INFRARED OPTICAL SENSOR FOR COMBUSTION DIAGNOSTICS USING WAVELENGTH MODULATION SPECTROSCOPY
Buckley, Steven G
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In this dissertation, an optical sensor for combustion diagnostics, based on near-infrared distributed feedback (DFB) tunable diode lasers, is developed. The sensor is implemented to perform simultaneous species and temperature measurements in a combustion system. The use of optical sensing allows non-intrusive measurements, which are useful particularly in systems sensitive to perturbations caused by external probes. The tunable diode laser sensor is based on absorption spectroscopy, where absorption of laser light by a particular rotational-vibrational absorption feature of a molecule is related to the molecular concentration. In addition, based on absorption ratio of two particular absorption features of a molecule, temperature is determined. A TDL sensor was designed, built, and tested in various conditions in the flame and a static cell for CH4 and H2O concentration and for temperature measurement. For high sensitivity, Wavelength Modulation Spectroscopy (WMS) was used. In general, quantification of WMS, particularly in combustion systems, requires detailed spectroscopic information of all absorption features probed by the sensor. Typical absorption features are often overlapped, and therefore measurement of spectroscopic information for individual transitions may be very difficult, or sometimes impossible. The lack of detailed spectroscopic information for the near-infrared transitions of interest, combined with the complexity of WMS technique itself, are the main issue for quantification WMS in combustion systems. In this dissertation, these problems and their solutions are discussed. Following a detailed theoretical study of WMS, new approaches and techniques for concentration and temperature measurement in combustion systems are developed.