OH radical measurements in combustion environments using wavelength modulation spectroscopy and dual-frequency comb spectroscopy near 1491 nm

Abstract

Hydroxyl radical (OH) is a key intermediate reactive species during combustion processes relevant to power production, transportation, and manufacturing. We demonstrate an OH sensor based on in situ laser absorption spectroscopy for deployment in industrial conditions. The sensor relies on telecommunications-fiber-coupled, tunable-diode-laser absorption spectroscopy of an OH transition near 1491 nm. By employing wavelength modulation spectroscopy, the sensor is capable of in situ, quantitative detection of OH down to mole fraction values of 10−5 over a 75-cm pathlength. To increase the accuracy of the OH sensor, we perform the first dual-comb spectroscopy measurement above a flame and use the results to create an absorption database of water vapor transitions from 1489.2 to 1492.5 nm at temperatures up to 2165 K. The database is included in the analysis procedure for the tunable diode laser sensor to account for the water vapor absorption that overlaps with the OH absorption. The utility of the laser sensor is demonstrated by characterizing the concentration of OH radical above a catalytic combustor under different operating conditions.