In situ combustion measurements of CO with diode-laser absorption near 2.3mu m

In situ measurements of CO concentration were recorded with tunable diode-l aser absorption spectroscopy techniques in both the exhaust and the immedia te post-flame regions of an atmospheric-pressure flat-flame burner operatin g on ethylene air. Two room-temperature cw single-mode InGaAsSb/AlGaAsSb di ode lasers operating near 2.3 mu m were tuned over individual transitions i n the CO first overtone band(nu' = 2 <-- nu" = 0) to record high-resolution absorption line shapes in the exhaust duct [79 cm above the burner, simila r to 470 K; R(15) transition at 4311.96 cm(-1)] and the immediate postflame zone [1.5 cm above the burner, 1820-1975 K; R(30) transition at 4343.81 cm (-1)]. The CO concentration was determined from the measured absorption and the gas temperature, which was monitored with type-S thermocouples. For me asurements in the exhaust duct, the noise-equivalent absorbance was similar to 3 X 10(-5) (50-kHz detection bandwidth, 50-sweep average, 0.1-s total m easurement time), which corresponds to a CO detection limit of 1.5 ppm m at 470 K. Wavelength modulation spectroscopy techniques were used to improve the detection limit in the exhaust to similar to 0.1 ppm m (similar to 500- Hz detection bandwidth, 20-sweep average, 0.4-s total measurement time). Fo r measurements in the immediate postflame zone, the measured CO concentrati ons in the fuel-rich flames were in good agreement with chemical equilibriu m predictions. These experiments demonstrate the utility of diode-laser abs orption sensors operating near 2.3 mu m for in situ combustion emission mon itoring and combustion diagnostics. (C) 2000 Optical Society of America. OC IS codes: 300.1030, 300.6260, 300.6340, 300.6380, 280.1740, 280.3420.