The use of chemiluminesence for light-off detection of flames

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Virginia Polytechnic Institute and State University

A fast response method for detection of light-off in gaseous flames and liquid spray flames has been developed. The method used chemiluminescent signals from the 2Σ - ²π OH system centered at 309 nm and the ²Δ - ²π CH system centered at 430 nm to indicate the presence of a flame. Spectral scans (performed on gaseous methane, liquid hexane and liquid Jet-A aircraft fuel) from 280 nm to 610 nm indicated that these two species produced the strongest signals available for flame detection. As their light is emitted in the ultraviolet spectrum, using the OH and CH radicals will potentially provide a good signal-to-noise ratio since, in combustion chambers, most of the broadband background emissions are in the infrared and visible wavelengths. These scans also showed that the hexane and Jet-A gave OH and CH signals of approximately equal intensity. The transient histories of OH and CH were investigated by performing light-off ignition tests and intermittent light-off ignition tests. These various flame conditions showed that both signals were good indicators of flame presence, showing on average, a response time of better than 3 milliseconds. It was found that when the Hydrogen to Carbon ratio of the fuel was decreased, the CH signal strength increased as a percentage of OH signal intensity. Finally, the output signal intensity was found to be sensitive to both the flame image magnification and to the part of the flame observed.