Compact optical probe for flame temperature and carbon dioxide using interband cascade laser absorption near 4.2 µm

An in situ thermometry technique was developed for low-pressure flames based on laser absorption spectroscopy of nascent carbon dioxide. Discrete, rovibrational lines within the ν3 fundamental band near 4.2 µm were utilized due to their superior strength, large temperature sensitivity and isolation from interfering absorption lines of other combustion species. These transitions were simultaneously accessed with a single interband cascade laser using scanned-wavelength direct absorption and Voigt lineshape fitting to infer temperature from the ratio of integrated absorbance. A single-ended optical probe constructed of sapphire rods was used to deliver the laser light to the flame. Measurements of temperature and CO2 were conducted in burner-stabilized flames for methane-oxygen and ethylene-oxygen reactant mixtures over a pressure range of 25-60 torr and from a distance of 3-23 mm above the burner face. Measurements were compared to a 1-D kinetic model as well as thermocouple measurements. The sensor was also used for path-averaged thermometry on a propane torch at 760 torr to demonstrate the potential for measurement at atmospheric pressure.