussed. It is suggested that the C 2 and CH emission occurs under conditions when free atoms and radicals diffuse back from the burnt mixture, while the HCO emission is associated with chain termination or thermal initiation processes. This supports the view that radical diffusion is important in flame propagation. No pre-ignition glow was observed with methane or formaldehyde. The pre-ignition glow of CO occurs readily at both rich and weak limits at atmospheric and low pressure and shows CO-flame bands (attributed to CO 2 ). Addition of methane or formaldehyde to the CO modifies the ignition limits and reduces the glow region, but with a little methane added to rich CO + O 2 , clear spectra were obtained which showed the cool flame bands of formaldehyde superposed on the CO-flame bands. Addition of formaldehyde instead of methane did not produce the formaldehyde bands. The reaction processes are discussed, and it is suggested that during oxidation of CO, free O atoms or O 3 are produced which may react with methane to result in cool flame processes. The relation of these observations to previous work on the auto-ignition of methane is discussed.