Determination of gas plume temperature from molecular emission spectra

Spectroscopic measurements of infrared molecular transi- tions in gas plumes are evaluated for their potential to yield a reliable remote sensing technique for determination of plume temperature. Un- der optically thin conditions, the ratio of intensities from two different CO2 transitions has no dependence on molecular concentration, but has suf- ficient thermal sensitivity to be used as a temperature diagnostic. Fine spectral resolution is not required. Experimental results involving simul- taneous thermocouple and spectroscopic measurements support the use of spectroscopic intensity ratios to determine stack plume tempera- tures. Measurements involve a plume from a vehicle exhaust and a stack plume from a propane-burning portable plume generator. Observed CO2 emission near 4.26 m is affected by optical thickness and by self- absorption of emitted radiation by the cool outer regions of the plume. Measurements on the wings of molecular bands mitigate these absorp- tion effects. Relationships between absorptive and emissive spectral in- tensities are developed that allow calibration of the temperature diagnos- tic using readily available absorption spectra. Based on data spanning a range of plume temperatures from 310 to 606 K, the root-mean-square difference between the spectroscopically derived temperatures and ther- mocouple data is 22 K. © 2006 Society of Photo-Optical Instrumentation