Experimental study of signal trapping of OH laser induced fluorescence and chemiluminescence in flames

The absorption of OHau chemiluminescence and laser-induced fluorescence (LIF) in the exhaust gas of confined premixed laminar CH4/air flames at atmospheric pressure was investigated. One flame was used as source and a second as absorber. OH LIF was excited in the nu aEuro(3)=0 ->nu'=1 band of the A-X electronic system around a parts per thousand 283 nm and spectrally resolved detected in the (0,0) and (1,1) vibrational bands around 305-320 nm. For OHau chemiluminescence, spectrally resolved detection was performed in the wavelength range 280-340 nm. For an absorption path of 54 mm and at Ta parts per thousand 2000 K, signal trapping on the order of 10-40% was observed. Signal trapping was most pronounced in the (0,0) band, as expected from the thermal population distribution of OH in the electronic ground state. The spectral distribution of the signals and the wavelength dependence of the signal trapping are addressed in this paper. Implications from the results with respect to detection strategies and chemiluminescence-based equivalence ratio measurements are discussed.