Advanced diode laser absorption sensor for in situ combustion measurements of CO2, H2O, and gas temperature

A diode-laser sensor system based on absorption spectroscopy techniques has been developed to measure CO₂, H₂O, and temperature nonintrusively in high-temperature combustion environments. An external-cavity diode laser operating near 2.0 μm was used to scan over selected CO₂ [(12°1)–(00°0) band] and H₂O transitions [(011)–(000), (021)–(010) bands] near 1.996 and 1.992 μm for measurements of CO₂ and H₂O concentration and gas temperature. Gas temperature was determined from the ratio of integrated line intensities. Species concentration was determined from the integrated line intensity and the measured temperature. The system was applied to measure temperature and species concentrations in the combustion region of a premixed C₂H₄-air flat-flame burner operating at fuel-lean conditions. The laser-based temperature measurements were in agreement with values determined using a (type S) thermocouple to within 3%. In addition, the measured CO₂ and H₂O concentrations agreed to within 6% and 3%, respectively, with calculated equilibrium values at measured temperatures. The minimum CO₂ detectivity was 200 ppm (for =0.51, 1470 K, a l-m path length, 200-Hz detection bandwidth). These results represent the first in situ combustion measurements of CO₂ concentration using room-temperature near-IR diode lasers. Furthermore, the results demonstrate the utility of diode-laser absorption sensors, operating near 2.0 μm, as attractive diagnostic tools for in situ combustion measurements of temperature and the concentrations of CO₂ and H₂O.