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Near-infrared diode laser absorption spectroscopy with applications to reactive systems and combustion control

Posted on:2008-10-06Degree:Ph.DType:Dissertation
University:Stanford UniversityCandidate:Li, HejieFull Text:PDF
GTID:1448390005451746Subject:Engineering
Abstract/Summary:
Near-infrared tunable diode laser (TDL) absorption spectroscopy based on H2O provides a sensitive method for reliable detection of gas parameters. Absorption lineshapes of selected H2O transitions are recorded in a heated cell to determine spectroscopic data to enable quantitative measurements. Strong collisional-narrowing effects are observed in the Ar-broadened H2O spectra due to the relatively weak collisional broadening. Temperature dependences of the Ar-broadening, -narrowing, and -shift coefficients are determined using Galatry fits to the absorption data. A TDL sensor for temperature and H2O based on direct-absorption spectroscopy is developed and demonstrated in coal-fired power plants.; WMS measurements at high pressures require large modulation depths for optimum detection of blended spectra. Real diode laser performance, including the phase shift between frequency modulation and intensity modulation and nonlinear intensity modulation, becomes important. These parameters are incorporated into an improved WMS model. The influence of these laser effects is investigated via measurements on H2O transitions near 1388 nm.; A fast-response (100 kHz) TDL sensor is developed for studies of combustion chemistry in shock tubes. Temperature is determined from the ratio of fixed-wavelength laser absorption of two H2O transitions near 7185.60 and 7154.35 cm-1. WMS is employed with 2f detection to improve the sensitivity. Normalization of the 2f signal by the 1f signal is used to remove the need for calibration and minimize interference.; A simple gasdynamic model called CHEMSHOCK is developed to predict gas properties behind reflected shock waves with significant energy release. CHEMSHOCK is based on combining constant-volume reaction with isentropic adjustment to measured pressure for a control mass of gas mixture in infinitesimal time steps. The model is validated with 1-D reacting computational fluid dynamics (CFD) calculations, and then compared to the measurements by the TDL sensor. The computational time for CHEMSHOCK is significantly reduced relative to the CFD model.; Detailed experiments are conducted to optimize the sensor position for thermoacoustic instability and lean blowout (LBO) detection in a swirl-stabilized combustor using a real-time TDL temperature sensor. The intensity of low-frequency fluctuations is used to detect the proximity to LBO and as a control variable for active LBO suppression without knowing the LBO equivalence ratio limit.
Keywords/Search Tags:Diode laser, Absorption, H2O, TDL, Spectroscopy, LBO, Detection
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