Emission Spectra of Hydrocarbon Flames Doped with Phosphorus-Containing Compounds

Phosphorus-containing compounds, including organophosphorus substances, are valuable laboratory flame additives for understanding the combustion behavior of sarin. Chemiluminescence features of excited intermediate species are useful markers of combustion, but an effective chemiluminescence-based combustion diagnostic for phosphorus-based compounds is yet to be developed. In this study, chemiluminescence spectra of atmospheric-pressure flames doped with three phosphorus-containing compounds, namely, dimethyl methylphosphonate, diisopropyl methylphosphonate, and phosphoric acid (H3PO4), were obtained in two flame configurations, namely, spherically expanding flames and piloted liquid-spray flames. The spectra are compared with those obtained from undoped flames fueled by methane and methanol, respectively. Spectra were recorded for the wavelength range of 200-800 nm and for three equivalence ratios (phi=0.8, 1.0, 1.2) of spherical flames. For both flame configurations, the intensity of the doped flame emission increased relative to the neat flames by over an order of magnitude, but no clear spectral features besides a broad, featureless continuum emission could be identified. No significant difference was observed between the spectra of the spray flames doped with H3PO4 and with diisopropyl methylphosphonate. The observance of a continuum emission for all the doped flames is attributed to blackbody emission, likely resulting from high-temperature soot particles in the flame originating from condensed, phosphorus-containing combustion products. However, unique, narrow chemiluminescence features specific to phosphorus combustion could not be identified at this time.